ARTICLE IN PRESS

Physica B 401–402 (2007) 433–436
www.elsevier.com/locate/physb

Optical, electrical, and X-ray-structural studies on Verneuil-grown
SrTiO3 single crystal: Annealing study
S. Mochizukia,, F. Fujishiroa, K. Shibataa, A. Ogib, T. Konyab, K. InababaDepartment of Physics, College of Humanities and Sciences, Nihon University, 3–25–40 Sakurajosui, Setagaya-ku, Tokyo 156–8550, Japanb
Rigaku Corporation, 3–9–12 Matsubara-cho, Akishima, Tokyo 196–8666, Japan

Abstract
In order to clarify crystal defect effects on the physical phenomena observed for a SrTiO3 single crystal grown by Verneuil method, theoptical density and photoluminescence spectra, complex impedance spectra, and crystal structure were fully studied for the as-grown
crystal boule. The as-grown crystal boule consists of a shell (which is colorless transparent and electrically good insulator) and a core(which is dark blue and has a high electrical conductivity (410–3 O–1cm–1), and a colossal static dielectric constant (4106) at room
temperature). The as-grown single crystal was then annealed at 973 K in an Ar–H2 gas stream. With the progressing of annealing, the asgrown single crystal becomes colorless-transparent insulator and the static dielectric constant decreases down to approximately 300 atroom temperature. The X-ray crystallographic studies indicate that the crystallinity is almost independent of the annealing, while thedielectric property is considerably affected by the annealing. A model on the basis of a (Ti3+–oxygen vacancy) complex defect is
proposed for explaining the observed properties of SrTiO3.
r 2007 Elsevier B.V. All rights reserved.
Keywords: Oxygen defect; Non-stoichiometry; X-ray topogragh; Photoluminescence; Quantum paraelectricity

1. Introduction
There has been a considerable number of experimental
research on the quantum paraelectricity [1,2], photoexcitation [3–5] and photo-induced phenomena [3,5] in
SrTiO3. Like cases for other transition metal oxides, a
SrTiO3 single crystal contains crystal defects which affectconsiderably its physical properties. For example, we havefrequently observed specimen-dependent-static dielectric
constant, photoluminescence (PL) spectra in its shape andintensity. Most of the recent workers use commerciallyavailable colorless transparent SrTiO3 crystals as specimens. The crystals are usually prepared as follows. Using
nominally pure SrTiO3 powder (approximately 99.99%) asa starting material, a SrTiO3 single crystal boule is grownby the Verneuil method. The obtained single crystal bouleis black at a glance. After annealing at about 1000 K in a
Corresponding author. Tel.: +81 3 5317 9771; fax: +81 3 5317 9771.

E-mail address: motizuki@physics.chs.nihon-u.ac.jp (S. Mochizuki).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2007.08.205

reducing gas atmosphere, the crystal boule becomes colorless transparent [6]. The annealed crystal boule is cut into
several parallel plates followed by mechanical polishing to
optical ﬂats, which is supplied to many workers as a
substrate crystal for thin ﬁlm deposition and as a specimen
for different studies. The purity of starting material powder,
the crystal growth apparatus, and the cutting and polishingprocesses depend on the crystal manufacture company.
Such differences are thought to be the main causes for theobserved specimen-dependent properties of real SrTiO3
single crystals. Crystal defects may also affect the photoelectric and photodielectric properties. For instance, photogenerated carriers (electrons and holes) tend to stay and toaccumulate around crystal defects, and they may display
apparent phenomena, which are not present in a perfect
SrTiO3 single crystal. Such specimen-dependent properties
and extrinsic phenomena have led many researchers astray.
In the present study, we study systematically the annealing
effects on the structural, electrical, and optical properties of
as-grown SrTiO3 crystal.

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S. Mochizuki et al. / Physica B 401–402 (2007) 433–436

2. Experimental

3

The optical density (OD) and PL spectra were measured
by two optical multi-channel analyzer systems. The opticalmeasurement systems are previously described in Ref. [3].
The complex impedance measurements were performed
with a LCR meter (Kokuyo LC–605). Advanced Thin ﬁlm
X-ray Diffractometer (Rigaku ATX–E) and Curved IP
2-Dimensional X-ray Diffractometer (Rigaku D/MAX
RAPID) were used for X-ray diffraction studies.

-Im Z (Ω)

40

T = 4.2 K
T = 77 K
T = 294 K

30

2

20

T = 294 K
1

10
0

0
0

10

20
30
Re Z (Ω)

40

0

1

2

3

Re Z (Ω)

3. Results and discussion
Fig. 1(a) shows the side view of a SrTiO3 single crystal
boule grown by the Verneuil method. The chemical purityof the starting material SrTiO3 powder was 99.99%.
Main impurities are Al, Ca, Ba, Fe, and Cu. Since Ba isvolatile at high temperature, the concentration of Ba in theboule is smaller than that in the starting material powder.
The boule looks black. When the SrTiO3 boule was cut
perpendicular to the growth axis [1 0 0] using a wire saw,
the boule tends to fracture. The brittleness is due to athermal stress whose magnitude depends on the crystal
growth condition. Fig. 1(b) shows one of the fractured
surfaces of the fragment, as a typical result. We can see acolorless transparent shell, which is electrically insulating(conductivity so10–8 O–1cm–1) and a dark blue core whichis conductive (sX10–3 O–1cm–1). The thickness of the shell
was about 2 mm for this boule. Both the degree of suchblue-color unevenness and the shell thickness also dependon the crystal growth condition. The blue color thins downwith leaving from the core center. The electrical conductivity also tends to decrease with leaving from the corecenter.
The complex impedance (Z ¼ Re Z+i Im Z) was measured for an as-grown SrTiO3 single crystal with Auelectrodes. It is found that both the real part of the
conductivity, Re s, and the real part of the dielectric
constant, Re e, tend to decrease with leaving from the coreFig. 1. The as-grown SrTiO3 single crystal boule: (a) side view and (b)fractured surface.

Fig. 2. Impedance complex plane plots for the as-grown SrTiO3 crystal atdifferent temperatures.

center. Fig. 2(a) shows the impedance complex plane plots
for a highly conductive as-grown SrTiO3 single crystal
plate at 294, 77, and 4.2 K. Each curve is not a perfectsemicircle but an elliptical arc without the zero intercept,
which implies that the impedance curve consists of somecomponents of arc. This suggests that the equivalentcircuit should be considered as two parallel capacitance
C–resistance R elements connected in serial at least.
Fig. 2(b) shows the expanded view of the high frequency
Z data near the origin. As seen in the left-hand side of thearc, the impedance plot shifts to the origin (the highfrequency limit) with decreasing temperature, growing an
extra arc at the high frequency side. The Re e and Re s
at 294 K are 1.2  106 and 1.3  10–2 O–1cm–1 at 1 kHz,
respectively. Similar colossal dielectric constant (X105) has
been already observed for oxygen-deﬁcient BaTiO3 which is
conductive [7]. The complex impedance plot has been carriedout for a colorless transparent SrTiO3 crystal at 294 K. This
crystal was obtained by fully annealing at 973 K in Ar–H2
mixture gas stream, which is almost the same crystal as thecommercially available one. The Re e and Re s of thiscrystal at 1 kHz are 342 and 1.6  10–9 O–1cm–1, respectively. The results obtained for the as-grown and fully
annealed SrTiO3 crystals indicate that the ellipticity ofthe Z curve relates to oxygen deﬁciency in the as-grown
SrTiO3 crystal.
The as-grown crystal plates cut from the core wereannealed at 973 K in a Ar–H2 mixture gas stream fordesired times. The silver was evaporated on both sides ofthe pate surfaces as electrodes. The cross-section andthickness of the specimens are 1 cm2 and 0.5 mm, respectively. At different stages of annealing, the complex
impedance was measured at room temperature in afrequency range 42 Hz–1 MHz. The results of R and C at
1 kHz are shown in Fig. 3.
These results suggest that the enhanced conductivity andcolossal dielectric constant are closely connected to crystal
defects. Through the annealing study, it is found that fully
annealed transparent crystal has different values of Re s
and Re e, depending on the original position in the crystal
boule. Incidentally, the ferroelectric-like hysteresis loop has
been clearly observed for the as-grown crystal and it comesARTICLE IN PRESS
109

4

107

3

Optical density

R (Ω)

S. Mochizuki et al. / Physica B 401–402 (2007) 433–436

105
103
101

a
as-grown

2
1
fully-annealed

10-1
104

0
3. 0

103

3

2. 5

2. 0

100
10-1
0

10

20
30
40
Annealing time (min)5060

Fig. 3. Annealing time dependence of the C and R for the as-grown
SrTiO3 crystal. The crystals were sputtered with Au electrodes.

to disappear at the early stage of annealing. Such strangechanges seem to be similar to the self-healing phenomenonof Re s during thermal reduction of a colorless transparent
SrTiO3 crystal [8]. It is interpreted the phenomenon to berelated to the redistribution of Sr and/or SrO complexes inclose neighborhoods at the surfaces and subsequent solidstate reactions leading to a chemical inhomogeneous
microscopic near-surface region which is characterized bynon-Perovskite phases.
The OD and PL spectra were also measured at different
temperatures as a function of annealing time: preliminary
results were reported in Ref. [6]. The spectra for the asgrown crystal and fully annealed one are shown in Fig. 4.
Below the energy gap of 3.2 eV, OD spectrum of the asgrown crystal shows many absorptions due to in-gap states
which can act as luminescence center. The oxygen defects(for example, Ti3+–oxygen vacancy complex) act as PL
center and some portion of them may remain even in fully
annealed crystal. In the complex, Ti3+ ions act as hole trap,
while oxygen vacancies act as electron trap. Thus, complex
defects are responsible for the Stokes-shifted greenish white
luminescence observed in SrTiO3 crystal [3].
Using a 3.8 eV laser light, we have measured thephotoenhancement effect on the complex impedance at
4.2 K for the as-grown crystal and different annealed single
crystals. Two Au electrodes with a gap of about 1 mm weresputtered on the one side of each crystal surface. The
applied voltage between the surface electrodes was 3 V. The
laser light ﬂuence was at least 1.2 W/cm2. The as-grown
crystal and crystals annealed for less than 2 h in Ar–H2 gas
stream did not exhibit any enhancement of the capacitanceby light irradiation, as shown in Fig. 5(a). However, after asubsequent annealing at 973 K in air for 12 h, this crystal
showed considerable photoenhancement, as shown in

1. 0

b
10 K

288 K

2

101

1. 5

as-grown

102

PL intensity (arb. units)

C (nF)

435

1

0
2

c

fully-annealed
12 K

1
297 K

0
3.2

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

1.8

Fig. 4. The OD and PL spectra of the as-grown and fully annealed
SrTiO3 crystals.

Fig. 5(b). A result for the fully annealed crystal is shownin Fig. 5(c). The subsequent annealing in air may enhancephotocarrier accumulation around crystal defects (forexample, Ti3+–oxygen vacancy complex).
The X-ray ﬂuorescence spectroscopic analysis has shownthat the oxygen content of as-grown crystal is smaller by
0.2% than that of fully annealed crystal. The X-ray
diffraction pattern, rocking curve, X-ray reﬂectivity, and
X-ray topograpgh were measured for the as-grown andfully annealed crystals. The lattice parameters of the both
crystals are almost same, which are 0.390498 and
0.390497 nm, respectively. The half-widths of (0 0 5) lineof the both crystals are also almost identical, 0.05011 and
0.04981. Non-Perovskite line, for instance, Ruddlesden–
Popper phases Srn+1TinO3n+1 (n ¼ 1–5) [9] are notobserved.
Figs. 6(a) and (b) are the X-ray topographs of the asgrown crystal and fully annealed one, respectively. These
topographs were obtained by the o scan around the centerof the (1 2 3) diffraction line. The o value is given by each
topograph. Although the o dependence (uneven coloring)
seen in the topographs indicates the structural inhomogeneity at their crystal surfaces, the crystallinity is almost independent of annealing. The two-dimensional

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S. Mochizuki et al. / Physica B 401–402 (2007) 433–436

436

103
102
101

annealing conditionat 973K in Ar-H2 gas for 2h

100

C (nF)

103
102

annealing condition
Fig. 6. X-ray topographs of the as-grown SrTiO3 crystal (a) and fully
annealed SrTiO3 crystal (b). The o value is given by each topograph.

at 973K in Ar-H2 gas for 2h101+ 973 K in air for 12 h100

Acknowledgements

103
fully-annealed102
UV light

101
100
102

103

104
Frequency (Hz)

105

106

Fig. 5. Photoenhancement of the C of different SrTiO3 crystals: (a)crystal annealed at 973 K in Ar–H2 gas stream for 2 h, (b) (a)+annealed
at 973 K in air for 12 h, (c) the fully annealed crystal. Solid circle and open
circle denote the C with and without UV light, respectively.

rocking-curve mapping study also proves that the crystallinity is almost independent of the annealing, regardless ofremarkable differences on the electrical and optical properties. Namely, the annealing decreases oxygen defect innumber and reduces microscopic semiconducting domain
in both number and size. The Maxwell–Wagner relaxation[10] at the interfaces between an insulating stoichiometricdomain and a semiconducting non-stoichiometric one isthus suppressed by annealing, which extinguishes thecolossal dielectric constant.

This work was partially supported by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education,
Science, Sport Culture, and Technology, Japan. This work
is supported by an Interdisciplinary General Joint Research Grant (Nihon University). This work is also
partially supported by a Research Grant from the Collegeof Humanities and Sciences (Nihon University).
References[1] K.A. Müller, H. Burkard, Phys. Rev. B 19 (1979) 3593.
[2] K.A. Müller, W. Berlinger, E. Toatti, Z. Phys. B 84 (1991) 277.
[3] S. Mochizuki, F. Fujishiro, S. Minami, J. Phys.: Condens. Matter 17(2005) 923.
[4] T. Hasegawa, M. Shirai, K. Tanaka, J. Lumin. 87–89 (2000)
1217.
[5] S. Mochizuki, S. Minami, F. Fujishiro, J. Lumin. 112 (2005) 267.
[6] S. Mochizuki, F. Fujishiro, K. Ishiwata, K. Shibata, Physica B 376(2006) 816.
[7] J. Yu, P.F. Paradis, T. Ishikawa, S. Yoda, Appl. Phys. Lett. 85 (2004)
2899.
[8] K. Szot, W. Speier, R. Carius, U. Zastrow, W. Beyer, Phys. Rev. Lett.
88 (2002) 075508.
[9] K. Szot, W. Speier, Phys. Rev. B 60 (1999) 5909.
[10] R. von Hippel, Dielectrics and Waves, MIT Press, Cambridge, MA,
1954.

ARTICLE IN PRESS

Physica B 401–402 (2007) 426–429
www.elsevier.com/locate/physb

Photo-induced defects of metal oxides: MgO and rutile TiO2
S. Mochizukia,, F. Fujishiroa, A. Iinoa, K. Shibataa, H. Yamamotoba

Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japanb
Department of Electronics and Computer Science, College of Science and Technology, Nihon University, 7-24-1 Narashinodai,
Funabashi-shi, Chiba 274-8501, Japan

Abstract
When MgO and rutile TiO2 are irradiated with ultraviolet laser light (l ¼ 325 nm, hn ¼ 3.81 eV) in vacuum, their photoluminescence
spectra change in intensity and/or in spectral structure with increasing irradiation time. By irradiating with the same laser light at room
temperature in O2 gas, their original photoluminescence re-appear. Such reversible phenomena are results of photo-induced oxygen
desorption and adsorption at surfaces. The mechanism and dynamics of the observed photo-induced spectral transitions are discussed on
the basis of the exciton theory.
r 2007 Elsevier B.V. All rights reserved.
Keywords: Photo-induced phenomena; Oxygen defect; Exciton instability; Oxygen desorption; Photoluminescence

1. Introduction
The photo-induced oxygen desorption and adsorption of
metal oxides have attracted much interest, since these
phenomena may be used for a crystallinity improvement
and for searching new functional materials. For 10 years,
we have studied these phenomena for different metaloxides, using the bulk single crystal, ceramics, ﬁlm andnanocrystal specimens [1–7]. When metal oxides areirradiated with ultraviolet (UV) laser light in vacuum,
their photoluminescence (PL) spectra change in intensity
and/or in spectral structure with increasing irradiation time
tir. After removing the laser light, these PL properties are
retained for long times at room temperature under room
light, regardless of any changes of atmosphere. By
irradiating with the same laser light at room temperaturein O2 gas, their original PL re-appear. Some of the results
are collectively shown in Fig. 1 for BaTiO3 single crystal,
Eu2O3 ceramics, KTaO3 single crystal, SiO2 glass, Sm2O3
ceramics, SrTiO3 single crystal, anatase TiO2 ceramics,

Corresponding author. Tel./fax: +81 3 5317 9771.

E-mail address: motizuki@physics.chs.nihon-u.ac.jp (S. Mochizuki).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2007.08.203

Y2O3 ceramics and ZnO ceramics. All the measurements
were carried out at room temperature. The tir (in min) oflaser light (l ¼ 325 nm, hn ¼ 3.81 eV) is given by arrows
which indicate the direction of the spectral change. Eachoxide shown in this ﬁgure shows a similar spectral change
for bulk single crystal, ceramic, ﬁlm and nanocrystal
specimens. Such reversible phenomena are the results ofphoto-induced oxygen desorption and adsorption at
surfaces, which also creates/annihilates oxygen defects,
and they may well yield materials for white-light emitting
devices and erasable optical storage. The photo-induced
phenomena are observed also at low temperatures andtherefore they may be not phonon-assisted phenomena but
purely electronic ones. Incidentally, the incident photon
energy (hn ¼ 3.81 eV) is smaller than the band gap energies
of Eu2O3, SiO2 glass, Sm2O3 and Y2O3. It indicates that
there are other electronic oxygen desorption and adsorption processes other than interband electronic excitation inbulk. Recently, many workers have been re-considering thephoto-induced oxygen desorption from MgO surface
taking into account the surface atomic and electronic
structures in detail [8]. Recently, we observed remarkable
photo-induced spectral change at room temperature undera 3.81 eV-laser-light irradiation. In the present paper,
results of MgO are presented comparing with those ofrutile.

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S. Mochizuki et al. / Physica B 401–402 (2007) 426–429

1

SiO2 glass
90 min0
KTaO3

1
210 min01

anatase TiO2
30 min0
ZnO

427

crystal (99.99% purity) was grown by the Verneuil method
at Nakazumi Crystal Co. Rutile ceramics was prepared bypressing rutile powder (Wako Pure Chemical Industry
Ltd., 99% purity) under a pressure of 0.2 GPa for 15 min atroom temperature followed by sintering at 673 K in air for
100 h. Irradiation is carried out with a continuous wave ofa He–Cd laser line (l ¼ 325 nm, hn ¼ 3.81 eV) with power
densities between 31 and 840 kW/m2. The illumination area
is between 2.54  10–6 and 1.13  10–8 m2. The He–Cd laserline also excites luminescence. Emitted light is dispersedand detected by using a grating spectrograph equippedwith a multichannel photodetection system.

3. Results and discussion

1

PL intensity (arb. units)

30 min0
SrTiO3

1
90 min0
1

Y2O3
60 min0
1

BaTiO390 min0

Figs. 2(a) and (b) show the 3.81 eV-laser-light-induced PL
spectral changes of MgO (1 0 0) single crystal at room
temperature in vacuum and O2 gas atmosphere, respectively.
The PL spectrum consists of at least two components
above 3.0 eV and around 2.5 eV. The intense PL due toCr3+ impurities is observed below 1.9 eV. Like for otheroxides shown in Fig. 1, the PL intensity of MgO single
crystal is enhanced by a 3.81 eV-laser-light irradiation in
vacuum, while the PL spectrum returns to the original one
under a 3.81 eV-laser-light irradiation in O2 gas. The
observed reversible photo-induced PL spectral change inbetween vacuum and O2 gas for MgO single crystal
indicates that oxygen desorption and absorption can occur
even by photons of lower energy than the bulk MgO bandgap (7.7 eV). The difference between the curve attir ¼ 0 min and that at tir ¼ 60 min in Fig. 2(a), is shownin Fig. 3.

Sm2O3
1

4

a

90 min

3

0
Eu2O3

2
90 min

0
3.2

3.0

2.8

2.6
2.4
2.2
Photon energy (eV)

2.0

1.8

Fig. 1. Photo-induced PL spectral changes of different oxides at room
temperature. The irradiation was carried out with a UV laser light(l ¼ 325 nm, hn ¼ 3.81 eV) in vacuum.

PL intensity (arb. units)

1

60
20
10
5
2
0.02
tir = 0 min

1
in vacuum
04btir = 0 min
0.02
2
5
10
20
60

3
2

2. Experimental
1

MgO single crystal (99.98% purity) was grown by thearc-melting method at Tateho Chem. Co. MgO ceramics
was prepared by pressing MgO powder (Wako Pure
Chemical Industry Ltd., 99.9% purity) under a pressureof 0.2 GPa at room temperature for 1 h followed bysintering at 1673 K in air for 100 h. Rutile TiO2 single

in O2 gas
0
3.2

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

1.8

Fig. 2. UV-laser-light-induced PL spectral changes of MgO single crystal
at room temperature: (a) in vacuum and (b) in O2 gas.

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S. Mochizuki et al. / Physica B 401–402 (2007) 426–429

As shown in this ﬁgure, a broad band around 2.5 eV,
accompanied by unresolved shoulder luminescence above
2.8 eV, appears. Coluccia [9] and Spoto observed theluminescence band at 3.3 eV under 5.7 and 4.6 eV excitations for MgO powder [10]. We have also carried out thesame photo-induced experiments for MgO ceramics. The
results are shown in Fig. 4.
It is noted that the PL intensity and speed of spectralchange for MgO ceramics are larger than those for MgO
single crystal. The same photo-induced experiments arealso carried out for rutile TiO2 single crystal, which isshown in Fig. 5.
The difference between the curve at tir ¼ 0 min and thatat tir ¼ 8 h in Fig. 5(a), is shown in Fig. 6.
The spectral components between 2.5 and 1.6 eV are dueto oxygen desorption. The spectral change is very similar
to that of anatase TiO2, which is shown in Fig. 1 andRef. [11]. It is found that the spectral changes shown inin vacuum2tir = 0 min
1h
8h

1PL intensity (arb. units)

428

0
1 min

2
in O2 gas

20 s

5 min

tir = 0 min

1
7h
60 min1
PL intensity

0
3.2

3.0

2.8

2.6

2.4
2.2
2.0
Photon energy (eV)

1.8

1.6

1.4

Fig. 5. UV-laser-light-induced PL spectral changes of rutile TiO2 single
crystal at room temperature: (a) in vacuum and (b) in O2 gas.
0
3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

1.8

Fig. 3. Difference spectrum between the curve at tir ¼ 0 min and that attir ¼ 60 min in Fig. 2(a).

6

PL intensity (arb. units)

4

2.8

2.6

2.4
2.2
2.0
Photon energy (eV)

1.8

1.6

1.4

Figs. 5(a) and (b) are hard to be induced by photons withenergies lower than the band gap (3.0 eV) of rutile.
Therefore, it may be concluded that oxygen desorption
from rutile surface can be induced only by the interband
electronic excitation. We have also carried out the same
photo-induced experiments for rutile TiO2 ceramics. Fig. 7
shows the photo-induced PL spectral change of rutile
ceramics in vacuum.
It is noted that, like the case of MgO, the PL intensity
and speed of spectral change of rutile ceramics are larger
than those of rutile single crystal.

tir = 0 min
0.02
2
5
10
20
40
60

4

3.0

Fig. 6. Difference spectrum between the curve at tir ¼ 0 min and that attir ¼ 8 h in Fig. 5(a).

0in O2 gas

-1

3.2

2

6

0

-2

60
40
20
10
5
2
0.02
tir = 0 min

in vacuum

1
PL intensity

3.2

2

4. Exciton model of photo-induced oxygen defect and photoenhanced white luminescence
0
3.2

3.0

2.8

2.6
2.4
2.2
2.0
Photon energy (eV)

1.8

1.6

1.4

Fig. 4. UV-laser-light-induced PL spectral changes of MgO ceramics atroom temperature: (a) in vacuum and (b) in O2 gas.

For MgO, conventional optical absorption measurements do not exhibit any absorption at photon energies
below 7.7 eV (which is the band gap of MgO). However, it
has been theoretically concluded that the surface oxygen

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S. Mochizuki et al. / Physica B 401–402 (2007) 426–429

PL intensity (arb. units)

3

2
10 min1tir = 0 min
0
3.2

3.0

2.8

2.6

2.4
2.2
2.0
Photon energy (eV)

1.8

1.6

1.4

Fig. 7. UV-laser-light-induced PL spectral change of rutile TiO2 ceramics
at room temperature in vacuum.

atomic structures (steps, kinks and corners) with low
crystal symmetry, which are regarded as clusters, create ingap states. The energy calculation of such surface clusters
gives the highest occupied molecular orbital (HOMO)thelowest unoccupied molecular orbital (LUMO) gap energy
smaller than 7.7 eV, for example, 4.5 eV [12]. Since a
3.81 eV (l ¼ 325 nm) laser photon can induce oxygen
desorption, there is a HOMO–LUMO gap of 3.81 eV.
The HOMO state is mainly localized on the surface
oxygen, while the LUMO state is delocalized over theadjacent Mg2+ ions. The optical transition from the
HOMO to the LUMO creates O–1 ion at surfaces. This
does not result in spontaneous oxygen emission. However,if the lifetime of the excited state is long and photon ﬂuenceis high, the second excitation of oxygen ion (O–1-O0+e–)
may occur and result in spontaneous oxygen desorption
and oxygen defect formation.
On the other hand, rutile TiO2 can be excited by the
3.81 eV-laser light. After such photoexcitation, some of thephoto-produced electrons and holes recombine directly,
displaying luminescence with a short lifetime. Anotherportion of photo-produced electrons may move itinerantly,
undergoing repeated collisions by phonons and trapping by
oxygen defects. When some portion of photo-produced
electrons and holes are trapped at distant traps, luminescence arises from accidental recombination of shallowly
trapped electrons with distant holes localized arounddefects, exhibiting a long-lasting luminescence. Possible
oxygen defects in rutile include a complex defect, Ti3+—(oxygen vacancy)—Ti3+, and a point defect, (oxygen
vacancy) +2e–. In the complex defect, Ti3+ ions act as

429

hole trap, while oxygen vacancies act as electron trap.
Toyozawa [13] proposed three types of symmetry-breaking
instabilities of excitons in the phonon ﬁeld. According tothis theory, if the electron and the hole have deformation
potentials of opposite sign, the decomposition into a pairof self-trapped particles occur. In other words, electron(e) and hole (h) combine, forming a trapped electron (Se)and a trapped hole (Sh) on adjoining sites, i.e. Se+Sh state.
In this case, the (e+Sh) and (h+Se) states are thought to beunstable, so that this exciton instability may result inoxygen desorption and oxygen defect formation at rutile
surface, like the case discussed for photo-induced breakingof Si–O bond in silica [14]. In both MgO and rutile TiO2,
the oxygen defects act as luminescence centres.
Acknowledgements
This work was partially supported by a Grant-in-Aid
for Scientiﬁc Research from the Ministry of Education,
Science, Sport Culture and Technology, Japan. This
work is supported by an Interdisciplinary General Joint
Research Grant (Nihon University). This work is also
partially supported by a Research Grant from the Collegeof Humanities and Sciences (Nihon University).
References[1] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl. Phys. Lett. 79(2001) 3785.
[2] S. Mochizuki, F. Fujishiro, T. Mochizuki, H. Yamamoto, Phys.
Status Solidi C 4 (2007) 518.
[3] S. Mochizuki, Physica B 340–342 (2003) 944.
[4] S. Mochizuki, H. Araki, Physica B 340–342 (2003) 969.
[5] S. Mochizuki, F. Fujishiro, S. Minami, J. Phys.: Condens. Matter 17(2005) 923.
[6] S. Mochizuki, S. Minami, F. Fujishiro, J. Lumin. 112 (2005) 267.
[7] F. Fujishiro, S. Mochizuki, J. Phys.: Conf. Ser. 21 (2005) 142.
[8] W.P. Hess, A.G. Joly, K.M. Beck, M. Henyk, P.V. Sushko,
P.E. Trevisanutto, A.L. Shluger, J. Phys. Chem. B 109 (2005) 19563.
[9] S. Coluccia, Stud. Surf. Sci. Catal. 21 (1985) 59.
[10] G. Spoto, E.N. Gribov, G. Ricchiardi, A. Damin, D. Scarano,
S. Bordiga, C. Lamberti, A. Zecchina, Prog. Surf. Sci. 76 (2004) 71.
[11] S. Mochizuki, T. Shimizu, F. Fujishiro, Physica B 340–342 (2003)956.
[12] P.E. Trevisanutto, P.V. Sushko, A.L. Shluger, K.M. Beck,
M. Henyk, A.G. Joly, W.P. Hess, Sur. Sci. 593 (2005) 210.
[13] Y. Toyozawa, Physica B+C 117&118 (1983) 23.
[14] A. Shluger, E. Stefanovich, Phys. Rev. B 42 (1990) 9664.

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current topics in solid state physics

Optical and dielectric studies on nanoparticles
and atomically-engineered surfaces of superparaelectric SrTiO3
Shosuke Mochizuki and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University,
3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Received 30 July 2006, revised 10 August 2006, accepted 10 August 2006
Published online 7 February 2007
PACS 68.35.-p, 77.84.Dy, 78.55.Kz
In order to find useful surface properties of SrTiO3 (STO) crystal, the photoluminescence and dielectric
properties of STO nanoparticles and STO bulk single crystal with nanometer steps have been measured at
different temperatures. The obtained results are compared with those obtained for STO bulk single crystal.

phys. stat. sol. (c) 4, No. 2, 515 – 517 (2007) / DOI 10.1002/pssc.200673302

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phys. stat. sol. (c) 4, No. 2, 515– 517 (2007) / DOI 10.1002/pssc.200673302

Optical and dielectric studies on nanoparticles
and atomically-engineered surfaces of superparaelectric SrTiO3
Shosuke Mochizuki* and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University,
3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Received 30 July 2006, revised 10 August 2006, accepted 10 August 2006
Published online 7 February 2007
PACS 68.35.-p, 77.84.Dy, 78.55.Kz
In order to find useful surface properties of SrTiO3 (STO) crystal, the photoluminescence and dielectric
properties of STO nanoparticles and STO bulk single crystal with nanometer steps have been measured at
different temperatures. The obtained results are compared with those obtained for STO bulk single crystal.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Strontium titanate (SrTiO3, STO) is a well-known quantum paraelectrics. STO bulkcrystal has a large dielectric constant [1], excellent optical transparency and remarkable photo-induced
effects [2, 3]. On the other hand, STO is also a well-known photocatalyst and the surface property has
attracted great attention. The nanosized particles and engineered nanosized-surface of STO are particularly interesting for the development of novel optoelectronic devices. We pointed out previously an important role of (Ti3+-oxygen vacancy-Ti3+) complex defects in the optical properties of STO single crystal[3, 4]. Recently, we have studied the photoluminescence and dielectric properties of STO nanoparticles
and STO single crystal with molecular layer steps on the surfaces. Here, we report the results.
2 Experimental The surface of a molecular-layer-step (MLS) STO single crystals were supplied by
Sinkosya Co. They consist of a flat terrace (200 nm in width) and molecular layer steps (0.4 nm inheight). STO nanoparticle specimens were prepared by compressing nominally pure STO nanoparticle
powder (99.9 % in purity and 50-90 nm in diameter), under 0.4 GPa for 10 min at room temperature.
They were then treated by 2.4 GHz microwave in air for 5 min. X-ray diffraction analysis shows that
STO specimens are a cubic perovskite phase STO. The experimental setup for photoluminescence measurements is presented in detail elsewhere [3].
3 Results and discussion Figure 1(a) shows the photoluminescence (PL) spectra of pristine STO
single crystal without any engineering at 13 K under different photoexcitation intensities. The photoexcitation is carried out with a 355 nm laser light from a Nd3+-YAG laser and the intensity (mJ/cm2) is givenby each curve. Under weak excitation, only a white luminescence around 2.45 eV is observed. With
increasing laser fluence, the white luminescence intensity increases monotonously and a near-band-edge
luminescence (NBE luminescence) around 3.23 eV appears. On the laser fluence further increasing up to
4 mJ/cm2, the white luminescence attains a maximum, while both the NBE luminescence and a broad
blue luminescence between the NBE luminescence and the white one become prominent. We have already clarified, for one-photon excitation PL, that the white luminescence arises mainly from oxygen*

Corresponding author: e-mail: motizuki@physics.chs.nihon-u.ac.jp, Phone: +81 3 5317 9771, Fax: +81 3 5317 9771

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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S. Mochizuki and F. Fujishiro: Optical and dielectric studies on nanoparticles

defects at the crystal surfaces [3], and the blue luminescence and the NBE luminescence are the freecarrier-enhanced defect luminescence and the free-carrier-enhanced band edge emission, respectively.
On laser fluence exceeding 4 mJ/cm2, the white luminescence intensity begins to decrease monotonously, while both the NBE luminescence band and the blue luminescence band continue to grow more,deteriorating the white luminescence band. Incidentally, we have found that the white luminescence intensity maximum appeared at 0.4 mJ/cm2 for the sintered STO powder (several hundreds micrometres in size)
compact. This suggests that most of the white-luminescence centres are at specimen surface [3]. The samemeasurement was carried out for the MLS STO single crystal. The result is shown in figure 1(b).
20

PL intensity (arb. units)

8

(a)

2

4.0 mJ/cm(b)2

1.4 mJ/cm

15

6

2.6

1

1.0 x 101041

1.4 x 10-2

7.7 x 10-2

8.1 x 10
5

2

9.8
5.2
-3

3.0 x 10
0

3.2

3.0

2.8

2.6

2.4

2.2

Photon energy (eV)

2.0

0

1.8

3.2

3.0

2.8

2.6

2.4

2.2

2.0

Photon energy (eV)

1.8

Fig. 1 The excitation light intensity dependence of the PL spectra of different STO single crystals: (a) Pristine
STO at 13 K, (b) MLS STO at 10 K. The excitation laser light wavelength is 355 nm.

(a)

2

2.5

6.8 mJ/cm

2.0
1.5

4.06

1.0

1.99

0.5

1.09
0.45

0.0

3.2

3.0

2.8

0.25
0.09

2.6

2.4

2.2

Photon energy (eV)

2.0

1.8

1.2

(b)
2

0.02 mJ/cm

(c)

1.0

PL intensity (arb. units)

PL intensity (arb. units)

3.0

Normalized PL intensity

As seen in this figure, the white luminescence intensity maximum appears at 1.4 mJ/cm2 which is smaller
than that (4.0 mJ/cm2) of the pristine STO single crystal. Clear white-blue luminescence colour change isobserved at an intensity of 8 mJ/cm2 with the naked eye. The white luminescence observed for both the
STO single crystals almost disappears above 40 K.
Figure 2(a) shows the photoexcitation intensity dependence of the PL spectrum of STO nanoparticles
at 10 K. Photoexcitation was carried out with a 355 nm laser light. In Fig. 2(b), after normalizing thespectra in intensity, the spectrum under intense excitation is compared with that under weak excitation.

0.8

10 K

0.6

2

6.80 mJ/cm

0.4

293 K

0.2

3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8

Photon energy (eV)

0.0

3.2

2.8

2.4

2.0

Photon energy (eV)Fig. 2 The excitation light intensity dependence of the PL spectrum of STO nanoparticle specimen at 10 K: (a),(b) comparison of the spectra under intense excitation with that under weak excitation, (c) temperature dependenceof PL spectrum.

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517

As seen from this figure, the white luminescence is merged in the blue luminescence band, which suggests that the white luminescence maximum intensity appears at lower excitation intensity than 0.02
mJ/cm2. The NBE luminescence is faintly seen at the high energy side of the blue luminescence as ashoulder. It is found that, at room temperature, the blue-luminescence intensity of the STO nanoparticles
is three times larger than that of the STO bulk single crystal. This suggests that the free carrier as anactivator of the blue-luminescence and/or the defect as a luminescence centre increase in number. Incidentally, a relatively high electrical conductivity (> 10-3 S/m) has been observed for the STO nanoparticle specimen. Figure 3(c) shows the PL spectra of the STO nanoparticles at 10 and 293 K. Unlike thepristine STO single crystal and MLS one, the temperature dependence of the blue luminescence is small.
As discussed in the previous paper [3], the one-photon-excitation PL of the pristine STO single crystal
is explained as follows. Most of the photo-generated holes may be deeply trapped at Ti3+ sites of thecomplexes. Unlike such holes, photo-generated electrons may move itinerantly, experiencing repeatedly
collisions by phonons and trapping by oxygen defects. When some portion of photo-generated holes and-electrons are trapped at distant complexes, the white luminescence arises from accidental recombinationof shallowly trapped electrons with distant holes localized around the defects, exhibiting a long-lasting
white luminescence. With increasing laser fluence, the numbers of photo-generated electrons and holes
increase and then the PL intensity may increase. At further increasing laser fluence, photo-generated
electrons and holes become diffused more rapidly inside crystal (bulk), and therefore the PL intensity
decreases. This is a reason why the white PL intensity maximum appears. In other word, this means thatmost of the defect luminescence centres ((Ti3+-oxygen vacancy-Ti3+) complex defects) are at the specimen surface. In the complex, the Ti3+ ions act as hole traps, while the vacancy tends to trap electron.
Since nanoparticle has large specific surface area, the STO nanoparticle specimen may exhibit a PL
intensity maximum at the laser fluence smaller than that of the bulk single crystal specimen. In addition
to these defect effects, it should be taken into the consideration that, at higher laser fluence, direct recombination of photo-generated carriers enhances the NBE and blue luminescence, deteriorating thewhite luminescence. Assuming the nonradiative mutiphonon relaxation, the observed small temperaturedependence of the blue luminescence intensity indicates that the height of an average potential barrier
separating the luminescent state from ground state in STO nanoparticles is small compared with that of
STO bulk crystals.
The dielectric constants (ε) of the three types of STO specimens were also measured with an impedance meter in a frequency range between 0.01 Hz and 3 MHz. The ε (1 kHz) of both the pristine STO
single crystal and the MLS STO one are about 300 at room temperature, while ε (1 kHz) of STO
nanoparticle specimens are between 340 and 300, regardless of a high porosity (about 50 %). Such theenhanced ε may be due to the polar ((Ti3+-oxygen vacancy-Ti3+) complex defects at nanoparticle surfaces
or due to the Maxwell-Wagner effect [5] contributed by the heterogeneity within nanoparticles, or due tothe Maxwell-Wagner effect contributed by the depletion layer between electrode and nanoparticle.
Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport Culture and Technology, Japan. This work is supported by an Interdisciplinary
General Joint Research Grant (Nihon University). This work is also partially supported by a Research Grant from theCollege of Humanities and Sciences (Nihon University), and by a Collaboration Research Grant from the Institute of
Natural Science (Nihon University).

References[1] K. A. Müller and H. Burkard, Phys. Rev. B 19, 3593 (1979).
[2] H. Katsu, H. Tanaka, and T. Kawai, J. Appl. Phys. 90, 4578 (2001).
[3] S. Mochizuki, F. Fujishiro, and S. Minami, J. Phys.: Condens. Matter 17, 923. (2005).
[4] S. Mochizuki, F. Fujishiro, K. Shibata, and K. Ishiwata, Physica B 376/377, 816 (2006).
[5] J. Yu, P. F. Paradis, T. Ishikawa, and S. Yoda, Appl. Phys. Lett. 85, 2899 (2004).

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current topics in solid state physics

Erasable photomemory phenomena in Eu2O3-, Y2O3and SrTiO3 nanoparticles at room temperature
Shosuke Mochizuki1, Fumito Fujishiro1, Teruyuki Mochizuki2, and Hiroshi Yamamoto3
1

Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan
2
Material Design Division, G. P. I. Japon, 271-12 Kunugida, Hachioji, Tokyo 193-0942, Japan3
Department of Electronics and Computer Science, College of Science and Technology,
Nihon University, 7-24-1, Narashinodai, Funabashi-shi, 274-8501, Japan
Received 30 July 2006, revised 25 September 2006, accepted 25 September 2006
Published online 7 February 2007
PACS 71.35.-y, 78.55.Kz, 78.67.Bf, 81.40.Tv
Reversible ultraviolet-laser-light-induced spectral changes have been observed for Eu2O3-, Y2O3- andSrTiO3 nanocrystal specimens at room temperature. Comparing with their bulk specimens, the speed of
the changes in the nanoparticle specimens tend to increase. The transitions are explained by photoinduced reduction and photo-induced oxidation at nanocrystal surfaces due to exciton instability.

phys. stat. sol. (c) 4, No. 2, 518 – 520 (2007) / DOI 10.1002/pssc.200673303

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phys. stat. sol. (c) 4, No. 2, 518– 520 (2007) / DOI 10.1002/pssc.200673303

Erasable photomemory phenomena in Eu2O3-, Y2O3and SrTiO3 nanoparticles at room temperature
Shosuke Mochizuki*, 1, Fumito Fujishiro1, Teruyuki Mochizuki2, and Hiroshi Yamamoto3
1

Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan
2
Material Design Division, G. P. I. Japon, 271-12 Kunugida, Hachioji, Tokyo 193-0942, Japan3
Department of Electronics and Computer Science, College of Science and Technology,
Nihon University, 7-24-1, Narashinodai, Funabashi-shi, 274-8501, Japan
Received 30 July 2006, revised 25 September 2006, accepted 25 September 2006
Published online 7 February 2007
PACS 71.35.-y, 78.55.Kz, 78.67.Bf, 81.40.Tv
Reversible ultraviolet-laser-light-induced spectral changes have been observed for Eu2O3-, Y2O3- andSrTiO3 nanocrystal specimens at room temperature. Comparing with their bulk specimens, the speed of
the changes in the nanoparticle specimens tend to increase. The transitions are explained by photoinduced reduction and photo-induced oxidation at nanocrystal surfaces due to exciton instability.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Several years ago, we found a reversible photo-induced spectral change in Eu2O3 atroom temperature [1, 2]. When Eu2O3 ceramic and film are irradiated with a 325 nm laser light in vacuum, their photoluminescence (PL) spectrum changes from a red sharp-line structure to a white broad
band, which can be clearly seen with the naked eye. After removing the ultraviolet (UV) laser light, thewhite PL state was stored for several years at room temperature under room light, regardless of any
changes of atmosphere. By irradiating with the same UV laser light at room temperature under O2 gas
atmosphere, the original red PL state re-appeares. Such reversible phenomenon may well yield materials
for white-light-emitting devices and erasable optical storage. Expecting the improvement in the speed ofsuch spectral change due to reduction of system size, we have studied the reversible UV-laser-lightinduced spectral change in several kinds of oxide nanoparticle specimens. In the present paper, we reportupon the results of Eu2O3 nanoparticles, Y2O3 nanoparticles and SrTiO3 nanoparticles.
2 Experimental Eu2O3 fine particles (several nanometers in size) were deposited on a roomtemperature silica substrate in O2 gas of a pressure (5.1 Pa) by laser ablation method with a pulsed Nd3+YAG laser light (wavelength λ = 355 nm). X-ray diffraction (XRD) shows that the as-deposited film
displays halo XRD pattern which is reproduced by broadening the B-form- and C-form Eu2O3 XRDlines. The particle size is several ten nanometers. Y2O3 and SrTiO3 nanocrystal specimens are preparedby pressing nominally pure Y2O3 nanocrystal powder (99.9 % in purity and 27 nm in average size) andnominally pure SrTiO3 nanocrystal powder (99.9 % in purity and 50-90 nm in diameter), respectively,
under 0.4 GPa for 10 min at room temperature. The XRD analysis shows that Y2O3 specimen and SrTiO3
one are a cubic phase Y2O3 and a cubic perovskeite phase SrTiO3, respectively. UV-laser-light irradiation is carried out with a continuous wave of a He-Cd laser line (λ = 325 nm) with a power of 30 mW.
The irradiated optic area is about 3 x 10-6 m2. The same laser light excites luminescence. Emitted light is*Corresponding author: e-mail: motizuki@physics.chs.nihon-u.ac.jp, Phone: +81 3 5317 9771, Fax: +81 3 5317 9771

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (c) 4, No. 2 (2007)519

dispersed and detected using a grating spectrograph equipped with an image-intensified photodetection
system. To avoid oil-vapour effects, the evacuation of the specimen chamber is carried out with an oilfree vacuum pumping system.
3 Results and discussion Figure 1(a) shows the luminescence of the Eu2O3 nanoparticle film which
was fully-irradiated with a 325 nm laser light in O2 gas of 1.01 x 105 Pa for 1h. The red colour is due tothe luminescent 5D0-7FJ (J = 0, 1, 2,…) transitions in Eu3+ ion. Under irradiation of the laser light, thespecimen chamber was then evacuated below 1.33 x 10-4 Pa and the red luminescence changes graduallyto white one. Figure 1(b) shows the luminescence of the same film irradiated with the same laser light invacuum for 1h. The measured spectral changes under the irradiation in vacuum and in O2 gas are shownin Fig. 1(c) and Fig. 1(d), respectively. The irradiation time tir is given by each curve in min.

(a)

(b)
in vacuum

PL intensity (arb. units)

in O2 gas

0.5

(c)

0.4

5

7

D0- FJ

0.5

in vacuum

0.4

0.3

7

D0- FJin O2 gas

0.3

0.2

0.2

tir = 30 min
10
2

0.1
0.0
3.2

5

(d)

2.8

2.6

1
4

0.1

0
3.0

tir = 0 min

2.4

2.2

Photon energy (eV)

2.0

1.8

0.0
3.2

30
3.0

2.8

2.6

2.4

2.2

Photon energy (eV)

2.0

1.8

Fig. 1 325 nm-laser-light-induced photoluminescence changes of Eu2O3 nanoparticle specimen: (a) and (b) show thecolour change, (c) and (d) show the spectral changes in vacuum and in O2 gas, respectively.

It is found that, after removing the 325 nm laser light, the white PL state is stored for more than several
years at room temperature under room light, regardless of any changes of atmosphere. As seen in Fig.
1(c), the PL due to the 5D0-7F2 transition decreases in intensity with irradiation time. The red-white luminescence change arises from a photo-induced detachment of oxygen near film surface. The photoinduced oxygen detachment may be accompanied both by the valence number change (Eu3+ → Eu2+) andby oxygen defect, as follows.
325 nm laser photon
Æ (Eu 2+ - oxygen vacancy - Eu 2+ ) complex +Eu 2 O3 in surface lattice ææææææ

1
O2 ≠
2

(1)

In the complex, the Eu2+ ions act as hole traps (Sh), while the oxygen vacancies behave as electron traps(Se). Toyozawa [3] proposed three types of symmetry-breaking instabilities of excitons in the phonon
field and, when the electron and the hole have deformation potentials of opposite sign, the decomposition
into a pair of self-trapped particles occur. Therefore, such exciton instability may result in oxygen desorption at Eu2O3 surfaces. The laser photon energy is close to the charge transfer exciton energy of
Eu2O3. In the present Eu2O3 specimens, the seeds of the observed photo-induced spectral change may be
the defects which had already existed at particle surfaces and can absorb 325 nm laser photons.
Figures 2(a) and (b) show the 325 nm-laser-light-induced spectral change of Y2O3 nanoparticle specimen in vacuum and O2 gas, respectively. The irradiation time is given by each curve in min. A reversible
photo-induced spectral change of the white PL has been clearly observed. The intense PL state createdunder 325 nm laser light irradiation is found to be stored for at least several days at room temperature,
regardless of any changes of atmosphere. The spectral changes are accompanied by some spectral redshift. It is found that the spectral change speed of Y2O3 nanoparticle specimen is about three times larger
than that of Y2O3 bulk crystal. Since the energy gap of Y2O3 crystal is about 5 eV [4] which is larger than
the incident photon energy (3.81 eV), the optical absorption due to surface defects should be considered.
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S. Mochizuki et al.: Erasable photomemory phenomena in serveral metal-oxide nanoparticles

PL intensity (arb. units)

450x103

400
350

in vacuum

tir= 60 min

450x10

(a)

3

400

41
30

350

300
250

20

250

200

0

10

150

20

100

2

50

5

200

5

100

2

300

10

150

(b)

tir= 0 min

in O2 gas

30
80

50

0

0

3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

Photon energy (eV)

3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

Photon energy (eV)Fig. 2 325 nm-laser-light-induced photoluminescence spectral change of Y2O3 nanoparticle specimen.

Quite similar photo-induced spectral changes have been observed for SrTiO3 nanoparticle specimen.
Figures 3(a) and (b) show the spectral changes of SrTiO3 nanoparticle specimen at room temperature.
PL intensity (arb. units)

60x10

3

in vacuum

50

tir= 260 min

(a)

80x10

3

in O2 gastir= 0 min

60

0.5

40
120
60

30

40

5

20

20
10
0

0
3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

Photon energy (eV)

0

(b)

1
2
5
60
3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

Photon energy (eV)Fig. 3 325 nm-laser-light-induced photoluminescence spectral change of SrTiO3 nanoparticle specimen.

It is noted that, unlike Eu2O3- and Y2O3 nanoparticles, the fully-photo-reduced SrTiO3 nanoparticles
became more luminescent by introducing O2 gas (compare the uppermost curve in Fig. 3(a) with theuppermost one in Fig. 3(b)). This suggests that a successive photoreduction-photooxidation treatment
removes nonradiative traps. As pointed out previously for SrTiO3 bulk single crystal [5], (Ti3+-oxygen
vacancy-Ti3+) complex defects at surfaces are luminescence centres. It is found that the speed of thespectral change is larger than that of SrTiO3 single bulk crystal [5].
Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport Culture and Technology, Japan. This work was supported by an Interdisciplinary
General Joint Research Grant (Nihon University). This work is also partially supported by a Research Grant from theCollege of Humanities and Sciences (Nihon University).

References[1] S. Mochizuki, T. Nakanishi, Y. Suzuki, and K. Ishi, Appl. Phys. Lett. 70, 3785 (2001).
[2] S. Mochizuki, Y. Suzuki, T. Nakanishi, and K. Ishi, Physica B 308-310, 1046 (2001).
[3] Y. Toyozawa, Physica B+C 117-118, 23 (1983).
[4] J. Robertson, J. Vac. Sci. Technol. B 18, 1785 (2000).
[5] S. Mochizuki, F. Fujishiro, and S. Minami, J. Phys.: Condens. Matter 17, 923 (2005).

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current topics in solid state physics

Structural and optical properties of as-deposited and annealed
SrTiO3 films prepared by laser ablation
Ken’ichiro Ishiwata and Shosuke Mochizuki
Department of Physics, College of Humanities and Science, Nihon University,
3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Received 20 June 2006, revised 18 July 2006, accepted 21 July 2006
Published online 2 November 2006
PACS 68.55.-a, 78.20.Ci, 78.55.Hx, 81.15.Fg, 81.40.Ef
In order to clarify the large band gap energy reported for thin SrTiO3 film (of which thickness is smaller
than 200 nm) and to examine the optical properties reported for bulk SrTiO3 crystals, different thick
SrTiO3 films were prepared by a pulsed laser ablation method. The morphology and structure of the asdeposited and annealed films were studied with the optical- and scanning electron microscopes and with
an x-ray diffraction method. The optical absorption and photoluminescence properties of the films arethen measured at temperatures between 6.5 and 300 K. A large band gap energy (about 4.0 eV) has been
observed even for thick amorphous SrTiO3 films (of which thicknesses are approximately 1.3 µm).

phys. stat. sol. (c) 3, No. 10, 3516 – 3519 (2006) / DOI 10.1002/pssc.200672127

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phys. stat. sol. (c) 3, No. 10, 3516 – 3519 (2006) / DOI 10.1002/pssc.200672127

Structural and optical properties of as-deposited and annealed
SrTiO3 films prepared by laser ablation
Ken’ichiro Ishiwata* and Shosuke Mochizuki
Department of Physics, College of Humanities and Science, Nihon University,
3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Received 20 June 2006, revised 18 July 2006, accepted 21 July 2006
Published online 2 November 2006
PACS 68.55.-a, 78.20.Ci, 78.55.Hx, 81.15.Fg, 81.40.Ef
In order to clarify the large band gap energy reported for thin SrTiO3 film (of which thickness is smaller
than 200 nm) and to examine the optical properties reported for bulk SrTiO3 crystals, different thick
SrTiO3 films were prepared by a pulsed laser ablation method. The morphology and structure of the asdeposited and annealed films were studied with the optical- and scanning electron microscopes and with
an x-ray diffraction method. The optical absorption and photoluminescence properties of the films arethen measured at temperatures between 6.5 and 300 K. A large band gap energy (about 4.0 eV) has been
observed even for thick amorphous SrTiO3 films (of which thicknesses are approximately 1.3 µm).
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Strontium titanate (SrTiO3, STO) crystal is a well-known perovskite-type paraelectricoxide with a large dielectric constant (300 at 300 K, 104 at 10 K) [1]. In view of the large dielectric constant, excellent optical transparency and remarkable photo-induced effects [2, 3], a thin STO film has
many important applications in the field of semiconductor device. So far, various growth techniques
have been applied to prepare thin STO films and several workers [4, 5] reported excessively large opticalband-gap energy Eg (3.4 eV < Eg < 3.8 eV) of thin STO films, while the Eg of a bulk STO crystal is approximately 3.2 eV. However, the cause of such a large Eg of the thin films has been not fully elucidated.
Recently, we have prepared different thick STO films and observed also excessively large Eg and intense
luminescence. The morphology, crystal structure and optical (absorption and photoluminescence) properties have been studied before and after thermal annealing. The obtained results are compared with thosereported for thin STO films.
2 Experimental STO films were prepared on different kinds of optically-flat substrates (silica glass,Au-coated silica glass, and C-cut sapphire) by a pulsed laser ablation method. The conditions of the laser
ablation are shown in Table 1.
Table 1 Conditions of the laser ablation.

laser
targetsubstrate temperature
atmosphere
substrates
spacing between target and substrate*

third-harmonic wave (λ = 355 nm) of Nd3+-YAG laser
as-grown STO single crystal grown by Verneuil methodroom temperature
vacuumsilica glass, Au-coated silica glass, and C-cut sapphire
110 mmCorresponding author: e-mail: ishiwata@phys.chs.nihon-u.ac.jp, Phone: +81 3 5317 9771, Fax: +81 3 5317 9771

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (c) 3, No. 10 (2006)

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After deposition, the half pieces of the as-deposited films were annealed at 873 K in air for 6 h. The
surface morphology of the films was studied with an optical microscope (OM) (Nano Scale Hybrid Microscope, KEYENCE VN-8000), an atomic force microscope (AFM) (Nano Scale Hybrid Microscope,
KEYENCE VN-8000) and a scanning electron microscope (SEM) (3D real surface microscope,
KEYENCE VE-9800). The structural data of the films were gathered at 300 K using an x-ray diffraction(XRD) system with Cu Kα1 radiation. The optical absorption, the transmittance and the reflectance of thefilms were measured using an optical multi-channel analyser (OMA). The photoluminescence (PL) andPL-excitation spectra were measured another OMA equipped with an image-intensified photodiode-array
detector. A He-Cd laser (λ = 325 nm) and a Nd3+-YAG laser were used as excitation sources. A Heclosed-cycle-refrigerator-type cryostat was used to control sample temperature.

(300)
(310)

(220)

(210)

(200)

(111)

1µm

(100)

annealed film

XRD intensity (arb. scale)(110)

3 Results and discussion The morphologic and structural studies were carried out at room temperatureon all of the fabricated films with the above-described three microscopes and the XRD apparatus. The
results were almost independent of the kinds of substrates. As typical results, the SEM images and XRD
patterns of the as-deposited and annealed STO films on the silica glass substrate are shown in
Fig. 1. As seen from these figures, the as-deposited film is a smooth-surface amorphous film, while theannealed film is a STO crystalline film consisting of grains. The grain sizes are characterized to be several hundreds nanometers by the SEM observations with a higher resolution. The lattice constant of theannealed film is estimated to be 0.390 ± 0.001 nm (which is close to the bulk crystal value) from the
XRD pattern. On the other hand, the XRD pattern of the as-deposited film can be reproduced by broadening and shifting the XRD pattern of the annealed film to lower angles, indicating that the average
atomic separation of the as-deposited film is increased. It is also seen that the thermal annealing process
makes many crevices in the uniform film layer and divides it into domains of several ten micronmeters
sizes, indicating that the strain in the film is almost released by creating crevices. Some large particles(whose sizes are between several hundreds nanometers and several micronmeters) are also observed.
They may be explosively spouted out from the STO target crystal during the laser ablation.

annealed film

as-deposited film

as-deposited film

1µm

10

20

30

40
50
2θ (degrees)

60

70

80

Fig. 1 The SEM images and X-ray diffraction patterns of the as-deposited and annealed STO films.

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3518

K. Ishiwata and S. Mochizuki: Structural and optical properties of as-deposited and annealed SrTiO3 films

The optical density (OD) and PL spectra of all the fabricated STO films were measured at temperatures between 6.5 and 300 K. As typical results, the OD spectra of the as-deposited and annealed STO
film on the silica glass and sapphire substrates at room temperature are shown in Fig. 2. The film thicknesses d estimated from the interference fringes are given by each curves. It is noted that the absorption
edge of the as-deposited film is blue-shifted to about 4.0 eV (which was estimated by plotting the squareof the OD against photon energy) and then it is red-shifted to about 3.3 eV (which is close to the Eg of
STO bulk crystal [6]) by thermal annealing. Such behaviour is independent of the kind of substrate material.

Optical density

2.0

STO/sapphire

STO/silica glass

1.5

as-deposited(d=1.32 µm)

as-deposited(d=0.84 µm)
annealed(d=0.93µm)

1.0annealed(d=1.43 µm)

0.5
0.0
4.5

4.0

3.5

3.0

2.5 4.5

4.0

Photon energy (eV)

3.5

3.0

2.5

Photon energy (eV)Fig. 2 The OD spectra of the as-deposited and annealed STO films on different kinds of substrates at 300 K.

Although it has been believed that the Eg of STO film shows a strong dependence on film thickness andthere is a critical film thickness (about 200 nm) [4, 5], the present results on the thick films clarify that,even if it is thick film, the STO film exhibiting a halo-XRD pattern has always large band gap energy.
We have measured the PL spectra of all the fabricated films at different temperatures between 7.4 and300 K. The PL spectra of the as-deposited and annealed STO films on the silica glass, Au-coated silica
glass and sapphire substrates at several temperatures are shown in Fig. 3. In this figure, the PL intensitiesof the as-deposited STO films on the silica glass are tripled. A 325 nm laser light was used for the excitation. It is found that the PL intensity is nearly proportional to the excitation light intensity. The timeresolved PL measurements showed that all the PL components have the lifetimes of about 10 ns.
3

35x10

PL intensity (arb. units)

30

200K

25

230K

STO/silica glass(as-deposited)
d= 0.84 µm

20
15

0
3
25x10

300K

STO/silica glass(annealed)
d=0.93 µm

300K

STO/Au(annealed)
d=1.95 µm

200K

10

2.8

190K

STO/sapphire(annealed)
d=1.43 µm

7.5K

7.5K
300K
3.2

7.5K

300K

200K

15

0

d=1.32 µm

7.4K

20

5

STO/sapphire(as-deposited)

7.4K

10
5

230K

STO/Au(as-deposited)
d=1.56 µm

7.4K
300K
2.4

2.0

3.2

300K
2.8

2.4

2.0

Photon energy (eV)

3.2

2.8

2.4

2.0

Fig. 3 The PL spectra of different STO film specimens at several temperatures.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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phys. stat. sol. (c) 3, No. 10 (2006)

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The observed PL spectral features are summarized as follows.
(1) The PL spectra of the as-deposited films consist of several ultraviolet (UV) components between 3.3
and 2.8 eV and several white luminescence components between 2.7 and 2.0 eV.
(2) The PL intensity is decreased by thermal annealing. Especially, the intensity decrease of the white
luminescence at low temperatures (< 8 K) is remarkable.
(3) The white luminescence components of the as-deposited STO films remain at room temperature,
while those of the annealed films almost disappears at room temperature.
(4) With increasing temperature from 7.5 K to about 180 K, the PL intensity monotonously decreases.
With further increasing temperature, the PL components above 2.8 eV begin to increase in intensity. The
PL intensity of each specimen attains each maximum at an intermediate temperature range between 190
and 230 K. On further increasing temperature to 300 K, all of the PL components decrease in intensity.
Similar UV luminescence and white one were observed for a bulk STO single crystal [7, 8], but thewhite luminescence of the bulk specimen almost disappears above 50 K. Pontes et al. [9] found an intense white luminescence band centred at 2.3 eV at room temperature in highly disordered STO films
and the luminescence disappeared with progressing thermal annealing (which induces crystallization).
The behaviour is very similar to the features (2) and (3). They tried to assign the white luminescence tonon-bridging oxygen defect centres. In the previous paper [8, 10], we pointed out the important role ofoxygen defects (for example, (Ti3+-oxygen vacancy-Ti3+) complex) for the white luminescence observedfor STO bulk single crystal. The temperature-enhanced luminescence at the intermediate temperature
range, which is given in the feature (4), may be due to some thermal release of trapped electrons. Comparing the considerably-blue-shifted optical absorption edge energies (see Fig. 2) with the excitation
laser photon energy (3.81 eV), a basic problem is raised about the PL of the as-deposited films. How
does a 325 nm (= 3.81 eV) laser photon excite the luminescence of the as-deposited films having large
band gap (> 3.8 eV)? A possible mechanism is the photoexcitation of mid-gap states due to structuraldefects and/or surface defects which are missed in a conventional optical transmission measurement. On
the other hand, since the interband excitation is effectively achieved in the annealed films by the 325 nm
laser photon, the PL components of the annealed films may be assigned to the recombination luminescence of the electron and hole trapped around crystal defects (which decrease in number by annealing).
Finally, though the blue-shifted energy gap of the film (which displays a halo XRD pattern) has beenfrequently attributed to the quantum-size effect or the amorphous structures, the present study suggeststhat the blue-shift originates mainly from large interatomic spacing in amorphous structure which contains a large number of oxygen defects. The PL spectra before and after annealing procedure clarify thatthe crystal defects in STO films act as the white-luminescence centres. More detailed analysis of theexperimental data shown in the present paper is now in progress. The results will be reported in a separate paper.

References[1] K. A. Müller and H. Burkard, Phys. Rev. B 19, 3593 (1979).
[2] H. Katsu, H. Tanaka, and T. Kawai, J. Appl. Phys. 90, 4578 (2001).
[3] T. Hasegawa, S. Mouri, Y. Yamada, and K. Tanaka, J. Phys. Soc. Jpn. 72, 41 (2003).
[4] D. Bao, X. Yao, N. Wakiya, K. Shinozaki, and N. Mizutani, Appl. Phys. Lett. 79, 3767 (2001).
[5] Y. Gao, Y. Masuda, and K. Koumoto, J. Korean Ceram. Soc. 40(3), 213 (2003).
[6] M. Cardona, Phys. Rev. 140, A651 (1965).
[7] T. Hasegawa, M. Shirai, and K. Tanaka, J. Lumin. 87-89, 1217 (2000).
[8] S. Mochizuki, F. Fujishiro, and S. Minami, J. Phys.: Condens. Matter 17, 923 (2005).
[9] E. M. Pontes, E. Longo, E. R. Leite, E. J. H. Lee, J. A. Varela, P. S. Pizani, C. E. M. Campos, F. Lanciotti, V.
Mastellaro, and C. D. Pinheiro, Mater. Chem. Phys. 77, 598 (2002).
[10] S. Mochizuki, F. Fujishiro, K. Shibata, and K. Ishiwata, Physia B 376/377, 816 (2006).

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current topics in solid state physics

Excitons in pristine silver iodide crystals
Shosuke Mochizuki and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan
Received 7 June 2006, revised 14 July 2006, accepted 17 July 2006
Published online 2 November 2006
PACS 61.72.Nn, 71.35.–y, 78.55.Hx
We prepared the film and bulk specimens of chemically-pure pristine silver iodide and their crystal structures were investigated by the x-ray diffraction method. The photoluminescence properties of these
specimens were measured at different temperatures under different excitation powers. The luminescence
due to the radiative decay of free exciton is observed at about 2.936 eV. Several specimen-dependent luminescence components (which increase sublinearly in intensity with increasing excitation power) arealso observed and they are discussed in the light of the x-ray diffraction data.

phys. stat. sol. (c) 3, No. 10, 3586 – 3591 (2006) / DOI 10.1002/pssc.200672144

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phys. stat. sol. (c) 3, No. 10, 3586 – 3591 (2006) / DOI 10.1002/pssc.200672144

Excitons in pristine silver iodide crystals
Shosuke Mochizuki* and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan
Received 7 June 2006, revised 14 July 2006, accepted 17 July 2006
Published online 2 November 2006
PACS 61.72.Nn, 71.35.–y, 78.55.Hx
We prepared the film and bulk specimens of chemically-pure pristine silver iodide and their crystal structures were investigated by the x-ray diffraction method. The photoluminescence properties of these
specimens were measured at different temperatures under different excitation powers. The luminescence
due to the radiative decay of free exciton is observed at about 2.936 eV. Several specimen-dependent luminescence components (which increase sublinearly in intensity with increasing excitation power) arealso observed and they are discussed in the light of the x-ray diffraction data.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Silver iodide (AgI) crystal has three phases designated as α, β and γ at normal pressure in the order of decreasing temperature [1], and it is a well-known superionic conductor and a solidbattery material. It is known that the dispersion of oxide fine particles into AgI enhances considerably
the ionic conductivity [2, 3]. A special basic problem about the AgI-based composites is raised. What are
the atomic and electronic structures of the AgI/oxide particle interfaces? Since constituent material AgI
shows clear exciton spectra even above room temperature [4] and the spectra are very sensitive to theenvironment in which excitons are moving, the optical spectra due to excitons may provide useful information about the structure of the AgI/oxide particle interfaces [3]. However, the information on the
structural and optical-spectral characteristics of pristine AgI remains somewhat fragmentary. Cardona [5]reported the optical absorption spectra of γ- and βAgI films. As seen from his spectrum (Fig. 1), thespectrum of γAgI film contains the spectral component of βAgI, and therefore the detailed structure nearabsorption edge of AgI remains to be clarified. Mochizuki and Ohta [4] measured the optical absorption
and photoluminescence (PL) spectra of AgI films at different temperatures from 7 K, through the superionic phase transition point (419 K), to 472 K, for the first time. The observed low-temperature exciton
spectra were still affected by crystal defects and impurities. To date, the detailed optical study on thewell-characterised γ- and βAgI crystals has been not reported. Although βAgI crystal can be easily
grown from a saturated solution, the crystal displays very complicated PL spectra. Very recently, wefabricated purer AgI crystals by the AgI-I2 composite technique and the melt-growth one, and studied
both their crystal structures and their PL properties. In the present paper, we report the results and clarify
the exciton nature of pristine AgI, taking account of their structural properties.
2 Experimental AgI powder of 99.999 % purity was produced by Alpha Aesar (U. S. A). Detected
impurities are B (1 ppm) and Si (4 ppm). The AgI powder was evaporated onto a low-temperature optically-flat sapphire substrate (R-cut) in vacuum to grow an AgI-I2 composite film, and the film was annealed at room temperature. The detail of the film fabrication has been reported previously [6]. Two*

Corresponding author: e-mail: motizuki@physics.chs.nihon-u.ac.jp, Phone: +81 3 5317 9771, Fax: +81 3 5317 9771

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phys. stat. sol. (c) 3, No. 10 (2006)

3587

types of bulk AgI crystals were prepared as follows. The powder was fully melted in a lidded alumina
crucible at 873 K for 12 h. The melt was cooled by the rapidly-cooling method with a twin roller. The
cooling speed was approximately 100 K/s. We call this specimen ‘the rapidly-cooled AgI’. Anothercooling method was also used. The melt was naturally cooled to room temperature in a furnace. We callthis specimen ‘the naturally-cooled AgI’. The obtained crystals were characterized by a conventional xray diffraction (XRD) method using a Cu Kα1 radiation. The PL spectra were measured by an opticalmulti-channel analyser consisting of a grating monochromator (focal length = 32 cm) and an intensified
diode-array detector (gate time > 5 ns). A Q-switched Nd3+–YAG laser (λ = 355 nm, pulse width < 5 ns,
repetition frequency = 10 Hz) was used as an excitation source. The PL excitation spectra were alsomeasured by varying the excitation-light wavelength. A closed-cycle helium refrigerator equipped with atemperature controller was used for changing specimen temperature between 10 and 300 K.
3 Results and discussion Figures 1(a) and (b) show the room-temperature XRD patterns of the naturally-cooled AgI and rapidly-cooled one, respectively. All of the diffraction lines observed for the rapidly-cooled AgI are assigned to those of the hexagonal AgI (βAgI). As seen in these figures, the XRD
pattern of the naturally-cooled AgI differs from that of the rapidly-cooled one. The (h0l) lines related tothe hexagonal stacking are weaken and broadened, which indicates the stacking disorder in the βAgI
structure. It is also noted that extra diffractions appear between 38 and 48 degrees, creating a broad background band. Several periodically-stacking faults (for example, 4H, 7H, 8H, 12H, 16H, 80H polytypes)of AgI were reported [7] for βAgI. The wurtzite structure of βAgI (2H) and zincblende structure of themetastable γAgI (3C) differ in the stacking sequence, and they can be regarded as different polytypes.

20

Fig. 13040

2θ (degrees)

50

(b)

20

40

2θ (degrees)50

β(203)β(202)β(103)β(102)

30

β(200)β(112), γ(311)β(201)β(100)β(110), γ(220)60

β(002), γ(111)β(101)

Intensity (arb. scale)β(112), γ(311)β(100)β(110), γ(220)Intensity (arb. scale)β(002), γ(111)

(a)

60

X-ray diffraction patterns: (a) the naturally-cooled AgI and (b) rapidly-cooled one.

So-called γAgI film displays almost the same XRD pattern observed for the naturally-cooled AgI, except
for weak β(100) line. Real γAgI film contains more or less stacking disorder, exhibiting traces of theβ(100) line and the extra diffractions between 38 and 48 degrees. It is said that γ phase is stabilized by
nonstoichiometry and strain due to a difference in the thermal expansion coefficients between the AgI
film and substrate. The naturally-cooled AgI is thin reddish, suggesting iodine deficiency, while therapidly-cooled AgI is thin yellowish. The naturally-cooled AgI is more iodine-deficient than the rapidlycooled one, becoming reddish, since the former AgI was heated at high temperature for longer time(about 10 h) than the latter one. This view agrees with the known result that a silver-rich AgI solution
crystallizes in the γ phase. Anyway, the naturally-cooled AgI should be designated as γAgI.

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3588

S. Mochizuki and F. Fujishiro: Excitons in pristine silver iodide crystals

We show first the optical spectra of different AgI films and look over the exciton structure of AgI.
Figures 2(a), (b) and (c) show the optical density and PL spectra of three different AgI films at about 12
K. The three films were evaporated on the sapphire substrates of different substrate temperatures Ts: (a)Ts = 200 K, (b) Ts = 109 K and (c) Ts = 300 K. The spectra shown in Fig. 2(a) correspond to the wurtzite
phase (βAgI). Near the band edge of AgI, sharp W1- and W2 exciton absorption bands, and a very sharp
luminescence band due to the radiative decay of the W1 exciton, appear. With introducing stacking disorder, the W1- and W2 exciton bands become broaden as shown in Fig. 2(b). With further introducing
stacking disorder, several polytype-structure-related Hj (j = 1, 2) and H’j (j = 1, 2) exciton absorptions,and two Z1 and Z2 exciton absorptions [8] also appear, as shown in Fig. 2(c). It is also noted that, withintroducing stacking disorder, new luminescence bands, G1 and G2, appear, merging with the W1 excitonband. The G1 and G2 luminescence bands may be assigned to the radiative decays of excitons trapped at
different stacking faults. As discussed previously [6, 8], three types of the spectral structures shown here
may arise mainly from iodine deficiency and/or thermal strain.

(a)

W1
W2

PL intensity, OD

(b)

W1
W2

H''1
H''2

H'1

OD

OD
G1

H'2

W1
W1

H1
H2

G2

PL

PL

PL

3.5

3.0

2.5 3.5

3.0

OD

Z2, W2

G1

G2

(c)

Z1, W1

2.5 3.5

3.0

Photon energy (eV)

2.5

Fig. 2 Optical spectra of different AgI films at about 12 K.

Figure 3 shows the PL spectra of the naturally-cooled AgI bulk at different temperatures between 9
and 288 K. The PL spectra at different temperatures above 178 K are also shown in the inset.
3

PL intensity (arb. units)

9K
4

17 K
37 K

3

57 K
78 K

2

1

0
3.1

98 K
118 K
139 K
158 K
178 K
228 K
3.0

350

PL intensity (arb. units)

5x10

178 K

300

198 K

250
200
150
100
50
0
3.1

2.9

228 K
258 K
288 K
3.0

2.9

Photon energy (eV)2.8

Photon energy (eV)

2.7

2.8

2.6

Fig. 3 Photoluminescence spectra of the naturally-cooled AgI at different temperatures.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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phys. stat. sol. (c) 3, No. 10 (2006)

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The PL spectrum at 9 K is characterized by an intense luminescence centred at 2.936 eV. This luminescence is on a skirt extending up to about 3.1 eV, and it is also accompanied by several shoulder components. The peak energy of 2.936 eV is higher than the Z1,2 exciton energy (2.919 eV) determined fromthe optical absorption spectrum of γAgI (zincblende structure). The peak energy is close to the Z1 exciton
energy and W1 exciton one determined from the optical absorption and reflection spectra [4–6, 8, 9].
With increasing temperature, the free exciton luminescence decreases in intensity, shifting toward 2.938
eV. With further increasing temperature above 78 K, the centre energy begins to red-shift and then it
attains 2.907 eV at 288 K. This energy is very close to the optical absorption peak energy of γAgI film at
4 K [4, 6]. Through the PL excitation measurements, it is found that the PL components below 2.9 eV
follow the free exciton formation. This suggests that the naturally-cooled AgI contains several kinds ofshallow exciton traps.
Figure 4 shows the PL spectra of the naturally-cooled AgI at 9 K under different excitation powers. In
the inset, the spectra obtained under I0 and 0.0057I0 are shown. The maximum power I0 is 50 µJ (~0.7
mJ/cm2). The PL intensity at 2.936 eV increases linearly with increasing excitation intensity up to 0.8I0,
while the PL components below 2.9 eV increase sublinearly in intensity. In the inset, after rescaling, thePL spectrum under the maximum excitation power is compared with that under the minimum one. This
indicates that the PL components below 2.9 eV arise from finite number of luminescence centres (forexample, crystal defects and residual impurities). On further increasing excitation power from 0.8I0, thePL intensity at 2.936 eV has become superlinearly increased and narrowed in spectral shape. In general,a superlinearly-dependent exciton luminescence is explained by the stimulated emission due to the exciton-exciton collision process [10, 11] or by the amplified spontaneous emission (so-called ASE), as reported for PbS films [12] and polymer film [13]. Since the observed lower-energy shift is too small to
explain the result by such exciton-exciton collision process, both the observed superlinear dependence
and the observed spectral narrowing may be assigned to the amplified spontaneous emission process.
The threshold pump intensity is estimated to be about 0.57 mJ/cm2. The skirt extending up to 3.1 eV
remains even at the highest photoexcitation and it may be ascribed to the bound exciton background due
to crystal imperfections (for example, stacking faults) or several combinations of exciton and phonons.
The LO and TO phonon energies of AgI are 15.4 and 13.2 meV, respectively [14]. In particular, thehigher energy portion of the skirt overlaps in energy of the W1 exciton absorption, polytype-related exciton absorptions [6, 8] and Z2 exciton absorption. Therefore, the higher energy portion (of which intensity
is decreased by reabsorptions) may consist of several kinds of free-exciton luminescence components
and it may be assigned to the radiative decays of these excitons. We carried out also the time-resolved
PL measurements.
400

300

I0

PL intensity (arb. units)

PL intensity (arb. units)

400
300

0.0057I0 (x 332)

200

200

0.81I00.54I0100

03.1

0.19I00.10I0

3.0

100
0
3.1

0.03I0

2.9

I0
3.0

2.9

2.8

Photon energy (eV)2.8

Photon energy (eV)

2.7

2.7

2.6

2.6

Fig. 4 Photoluminescence spectra of the naturally-cooled AgI at 9 K under different excitation powers.

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S. Mochizuki and F. Fujishiro: Excitons in pristine silver iodide crystals

It is found that the free exciton luminescence decays within several nanoseconds, while the PL components below 2.9 eV remain after 30 ns. This suggests that the PL components below 2.9 eV arise fromexcitons trapped at crystal defects or/and impurities.
Figure 5 shows the PL spectra of the rapidly-cooled AgI (βAgI) at different temperatures. A sharp luminescence appears at 2.930 eV, which is slightly lower than that of the naturally-cooled AgI. With
increasing temperature up to 118 K, the luminescence decreases in intensity and it shifts slightly toward
2.937 eV and, on further increasing temperature, the luminescence begins to red-shift, which suggests a
coexistence of the heavy-hole exciton and light-hole one.

PL intensity (arb. units)

8K
27 K

8

47 K
67 K

6

4

98 K

118 K
138 K
158 K

2

PL intensity (arb. units)

3

10x10

3000

158 K
2000

178 K
1000

258 K
288 K

228 K

0
3.1

3.0

2.9

2.8

Photon energy (eV)

178 K
228 K

0
3.1

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

Photon energy (eV)Fig. 5Photoluminescence spectra of the rapidly-cooled AgI at different temperatures

Like as the naturally-cooled AgI, the sharp luminescence is accompanied by several PL components
below 2.9 eV. With further increasing temperature, the PL components also decrease in intensity. Unlike
the naturally-cooled AgI, a broad luminescence appears at 2.35 eV. Through the PL excitation measurements, it is found that all the PL components below 2.9 eV follow the free exciton formation. Thissuggests that the rapidly-cooled AgI contains deeply exciton traps due to excessively high cooling rate.
3

14x10

I0

PL intensity (arb. units)

14x10

PL intensity (arb. units)

12
10
8
6
4

0.81I00.67I00.44I0Fig. 612
100.0057I0 (x 274)

8
6

I0

4
2
0

3.0

2.8

2.6

2.4

Photon energy (eV)0.19I02.2

0.03I0

2
0

3

3.0

2.8

2.6

Photon energy (eV)

2.4

2.2

Photoluminescence spectra of the rapidly-cooled AgI at 8 K under different excitation power.

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phys. stat. sol. (c) 3, No. 10 (2006)

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Figure 6 shows the PL spectra of the rapidly-cooled AgI under different excitation powers at 8 K. The
maximum intensity I0 is 50 µJ (~0.7 mJ/cm2). The 2.931 eV peak intensity increases linearly with increasing excitation power up to 0.7I0 and, on further increasing excitation power, it becomes to increasesuperlinearly, which suggests the onset of an amplified spontaneous emission. On the other hand, the PLcomponents below 2.9 eV increase sublinearly with increasing excitation power. Like as the naturallycooled AgI, the result indicates that the PL components below 2.9 eV arise from finite number of luminescence centres (crystal defects and impurities). It is noted that the PL spectra are almost independent of
the cooling rate, though their crystal structure and absorption spectra differ each other. We carried out
also the time-resolved PL measurements at 8 K for the rapidly-cooled AgI. It is found that the PL components above 2.8 eV disappear within several nanoseconds, while the 2.35 eV luminescence remains
after 20 ns.
In conclusion, we prepared different chemically-pure film- and bulk AgI specimens and we have studied their structures and optical properties. The followings are found.
(i) Depending on the preparation methods, AgI crystallizes in a wurtzite structure (βAgI) or zincblendelike one (γAgI) which is stabilized by stacking faults, leading to several kinds of polytype structures.
(ii) The spectral structures of optical absorption and the photoluminescence properties (temperature- andexcitation-power dependencies of the photoluminescence spectra) clarify the intrinsic exciton nature andcrystal-defect-induced exciton one in real AgI specimens. The stacking faults lead to the splitting of thedegenerated Z1,2 exciton state and the creation of new polytype-structure-related HJ and H’J exciton
states.
(iii) The enhanced luminescence and spectral narrowing have been observed at high excitation powers(> 0.56 mJ/cm2) for the AgI bulk crystals. It can be explained by the amplified spontaneous emission
process.
Finally, we would like to draw an attention to similar splitting of the Z1,2 exciton state for AgI microcrystal [15]. One of the causes for the Z1,2 splitting in AgI microcrystal is some crystal-field change at
the crystallite surfaces. The iodine deficiency may affect on the crystal structure (namely, crystal filed
around Ag+ ion) of AgI, as discussed in the present paper.
Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport Culture and Technology, Japan. This work is supported by an Interdisciplinary
General Joint Research Grand (Nihon University). This work is also partially supported by a Research Grant fromthe College of Humanities and Sciences (Nihon University).

References[1] P. V. Smith, J. Phys. Chem. Solids 37, 589 (1976).
[2] J.-S. Lee, S. Adams, and J. Maier, J. Electrochem. Soc. 147, 2407 (2000).
[3] S. Mochizuki and F. Fujishiro, J. Phys.: Condens. Matter 15, 5057 (2003).
[4] S. Mochizuki and Y. Ohta, J. Lumin. 87–89, 299 (2000).
[5] M. Cardona, Phys. Rev. 129, 69 (1963).
[6] S. Mochizuki and F. Fujishiro, J. Phys.: Condens. Matter 16, 3239 (2004).
[7] P. R. Prager and P. Krishna, Crystal Growth and Characterization of Polytype Structures (Pergamon, Oxford,1983), p. 451.
[8] S. Mochizuki, Physica B 308–310, 1042 (2001).
[9] M. Bettini, S. Suga, and R. Hanson, Solid State Commun. 15, 1885 (1974).
[10] C. Klingshirn, phys. stat. sol. (b) 74, 547 (1975).
[11] I. Tanaka and M. Nakayama, J. Appl. Phys. 92, 3511 (2002).
[12] V. Sukhovatkin, S. Musikhin, I. Gorelikov, S. Cauchi, L. Bakueva, E. Kumacheva, and E. H. Sargent, Opt.
Lett. 30, 171 (2005).
[13] M. D. McGehee, R. Gupta, S. Veenstra, E. K. Miller, M. A. Diaz-Garcia, and A. J. Heeger, Phys. Rev. B 58,
7035 (1998).
[14] G. L. Bottger and C. V. Damsgard, J. Chem. Phys. 57, 1215 (1972).
[15] S. Mochizuki and K. Umezawa, Phys. Lett. A 228, 111 (1997).
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current topics in solid state physics

Excitons in AgI–oxide particle composites: AgI–SrTiO3Fumito Fujishiro and Shosuke Mochizuki
Department of Physics, College of Humanities and Sciences, Nihon University, 3–25–40 Sakurajosui,
Setagaya-ku, Tokyo 156–8550, Japan
Received 7 June 2006, revised 8 July 2006, accepted 8 July 2006
Published online 2 November 2006
PACS 71.35.-y, 72.80.Tm, 78.55.Hx
We fabricated (x)AgI–(1–x)SrTiO3 fine particle composites over a wide composition range of 0–100 mol% AgI. It is found that the dispersion of SrTiO3 fine particles enhances the ionic conductivity and(0.6)AgI–(0.4)SrTiO3 has the highest ionic conductivity (1.68 x 10–4 S/cm which is two hundreds times incomparison with that of pristine AgI), for the first time. In order to clarify such ionic conductivity enhanced by dispersing SrTiO3 fine particles into AgI, the photoluminescence measurements were carriedout at different temperatures between 10 K and room temperature under different photoexcitation intensities, together with the structural and morphological studies (X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray fluorescence spectroscopy). The spectra consist of free exciton luminescence band and several broad luminescence bands due to the excitons trapped at crystal defects andresidual impurities. The free exciton luminescence band almost disappears at x = 0.6, which may suggest
the existence of considerable number of non-radiative traps (crystal defects) at the AgI/SrTiO3 particle interfaces. Such crystal defects may act as ionic pathways. The structural and morphological studies confirm the randomly-stacked γAgI/βAgI heterostructures at the AgI/SrTiO3 particle interfaces.

phys. stat. sol. (c) 3, No. 10, 3592 – 3597 (2006) / DOI 10.1002/pssc.200672112

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phys. stat. sol. (c) 3, No. 10, 3592 – 3597 (2006) / DOI 10.1002/pssc.200672112

Excitons in AgI–oxide particle composites: AgI–SrTiO3Fumito Fujishiro* and Shosuke Mochizuki
Department of Physics, College of Humanities and Sciences, Nihon University, 3–25–40 Sakurajosui,
Setagaya-ku, Tokyo 156–8550, Japan
Received 7 June 2006, revised 8 July 2006, accepted 8 July 2006
Published online 2 November 2006
PACS 71.35.-y, 72.80.Tm, 78.55.Hx
We fabricated (x)AgI–(1–x)SrTiO3 fine particle composites over a wide composition range of 0–100 mol% AgI. It is found that the dispersion of SrTiO3 fine particles enhances the ionic conductivity and(0.6)AgI–(0.4)SrTiO3 has the highest ionic conductivity (1.68 x 10–4 S/cm which is two hundreds times incomparison with that of pristine AgI), for the first time. In order to clarify such ionic conductivity enhanced by dispersing SrTiO3 fine particles into AgI, the photoluminescence measurements were carriedout at different temperatures between 10 K and room temperature under different photoexcitation intensities, together with the structural and morphological studies (X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray fluorescence spectroscopy). The spectra consist of free exciton luminescence band and several broad luminescence bands due to the excitons trapped at crystal defects andresidual impurities. The free exciton luminescence band almost disappears at x = 0.6, which may suggest
the existence of considerable number of non-radiative traps (crystal defects) at the AgI/SrTiO3 particle interfaces. Such crystal defects may act as ionic pathways. The structural and morphological studies confirm the randomly-stacked γAgI/βAgI heterostructures at the AgI/SrTiO3 particle interfaces.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction It is known that a dispersion of oxide fine particles into silver iodide (AgI) (which iswell known as a typical superionic conductor) enhances considerably its ionic conductivity at room temperature by several orders of magnitude, in comparison with that of pristine AgI. Theoretically, in thecomposites, cations (which are the component of the ionic crystal) are adsorbed at the oxide surfaces,
creating the space charge layer at the ionic conductor/oxide particle interfaces and enhancing the electrical conductivity and, moreover, the smaller particle enhances more ionic conductivity and the oxide
having the higher dielectric constant does also [1]. To date, many studies on the conductivity enhancement have been reported for AgI-based composites. In particular, AgI–γAl2O3 composites were widely
investigated. A polytype structure and stacking disorder in the AgI domain were confirmed by X-ray
diffraction (XRD) and differential scanning calorimetry, which also accounts for the observed electricalproperties of the composites [2]. A special basic problem about AgI-based composites is raised. What are
the atomic and electronic structures of the AgI/oxide particle interfaces? Fortunately, constituent material
AgI shows clear optical absorption and photoluminescence (PL) spectra due to excitons even at room
temperature [3–5]. Moreover the exciton behaviour is very sensitive to the environments in which excitons are moving, and therefore the exciton spectra may provide useful information about the microscopic
structure at the AgI/oxide particle interfaces. Previously, we fabricated several kinds of AgI–oxide fineparticle composites and observed considerable conductivity enhancement. In addition to measurementsof the XRD patterns and electrical conductivities, we measured the optical spectra due to excitons anddiscussed the pathway for the mobile Ag+ ion (for example, AgI–anatase TiO2 composites [6]). Inciden*

Corresponding author: e-mail: fumito@phys.chs.nihon-u.ac.jp, Phone: +81 3 5317 9771, Fax: +81 3 5317 9771

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phys. stat. sol. (c) 3, No. 10 (2006)3593

tally, strontium titanate (SrTiO3) is known as the quantum paraelectrics [7] and has a high dielectric
constant (300) at room temperature. In addition, SrTiO3 particles tend to contain oxygen defects (a point
defect as an oxygen vacancy and a complex defect represented as the Ti3+–oxygen vacancy–Ti3+) andshow a catalysis. Therefore, the dispersion of SrTiO3 particles into AgI may easily affect the electronic
and atomic structure at/near the AgI/SrTiO3 particle interfaces and hence we can anticipate a large conductivity enhancement for the AgI–SrTiO3 fine particle composites.
In present paper, we show the results of the structural, electrical and optical measurements for the
AgI–SrTiO3 composites and discuss the structure and exciton behaviour based on the data obtained.
2 Experimental The AgI–SrTiO3 composites, (x)AgI–(1–x)SrTiO3, were prepared by fully mixing
AgI powder (Alfa Aesar, 99.999%) with SrTiO3 nanoparticles (Aldrich, 99.5%) in air. The mixture wasuniaxially compressed under 0.2 GPa for 30 minutes at room temperature to fabricate pellets, and thenthe pellets were heated in the lidded crucible at 673 K for 12 h in air. The pristine SrTiO3 (x = 0) andpristine AgI (x = 1) pellets were prepared by the same method.
The composites were characterized at 295 K by XRD analysis with Cu Kα1 radiation. The morphology of the composites was analyzed by using a scanning electron microscope (SEM). Distribution mapsof Ag, I, Sr, Ti and O atoms in the composites were obtained by means of a SEM with an energy dispersive X-ray (EDX) fluorescence spectrophotometer. The electrical conductivity was measured by using
the complex impedance method with silver electrodes evaporated onto both sides of the pellets. The PL
spectra were measured by an optical multi-channel analyzer consisting of a grating monochromator (focal length = 32 cm) and an image-intensified diode-array detector (gate time ≥ 5 ns). A CW He–Cd laser(λ = 325 nm) and a Nd3+:YAG laser (λ = 355 nm, pulse width ≤ 5 ns, repetition frequency = 10 Hz) wereused as the excitation source. A closed cycle helium-gas refrigerator equipped with a temperature controller was used for changing specimen temperature between 10 K and 288 K.

Electrical conductivity (S/cm)

3 Results and discussion The electrical conductivity variation against AgI content is shown in Fig.
1. There is the conductivity enhancement, which shows a maximum (about 1.68 x 10–4 S/cm) at x = 0.6.
Since the conductivity for pristine AgI (x = 1) is about 8.04 x 10–7 S/cm, the conductivity at x = 0.6 isenhanced about two hundreds times in comparison with that of pristine AgI.
10

-3

10

-4

10

-5

10

-6

0

0.2

0.4

0.6

AgI content, x

0.8

1
1.0

Fig. 1 Electrical conductivity for different (x)AgI–(1–x)SrTiO3 composites at room temperature.

This conductivity enhancement may be caused by Ag+ ion conduction through the randomly-stacked
γAgI/βAgI heterogeneity at/near the AgI/SrTiO3 particle interfacial region [8]. However, (0.1)AgI–
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3594

F. Fujishiro and S. Mochizuki: Excitons in AgI–oxide particle composites(0.9)SrTiO3 shows the conductivity decrease. Since SrTiO3 is an electronic conductor, dopant AgI isthought to be an electronic-carrier scattering centre.
We performed observations of the fractured surfaces of (x)AgI–(1–x)SrTiO3 composites with a SEM
attached with an EDX fluorescence analyser. The low- and high-resolution-SEM images of the (0.6)AgI–(0.4)SrTiO3 are shown in Figs. 2(a) and (b), respectively, as typical results.

Fig. 2 Scanning electron microscope images of the (0.6)AgI–(0.4)SrTiO3: (a) low- and (b) high resolution.

(a)β(202)

(b)

x= 1

(AgI)

x= 0.8x= 0.6x= 0.4x= 0.2

Intensity (arb. scale)β(201)β(200)β(103)β(112),γ(311)β(110), γ(220)β(102)β(101)β(100)

Intensity (arb. scale)β(002), γ(111)

Rough-face blocks, smooth-face them and voids are seen in these figures. The rough-face blocks contain fine particles which aggregate to make larger domain. From the EDX measurement, we found that
AgI and SrTiO3 fine particles distributed uniformly over both the rough-face blocks and smooth-facethem. This indicates that the rough-face blocks are the fractured surfaces of the smooth-face blocks, andSrTiO3 particles are densely packed within the large AgI blocks, which makes the three-dimensionally
networks, as the AgI–anatase TiO2 composites [6].
Figures 3(a) and (b) show the XRD patterns for different (x)AgI–(1–x)SrTiO3 composites at 295 K.
All XRD patterns in Fig. 3(a) are normalized in intensity.

20

30

40

2θ (degrees)

(210)

(200)

(111)

(110)

(100)

x= 0.1x= 0(SrTiO3)

50

60

20

30

40

2θ (degrees)

50

60

Fig. 3 X-ray diffraction patterns at 295 K: (a) for different (x)AgI–(1–x)SrTiO3 composites and (b) of the(0.6)AgI–(0.4)SrTiO3.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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phys. stat. sol. (c) 3, No. 10 (2006)

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It is found that the full width at half maximum of the diffraction lines due to SrTiO3 hardly changes
with increasing AgI contents, which indicates that the average size of SrTiO3 particles is not changed in
the composites. Pristine AgI shows clear lines due to βAgI (wurtzite structure), and with decreasing x,
the (h0l) lines due to the stacking sequence of the hexagonal βAgI are weakened in intensity and widened. In small x region, the diffraction lines related to AgI become γAgI-like them. The diffraction profile is similar to that of Type II AgI crystal reported by Lee et al. [9]. Thus, it is found that, with increasing SrTiO3 content, an amount of γAgI component increases and some stacking faults of βAgI are created, in addition to the preferential orientation of the crystallites. It is also noted that a broad background(which is shown by a dotted curve ranging from 27 to 52 degrees) grows in middle x region, as shownfor (0.6)AgI–(0.4)SrTiO3 in Fig. 3(b). Unlike to the present AgI–SrTiO3 composites, several diffractionlines related to the 7H-polytype AgI were clearly observed for AgI–γAl2O3 composites [2] in the same2θ range. The 7H-AgI is partially composed of the stacking sequence of γ- and βAgI [10]. Although the
XRD pattern does not give the information about the microstructure at the interfaces but about the macroscopically-averaged structure, we can assume that the amorphous like AgI domain and stacking disordered AgI one exist in the AgI/SrTiO3 particle interfacial region. Such stacking disorder may induce
some randomly stacked heterogeneities consisting of βAgI crystallites and γAgI them, creating ionicpathways.
Figure 4 shows the PL spectra for different (x)AgI–(1–x)SrTiO3 composites at 10 K. The specimens
were excited with a 325 nm laser light (intensity is 0.3 W/cm2). All curves in this figure are normalized
at the each peak intensity. The rescaling factor is given by each curve.

PL intensity (arb. scale)

FE
x1

x= 1
(AgI)

x 0.36x= 0.8x8x= 0.6x 28

x= 0.4x 159

x= 0.2x 362

x= 0.1

x 110

3.0

2.8

2.6

x= 0
(SrTiO3)
2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 4 Photoluminescence spectra for different (x)AgI–(1–x)SrTiO3 composites at 10 K.

As seen from this figure, the sharp free exciton (FE) luminescence band due to the wurtzite phase AgI
appears at about 2.94 eV for pristine AgI. A broad luminescence band is also observed at 2.78 eV and it
may be assigned to the radiative decay of excitons trapped at crystal defects, because most of the pristine
AgI bulk specimens show specimen-dependent luminescence components in energies between 2.9 and2.6 eV [4, 11]. With adding SrTiO3 particles of 20 mol % (x = 0.8), the FE luminescence intensity decreases considerably. The broad luminescence band shifts toward lower energy side, enhancing intensity,and at least two shoulders are observed at the high energy side between 2.94 and 2.82 eV. At x = 0.6 (thiscomposite has a maximum ionic conductivity), the FE luminescence band almost disappears and a newwww.pss-c.com© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3596

F. Fujishiro and S. Mochizuki: Excitons in AgI–oxide particle composites

band begins to appear at 2.34 eV. The depression of the FE luminescence suggests a creation of the nonradiative traps at/near the AgI/SrTiO3 particle interfaces. A candidate for the non-radiative traps is anamorphous structure. Possible amorphous structure is the randomly-stacked γAgI/βAgI heterostructures
which is already shown in the XRD components ranging from 27 to 52 degrees (Fig. 3(b)). On the other
hand, since the luminescence intensity at 2.34 eV is higher than that of pristine SrTiO3 (note the rescaling factor), the origin of the luminescence band may be related to the AgI/SrTiO3 particle interfaces.
With further adding SrTiO3 particles, a new luminescence band appears at 2.92 eV (see the spectra of x =0.4 and 0.2). For pristine SrTiO3, the luminescence components centred at about 2.29 and 2.49 eV are
assigned to oxygen defect structures, for example (Ti3+–oxygen vacancy–Ti3+) complex. Another luminescence band is seen around 1.95 eV at x = 0.4 and 0.2. Since this luminescence band is not observedfor pristine AgI and pristine SrTiO3, it is also assigned to the radiative decay of the exciton trapped at the
AgI/SrTiO3 particle interfaces.
We measured the PL spectra for different (x)AgI–(1–x)SrTiO3 composites under more intense pulsed
355 nm laser light at different temperatures between 10 and 288 K. PL spectra at 10 K are shown in Fig.
5. Solid curves are the spectra measured under laser intensity of 0.5 mJ/cm2 (~105 W/cm2) and dottedcurves are the spectra measured by attenuating the laser light intensity.
FE

x1

2

x 86 (0.01 mJ/cm )PL intensity (arb. scale)

x 11
x 28

x= 1
(AgI)
2

x 547 (0.01 mJ/cm )x= 0.82

x 2691 (0.01 mJ/cm )x 75

x= 0.62

x 2695 (0.04 mJ/cm )x= 0.4x 1737x 16762

x 9864 (0.1 mJ/cm )x= 0.2
2

x 20379 (0.1 mJ/cm )x= 0.1
2

x= 0(SrTiO3)x 34435 (0.03 mJ/cm )x 2542
3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 5Photoluminescence spectra for different (x)AgI–(1–x)SrTiO3 composites at 10 K.

As shown in this figure, the FE luminescence (dotted curve) of pristine AgI is considerably broadened
and redshifted to 2.92 eV, with increasing laser intensity. The energy spacing between the FE luminescence peak energy (2.94 eV) and the broad luminescence band peak energy (2.92 eV) is about 20 meV,suggesting the exciton–exciton scattering to higher exciton states or inelastic scattering from the heavyhole exciton state to light-hole exciton one. Although the detailed exciton structure has not been clarified
at the present, a probable energy spacing between n = 1 and n = 2 excitons can be thus estimated 20
meV. On the other hand, all composites specimens (except for pristine AgI and pristine SrTiO3) exhibit
the luminescence components (which tend to saturate under higher laser intensity) between 2.92 and 2.70
eV. Therefore, it indicates that the luminescence centres responsible for these lower-energy luminescence components are limited in number and they arise from the radiative decay of trapped excitons at
the AgI/SrTiO3 particle interfaces.

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phys. stat. sol. (c) 3, No. 10 (2006)

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We summarize the structural, electrical and PL studies on the AgI–SrTiO3 composites, as follows. The
dispersion of SrTiO3 fine particles into AgI creates the randomly-stacked γAgI/βAgI heterostructures in
the hexagonal βAgI and also the Ag+ ion adions on SrTiO3 particle surfaces, enhancing Ag+ ion conductivity. Incidentally, it is known that an Ag+ ion-rich AgI solution crystallizes γAgI, while an Ag+ ion
deficient one does so βAgI. Therefore, the randomly-stacked γAgI/βAgI heterostructures and otheramorphous-like structures may be created at the interfacial region, providing the Ag+ ion conduction
pathway. It is generally known that the ionic conductivity of an amorphous phase is higher than that of acrystalline phase, because of the large free volume for the ionic migration. Hence, the AgI–SrTiO3 composites showing a higher ionic conductivity may contain a larger amount of the randomly-stacked
γAgI/βAgI heterostructures and amorphous-like structures which act as the non-radiative traps for AgI
free exciton. This is a reason why (0.6)AgI–(0.4)SrTiO3 (which has the maximum conductivity) hardly
shows FE luminescence.
Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport Culture and Technology, Japan. This work is supported by an Interdisciplinary
General Joint Research Grand (Nihon University). This work is also partially supported by a Research Grand fromthe College of Humanities and Sciences (Nihon University).

References[1] J. Maier, J. Phys. Chem. Solids 46, 309 (1985).
[2] J.-S. Lee, S. Adams, and J. Maier, J. Electrochem. Soc. 147, 2407 (2000).
[3] M. Cardona, Phys. Rev. 129, 69 (1963).
[4] S. Mochizuki and K. Umezawa, Phys. Lett. A 228, 111 (1997).
[5] S. Mochizuki and Y. Ohta, J. Lumin. 87–89, 299 (2000).
[6] S. Mochizuki and F. Fujishiro, J. Phys.: Condens. Matter 15, 5057 (2003).
[7] K. A. Müller and H. Burkard, Phys. Rev. B 19, 3593 (1979).
[8] F. Fujishiro and S. Mochizuki, Solid State Ionics (in preparation).
[9] J. -S. Lee, S. Adams, and J. Maier, J. Phys. Chem. Solids 61, 1607 (2000).
[10] B. L. Davis and L. R. Johnson, Crystal Lattice Defects 5, 235 (1974).
[11] S. Mochizuki and F. Fujishiro, phys. stat. sol. (c) 3(10), 3586 (2006); this conference.

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ARTICLE IN PRESS

Physica B 376–377 (2006) 816–819
www.elsevier.com/locate/physb

Defect-induced optical absorption and photoluminescence of Verneuilgrown SrTiO3 crystal
Shosuke Mochizuki, Fumito Fujishiro, Ken’ichiro Ishiwata, Kohei Shibata
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
In order to clarify the mechanism of the photoluminescence (PL) and other photo-induced effects observed for Verneuil-grown SrTiO3
crystal, a nominally pure dark-blue as-grown crystal, which has high DC conductivity ðs ’ 103 O1 cm1 Þ and high dielectric constant(e4106 at 1 kHz) at room temperature, was prepared and annealed at 973 K in a 90% Ar–10% H2 mixture gas stream. The opticaldensity (OD) and PL spectra were studied at the different stages of the annealing. In addition to the band gap absorption, at least eight
intense optical absorptions due to crystal defects are observed around about 2.9, 2.4, 2.2, 2.1, 1.7, 0.82 and 0.27 eV for the as-grown
crystal. With progressing annealing, these absorptions became weak and the crystal became a colorless-transparent insulator. We havefound a reasonable connection between the OD spectra and the PL ones.
r 2006 Elsevier B.V. All rights reserved.
PACS: 78.55.m; 78.40.q; 61.72.yKeywords: Optical absorption; Photoluminescence; Raman scattering; Crystal defects

The optical properties of strontium titanate (SrTiO3)
crystal have gained a lot of interest in recent years in
relation to photo-induced phenomena, for example, photoenhanced giant dielectric constant [1] and photoluminescence (PL). Up to present, most of the workers use thesingle crystal grown by the Verneuil method, so-called‘Verneuil-grown crystal’. The as-grown crystal (which isdark blue) is usually annealed at high temperature in
reducing atmosphere. The fully annealed crystals are cut
into parallel plate, polished and supplied to workers as a
stoichiometric crystal (which is colorless transparent) fromdifferent crystal manufacturing company. However, it iswell known that the Verneuil-grown crystal contains
chemical heterogeneity, different oxygen defects and many
dislocations with a density of approximately 109 cm2 [2,3].
One has to notice also about the purity of the starting
material SrTiO3 powder being at most 99.99%. Main
optical- and electrical-active impurity is known to be iron.
Although the greenish white PL band centered at 2.4 eV isCorresponding author. Tel./fax: +81 3 5317 9771.

E-mail address: motizuki@physics.chs.nihon-u.ac.jp (S. Mochizuki).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.12.204

well-known for low-temperature SrTiO3 crystal and it isdiscussed in connection with the suppression of ferroelectric instability by quantum ﬂuctuation [1]. However, anintense white luminescence has been observed even at room
temperature for amorphous SrTiO3 [4] and nanocrystal
SrTiO3 [5]. Incidentally, the PL intensity of the Verneuilgrown crystal is specimen-dependent. Recently, we havestudied the PL properties of the dark-blue as-grown andcolorless transparent SrTiO3 crystals [6]. In addition to thewhite luminescence band around 2.4 eV, we found a broadband around 2.9 eV, which displays a faint-blue luminescence, and a near-band-edge (NBE) emission at 3.2 eV at
low temperatures under intense photoexcitation, which areconsiderably enhanced for the as-grown crystal. It is an
important problem whether the luminescence bands at 2.4,
2.9 and 3.2 eV arise from intrinsic nature or not? In orderto clarify it, we have systematically studied both the opticaldensity (OD) and PL spectra at different stages ofannealing as-grown crystal.
A dark-blue Verneuil-grown SrTiO3 single-crystal
boule was cut into parallel plates with a size of
10 mm  10 mm  0.5 mm in thickness. After polishing

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S. Mochizuki et al. / Physica B 376–377 (2006) 816–819

their surfaces to optical ﬂat ones, the as-grown plate wasannealed at 973 K in a 10% H2–90% Ar mixture gas during
desired time. The as-grown crystal has high DC conductivity ðs ’ 103 O1 cm1 Þ and high dielectric constant(e4106 at 1 kHz) at room temperature. With progressive
annealing, the crystal becomes colorless transparent
insulator (s4108 O1 cm1 and e ’ 300). The OD spectrum measurements were ﬁrst performed at room temperature on the crystal recovered from different stages ofannealing. The results are shown in Fig. 1. The annealing
time (tan) is given by each curve. As seen in this ﬁgure, theas-grown crystal shows an intense background absorption
which increases gradually toward infrared (IR) region. As
superposing on the background absorption, there appear
several absorption peaks around about 2.9, 2.4, 2.2, 2.1,
1.7, 0.82 eV and un-resolved absorptions in the energy
regions between 3.1 and 2.6 eV, and between 1.9 and1.2 eV. The sharp OD increase at photon energies larger
than 3.1 eV is due to the band-gap absorption onset(3.2 eV) of cubic SrTiO3. With progressive annealing, theabsorption around 0.82 eV disappears rapidly and theabsorptions around 2.15 and 2.08 eV were merged in the
broad absorption band around about 1.7 eV, decreasing
the background absorption in intensity. It is noted that the
OD spectral change observed under annealing in a
reducing H2–Ar atmosphere progresses for the reverse ofthat observed under annealing transparent crystal in a
vacuum [7]. As pointed out by Szot et al. [2,3], the thermal
reduction creates not only homogeneous crystal defects(oxygen vacancies) but also non-Perovskite phases with the
Ruddlesden–Popper phases [(SrO)*(SrTiO3)n]. We have

d= 0.5 mm
T= 294 K
as grown

2

measured also the OD spectra in the IR region, excepting
the as-grown crystal of which IR transmittance is too low
to measure the OD. Combining the IR spectra with theUV–NIR spectra, the UV–IR OD spectra were obtained.
As a typical example, the OD spectra of the crystal
annealed for 11 min and fully annealed (commerciallyavailable) crystal are shown in Fig. 2. The spectrum for theannealed crystal ðtan ¼ 11 minÞ is expanded as shown in theinset. We can see clearly ﬁve absorption bands at photon
energies, 2.88, 2.36, 1.65, 0.27 eV, and weak phonon
absorptions below 0.27 eV. We have also measured the
OD spectra and electrical conductivity (s) of all the
specimens at different temperatures. With decreasing
temperature from room temperature to 6 K, both thebackground absorption intensity and s decrease, and otherabsorptions become sharpen. This indicates that thebackground absorption is closely related to free carriers.
However, it is not explained by conventional free carrier
absorption, since the absorption has peaked at 0.27 eV.
Similar absorption was also observed by heating thetransparent crystal in vacuum and it can be assigned to
the LO-phonon- or disorder-assisted intervalley scattering
between the two minima at the G and X points [8]. This
interpretation is compatible with the present results. Withdecreasing temperature, the fundamental absorption edge
shifts to higher energy side, and the 2.9, 2.4 and 1.6 eVabsorption band shift also to their higher energy sides. The
similar absorptions were also observed between 2.8 and1.8 eV for a stoichiometric SrTiO3 crystal by Szot et al [3].
It is also noted that several PL bands were observed for thetransparent SrTiO3 crystal in this photon energy region [6].
Wild et al. [9] studied pure and iron-doped SrTiO3 crystals
and found the optical absorptions at 2.92, 2.4, 2.1 and1.6 eV, and NIR absorptions in these crystals. They
assigned the 2.92 eV absorption and 2.1 eV one to theoptical transition from the Fe4+ donor to the conduction
band and the Fe3+–oxygen vacancy complex state to the
conduction band, respectively. They also assigned the 2.4
and 1.6 eV absorptions to crystal defects proper to SrTiO3.
Faughnan [10] identiﬁed the 2.9 eV peak as an absorption

3 min
3
0.5

9 min

1

15 min, 60 min

T= 294 K
2.88 eV

Optical density

Optical density

3817

11 minand fully-annealed211.65 eV
0.4

0.3

11 min
3.0

0
0

3.0

2.5

2.0
1.5
Photon energy (eV)1.0

Fig. 1. Room temperature OD spectra of the as-grown and annealed
SrTiO3 crystals.

3.0

0.27 eV

2.36 eV

2.5

2.5

2.0

1.5

1.0

0.5

2.0
1.5
Photon energy (eV)

11 min1.0

fullyannealed
0.5

Fig. 2. Room temperature UV–IR OD spectra of SrTiO3 crystal annealedfor 11 min and fully annealed crystal. The spectrum of the annealed crystal
for 11 min is expanded in the inset. The specimen thickness is 0.5 mm.

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S. Mochizuki et al. / Physica B 376–377 (2006) 816–819

818

by a Fe4+ ion in a cubic site, using the electron
paramagnetic resonance technique.
We have measured the PL spectra, PL excitation spectra
and time-resolved PL spectra of the crystal as functions oftir, excitation laser ﬂuence (Iex) and temperature. With
progressing tir, the PL intensity and the luminescence decay
time (td ) increase. The spectral shape observed for the asgrown and annealed crystals is similar to that observed forthe fully annealed crystal which is commercially available[6]. As a typical result, the PL spectra are shown in Fig. 3.
The measurements were carried out at 10 K withI ex ’ 1 mJ cm2 . To see the spectral change clearly, each
spectrum is appropriately rescaled in this ﬁgure. Each
rescaling factor is given by each curve. As seen in thisﬁgure, the as-grown crystal shows at least three PL bands.
The ﬁrst one is a sharp band around 3.22 eV which is close
to the band edge energy (3.27 eV) of SrTiO3. We call thisband ‘NBE luminescence band’. The second one is a broadand weak-background band extending from 3.2 to 2 eV. A
faint peak is seen at 3.11 eV in the weak broad band.
Although there are some structures in this band, we calltentatively this band ‘A luminescence band’. The third one
is an intense broad band around 2.45 eV. We call this band
‘B luminescence band’. The A and B luminescence peakenergies correspond well to the OD peak ones shown in
Fig. 1. This suggests that the luminescence centers
responsible for the A and B luminescence bands originate
from crystal defects. Since any luminescence is notobserved below 1.8 eV, the defects giving the OD peaks
at 1.67, 0.82 and 0.27 eV do not act directly as luminescence center. With progressive annealing, all the lumines-

cence bands increase in intensity. In particular, the B
luminescence increase is prominent, while the NBE
luminescence intensity is almost independent of annealing.
This indicates that non-radiative trap for the B luminescence decreases in number with increasing tir. The spectrumof the crystal annealed for 3 min is somewhat different
from others. The spectrum is dominated by the A
luminescence (which may be seen as faint blue luminescence), merging the NBE and B luminescence bands into
this band.
We have measured also the PL spectra of the as-grownand different annealed crystals, at about 10 K under
different Iex. As typical examples, the results for the asgrown crystal and the crystal annealed for 11 min are
shown in Fig. 4. With increasing Iex, the PL intensity of the
B luminescence increases for all the specimens. The B
luminescence intensity of the as-grown crystal attains amaximum at about 1 mJ cm2, while all of the annealed
crystals show their PL intensity maxima at about
4 mJ cm2, like the case for the colorless transparent crystal[6]. On Iex further increasing, the B luminescence intensity
decreases monotonically, while both the NBE luminescenceband and the A luminescence one continue to grow,without any saturation. This indicates that the NBE
luminescence and the A one arise from a great number ofcrystal defects. Different types of crystal imperfections
should be considered for real SrTiO3 crystal: for example,
surface textures, chemical heterogeneity (for example,
Ruddlesden–Popper phases), dislocations, Ti3+–oxygen
vacancy complexes and oxygen point defects. In the
Ti3+–oxygen vacancy complex, Ti3+ ion is one of the8Iex~ 1 mJ/cm2,T= 10 K

10.16 mJ/cm2 (x 1)

6fully-annealed

0.95 mJ/cm2 (x 20)

x10.25 mJ/cm2 (x 20)

x 2.3311 minx 2.259 min

x 3.65PL intensity (arb. units)

PL intensity (arb. scale)4

60 minas grown

0.03 mJ/cm2 (x 20)

2

0
8

6

3.99 mJ/cm2

11 min17.69 mJ/cm2

4
3 min

9.96 mJ/cm2

x 18.62

as grown
3.2

3.0

0.22 mJ/cm2

x 47.22.8

2.6
2.4
Photon energy (eV)

2.2

2.0

0
1.8

Fig. 3. PL Spectra of the as-grown and different annealed SrTiO3 crystals.

3.2

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

1.8

Fig. 4. Excitation-intensity dependence of the PL of the as-grown andannealed SrTiO3 crystals for different times.

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S. Mochizuki et al. / Physica B 376–377 (2006) 816–819

This work was supported by a Grant-in-Aid for
Scientiﬁc Research from Ministry of Education, Science,
Culture, Sport and Technology, Japan. This work was
supported by an Interdiciplinary General Joint Research

fully-annealed

T= 293K

5
fully-annealed
as grown

Raman intensity (arb. units)

candidate hole trap centers and the oxygen vacancy is oneof the candidate electron trap centers. When excitation
laser ﬂuence is low, photo-generated electrons and holes ata crystal surface drift inside the crystal, repeating trapping
at some shallow levels and releasing from them, while holes
may be rapidly trapped at crystal defects. Incidentally, no
phenomena related to free holes has been not observed.
Accidental electron–hole recombination at the crystal
defects may create the B luminescence. When Iex increases,
photo-generated electrons become energetic and, therefore,
most of the electrons diffuse rapidly inside the crystal(bulk), without recombining at surface. Therefore, theluminescence from the surface region begins decreasing in
intensity at a given laser ﬂuence. This may be a reason why
the 2.4 eV luminescence intensity maximum is observed on
the laser ﬂuence increasing. Generally, such crystal-defectconcentration at surface is higher than that inside thecrystal. The B luminescence from the inside of the crystal
may saturate in intensity with further increasing Iex.
Further information on the crystal defects inside the crystal(bulk) may be deduced from the two-photon-excitation PL
measurement in the transparent photon energy region.
Since the annealing enhanced the B luminescence, somekinds of crystal defects acted as non-radiative traps. The
defect structure of SrTiO3 is hard to be opticallydetermined and, therefore, it is not clear which type of
defect structure acts as such traps. On the other hand, since
the NBE and the A luminescence never show any
saturation at intense photoexcitation (410 mJ cm2), they
are hard to connect with the mechanism participating asmall number of iron impurity.
Finally, the room-temperature Raman-scattering spectra
at different stages of annealing are shown in Fig. 5. The
intensity increase with progressing annealing is due to thedecolorization. In the inset, the spectra of the as-grown andfully annealed crystal are compared after rescaling. As seen
below 40 cm1, the defect-induced mode appears clearly as
a Rayleigh-scattering tail in the as-grown crystal.

819

4
11 min
3
0

2

400

600

800

3 min

1

0

200

as grown

0

200

400
Raman shift (cm-1)

600

800

Fig. 5. Room-temperature Raman scattering spectra of SrTiO3 crystal at
the different stages of annealing.

Grant (Nihon University). This work was also supported
by a Research Grant for Science, College of Humanities
and Sciences (Nihon University).

References[1] K. Nasu, Rep. Prog. Phys. 67 (2004) 1607.
[2] K. Szot, W. Speier, Phys. Rev. B 60 (1999) 5909.
[3] K. Szot, W. Speier, R. Carius, U. Zastrow, W. Beyer, Phys. Rev. Lett.
88 (2002) 075508.
[4] F.M. Pontes, E. Longo, E.R. Leite, E.J.H. Lee, J.A. Varela, P.S.
Pizani, C.E.M. Campos, F. Lanciotti, V. Mastellaro, C.D. Pinheiro,
Mater. Chem. Phys. 77 (2002) 598.
[5] S. Mochizuki, F. Fujishiro, Nanaotechnology, submitted.
[6] S. Mochizuki, F. Fujishiro, S. Minami, J. Phys.: Condens. Matter 17(2005) 923.
[7] H.W. Gandy, Phys. Rev. 113 (1959) 795.
[8] P. Calvani, M. Capizzi, F. Donato, S. Lupi, P. Maselli, D.
Peschiaroli, Phys. Rev. B 47 (1993) 8917.
[9] R.L. Wild, E.M. Rockar, J.C. Smith, Phys. Rev. B 8 (1973) 3828.
[10] B.W. Faughnan, Phys. Rev. B 4 (1971) 3623.

ARTICLE IN PRESS

Physica B 376–377 (2006) 827–830
www.elsevier.com/locate/physb

Photo-induced reversible spectral change in several
AgI–oxide particle composites
Fumito Fujishiro, Shosuke Mochizuki
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
Different kinds of metal oxide (anatase, ZnO and SrTiO3) ﬁne particles were dispersed in silver iodide (AgI). These composites exhibit
considerable electrical conductivity enhancement. When these composites are irradiated with a 325 nm laser light in vacuum, theirphotoluminescence spectra are changed. By irradiating with same laser light at room temperature in O2 gas, the original spectral
structure reappears. The observed reversible phenomena are explained by some excitation energy transfer and taps at the interfaces
between AgI and oxide particles. Such reversible phenomena may yield materials for optical oxygen-partial-pressure sensors.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.72. Ji; 71.35.–y; 78.55.–m
Keywords: AgI-based composite; Photoluminescence; Oxygen defects

1. Introduction
Recently, considerable attention has been devoted toresearch for new materials to be used for optical sensor [1]and photomemory. The interest in photoluminescentmaterials showing photo-induced photoluminescence (PL)spectral change at room temperature has signiﬁcantly
grown. Hitherto, we have observed the photo-induced PL
spectral change on several metal oxides [2,3]. When metaloxide is irradiated at room temperature with a continuous
wave (CW) 325 nm laser light in a vacuum, a white
luminescence band appears and then it increases inintensity with increasing irradiation time (tir). Afterremoving the laser light, the white PL state is recorded
for more than several years at room temperature underroom light, regardless of any changes of atmosphere. By
irradiating with the same laser light at room temperature inO2 gas, the original PL property reappears. The observed
spectral changes are explained by the photooxidation andphotoreduction on the oxide surfaces, which induce thevalence-number change, and annihilation or creation ofoxygen defects.
Corresponding author. Tel./fax: +81 3 5317 9771.

E-mail address: fumito@phys.chs.nihon-u.ac.jp (F. Fujishiro).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.12.207

Till today, we have been studying the reversible photoinduced PL spectral change in different oxides anddifferent composites, in order to ﬁnd the material having
much superior photofunction. In the study, we haveobserved, for the ﬁrst time, the reversible photo-induced
PL spectral changes in several composites which consist ofsilver iodide (AgI) and metal oxide ﬁne particles. The
spectral change arises from some excitation energy transferfrom AgI to photo-generated crystal defect, at theinterfaces between AgI and oxide particle. In the present
paper, we summarize the recent results obtained in ourlaboratory.
2. Experimental procedure
The AgI–oxide ﬁne particle composites were prepared byfully mixing AgI powder (purity 499.999%, Alfa Aesar)and different kinds of metal oxide powder, SrTiO3 nanosized powder (purity499.5%, Aldrich), anatase powder(an averaged diameter of 50 nm, purity499.9%, Wako)
and ZnO powder (averaged diameter 20 nm, purity495.0%, Wako), by compressing under 0.2 GPa for
30 min to fabricate the pellets, and then the pellets were
heated at 673 K for 12 h in air. The photo-induced PL

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F. Fujishiro, S. Mochizuki / Physica B 376–377 (2006) 827–830

spectral change was measured by means of a CW He–Cdlaser (wavelength ¼ 325 nm) and an optical multichannel
analyzer. The specimens were set in a chamber connectedwith an oil-free vacuum system, which is able to exchange
the atmosphere around the specimens at room temperature.

Before the measurements on the photo-induced PL
spectral change, we studied the electrical conductivities
and the PL, PL excitation and time-resolved PL spectra atdifferent temperatures for some composites [4,5]. It isfound that (x)AgI–(1–x)anatase, (x)AgI–(1–x)ZnO
and (x)AgI–(1–x)SrTiO3 composites display the electricalconductivity (s) maximum against the AgI contents,
0.4, 0.5 and 0.6, respectively. It is also found thats of (0.4)AgI–(0.6)anatase, (0.5)AgI–(0.5)ZnO and(0.6)AgI–(0.4)SrTiO3 are enhanced by about three orders,
one order and two orders of magnitude in comparison withthat of pristine AgI, respectively. Possible causes of suchconductivity enhancement are thought to be the silver
vacancies and other crystal defects produced at the
AgI–oxide particle interfaces [6]. After fully irradiatingthese composites with 325 nm laser light in O2 gas and in
vacuum, the photo-induced experiments were performed.
The same experiments were carried out for pristine AgI,
pristine anatase, pristine ZnO and pristine SrTiO3 as thereference specimens. All of the measurements were carriedout at room temperature. Regardless of the sensitization of
AgI, the photo-induced PL spectral changes are clearlyobserved for AgI-based composites. As typical results foreach composite, the spectral change of (0.4)AgI–(0.6)anatase, (0.5)AgI–(0.5)ZnO and (0.4)AgI–(0.6)SrTiO3 composites are shown in the present paper, as follows.
Figs. 1(a) and (b) show the photo-induced PL spectralchanges of (0.4)AgI–(0.6)anatase composite and pristine
anatase, respectively. Curves 1 and 2 are obtained after
irradiating for 30 min in O2 gas and in vacuum, respectively. In O2 gas, the composite shows a maximum at
2.91 eV, which is close to the free exciton energy, E AgI
FE , ofpristine AgI [7]. The broad background emission band
centered at about 1.9 eV is due to oxygen defects in anataseparticles. The PL intensity peak energy of this band issmaller than that (2.2 eV) of pristine anatase (curve 3).
Generally, the energy of oxygen defect, Edefect, depends on
the site and local symmetry. Surely, the result indicates that
Edefect is affected by adding AgI. When the specimen
chamber is evacuated, the AgI free exciton luminescence
disappears with increasing tir, while a new anatase-related
emission grows at about 2.2 eV with increasing tir. Throughmany successive experiments, it is found that the spectralchanges (122 and 324) appear repeatedly. Since the PL
intensity peak energy of difference spectra, curve 2–curve 1
and curve 4–curve 3, differ from curves 1 and 3, the energyof photo-induced crystal defects (PID) in both the
AgI–anatase composite and pristine anatase is different

1.0

PL intensity (arb. units)

3. Results and discussion(a)(0.4)AgI-(0.6)anatase
2: in vacuum

0.5

1: in oxygen

0.0
1.5

(b)
pristine anatase

1.0

4: in vacuum

0.5
3: in oxygen
0.0
3.2

3.0

2.8
2.6
2.4
Photon energy (eV)

2.2

2.0

1.8

Fig. 1. The photo-induced spectral change AgI–anatase composite: (a)(0.4)AgI–(0.6)anatase composite and (b) pristine anatase.

from that of the original oxygen defect. Such photoinduced defect formation at anatase particle surfaces invacuum may be given by the following expressions
TiO2 in lattice

TiO2 in lattice

325 nm laser photon!

325 nm laser photon

 complex þ!

1O2 " ,
2ðOV þ 2eÞ þ

1
O2 " ,
2(1)

ðTi3þ  OV  Ti3þ Þð2Þwhere OV denotes the oxygen vacancy. In reduced anatase,
it is known for the near-band edge absorption due tooxygen defects, see, for example, Sekiya et al. [8]. The small
difference of PL intensity peak energy between curve 2 andcurve 4 indicates that the PID energy structure is almost
unchanged by adding AgI.
Figs. 2(a) and (b) show the photo-induced PL spectralchange for (0.5)AgI–(0.5)ZnO composite and pristine ZnO,
respectively. The spectra in the ﬁgures are obtained in
vacuum (dotted curve) and in O2 gas (solid curve), after
irradiating for 30 min in each atmosphere. In O2 gas, thiscomposite shows a maximum at 2.91 eV which is close to
E AgIFE . The broad background emission band centered atabout 2.2 eV is due to oxygen defects in ZnO particles. This
energy is smaller that (2.52 eV) of pristine ZnO particles. It
indicates that Edefect is affected by adding AgI. When thespecimen chamber is evacuated, the AgI free exciton
luminescence disappears with increasing tir, while theemission due to oxygen defects increases in intensity withincreasing tir. However, the magnitude of the spectralchange is smaller than that for (0.4)AgI–(0.6)anatase
composite. The photo-induced defect formation at ZnO
particle surfaces in vacuum may be given by the following

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829

(a)

(a)

(0.5)AgI-(0.5)ZnO(0.4)AgI-(0.6)SrTiO3

1.0

10: in vacuum

1.0
6: in vacuum

9: in oxygen
PL intensity (arb. units)

PL intensity (arb. units)

0.5
5: in oxygen
0.0(b)8: in vacuum

1.0

pristine ZnO

7: in oxygen

0.5

0.5

0.0
(b)
12: in vacuum

1.0

pristine SrTiO3

0.5
11: in oxygen

0.0
3.2

3.0

2.8

2.6
2.4
2.2
Photon energy (eV)

2.0

1.8

Fig. 2. The photo-induced spectral change AgI–ZnO composite: (a)(0.5)AgI–(0.5)ZnO composite and (b) pristine ZnO.

expressions
ZnO in lattice

ZnO in lattice

325 nm laser photon!

325 nm laser photon!

1ðOV þ 2eÞ þ O2 " ,
2

(3)

1
O2 " .
2

(4)

Zni þ

It is well-known that the Zn interstitial (Zni) and OV create
to shallow donor levels [9]. Through many successive
experiments, it is found that the spectral changes (526
and 728) appear repeatedly. Small energy difference ofoxygen-defect-related PL intensity peak between curve 5and curve 6 indicates that Edefect of (0.5)AgI–(0.5)ZnO
composite is almost unchanged by photo-irradiation.
Figs. 3(a) and (b) show the photo-induced PL spectralchange of (0.4)AgI–(0.6)SrTiO3 and pristine SrTiO3,
respectively. The spectra in the ﬁgures are obtained in
vacuum (dotted curve) and in oxygen (solid curve), after
irradiating for 30 min in each atmosphere. By evacuating
O2 gas from the specimen chamber, the broad emission
band grows at about 2.3 eV, which arises from oxygen
defects in SrTiO3 particles. Unlike the case of(0.4)AgI–(0.6)anatase and (0.5)AgI–(0.5)ZnO, AgI exciton
emission increases in intensity in vacuum with increasingtir. Through many successive experiments, it is found thatthe spectral changes (9210 and 11212) appear repeatedly. As seen in curve 11, there are at least four
luminescence components at about 3.0, 2.93, 2.85, 2.67
and below 2.6 eV in pristine SrTiO3 particle. Rescaling andthan comparing curve 9 and curve 11, the following are
obtained. The PL component higher than 3 eV is suppressed by adding AgI, which is due to the opticalabsorption by AgI domain. The broad PL component
below 2.6 eV, which is centered at 2.35 eV, is considerablyenhanced by adding AgI. The luminescence observed

0.0
3.2

3.0

2.8

2.6
2.4
2.2
Photon energy (eV)

2.0

1.8

Fig. 3. The photo-induced spectral change AgI–SrTiO3 composite: (a)(0.4)AgI–(0.6) SrTiO3 composite and (b) pristine SrTiO3.

around 2.4 eV is known to arise from oxygen defects [3].
The process of the photo-induced defect formation at
SrTiO3 particle surfaces in vacuum may be expressed asfollows
SrTiO3 in lattice
SrTiO3 in lattice

325 nm laser photon!

ðOV þ 2eÞ þ

1
O2 " ,
2(5)

325 nm laser photon!

ðTi3þ  OV  Ti3þ Þcomplex þ

1O2 " .
2

ð6Þ

Before discussing the experimental results, we mention
brieﬂy about the structure of these composites. The X-ray
diffractograms, SEM observations and EDX measurements
show that oxide particles are packed, without alloying, ineach large AgI particle domain whose size is approximately
several micronmeters. The 325 nm (3.8 eV) laser light excites
not only AgI ðE AgI
FE ¼ 2:9 eVÞ but also anatase (whose energy
gap Eg is 3.2 eV), ZnO ðE g ¼ 3:2 eVÞ and SrTiO3(Eg ¼ 3.2 eV), at room temperature. In addition to theinterfacial oxygen defects created by adding oxide ﬁneparticles to AgI, the irradiation of 325 nm laser light invacuum creates extra oxygen defects at the surfaces of oxide
ﬁne particles. If E AgI
FE is close to Edefect, the excitation energyof AgI is transferred to oxygen defect in adjacent oxide
particle, exciting oxygen defect and then displaying luminescence, suppressing the AgI free exciton luminescence. Since
the number of photo-induced oxygen defect decreases under
325 nm laser light irradiation in O2 gas, the deexcitation ofphoto-excited AgI gives luminescence without exciting
oxygen defect. This is the reason why the AgI free exciton
luminescence reappears under 325 nm laser light irradiationin O2 gas for (0.4)AgI–(0.6)anatase and (0.5)AgI–(0.5)ZnO
composites.

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However, in the case of (0.4)AgI–(0.6)SrTiO3 composite,
the AgI free exciton luminescence in O2 gas is considerably
decreased by adding SrTiO3. This suggests that the oxygen
defects generated by adding AgI act as nonradiative traps
for AgI free exciton. Many optical absorption bands due tooxygen deﬁciency are also known for SrTiO3. Because oflarge dielectric constant (approximately 300) of SrTiO3,
photo-generated electrons and holes do not form excitons
but they move independently throughout AgI/oxide interfaces. It is also known that, in SrTiO3 crystal, electrons
move freely, while holes are self-trapped. In the(Ti3+–OV–Ti3+) complex, the Ti3+ ions act as hole traps,
while the OV act as electron traps. Accidental recombination of electron and hole at photo-generated (Ti3+–OV–
Ti3+) complex induces the 2.4 eV luminescence. On theother hand, another portion of photocarriers may compensate the traps, enhancing the AgI free exciton luminescence. This may be the reason why the AgI free exciton
luminescence is enhanced under 325 nm laser light irradiation in vacuum for (0.4)AgI–(0.6)SrTiO3 composite. Since
the defect structure is hard to be optically determined, it is
not clear which type of defect acts as such traps.
Finally, we are now studying the reversible photoinduced PL spectral change as a function of oxygenpressure, for applying the phenomena to the optical
oxygen-partial-pressure sensor.
Acknowledgements
This work was supported by a Grant-in-Aid for
Scientiﬁc Research from Ministry of Education, Science,
Culture, Sport and Technology, Japan, Interdiciplinary
General Joint Research Grant (Nihon University) andResearch Grant for Science, College of Humanities andSciences (Nihon University).
References[1] Y. Amao, K. Asai, I. Okura, Anal. Commun. 36 (1999) 179.
[2] S. Mochizuki, T. Nakanishi, Y. Suzuki, Appli. Phys. Lett. 79 (2001)
3785.
[3] S. Mochizuki, F. Fujishiro, S. Minami, J. Phys.: Condens. Matter 17(2005) 923.
[4] S. Mochizuki, F. Fujishiro, J. Phys.: Condens. Matter 15 (2003) 5057.
[5] F. Fujishiro, S. Mochizuki, Physica B 340–342 (2003) 216.
[6] T. Jow, J.B. Wagner Jr., J. Electrochem. Soc. 126 (1979) 1963.
[7] S. Mochizuki, Physica B 308–310 (2001) 1042.
[8] T. Sekiya, K. Ichimura, M. Igarashi, S. Kurita, J. Phys. Chem. Solids
61 (2000) 1237.
[9] S.B. Zhang, S.-H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205-1.

Institute of Physics Publishingdoi:10.1088/1742-6596/21/1/023

Journal of Physics: Conference Series 21 (2005) 142–148
Second International Conference on Photo-induced Phase Transitions

Photo-induced phenomena in SrTiO3Fumito FUJISHIRO, Shosuke MOCHIZUKI
Department of Physics, College of Humanities and Sciences, Nihon University, 3-2540 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
E-mail: fumito@phys.chs.nihon-u.ac.jp
Abstract. When different SrTiO3 single crystals are irradiated at room temperature with acontinuous wave 325 nm laser light in an evacuated specimen chamber, their luminescenceincrease in intensity, creating a broad visible luminescence band centred at about 2.4 eV. Then,
introducing oxygen gas into the specimen chamber, the photoluminescence spectra return
reversibly to the original weak luminescence with the same laser light irradiation. Afterremoving the laser light irradiation, each photoluminescent state is stored for a long time atroom temperature under room light, regardless of any changes of atmosphere. Such photoinduced spectral change has been also observed at different temperatures from 6 K to room
temperature. The observed phenomenon is explained by the photo-induced oxygen defect at thesurfaces of SrTiO3 crystal. The observed photo-induced optical phenomena are discussed in thelight of both the crystal defect chemistry of SrTiO3 and the exciton theory.

1. Introduction
In recent years, considerable attention has been devoted to the search for new photo-induced
phenomena to be used as erasable photomemory materials [1]. We have observed the reversible photoinduced spectral transitions in the photoluminescence of Eu2O3 [2], Sm2O3 [3], anatase TiO2 [4],vitreous SiO2 [5] and other materials [6]. When these materials are irradiated at room temperature witha continuous wave (CW) 325 nm laser light in an evacuated specimen chamber, their luminescencechange in intensity, creating a broad visible luminescence band. Then, introducing oxygen gas into thespecimen chamber, the photoluminescence (PL) spectra return reversibly to the original luminescencewith the same laser light irradiation. After removing the laser light irradiation, each PL state is stored
for a long time at room temperature under room light, regardless of any changes of atmosphere. Such
photo-induced spectral change has been also observed at different temperatures from 6 K to room
temperature. Such reversible phenomena may well yield materials for erasable optical storage andwhite-light-emitting devices. Very recently, we have also observed quite similar photo-induced PL
spectral change in strontium titanate SrTiO3 single crystal. Grabner [7] measured the PL properties of
doped and undoped SrTiO3 crystals, and observed a near-infrared luminescence band around about 1.6
eV and a visible luminescence band around 2.4 eV. In addition to these PL bands, we have found anear-band-edge emission at 3.2 eV, accompanied by a broad PL band around 2.9 eV [8, 9]. In
particular, the 2.4 eV luminescence shows clear spectral change.
In the present paper, we summarize our recent studies on the photo-induced PL spectral change
observed for different SrTiO3 crystals and discuss them in the light of both the crystal defect chemistryof SrTiO3 and the exciton theory.

© 2005 IOP Publishing Ltd

142

143

2. Experimental
The SrTiO3 crystal was grown by the Verneuil method and the as-grown crystal is dark blue. The asgrown crystal became perfectly colourless and transparent by annealing in reducing Ar-H2 mixture gas
atmosphere at high temperature. We have measured the optical density (OD), PL, PL excitation andtime-resolved PL spectra for the as-grown and annealed crystals at different temperature between 6 K
to room temperature. The details of experimental procedure are described in our previous papers [8, 9].
The reversible photo-induced PL spectral changes were measured by means of a CW He–Cd laser(wavelength= 325 nm) as the excitation source and an optical multichannel analyzer. The specimens
were set in a chamber connected with an oil-free vacuum system, which is able to exchange theatmosphere around the specimens at room temperature. The experimental setup is shown in figure 1.
4

SrTiO3 crystal
Optical density

T= 294 K
3

as-grown2

10

fully-annealed

3.0

2.0

1.0

Photon energy (eV)Figure 1. The experimental setup.

Figure 2. Optical density spectra of
the as-grown and fully-annealed
SrTiO3 crystals at 294 K.

3. Results and discussion
Figure 2 shows the OD spectra of the as-grown and fully-annealed SrTiO3 crystals at room
temperature. As seen in this figure, the as-grown crystal shows an intense background absorption
which increases gradually toward IR region. We name tentatively this background NIR absorption. As
superposing on the NIR absorption, there appear several absorption peaks around 2.4, 2.15, 2.08 and0.82 eV and un-resolved absorptions in the energy ranges between 3.1 and 2.6 eV, and between 1.9
and 1.2 eV. The OD spectra increased steeply at photon energies larger than 3.1 eV is ascribed to thefundamental electronic absorption onset (3.2 eV) of cubic SrTiO3 [8]. With progressing annealing, theabsorption around 0.82 eV disappears rapidly and the absorptions around 2.15 and 2.08 eV merge into
the broad absorption band around 1.65 eV, decreasing the NIR absorption in intensity. As seen above,
many absorptions appear at energies lower than the bandgap energy (3.2 eV), which indicates that
there are many crystal-defect-induced levels in the energy gap. The midgap absorptions below 3.1 eV
may arise not from homogeneously crystal defects (oxygen vacancies) but from non-Perovskite phases
3+3+with the Ruddlesden-Popper phases [(SrO)*(SrTiO3)n] [10] and (Ti -oxygen vacancy-Ti ) complex[11,12].
During a CW 325 nm laser light irradiation in vacuum (or in oxygen) at room temperature, we havemeasured the PL spectral change of the as-grown and fully-annealed crystals as a function of theirradiation time. The result for the as-grown crystal at room temperature in vacuum is shown in figure3(a). The irradiation time is shown by each curve. As seen in figure 3 (a), the initial PL spectrum isvery broad, and it has an intensity peak at about 2.8 eV and two faint kinks at about 2.4 and 2.15 eV.
With increasing irradiation time, the intensity increases, growing the PL component below 2.8 eV andthe speed of the spectral change becomes deduced. A difference spectrum between the spectrum
obtained at 60 min and rescaled spectrum obtained at 0 min is calculated, which is shown in figure 3

144(b). This figure shows clearly that a broad PL band has grown at 2.4 eV under CW 325nm laser lightirradiation in vacuum.
(a)

SrTiO3 crystal
60 min50 minas-grown
40 minT= 295 K
30 min20 minPL intensity (arb. units)

2

1

0
3

10 min
2 min
0 min

(b)
60 min2

1

0 min(rescaled: x 2.5)

15

(a)

50 min 60 min
40 min30 minSrTiO3 crystal
fully-annealed
T= 295 K

10

PL intensity (arb. units)

3

5

0
15

20 min10 min0 min(b)0 min (rescaled: x 1.3)
60 min105difference

difference
0
3.0

2.8

2.6

2.4

2.2

2.0

1.8

0
3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Photon energy (eV)Figure 3. Photo-induced PL spectralchange of the as-grown SrTiO3 crystal at
295 K. (a) Spectral change in vacuum. (b)Extraction of photo-induced component.

Figure 4. Photo-induced PL spectralchange of the fully-annealed SrTiO3 crystal
at 295 K. (a) Spectral change in vacuum.
(b) Extraction of photo-induced component.

Figure 4 (a) shows the photo-induced PL spectral change of the fully-annealed crystal at room
temperature in vacuum. The PL component below 2.8 eV grows with increasing irradiation time. Withfurther increasing irradiation time, the rate of the change becomes small. As done for the results of the
as-grown crystal, the difference spectrum is also calculated, which is given in figure 4 (b). It clearly
indicates that a broad PL band has grown at 2.4 eV under 325nm laser light irradiation in vacuum.
Then we have introduced the oxygen gas to the specimen chamber and irradiated the same laser light.
The 2.4 eV band decreases in intensity for both the as-grown and fully-annealed crystals, returning tothe original weak PL state. The spectral changes observed for the SrTiO3 single crystals are mainly
characterized by an intensity change of the 2.4 eV luminescence. Since the 325nm laser light neverdissociates directly free O2 molecule, the observed spectral changes arise from the photo-induced
associative detachment and photo-induced dissociative adsorption of O2 molecules near the SrTiO3
surfaces. In the other word, these phenomena correspond to the photo-reduction and the photooxidation, respectively. It has also found that the speed of the spectral change accelerates on
increasing the intensity of the activating light. The same experiments were carried out by changing the
wavelength of the irradiating laser light. As results, it has been found that no spectral change wasobserved except for the CW 325 nm laser light.
The PL spectra of the as-grown and different annealed SrTiO3 crystals were measured at different
temperatures between 295 and 6 K under a CW 325 nm and a pulsed 355 nm laser light of different
laser fluences. With decreasing temperature, the 2.4 eV band merged in the broad band at room
temperature becomes prominent and then the PL spectra of these crystals below 30 K are characterized
by the 2.4 eV band for weak excitation. As typical results, the PL spectra of the as-grown and fully
annealed crystals are shown in figure 5. It has also found that the photo-induced PL spectral change
has been more clearly observed at 13 K. The result for the fully-annealed SrTiO3 single crystal isshown in figure 6 (a), as a typical result. The irradiation time is given by each curve. The peakintensity of the 2.4 eV luminescence is plotted against irradiation time in figure 6 (b). The PL
increases in intensity and then tends to saturate at 90 min without any change of the spectral shape. It

145

PL intensity (arb. units)

has been found that the magnitude of the spectral change at 2.4 eV for 60 min is nearly independent of
temperature.
SrTiO3 crystal

10

as-grown
x 58
5fully-annealed
x1

0

3.2

3.0

2.8

(a)

40

SrTiO3 crystal
fully-annealed
T= 13 K

90 min30

60 min20 min

20

6 min

10

0 min
0
3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

PL intensity (arb. units)

T= 13 K-2
Iex= 0.8 mJcm

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 5. PL spectra of the as-grown andfully-annealed SrTiO3 crystals at 13 K. The
excitation laser wavelength is 355 nm and-2
laser fluence is 0.8 mJcm .

(b)

40

T= 13 K

30

20

under irradiation
050

without irradiation
100

150

Irradiation time (min)Figure 6. Photo-induced PL spectralchange of the fully-annealed SrTiO3 crystal
at 13 K. (a) Spectral change in vacuum. (b)Irradiation time vs the peak PL intensity.

4. Mechanism of the photo-induced photoluminescence spectral change in SrTiO3
Through our many optical experiments [8, 9, 13] on the as-grown SrTiO3 crystal and the SrTiO3
crystals recovered from different annealing stages, it has found that the PL of real SrTiO3 crystals is
considerably affected by crystal defects. All of the results on the reversible photo-induced PL spectralchange of the as-grown and different annealed SrTiO3 single crystals may be summarized as follows.
(1) The reversible photo-induced PL spectral change can be observed at different temperatures,from 6 K to room temperature. The spectral change takes time and the speed of change isnearly independent of temperature, but it increases with increasing light intensity.
(2) The spectral change tends to saturate at long light irradiation time (more than 90 min).
(3) The spectral change is induced by changing the specimen atmosphere between oxygen gas andvacuum only under a CW 325 nm laser light.
(4) After removing the light irradiation, each PL property persists for a long time even at room
temperature under room light, regardless of any changes of atmosphere.
The result (1) indicates that the observed spectral changes are not phonon-assisted phenomena but
purely electronic ones and that the PL band at 2.4 eV is an extrinsic luminescence due to theluminescence centres introduced by a CW 325 nm laser light. Since the 325 nm (= 3.81 eV) laser lightnever dissociates directly free O2 molecule, the result (3) indicates that the phenomena arise from thephoto-induced associative detachment and the photo-induced dissociative adsorption of O2 molecules
near the SrTiO3 surface. In this case, photo-excited SrTiO3 crystal acts as a photocatalyst fordecomposing O2 molecule and also as an oxygen reservoir. The results also summarize how spectralchange arises naturally from the photo-activated oxidation and reduction. The reduction of SrTiO3

146

surface under a CW 325 nm laser light in vacuum may be accompanied by creation of electroncaptured oxygen vacancy V”O. Using a local spin density approximation plane-wave pseudo potential
method, Astala and Bristowe [14] have shown that the doubly positively charged state is most stable.
It is well in agreement with the experimental result that oxygen deficiency in SrTiO3 enhances theelectrical conductivity even at low temperatures [15]. This means that most of the electrons aroundoxygen vacancies are released and, therefore, such oxygen vacancy sites are relatively positively
charged. Therefore, the oxygen vacancies tend to trap photo-generated electrons. Several authors [11,3+3+
12, 16] proposed another type of defect, (Ti -VO”-Ti ) complex, at SrTiO3 surface. Taking account of
this complex, the photo-induced oxygen desorption can be explained as follows.

1
CW 325 nm laser photon
SrTiO3 in lattice ←⎯ ⎯ ⎯ ⎯ ⎯ ⎯ ⎯⎯→ (Ti 3+ − VO" − Ti 3+ ) complex + O2
2

(1)

In the complex, the Ti3+ ions act as hole traps (Sh), while the vacancy V”O do as electron traps (Se).
In SrTiO3 crystal, it has been known that electrons determine well the transport properties, forexample, electrical conductivity and photoconduction, while any phenomena related to holes havebeen not observed. This indicates that holes are almost trapped around crystal defects. Toyozawa [17]proposed three types of symmetry-breaking instabilities of excitons in the phonon field and when theelectron and the hole have deformation potentials of opposite sign, the decomposition into a pair ofself-trapped particles occurs. This instability of an exciton leads to lattice decomposition into anelectron centre (an anion vacancy) Se and a hole centre (an anion interstitial) Sh, if exciton is formed in
the bulk. On surfaces, the hole center is emitted out of the surface, thus leaving only the electron
center. Therefore, such exciton instability may result in oxygen desorption at SrTiO3 surface invacuum and thus the oxygen vacancies produced give rise to the 2.4 eV luminescence. It is noted that
the CW 325 nm laser light is indispensable for the spectral change due to the photo-induced oxidation-2and -reduction in SrTiO3. The CW 325 nm laser fluence in our experiments was at most 0.8 Wcm ,and therefore any bond breaking mechanism based on the instability under high-density excitation
may be rejected from the present discussion. Unlike alkali halides, a single interband excitation neverinduces the desorption of surface atom in semiconductors, since the binding energy of atoms is larger
than the energy gap. In the case of photo-excitation of such semiconductor, only atom around crystal
defect at the surfaces can be released [18, 19, 20, 21]. Astala and Bristowe[14]calculated the oxygen
vacancy formation energy of about 7 eV, which are approximately twice of the bulk energy gap andare slightly smaller than twice of the CW 325 nm laser photon energy, at the TiO2-terminated (100)3+3+surface of SrTiO3. Both the oxygen defects (for example, Ti -O”V -Ti and O”V) inherent to SrTiO3
surfaces and the infinite continuity of the laser light may enable the 325 nm laser photon to release O2
molecule from the surface. Incidentally, the pulsed 355 nm laser light (the pulse width = 4~5 ns),though the photon energy is close to the CW 325 nm laser light, never induced the spectral change-2even at 14 mJcm [8]. The oxygen desorption via such excited state may occur immediately after alaser pulse incidence. However, it is found that relatively long time is taken to complete the spectralchange in SrTiO3, as the result (2). This indicates that the photo-induced oxygen desorption probability
is very low at SrTiO3 surface. The observed saturation tendency with further increasing irradiation timesuggests that the photo-induced oxygen defect increases the potential energy of the surface irradiatedby light. In such case, equilibrium may be realized in a system composed of a matter (SrTiO3 crystal)and a radiation field (325 nm laser photon field). It can be assumed that the intensity, coherency andcontinuity of laser light determine the degree of fluctuation in the system and the dynamics of thephoto-induced PL spectral change.
Recently, there are several reports on the photo-induced change of dielectric and transport
properties of SrTiO3 crystal under ultraviolet irradiation and the change is frequently explained as theintrinsic property of SrTiO3 crystal. However, if some of photo-generated carriers are trapped at theoxygen defects, large enhancement of the dielectric constant may be observed as an extrinsic effect. In
the past, Uwe [22] pointed out the importance of the ferroelectric microregion [23] in the photo-

147

excited SrTiO3 crystal, as follows. Photo-generated electrons create Ti3+ ions and then these ionsproduce electric dipoles with oxygen defects. The dipoles may produce ferroelectric microregions
which give large enhancement of the dielectric constant.
Finally, since some chemical heterogeneity on the surface has been pointed out for the surfaces of
SrTiO3 crystal reduced and oxidized at high temperatures [10] and small amount of OH ions are
introduced inevitably during crystal growth by the Verneuil method, these effects must be considered
for more detailed discussion.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education,
Science, Sports, Culture and Technology, Japan. This work was also supported by an Interdisciplinary
General Joint Research Grant for Nihon University.
References[1] Nagaosa N, Ogawa T 1989 Theory of photoinduced structure change Phys. Rev. B 39 44724483.
[2] Mochizuki S, Nakanishi T, Suzuki Y and Ishi K 2001 Reversible photoinduced spectral change
in Eu2O3 at room temperature Appl. Phys. Lett. 79 3785–3787[3] Mochizuki S 2003 Intense white luminescence of Sm2O3 irradiated with ultraviolet laser lightunder vacuum Physica B 340–342 944–948[4] Mochizuki S, Shimizu T and Fujishiro F 2003 Photoluminescence study on defects in pristine
anatase and anatase-based composites Physica B 340–342 956–595[5] Mochizuki S and Araki H 2003 UV-laser-light-produced defects and reversible blue–whitephotoluminescence change in silica Physica B 340–342 969–973[6] Mochizuki S and Fujishiro F 2003 Structural, electrical and optical studies on AgI–anatase
composites J. Phys.:Condens. Matter 15 5057–5072[7] Grabner L 1969 Photoluminescence in SrTiO3 Phys. Rev. 177 1315–1323[8] Mochizuki S, Fujishiro F and Minami S 2005 Photoluminescence and reversible photo-induced
spectral change of SrTiO3 J. Phys.:Condens. Matter 17 923–948[9] Mochizuki S, Minami S and Fujishiro F 2005 The reversible UV-laser-light-induced spectralchange and origin of the 2.4 eV luminescence band in SrTiO3 J. Lumin. 112 267–270[10] Szot K and Speier W 1999 Surfaces of reduced and oxidized SrTiO3 from atomic force
microscopy Phys. Rev. B 60 5909–5926[11] Henrich V E, Dresselhaus G and Zeiger H J 1978 Surface defects and the electronic structure of
SrTiO3 surfaces Phys. Rev. B 17 4908–4921[12] Kimura S, Yamauchi J, Tsukada M and Watanabe S 1995 First-principles study on electronic
structure of the (001) surface of SrTiO3 Phys. Rev. B 51 11049–11054[13] Fujishiro F and Mochizuki S 2005 Optical spectra change of Verneuil-grown SrTiO3 single
crystal during decolorization process submitted.
[14] Astala R and Bristowe P D 2001 Ab initio study of the oxygen vacancy in SrTiO3 Modelling
Simul. Mater. Sci. Eng. 9 415–422[15] Jourdan M and Adrian H 2003 Possibility of unconventional superconductivity of SrTiO3−δ
Physica C 388/389 509–510[16] Cord B and Courths R 1985 Electronic study of SrTiO3(001) surfaces by photoemission Surf.
Sci. 162 34–38[17] Toyozawa Y 1983 Symmetry breaking excitonic instabilities in deformable lattice Physica B+C
117–118 23–29[18] Puchin V E, Shluger A L and Itoh N 1993 Theoretical studies of atomic emission and defectformation by electronic excitation at the (100) surface of NaCl Phys. Rev. B 47 10760–10768[19] Itoh N, Kansaki J, Okano A and Nakai Y 1995 Laser-Beam Interaction with Defects on
Semiconductor Surfaces: An Approach to Generation of Defect-Free Surfaces Annu. Rev.

148

Mater. Sci. 25 97–127[20] Itoh N and Stonham A M 2000 Material Modification by Electronic Excitation (Cambridge:
Cambridge University Press)[21] Song K S and Williams R T 1993 Self-Trapped Excitons (Berlin: Springer)[22] Uwe H 2003 Impurity effect in quantum paraelectrics Meeting Abstract of the Physical Societyof Japan 58 640[23] Abe H, Harada K, Matsuo R J, Uwe H and Ohshima K 2002 X-ray diffuse scattering associatedwith ferroelectric microregions in KTa1-xNbxO3 J. Phys.: Condens. Matter 13 3257

Institute of Physics Publishingdoi:10.1088/1742-6596/21/1/031

Journal of Physics: Conference Series 21 (2005) 189–194
Second International Conference on Photo-induced Phase Transitions

Photo-induced valence-number changes and defects in
Eu2O3 fine particle films
Shosuke Mochizuki1, Fumito Fujishiro, Ken’ichiro Ishiwata
Department of Physics, College of Humanities and Sciences, Nihon University, Tokyo
156-8550, Japan
E-mail: motizuki@physics.chs.nihon-u.ac.jp
Abstract. Different Eu2O3 fine particle films were prepared by the laser ablation method.
Under a continuous wave (CW) 325 nm laser light irradiation, their photoluminescence spectra
are measured both in vacuum and in O2 gas atmosphere. All the specimens display reversible
spectral change and clear luminescence colour changes (red luminescence ↔ white
luminescence) for changing specimen atmosphere. Moreover, the white-luminescent state isstored for more than several years at room temperature in air under room light. The obtained
results suggest that the spectral changes arise from both the photo-induced valence (Eu3+ →Eu2+) change of europium ions and the photo-generated oxygen defect structure at Eu2O3
particle surfaces in vacuum, which is expressed by the following reaction, Eu2O3 in lattice +CW 325 nm photon → (Eu2+-oyygen vacancy-Eu2+) complex + ½O2. The obtained results are
discussed in the light of the exciton theory.

1. Introduction
Europium sesquioxide Eu2O3 exhibits five crystal phases designated by X, H, A, B and C at normalpressure in the order of decreasing temperature [1]. Cubic X form transforms into hexagonal H form at
about 2553 K, to hexagonal A form at about 2413 K, to monoclinic B form at about 2313 K, to cubic
C form at about 1373 K. The B form contains six formulae per unit cell of which space group is C2/m.
The body-centered cubic C form contains 32 europium ions and 48 oxygen ions per unit cell of whichspace group is Ia3. Several years ago, we found reversible photo-induced spectral change in Eu2O3 atroom temperature [2, 3]. When Eu2O3 powder compacts, films and particles are irradiated with a 325
nm laser light in vacuum, their photoluminescence (PL) spectra change from a red sharp-line structure
to a white broad band, which can be clearly seen with the naked eye. After removing the UV laser
light, the white PL state is stored for more than several years at room temperature under room light,
regardless of any changes of atmosphere. By irradiating with the same UV laser light at room
temperature under oxygen gas atmosphere, the original red PL state re-appears. Such reversible
phenomenon may well yield materials for white-light-emitting devices and erasable optical storage.
Especially, Eu2O3 fine particles are thought to have potential application for high-density luminescentdevice and they lead to important modifications of some of the bulk properties. For example, particlesize reduction may give rise to the confinement of Eu3+ excitation and long-wavelength phonon modes
1

To whom any correspondence should be addressed.

© 2005 IOP Publishing Ltd

189

190

in the particle [4, 5]. In the present paper, we report upon the structural and optical studies on Eu2O3
fine particle films and we arrive at a view on the photo-induced spectral change.
2. Specimen preparation and characterization
The laser ablation target was prepared by compressing Eu2O3 powder 99.98 % in purity under apressure of 0.2 GPa for 1 h at room temperature and then by sintering at 1273 K for 24 h in air. Eu2O3
fine particle production and deposition were carried out in oxygen atmosphere of different pressureswith a pulsed Nd3+-YAG laser light (wavelength λ=355 nm, pulse width=3 ns, repetition =10 Hz). The
temperature of the silica glass substrate was room temperature. After deposition, the half of the asdeposited film specimens was annealed at 873 K in air for 24 h. The conditions of the specimen
preparation are listed in table 1.
Table 1. Summary of Eu2O3 films investigated.
Specimen

Laser ablation atmosphere

Comments

Oxygen pressure (Pa)
#1α

0.00

as deposited

#1β

0.00

annealed at 873 K in air

#2α

1.26

as deposited

#2β

1.26

annealed at 873 K in air

#3α

2.54

as deposited

#3β

2.54

annealed at 873 K in air

#4α

5.10

as deposited

#4β

5.10

annealed at 873 K in air

As standard specimens, the B-form and C-form specimens were also prepared by sintering Eu2O3
powder compacts at 1823 K and 1273 K in air, respectively. The films were characterized by the x-ray
diffraction (XRD) analysis with Cu Kα1 radiation at 300 K. Distribution maps of europium andoxygen atoms in the films were obtained by using a scanning electron microscope (SEM) with anenergy dispersive x-ray (EDX) fluorescence spectrophotometer (Shimadzu SSX-550). Figures 1(a),(b), (c) and (d) show the room-temperature XRD patterns of Eu2O3 specimens listed in table 1. Graybold and black solid curves correspond to as-deposited film and annealed one, respectively. The XRD
lines of the B-form and C-form Eu2O3 crystals [6] are given as histograms in figures 1(e) and (f),respectively. As seen in these figures, the as-deposited films display halo XRD pattern. The halo XRD
pattern is reproduced by rescaling and broadening these B-form- and C-form XRD lines and then by
superposing them. This suggests that the as-deposited films consist of very small crystallites or that theas-deposited films are amorphous. As-deposited films were fully transformed to crystalline Eu2O3 onesby heating at 873 K in air for 24 h. The crystalline Eu2O3 films are mixtures of a lot of C-form
crystallites and few B forms. The EDX fluorescence map of europium atoms coincides well with thatof oxygen atoms. Both the XRD patterns and the EDX fluorescence maps indicate that the produced
films are chemically uniform Eu2O3. The film morphology was analyzed by the SEM and an optical
microscope with a charge-coupled device camera system. As typical results, the SEM images of the
as-deposited Eu2O3 film (#4α) and annealed film (#4β) are shown in figures 2(a) and (b), respectively.
The uniform particle layer consists of particles smaller than several ten nanometers. Few particles withsize larger than several hundred nanometers are seen. These particles may be explosively spouted out

191from Eu2O3 target. As seen in figure. 2(b), annealing gives rise to many crevices in the uniform layerand divide into large number of domains of several micronmeter sizes.

C-form Eu2O3(f)

B-form Eu2O3 (e)(d)

Intensity (arb. scale)as deposited (#4α)
annealed (#4β)(c)
as deposited (#3α)
annealed (#3β)(b)
as deposited (#2α)
annealed (#2β)(a)
as deposited (#1α)
annealed (#1β)
20

30

40

50

60

70

80

2θ (degree)

Figure 1. Room-temperature x-ray
diffractograms of different Eu2O3 films
for CuKα1 radiation.

Figure 2. SEM images of Eu2O3 films:(a) as-deposited film (#4α), (b) annealed
film (#4β).

3. Results and discussion
Figures 3(a), (b), (c) and (d) show the PL spectra of the as-deposited and annealed Eu2O3 films. The
measurements were carried out at room temperature in air under CW 325 nm photoexcitation. These
specimens show the 5D0 → 7FJ (J=0, 1, 2, 3, 4) emissions at wavelengths between 570 and 720 nm [7].
With increasing oxygen pressure for the pulsed laser ablation, the 5D0→ 7F0 emission band becomes
sharpen. At wavelengths shorter than 550 nm, these specimens show also broad band emission. The
large PL intensity ratio of the broad band emission to the 5D0→ 7F2 one is observed for the#1α, #1β, #2α and #2β films which were fabricated in vacuum or extremely low oxygen pressure.
Except for the #1 specimen fabricated under vacuum, annealing depresses considerably the PL
intensity and makes the spectrum to resolve clearly into several emission bands. Since crystallites
grows in size and transform to predominant C-form film by annealing, the selection rule may become
rigorously effective to depress PL intensity. In figure 4, the room-temperature PL spectra of different
Eu2O3 specimens are compared. Although the spectra are rescaled to each maximum intensity value,
the C-form bulk specimen shows more intense emission than the B-form bulk one. These 5D0→ 7FJ
emission lines appear at almost the same wavelength in every specimen, but the bandwidths in the asdeposited fine particle films are larger than those in annealed films. Such broadening may arise fromboth random arrangement of crystallites and finite size of crystallites at which surfaces the electronic
excitations of Eu3+ ions are scattered. It is also found that the PL spectra of the as-deposited andannealed fine particle films consist of the emission lines observed at both the B-form and C-form
crystals. More high resolution measurements indicate that there are, at least, three distinct 5D0→ 7F0
emissions in the annealed films, while there is a very broad 5D0→ 7F0 emission in the as-deposited192

films. In addition, the presence of the 5D0→ 7F0,3 emissions and the J-degeneracy of 7F1,2 level suggestthat the local site environments of these Eu3+ ions may be described by different point groups, forexample, C1,2, Cs and C2v [8]. The effects of surface reconstruction of constituent atoms and surface
defects have been frequently pointed out for fine particle specimens.
0.6
5

0.5
0.4
0.3

5

annealed film (#2β)(d)

7

D0 - F 2

7(d)

D0 - F 2

as deposited (#4α)
annealed (#4β)

0.2

5

0.1

7

D0- F05

5

7

D0- F1

5

7

7

D0 - F 4

5

D0- F3

7

5

D0- F05

7

D0 - F 1
5

7

D0 - F 4

7

D0- F30.0

PL intensity (arb. units)

0.2

as deposited (#3α)
annealed (#3β)

0.1
0.0

(b)

0.2

as deposited (#2α)
annealed (#2β)
0.1

PL intensity (arb. scale)

(c)

0.3

as deposited film (#2α)(c)

B-form (1823K)(b)

C-form (1273K)

(a)

0.0

(a)

0.06

as deposited (#1α)
annealed (#1β)

0.04
0.02
0.00
400

450

500

550

600

Wavelength (nm)

650

700

Figure 3. Photoluminescence spectra ofdifferent Eu2O3 films before and after
annealing.

550

600

650

700

Wavelength (nm)Figure 4. Photoluminescence spectra ofdifferent Eu2O3 specimens.

As shown in figure 4(b), the B-form (monoclinic crystalline phase) with Cs point group can exist atroom temperature, as a metastable state. Moreover, Eu2O3 tends to be structurally degenerated at room
temperature. Therefore, some local structural mixture of the B and C forms may occur in Eu2O3fine
particles, as seen in figure1. This is compatible with the results obtained for Eu3+ doped Y2O3
nanocrystals and Eu2O3 nanocrystals [9, 10].
The as-deposited and annealed films were irradiated with the 325 nm laser light in oxygen gas of
1.01 x 105 Pa or in air, the specimens showed red emission which can be seen with the naked eye.
When the specimen chamber was evacuated, the PL colour changes from red to white. The intensity of
the white emission increases with increasing irradiation time, which can be clearly seen with the naked
eye. Oxygen gas of 1.01 x 105 Pa was again introduced in the chamber. The red PL colour appeared
again, while the white-colour PL decreased in intensity with increasing irradiation time. Such PL
colour change took place repeatedly with sufficient reproducibility, when the kind of atmosphereabout specimen was exchanged. After removing the 325 nm laser light irradiation, each PL-state isstored for more than several years at room temperature under room light, regardless of any change ofatmospheres (1.01x105 Pa O2 gas and air). The change to white PL occurs only as a result of the CW
325 nm laser light irradiation in vacuum, while we have not observed the colour change in any otherreducing atmosphere; for example, Ar-H2 mixture gas. The change to white PL can be observed at
different temperatures from 7 K to room temperature. This indicates the observed spectral changes are
purely electronic phenomena and are not phonon-assisted.
By irradiating with the same 325 nm laser light, we have then measured the PL spectral change of
all of the films listed in table 1 at room temperature under both vacuum of 1.33 x 10-4 Pa and193

atmospheric O2 gas, as a function of irradiation time. As typical results, the spectra obtained for the
specimens (#1α and #1β) and the specimens (#3α and #3β) are shown figures 5 and 6, respectively.
The spectral change for 30 minutes is shown in the figures at intervals of 2 minutes for each specimen.
The arrows indicate progress of irradiation time. The kind of atmosphere is given in each figure.
in oxygenin oxygen

(d)

(d)

V
O

#1β
400

#1β

in oxygen(b)#1α

in vacuum

(a)

600

700

in vacuum(c)PL intensity (arb. scale)

PL intensity (arb. scale)in vacuum

500

#3β

(c)
#3β

in oxygen

(b)
#3α

V
O
400

500

600

700

in vacuum

(a)
#3α

#1α

400

450

500

550
600
Wavelength (nm)

650

700

Figure 5. Photo-induced spectral changeof Eu2O3 films (#1α and #1β).

400

450

500

550

600

650

700

Wavelength (nm)Figure 6. Photo-induced spectral changeof Eu2O3 films (#3α and #3β).

The speed of the spectral change is increased with increasing laser fluence. The annealed films(#2β, #3β, #4β) show larger PL-intensity change than the as-deposited films (#2α, #3α, #4α).
Specimen fabricated under higher oxygen pressure shows more structure-less white luminescence. We
also studied the spectral transition in all the specimens for more than four hours. The results for the#3α- and #3β specimens are shown in the insets in figures 6(b) and (d), as typical results. In the insets,
V and O indicate vacuum and oxygen atmosphere, respectively. The irradiation time was about 120
minutes for each atmosphere. It is noted that the white PL band increases in intensity with depressing
5
D0→ 7FJ emissions. Then, the structure of the white band becomes pronounced.
4. Mechanism of the photo-induced spectral transition
The results given in the section 3 suggest that the reversible spectral transition observed in the Eu2O3
fine particle films arises from photo-activated oxidation and reduction near particle surfaces. Since the
325 nm laser light cannot dissociate directly free O2 molecules as atmospheric gas, the photo-induced
oxidation indicates that the photo-dissociation energy of O2 is considerably decreased at the particle
surfaces by the interactions between O2 and Eu2O3. The photo-activated reduction of Eu2O3 fineparticle is accompanied both by a valence-number change of europium ions (Eu3+→ Eu2+) and byoxygen defect formation. Broad band emission may be connected with Eu2+ ions and local structuralchanges arising from oxygen defects. We prepared also a EuO film by reducing Eu2O3 powder
compact at 1273 K in vacuum. The EuO film shows a broad PL band centred at about 1.03 µm. This
centre wavelength is far away from that of the observed white PL. Therefore, the photo-generated Eu2+
ions and oxygen defects do not form any structure based on EuO crystal, but they may form (Eu2+-

194

oxygen vacancy-Eu2+) complex. Therefore, the observed photo-induced phenomena can be described
the following reaction:
CW 325 nm laser photon
Eu 2 O3 in lattice ←⎯⎯⎯⎯⎯⎯⎯⎯→( Eu 2+ − oxygen vacancy − Eu 2+ ) complex +

1
O2
2

(1)

2+

In the complex, the Eu ions act as hole traps (Sh), while the oxygen vacancy do as electron traps(Se). Toyozawa [11] proposed three types of symmetry-breaking instabilities of excitons in the phonon
field and when the electron and the hole have deformation potentials of opposite sign, thedecomposition into a pair of self-trapped particles occur. In other word, the instability of an exciton
leads to lattice decomposition into an electron centre (an anion vacancy) Se and a hole centre (an anioninterstitial) Sh, if the exciton is formed in the bulk. On surfaces, the hole centre is emitted from thesurface, thus leaving only the electron centre. Therefore, such exciton instability may result in oxygen
desorption at Eu2O3 surfaces. Unlike alkali halides, a single interband excitation never induces thedesorption of surface atom in semiconductors, since the binding energy of atoms is larger than theenergy gap. In such semiconductor case, only atom around crystal defect at the surfaces can bereleased for single photoexcitation [12]. In the present Eu2O3 specimens, the seeds of the observed
photo-induced spectral change may be the defects which had already existed at particle surfaces. It has-13been said that the desorption via such excited state occurs within 10 s after a laser pulse incidence.
However, considerable time is taken to complete the spectral change in Eu2O3, as seen in figures 5 and6. This indicates that the photo-induced oxygen desorption probability is very low at the Eu2O3 particle
surfaces. The saturation tendency observed at further increasing irradiation time suggests that photoinduced oxygen defect structure increases the lattice potential energy of the light irradiated surface. In
such case, equilibrium may be realized in a system composed of a matter (Eu2O3 particles) and aradiation (laser photon) field.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of
Education, Science, Sports, Culture and Technology, Japan. This work was supported by an
Interdisciplinary General Joint Research Grant (Nihon University).
References[1] Gschneid K A Jr, Eyring L R 1979 Handbook on the Physics and Chemistry on Rare Earths ,vol. 3 (Amsterdam: North-Holland).
[2] Mochizuki S, Suzuki Y, Nakanishi T, Ishi K 2001 Physica B 308-310 1046.
[3] Mochizuki S, Nakanishi T, Suzuki Y, Ishi K 2001 Appl. Phys. Lett. 70 3785.
[4] Wakefield G, Keron H A, Dobson P J, Hutchison L H 1999 J. Colloide Interface Sci. 215 179.
[5] Hong K S, Meltzer R S, Bihari B, Willams D K, Tissue B M 1998 J. Lumin. 76&77 234.
[6] Powder Diffraction File: Inorganic Phases Sets 1-50, International Center for Diffraction Data,
Pennsylvania, 2000
[7] Dicke G H 1967 Spectra and Energy Levels of Rare Earth Ions in Crystal (New York:Interscience, p. vi).
[8] Caros L D, Videira A L L 1994 Phys. Rev. B 49 11721.
[9] Williams D K, Yuan H, Tissue B 1999 J. Lumin. 83-84 297.
[10] Eilers H, Tissue B M 1996 Chem. Phys. Lett. 251 74.
[11] Toyozawa Y 1983 Physica B 112&118 23.
[12] Itoh N, Stonenham A M 2000 Material Modification by Electronic Excitation (Cambridge:
Cambridge University Press).

INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 17 (2005) 923–948

doi:10.1088/0953-8984/17/6/011

Photoluminescence and reversible photo-induced
spectral change of SrTiO3
Shosuke Mochizuki1 , Fumito Fujishiro and Seiko Minami
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40
Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
E-mail: motizuki@physics.chs.nihon-u.ac.jp

Received 7 June 2004, in final form 4 January 2005
Published 28 January 2005
Online at stacks.iop.org/JPhysCM/17/923
Abstract
When strontium titanate (SrTiO3 ) single crystal is irradiated at room
temperature with a 325 nm laser light in an evacuated specimen chamber,the luminescence intensity increases, creating a broad visible luminescencecentred at about 2.4 eV. Then, introducing oxygen gas into the specimen
chamber, the photoluminescence spectrum returns reversibly to the original
weak luminescence under the same laser light irradiation. After removing thelaser light irradiation, each photoluminescent state is stored for a long time atroom temperature under room light, regardless of any changes of atmosphere.
Such photo-induced spectral change has been observed also at different
temperatures from 13 K to room temperature. The observed phenomenon
is explained by means of the photo-induced oxygen defect formation at thesurfaces of SrTiO3 crystal. For the same SrTiO3 single crystal, we have studied
the photoluminescence properties. Besides the 2.4 eV luminescence band, we
have observed new two luminescence bands centred at about 3.2 eV and about
2.9 eV. The energy, 3.2 eV, is close to both the photoluminescence excitationedge energy and the reported band edge energy of SrTiO3 crystal. Both the
3.2 eV luminescence and the 2.9 eV luminescence decay rapidly after a pulsed
photoexcitation, while the 2.4 eV luminescence lasts for several seconds at
13 K. The excitation light intensity dependence of these luminescence bands
has been also measured at 13 K. The 2.4 eV luminescence increases in intensitywith increasing excitation intensity up to 4 mJ cm−2 , and then it becomesdecreased with further increase in the excitation intensity. On the other hand,both the 3.2 eV luminescence and the 2.9 eV luminescence increase in intensitywith increasing excitation intensity, without any saturation. Although the
2.4 eV luminescence had been assigned to the radiative decay of intrinsic selftrapped excitons in a superparaelectric state by several workers, the present
studies have clarified that the luminescence originates mainly from crystal
defects (oxygen defects and chemical heterogeneity in the surface region).
1 Author to whom any correspondence should be addressed.

0953-8984/05/060923+26$30.00© 2005 IOP Publishing Ltd

Printed in the UK

923

924

S Mochizuki et al

Both the 3.2 eV luminescence and the 2.9 eV luminescence are discussed
qualitatively.

1. Introduction
Considerable attention has been given to clarifying the quantum paraelectric state in strontium
titanate (SrTiO3 ). The lattice of SrTiO3 has the cubic perovskite-type structure at room
temperature. At 105 K, it transforms into a tetragonal structure. With temperature decreasingfrom room temperature, the static dielectric constant increases prominently, and it attains
several ten thousands at 4 K without any paraelectric–ferroelectric transition (Müller and
Burkard 1979). Between 4 and 0.3 K, the dielectric constant was found to be independent of
temperature. Recently, a prominent enhancement of the dielectric constant has been observed
under ultraviolet light irradiation (Katsu et al 2001, Takesada et al 2003, Hasegawa et al 2003),
but it is now under dispute, involving some complicated problems relating to electrode/SrTiO3
interfaces. In order to explain the quantum paraelectric state, one may assume first that Ti4+
ions are fluctuating quantum mechanically in the SrTiO3 lattice, as if they are in the critical state
just before the paraelectric–ferroelectric transition in usual ferroelectrics. Müller et al (1991)
proposed that SrTiO3 is in a coherent quantum state associated with a rotonic minimum in thetransverse acoustic mode. For the photo-enhanced dielectric constant, Nasu (2004) assumed
that ultraviolet light broke the crystal inversion symmetry, creating ferroelectric microdomains.
He assumed also that such microdomains fluctuated and itinerated quantum mechanically in the
SrTiO3 crystal. SrTiO3 crystal has an indirect gap and a direct gap in an energy range between
about 3.2 eV and about 3.5 eV (Cohen and Blunt 1968, Blazey 1971, Capizzi and Frova 1970,
Capizzi et al 1972, Cardona 1965, Zollner et al 2000, Hasegawa et al 2000). It is known thatthe luminescence intensity is specimen dependent (Grabner 1969). Hasegawa et al (2000)
measured the optical absorption and photoluminescence spectra at temperatures between 10 K
and room temperature. They found that the crystal exhibited a broad photoluminescence bandaround 2.4 eV. Through time-resolved photoluminescence measurements, they assigned the
2.4 eV luminescence to the radiative decay of intrinsic self-trapped excitons. However, many
metal oxides display very similar broad photoluminescence between 2 and 3 eV: for example,
Sm2 O3 (Mochizuki 2003), Eu2 O3 (Mochizuki et al 2001), anatase TiO2 (Mochizuki et al 2003),
Al2 O3 (Mochizuki and Araki 2003a),vitreous SiO2 (Mochizuki and Araki 2003b) and LaAl2O3(Kawabe et al 2000). Moreover, their luminescence intensities are specimen dependent,
especially as regards the thermal history and crystal preparation method. Unfortunately, thisspecimen-dependent nature of the 2.4 eV luminescence has been accorded too little respect inthe optical study of SrTiO3 and it has led many researchers astray.
Very recently, we have found at room temperature that the 2.4 eV luminescence band of
SrTiO3 grows under a 325 nm laser light irradiation in vacuum, while it disappears under thesame laser light irradiation in oxygen gas. Through many successive experiments involving
replacing the specimen atmosphere, it was found that the spectral change occurs reversibly
under the 325 nm laser light irradiation. The observed phenomenon relates to the photoinduced oxygen defects at the surfaces of SrTiO3 . Now or never is the time to reinvestigate
the photoluminescence properties of SrTiO3 in detail, taking account of such oxygen defects.
In the present paper, we report the photoluminescence spectrum, its excitation intensity
dependence, the decay profile, the photoluminescence excitation spectrum, the time-resolved
photoluminescence spectrum and the reversible photo-induced spectral change of SrTiO3 single
crystal at different temperatures between 12 K and room temperature in detail.

Photoluminescence and reversible photo-induced spectral change of SrTiO3925

2. Experimental detail
The SrTiO3 crystal was well grown by the Verneuil method and the as-grown crystal is dark blue.
The as-grown crystal was then annealed under an appropriate reducing atmosphere. The crystal
became perfectly colourless and transparent under the annealing. Several as-grown (dark blue)
and annealed (colourless transparent) SrTiO3 single crystals, 10 × 10 × (0.5–1.0) mm in size,
were supplied by Sinkosya Corporation. The broad surfaces of the crystals are optically flatand they are oriented in the [100] direction. It should be noted that such transparent SrTiO3
crystals grown by the Verneuil method have been widely used by many workers for studying
electrical and optical properties. As reference specimens, we have used also several SrTiO3
powder compact discs which were sintered at 1273 K and annealed at different atmosphere.
The photoluminescence (PL) spectra were measured by using an optical multichannel
analyser consisting of a grating monochromator (focal length = 32 cm, grating = 1200 or
150 grooves mm−1 ) and an image-intensified diode-array detector (number of channels =1024, gate time = 5 ns). An Nd3+ :YAG laser (wavelength = 355 nm, pulse width = 4–5 ns,
repetition = 10 Hz), a continuous wave (CW) He–Cd laser (wavelength = 325 nm) and a
monochromatic light source consisting of a 150 W xenon lamp and a grating monochromator(focal length = 20 cm) were used as the excitation source. The excitation light intensity
dependence of the PL spectrum was measured by attenuating the incident laser light intensitywith glass attenuators calibrated in transmissivity. The PL excitation (PLE) spectra wererecorded by varying the excitation light wavelength λex with the same monochromatic lightsource as described above, and by detecting the luminescence light intensity at a desired
wavelength λob as a function of λex with a grating monochromator (focal length = 20 cm) anda synchronous light detection system. The time-resolved PL spectra were taken using the same
optical multichannel analyser to which two delay pulse generators were attached. The delay
time td , which is the measurement start time after a laser pulse incidence, and the gate time tg ,which is the time after td of the spectral measurement, were set with these delay pulse generators
which were controlled with a personal computer. The actual minimum gate time was 5 ns. The
decay curves were measured with an apparatus consisting of a grating monochromator (focal
length = 20 cm) equipped with a photodetection system (time constant = about 1 µs) andthe Nd3+ :YAG laser oscillating at 355 nm. The optical density (OD) spectrum was measured
using another optical multichannel analyser.
An optical cryostat was used for the specimen chamber, in which a closed-cycle
helium refrigerator equipped with a temperature controller was used to change the specimen
temperature between 12 K and room temperature. Another specimen chamber was also used
for the measurement of the reversible spectral change at room temperature. An oil-free vacuum
system was used for evacuating these specimen chambers.
3. Results
3.1. Photoluminescence spectra of SrTiO3 under weak excitation
Figure 1(a) shows the PL spectra of SrTiO3 at different temperatures under a pulsed 355 nm
laser light excitation (pulse width = 4–5 ns, repetition = 10 Hz, fluence = 0.079 mJ cm−2 ).
A broad luminescence band is observed around 2.4 eV at 12 K. The band seems slightly
asymmetrical in spectral shape and it consists of two luminescence bands centred at 2.46 and2.40 eV, which are labelled B and B , respectively. This indicates that this photoluminescencecomes from at least two kinds of luminescent centres. The spectral shape and intensity differ
slightly with the specimens. We measured also the PL spectra of an as-grown crystal as afunction of annealing time at 973 K in a reducing atmosphere. It has been found that the PL

926

S Mochizuki et al

PL intensity (arb. units)

70x10

3

B'

50
40

25 K

30

27 K

20

28 K

NBE

30 K

10

37 K
3.2

2500

(a)

12 K

0

PL intensity (arb. units)

B"
B

60

3.0

2.8

2.6

2.4

2.2

2.0

1.8

NBE
(b)

2000
1500

B
A

1000

37 K

38K

500

58K

0

PL intensity (arb. units)

3.2

3.0

2.8

2.6

2.4

Photon energy (eV)

2.2

2.0

1.8

1

(c)

0.1
0.01

B luminescence

0.001
20

30

40

50

-1

60

70

80

1000/T (K )Figure 1. The photoluminescence spectra of SrTiO3 at different temperatures: (a) T  37 K; (b)T  37 K. (c) The temperature dependence of the integrated intensity. The excitation laser fluence
was 0.079 mJ cm −2 . The spectral integration was performed in a photon energy range between
2.9 and 1.77 eV.

intensity increases with increasing annealing time, while the PL intensity peak energy andspectral width are slightly dependent on the annealing time. With increasing temperature, the2.46 eV luminescence band grows more and then this band becomes together with the 2.40 eV
luminescence band, displaying an asymmetrical band centred at about 2.44 eV above 27 K. We
call this luminescence band the ‘B luminescence band’. On further increasing the temperature,
the B luminescence intensity continues to decrease, while a new luminescence band appears at
2.89 eV and becomes prominent, as seen in figure 1(b). We tentatively call this luminescenceband the ‘A luminescence band’. The A luminescence band seems to extend above 3.2 eV.
The A luminescence band grows to the detriment of the B luminescence and it is superior
to the B luminescence band above 58 K. The integrated PL intensity of the B luminescenceband is plotted against inverse temperature on a semi-logarithmic scale in figure 1(c). The

Photoluminescence and reversible photo-induced spectral change of SrTiO3

927

spectral integration was performed in a photon energy region between 2.9 and 1.77 eV. In thisfigure, the maximum luminescence intensity is normalized to unity. As seen in this figure, theluminescence intensity decreases gradually with increasing temperature to 25 K and, on further
increasing the temperature above 28 K, the intensity decreases exponentially. Approaching
43 K, the intensity becomes constant. Although such temperature dependence approaching a
constant has been not reported for SrTiO3 , a slight sign of such constant intensity appearedin the reported curve (figure 3(b): Hasegawa et al 2000), as some deviation from the curve
fitting to the data. The constant intensity is due to the A luminescence component which
grows gradually at high temperatures. Although the PL band consists of several luminescencecomponents as described above, we subtract the constant intensity from the data and we assume
a phonon-assisted non-radiative decay process for the observed B luminescence. Then, we
perform a curve fitting by using the well-known expression for the integrated PL intensity I (T),I (T ) =1,
1 + C exp(−E/kT )(1)

where C is a constant and E is the height of an average potential barrier separating aluminescent state from the ground state. In addition to such a phonon-assisted non-radiative
process, thermal ionization of luminescence states is also considered to participate in theintensity decrease with increasing temperature. In such a case, E may correspond to theionization energy. In the fitting, the data obtained above 40 K are taken off to avoid the effectof the A luminescence. The best-fitted curve is obtained with a E = 32 meV. The value
is slightly smaller than the value (43 meV) reported by Hasegawa et al (2000). At present,
it is not clear whether the difference arises from the specimen-dependent nature or from theerror in the specimen surface temperature measurements or from the manner of the curve
fitting—performed probably ignoring the effect of the A luminescence.
As seen in figures 1(a) and (b), a sharp luminescence band is also observed at 3.23 eV
which is close to the observed PLE edge (3.26 eV) and the reported indirect band gap energy
Egid (3.27 eV) of SrTiO3 . We tentatively call this luminescence band appearing near the bandedge the ‘NBE luminescence band’. The steep rise above 3.26 eV is due to the parasitic
luminescence of the optical multichannel analyser used.
3.2. Photoluminescence excitation spectra of SrTiO3
The photoluminescence excitation (PLE) spectra of the same SrTiO3 single crystal have also
been measured at different temperatures. Some of the spectra are selected and shown in figure 2,
in which the PL spectrum observed at 13 K is also shown. The luminescence intensity wasmonitored at the intensity peak energy of the B luminescence band. The PLE spectrum displays
a sharp edge at 3.26 eV and a kink at 3.42 eV, which are close to the reported indirect gap
energy Egid (3.27 eV) and the direct gap energy Egd (3.46 eV) of SrTiO3 (Capizzi and Frova
1970), respectively. The PLE spectrum indicates that the B luminescence is caused mainlyby the interband excitations. The energy difference of the B luminescence band from the
PLE edge is about 0.80 eV. The shape of the PLE spectrum differs in an energy region higher
than the PLE edge from the absorption spectra reported by Blazey (1971), Capizzi and Frova(1970), Capizzi et al (1972), Cardona (1965) and Zollner et al (2000). Generally, there aremany crystal defects at the specimen surfaces where photoexcited states tend to decay nonradiatively. Such a luminescence quench effect appears more prominently in an energy region
higher than the fundamental absorption edge. The excitation with photons above the band gap
energy produces energetic electrons and holes, which are eventually thermalized by emitting
phonons. Therefore, the PLE above the band gap is governed not necessarily by the direct

928

S Mochizuki et al

T = 13 K

(b)

PLE

13 K

PL

λob = 2.45 eV

λex = 3.49 eV

25 K
5.0

4.5

4.0

B

3.5

3.0

Photon energy (eV)

Optical density

PL intensity (arb. scale)

20 K

(a)

2.0
1.5
1.0

13 K

0.5

NBE

0.0
5.0

4.0

3.0

2.0

Photon energy (eV)
5.0

4.5

4.0

3.5

3.0

2.5

2.0

Photon energy (eV)Figure 2. The photoluminescence (PL), optical density (OD) and photoluminescence excitation(PLE) spectra of SrTiO3 at 13 K. The luminescence is monitored at the peak energy (=2.45 eV) of
the B luminescence band. The OD spectrum at 13 K and the PLE spectra at different temperaturesare shown in inset (a) and inset (b), respectively.

quenching of highly excited electrons at defect sites but by the change in the probability of theinteraction of electron–hole pairs with defects.
As shown in inset (a) of figure 2, the OD spectrum has a faint tail below 3.2 eV. It isfound that the OD of the tail decreases with annealing in a reducing atmosphere. Although
the B luminescence band is well excited by the interband transition, the PLE spectra indicatethat the luminescence can also be excited even by photons having energies lower than indirect
gap. Under a 442 nm (=2.80 eV) laser light excitation, we have certainly detected the B
luminescence spectrum through a laser light sharp cut filter (HOYA: Y48). The B luminescence
can also be ascertained clearly by the naked eye through the same filter. Under the same
442 nm excitation, we have also found more intense luminescence for sintered SrTiO3 powder
compact specimens (Mochizuki and Fujishiro 2005). These results indicate that there aredifferent defect-related luminescent levels in the energy gap.
As seen in inset (b), the PLE spectral shape changes with increasing temperature,
except for the absorption edge profile. The suppression seen at photon energies higher than
3.7 eV becomes prominent with increasing temperature. The suppression may arise from thetemperature change in the probability of the interaction of electron–hole pairs with defects.
In the following sections, the observed energy difference (0.8 eV), together with the photoinduced spectral change, will be discussed, and it will be concluded that the B luminescenceband comes from the extrinsic luminescent centres around which photo-generated carriers (freeelectron or free hole, or both) are trapped.

Photoluminescence and reversible photo-induced spectral change of SrTiO3

929

(a)

PL intensity (arb. units)

2

10
8
6

30 K

14 K

4

2

59 K

1

49 K

40 K

B luminescence

8
6

10

-5

10

-4

10

-3

10

-2

Time (s)(b)

PL intensity (arb. units)

2

10
8
6

14 K

4

2

1

B luminescence

8
6

0

2

4

6

8

10

12

14

Time (ms)Figure 3. The photoluminescence decay profile of the B luminescence band of SrTiO3 : (a)log (PL intensity)–log t plots at different temperatures, (b) log (PL intensity)–t plots at 14 K.
The photoluminescence was monitored at 2.45 eV. The 355 nm excitation laser fluence wasapproximately 0.2 mJ cm −2 .

3.3. The photoluminescence decay profile of SrTiO3
We have measured the luminescence decay curves of the B luminescence of the same SrTiO3
single crystal at different temperatures. The luminescence was monitored at an intensity peakwavelength of the B luminescence band. Figure 3(a) shows the PL decay profiles of the B
luminescence at different temperatures. The PL intensity is plotted against time on a log–log
scale. The decay profile at 14 K is also displayed as a semi-log plot in figure 3(b). It is notedthat the crystal displays luminescence for times longer than several tens of milliseconds at
14 K. The profile at 14 K can be well expressed as a three-component exponential decay curve,(2)
I (t) = I1 exp(−t/τ1 ) + I2 exp(−t/τ2 ) + I3 exp(−t/τ3 ),with the following parameters:
I1 = 15.408,
τ1 = 2.6533 × 10−6 s,
I2 = 11.134,
τ2 = 5.4218 × 10−4 s,
I3 = 16.909,
τ3 = 2.8396 × 10−3 s.
Since the photodetection system used for decay curve measurements has a long time constant(about 1 µs), the shorter lifetime component is not resolved. Therefore, the B luminescence
may arise from at least three kinds of luminescence centres.

930

S Mochizuki et al
35x10

3

PL intensity (arb. units)

T = 13 K

4.0 mJ/cm

2

(a)

30
25
2

10 mJ/cm

20
15

2

14 mJ/cm
10

-2

5

2

8.1x10 mJ/cm

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

35x10

3

T = 13 K

B luminescence

30

(b)

25
20
15
10

NBE luminescence

5
0
0

2

4

6

8

2

10

12

14

Laser fluence (mJ/cm )Figure 4. The excitation light intensity dependence of the photoluminescence spectrum of SrTiO3
at 13 K: (a) spectra under intense, medium and weak excitations; (b) the excitation laser fluence
dependence of the peak intensities of the near-band-edge luminescence (NBE) band and the B
luminescence band. The NBE luminescence peak energy and the B luminescence peak energy
are 3.2 and 2.4 eV, respectively.

Alternatively, the same decay data were examined with a power law (t −n ). It is foundthat the curve cannot be uniquely expressed with the t −2 law (Tsang and Street 1979); rather
it has a t −1−2 dependence. Hasegawa et al (2000) performed also decay curve fitting with thet −n law, and they inferred suitability of the t −2 law for SrTiO3 . However, a caution should be
required for result to differ greatly by which data points are selected for the curve fitting withthe power law.
At the present stage, the observed decay curve should be regarded obediently as an
exponential decay curve consisting of different components. The long lifetime component
may arise from some radiative transitions of different deeply trapped states. After stopping thelaser irradiation of the as-grown and annealed SrTiO3 crystals, we frequently observed faintphosphorescence lasting for several seconds, with the naked eye. Unfortunately, the decay timeof such a phosphorescence component (t  10−3 s) cannot be determined from the decay
curve analysis, because the phosphorescence contribution to the decay curve is too small.
3.4. Photoluminescence spectra of SrTiO3 under intense excitation
We have measured the PL spectra of the same SrTiO3 single crystal at different excitation laser
fluences. A typical result is shown in figure 4(a). In figure 4(b), the peak intensities of thePhotoluminescence and reversible photo-induced spectral change of SrTiO3

PL intensity (arb. units)

50x10

B

3

40

931

12 K
17 K
19 K

NBE

(a)

21 K
23 K
25 K
27 K
29 K

30

20

A

10

31 K
39 K

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

PL intensity (arb. units)

Photon energy (eV)
300

NBE

T = 13 K

(b)

200
100
0
3.35

3.30

3.25

3.20

3.15

3.10

Photon energy (eV)Figure 5. The photoluminescence of SrTiO3 under intense photoexcitation: (a) photoluminescence
spectra at different temperatures (T  39 K). The 355 nm excitation laser fluence was
0.93 mJ cm −2 . (b) Near-band-edge (NBE) emission at 13 K. The 355 nm excitation laser fluence
was 12 mJ cm −2 .

NBE and the B luminescence bands are plotted against laser fluence. Solid squares and hollow
circles correspond to the NBE luminescence and the B luminescence, respectively. Under
weak excitation, only a B luminescence band is observed. With increasing laser fluence,
the B luminescence intensity increases monotonically, and the NBE and A luminescencebands appear. On the laser fluence further increasing up to 4 mJ cm−2 , the B luminescenceattains a maximum, while both the NBE luminescence and the A luminescence become
prominent. On laser fluence exceeding 4 mJ cm −2 , the B luminescence intensity begins todecrease monotonically, while both the NBE luminescence band and the A luminescence band
continue to grow, degrading the B luminescence band. Incidentally, we have found that the
B luminescence intensity maximum appeared at 0.4 mJ cm−2 for the sintered SrTiO3 powder
compact. This suggests that most of the B luminescence centres are at the specimen surface.
In order to study the NBE, A and B luminescence bands in detail, we have measured
the PL spectra at different temperatures under an intense excitation of 0.93 mJ cm−2 . This
excitation intensity is enough to show up all of the luminescence bands. The results are shownin figures 5(a), 6(a) and (b). We subtract the spectrum measured at 297 K from the spectrum
measured at 12 K, in an attempt to extract the B luminescence component. The difference
spectrum is given in figure 6(c). By comparing this spectrum with the spectrum (figure 1(a))
observed at the same temperature under a weak photoexcitation, it was found that the spectral

932

S Mochizuki et al

30x10

159 K

3

(a)

139 K

20

PL intensity (arb. units)

10

99 K

39 K

0
30x10

3

198 K
237 K
278 K

159 K

(b)

20
297 K

10
3

60x100
50

(c)

40
30
20
10
0

PL intensity (arb. units)

3.2

3.0

2.8

2.6

2.4

Photon energy (eV)

2.2

2.0

1.8

1

(d)
0.1
0.01

weak excitation
intense excitation

B luminescence

0.001
20

30

40

50

-1

60

70

80

1000/T (K )Figure 6. The photoluminescence of SrTiO3 under intense photoexcitation (0.93 mJ cm −2 ): (a)photoluminescence spectra at different temperatures (39 K  T  159 K); (b) photoluminescence
spectra at different temperatures (159 K  T  297 K); (c) the difference spectrum; (d) thetemperature dependence of the integrated intensity of the B luminescence band for weak andintense excitations which were 0.079 and 0.93 mJ cm −2 , respectively. The spectral integration was
performed in a photon region between 2.9 and 1.77 eV.

shape of the B luminescence band is almost independent of the photoexcitation intensity. With
temperature increasing up to 39 K, the B luminescence intensity decreases and then it disappears
at 39 K, while the NBE and A luminescence intensities are almost constant. With temperature
increasing above 39 K, the A luminescence band continues to grow more without changing
the spectral shape and then it attains a maximum at 159 K, while the NBE luminescence band
becomes weak with the spectral broadening and is merged in the A luminescence band at 198 K.
At present, it is not clear why such a phonon-assisted nature of the A luminescence band arose.
With the temperature further increasing above 159 K, the A luminescence intensity decreases.
This intensity decrease may be due to non-radiative traps. The PL spectrum measured at 297 K
was subtracted from that measured at different temperatures, and then the spectral integration
was performed over a photon energy range between 2.9 and 1.77 eV. The integrated intensities
thus obtained for the B luminescence band are plotted against inverse temperature, as hollow

PL intensity (arb. units)

Photoluminescence and reversible photo-induced spectral change of SrTiO3

1400
1200

933

2

(a)

T=13K: 4.0 mJ/cm

1000
0ns - 1ms

800
600

50ns - 1ms

400

0ns - 2µs

200
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

1400
2

(b)

T=13K: 10 mJ/cm

1200
1000

0ns - 1ms

800
600
400

0ns - 2µs

20050ns -1ms

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 7. The time-resolved photoluminescence spectra of SrTiO3 at 13 K: (a) laser
fluence = 4.0 mJ cm −2 ; (b) laser fluence = 10 mJ cm −2 .

squares, in figure 6(d). The data obtained for weak excitation, which are already shown infigure 1(c), are also plotted as solid circles in this figure.
Under a high laser fluence (12 mJ cm−2 ), more detailed measurements were performed
also on the NBE luminescence and the A luminescence, using a high resolution monochromator.
This laser fluence is enough to show up the NBE and A luminescence bands. The result obtained
at 13 K is given in figure 5(b). As seen in this figure, the NBE luminescence band is on the riseof the A luminescence band. The NBE luminescence peak is about 77 meV lower in energy
than the onset energy of the A band, and it is about 34 meV lower than the PLE edge energy.
The NBE luminescence band and the A luminescence have similar temperature and excitation
intensity dependences. This suggests that the NBE luminescence and the A luminescence havethe same origin.
Most of luminescent metal oxides show some photo-darkening effects under intense
photoexcitation. It is found that the PL spectrum and PL intensity of the SrTiO3 single crystal
used were almost unchanged, in spite of such intense 355 nm photoexcitation (14 mJ cm−2 ).
3.5. Time-resolved photoluminescence spectra of SrTiO3Time-resolved photoluminescence spectra of the same SrTiO3 single crystal have beenmeasured at 13 K under different excitation intensities. To study the time evolutions of
all of the luminescence bands, the measurements were performed under intense excitation.
The spectra obtained under excitations of 4.0 and 10 mJ cm −2 are shown in figures 7(a)and (b), respectively. The measurement times, td and tg , are given by each curve. At the934

S Mochizuki et al

initial stage of photoexcitation, the laser light pulse is seen at twice the wavelength, 710 nm(=355 nm × 2 = 1.745 eV), of each spectrum, which is common to every grating
monochromator. Both the NBE luminescence and the A luminescence disappear rapidly
within 50 ns after excitation. Taking account of the parasitic delay, timing jitter times of theapparatus used and the actual laser pulse duration (10 ns), the lifetimes of the NBE and A
luminescences are estimated to be less than 10 ns. The B luminescence band is growing overtime between 50 ns and 1 ms, and the spectral shape is almost unchanged with further increasein time. The B luminescence observed between 50 ns and 1 ms is slightly shifted toward
the lower energy side of the B luminescence observed between 0 ns and 1 ms, as shown infigure 7(a). The shift is due to the time integration effect on the PL intensity. The measured
intensity between td and tg is expressed as follows:  tgI (t) =
η j N j (t = 0)e−t/τ j dt,
(3)
j

td

where η j , N j (t = 0) and τ j are the quantum yield, number and lifetime of j th luminescencecentre including the extrinsic and intrinsic luminescence centres, respectively. If theluminescence centres responsible for the B luminescence band differ slightly in energy, asmall shift may be anticipated, as is in fact observed.
Both the PLE spectrum shown in figure 2 and these time-resolved spectra show clearly thatthe B luminescence follows the NBE and A luminescence. The long lifetime luminescence,
continuing for more than several tens of milliseconds, like the B luminescence, is not peculiar to
SrTiO3 but such long lasting luminescence is frequently observed for different metal oxides, forexample, ZrO2 (Fujishiro and Mochizuki 2005). This indicates that the B luminescence cannot
necessarily be connected with the nature of the quantum paraelectric state. Such long lifetime
luminescence observed for different oxides is thought to arise from accidental recombinationof shallowly trapped electrons with distant holes localized around defects. In such a case, theluminescence has slightly different long lifetimes, as seen in figures 3(a) and (b), and the B
luminescence spectral shape is almost unchanged with time, as seen in figures 7(a) and (b).
3.6. Reversible photo-induced spectral change in SrTiO3When the same SrTiO3 single crystal is irradiated with a CW 325 nm laser light at room
temperature in an evacuated chamber, the B luminescence intensity is enhanced to about
twice. Then, introducing oxygen gas into the specimen chamber, the intense PL state returns
to the original weak one under the same laser light irradiation, with increasing irradiation time.
Through many successive experiments, it was found that the spectral change is nearly reversible
under 325 nm laser light irradiation. After removing the 325 nm laser light irradiation, each
PL state is stored for a long time at room temperature under room light, regardless of any
changes of atmosphere. A typical example of the photo-induced spectral change obtained isshown in figure 8. The experiments were carried out in the following order: (a) → (b) → (c)→ (d). Irradiation times tir under a given atmosphere and the kind of atmosphere are indicatedby each curve. Irradiating under oxygen gas considerably decreases the PL intensity of thespecimens. In figure 9, the difference spectrum, curve 7 − curve 8, is shown. The PL spectralchange observed for the SrTiO3 single crystal is mainly characterized by an intensity changeof the B luminescence band. A slight degradation of the PL intensity is observed, which may
be caused by some photo-induced non-radiative centres. The same experiments were carriedout by changing the wavelength of the irradiating laser light. No spectral change was observed
except for the CW 325 nm laser light. It was found that the speed of the change accelerates
on increasing the power density of the activating light.

Photoluminescence and reversible photo-induced spectral change of SrTiO3935

in vacuum
1500

2: tir = 60 min

(a)

1000
500

1: tir = 0 min
0in oxygen

PL intensity (arb. units)

1500

3: tir = 0 min

(b)

1000
500

4: tir = 60 min
0in vacuum

(c)

1500

6: tir = 60 min

1000
500

5: tir = 0 min
0in oxygen

(d)

1500

7: tir = 0 min
1000
500

8: tir = 60 min
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 8. The photo-induced reversible spectral change of SrTiO3 at room temperature: (a) in
vacuum, (b) in oxygen gas, (c) in vacuum, (d) in oxygen gas. The experiments were carried out inthe following order: (a) → (b) → (c) → (d). Each irradiation was carried out for 60 min under agiven atmosphere. Each spectral change is indicated by an arrow. The 325 nm laser fluence was
0.79 W cm −2 .

Similar photo-induced spectral change has been observed at different temperatures from13 K to room temperature. The result obtained at 13 K in vacuum under irradiation with325 nm laser light is shown in figure 10(a), as a typical result. The tir in vacuum is indicatedby each curve. The peak intensity is plotted against tir in figure 10(b). With increasing tir , theintensity increases and then tends to saturate at 90 min. The intensity increase is proportional
to tirn (n < 1). It has been found that the magnitude of the spectral change for 60 min is nearly
independent of temperature. After stopping the laser light irradiation, the photoluminescenceintensity decreases slightly with increasing time and then reaches a steady state value, which
is a photomemory phenomenon.

936

S Mochizuki et al800

curve 7 - curve 8

PL intensity (arb. units)

600

400

200

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 9. The difference spectrum for curve 7 and curve 8. Curves 7 and 8 are shown in figure 8.

Like for Sm2 O3 (Mochizuki 2003), Eu2 O3 (Mochizuki et al 2001), anatase TiO2
(Mochizuki et al 2003) and vitreous SiO2 (Mochizuki and Araki 2003b), the observed spectral
transition may arise from photo-induced oxidation and photo-induced reduction.
3.7. Photoluminescence properties of as-grown SrTiO3
In order to elucidate the crystal defect effects on the photoluminescence more directly, we
have studied the reversible photo-induced spectral transition for an as-grown Verneuil SrTiO3
crystal at room temperature. The as-grown SrTiO3 crystal is dark blue. The luminescence was
excited also with a 325 nm laser line of the He–Cd laser. The result is shown in figure 11(a).
Curve 1 is the spectrum of the as-grown crystal. The PL intensity of the as-grown crystal isstronger than several thousands times that of the annealed crystal. The spectrum consists of atleast three luminescence bands, at 2.9, 2.3 and 2.2 eV. Curve 2 is the spectrum measured after
irradiating in oxygen gas for 60 min. Under 325 nm laser light irradiation in oxygen gas, thePL intensity is decreased considerably. The degradations of the 2.3 and 2.2 eV luminescencebands are especially prominent. The specimen chamber is evacuated again. Curve 3 andcurve 4 are the spectra measured at 60 and 480 min after beginning irradiation in vacuum,
respectively. Then, oxygen gas was introduced again into the specimen chamber. Curve 5 isthe spectrum measured at 840 min after beginning irradiation in oxygen gas. The integrated
PL intensity of curve 4 is increased more than 16 times compared with that of curve 5. In
figure 11(b), curve 4 is compared after rescaling curve 5 so as to fit to curve 4 at photon energies
higher than 2.8 eV. The rescaled curve is shown as curve 5 . Like for the annealed transparent

Photoluminescence and reversible photo-induced spectral change of SrTiO3

35x10

3

T = 13 K

30

PL intensity (arb. units)

937

(a)

tir = 90 min

25
60 min20

20 min
6 min

15

0 min

10
5
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)

PL intensity (arb. units)

25x10

T = 13 K

3

(b)

20

15

under irradiation
050

without irradiation
100

150

tir (min)Figure 10. The photo-induced spectral change of SrTiO3 at 13 K in vacuum: (a) photoluminescence
spectra for 325 nm laser light irradiation; the irradiation time is indicated by each curve; (b) theirradiation time dependence of the peak intensity of the photoluminescence. The 325 nm laser
fluence was 0.79 W cm −2 .

SrTiO3 crystal, an A-like luminescence appears in the as-grown crystal: compare curve 5 infigure 11(a) with curve 8 in figure 8(d) and the curve observed at 297 K in figure 6(b). To deducethe information on the B luminescence observed in the annealed SrTiO3 crystal, the difference
spectrum (curve 4 − curve 5 ) was calculated; it is shown in figure 11(b). We can find a broad
luminescence band centred at 2.3 eV, which is slightly shifted to the lower energy side of the
B luminescence band of the annealed crystal. It is noted that the B luminescence intensity
for the as-grown and the annealed crystals can be varied by 325 nm laser light irradiation in
vacuum and in oxygen gas. Since the light penetration depth is very small at 325 nm for
SrTiO3 (Zollner et al 2000), the observed spectral change of the B luminescence band could
be closely related to the change of surface electronic states.
Using the same as-grown crystal, we have measured the photoluminescence spectra atdifferent temperatures under an intense excitation (14 mJ cm−2 ). As a typical result, thespectrum obtained at 12 K is shown in figure 12. The spectrum obtained at 13 K under the938

S Mochizuki et al

30x10

3

(a)
4: in vacuum for 480 min

PL intensity (arb. units)

25

20
3: in vacuum for 60 min
15
1: as grown10
2: in oxygen for 60 min
5
5: in oxygen for 840 min

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

35x10

3

(b)

30

4

25

5'

20
15
10

curve 4 - curve 5'

5
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 11. The photo-induced reversible spectral change of as-grown SrTiO3 at room temperature:(a) photo-induced spectral change for replacing specimen atmosphere, (b) comparison of curve 4
with curve 5. The 325 nm laser fluence was 0.79 W cm −2 .

same excitation intensity for the annealed crystal is also shown; this was already shown infigure 4(a). The NBE and A luminescence are considerably enhanced in the as-grown crystal.
The fine structure and PL intensity of the NBE luminescence band depend on the annealing
time. The results suggest that the NBE and A luminescence are assisted by crystal defects.
3.8. Miscellaneous
We have measured the PL spectra of sintered SrTiO3 powder compacts at different temperaturesand different laser fluences. The powder specimens show clearly the NBE, A and B
luminescences. The NBE and A luminescences remain at room temperature, while the B
luminescence disappears above 40 K. Most of the PL properties were very similar to thoseof single-crystal specimens. Like for other luminescent oxides, the powder compacts wereless luminescent and their luminescence decay lifetimes were shorter than those of the single
crystals. The main difference between the cases for the powder compacts and the single
crystals is that the B luminescence of the powder compacts saturates at lower laser fluence(approximately 0.5 mJ cm−2 ), while that of the single crystals saturates at 4 mJ cm−2 .

Photoluminescence and reversible photo-induced spectral change of SrTiO3

50x10

939

3

2

T ~ 13 K, 14 mJ/cm

PL intensity (arb. units)

40

30

as-grown

20

annealed
10

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 12.
The photoluminescence spectra of the as-grown dark blue and annealedtransparent SrTiO3 crystals at about 13 K under an intense 355 nm photoexcitation (laser
fluence = 14 mJ cm −2 ).

4. Discussion
4.1. The reversible photo-induced spectral change
The present experimental results on the reversible photo-induced spectral change of SrTiO3
single crystal may be summarized as follows.
(1) The photo-induced reversible spectral change between weak PL and intense PL can beobserved at different temperatures, from 13 K to room temperature. The spectral changetakes time. The speed of the spectral change is nearly independent of temperature but it
increases with increasing light intensity.
(2) The spectral change tends to saturate at a long light irradiation time (more than 90 min).
(3) The spectral change is induced by changing the specimen atmosphere between oxygengas and vacuum only under a CW 325 nm laser light. On the other hand, intense pulsed
355 nm laser light (14 mJ cm−2 ) never induced such spectral change.
(4) After removing the 325 nm laser light irradiation, each PL property persists for a long timeeven at room temperature under room light, regardless of any changes of atmosphere.
The result (1) indicates that the observed photo-induced spectral changes are purely
electronic phenomena and they are not phonon-assisted ones. The result (1) indicates also thatthe B luminescence is an extrinsic luminescence due to the luminescence centres introduced

940

S Mochizuki et alby a CW 325 nm laser light. Since the 325 nm (=3.81 eV) laser light never dissociates
directly free O2 molecules, the result (3) indicates that the phenomena arise from the photoinduced associative detachment and photo-induced dissociative adsorption of O2 molecules
near the SrTiO3 surface. In this case, photoexcited SrTiO3 crystal acts as a photocatalyst fordecomposing O2 molecules and also as an oxygen reservoir. The results also summarize how
spectral change arises naturally from photo-activated oxidation and reduction.
Although some chemical heterogeneity on the surface has been pointed out for the surfacesof SrTiO3 crystal reduced and oxidized at high temperatures (Szot et al 1997, Szot and Speier1999), we discuss tentatively the observed photo-induced spectral changes at room temperatureby regarding the measured SrTiO3 crystal as chemically homogeneous, as follows. The
chemical heterogeneity effects will be discussed in the next subsection. The reduction of
the SrTiO3 surface under a CW 325 nm laser light in vacuum may be accompanied by thecreation of electron-captured oxygen vacancies VO , as follows:(O2− in lattice) + (CW 325 nm photon) → VO + (two electrons) + 12 O2 ,(4)

where Kröger–Vink notation is used (Kröger and Vink 1956). Using a local spin density
approximation plane wave pseudopotential method, Astala and Bristowe (2001) have shownthat the doubly positively charged state is the most stable. This is in good agreement withthe experimental result that oxygen deficiency in SrTiO3 enhances the electrical conductivity(Jourdan and Adrian 2003) even at low temperatures. This means that most of the electrons
around oxygen vacancies are released and, therefore, such oxygen vacancy sites are relatively
positively charged. Therefore, the oxygen vacancies tend to trap photo-generated electrons.
On the other hand, several authors (Henrich et al 1978, Cord and Courths 1985, Kimura et al1995) proposed another type of defect at the SrTiO3 surface as follows:(O2− in lattice) + (CW 325 nm photon) → (Ti3+ –VO –Ti3+ ) complex + 12 O2 .

(5)

In the complex, the Ti3+ ions act as hole traps, while the vacancy VO tends to trap electrons.
For SrTiO3 crystal, it is known that electrons determine the transport properties, for example,
electrical conductivity and photoconduction, well, while no phenomena related to holes havebeen observed. This indicates that holes are almost trapped around crystal defects. Incidentally,
the high dielectric constant (several tens of thousands) of SrTiO3 crystal suppresses, creating
excitons (ex). Toyozawa (1983) proposed three types of symmetry-breaking instability for
excitons in the phonon field and, when the electron and the hole have deformation potentials
of opposite sign, decomposition into a pair of self-trapped particles occurs. In other words,
the instability of an exciton leads to lattice decomposition into an electron centre (an anion
vacancy) Se and a hole centre (an anion interstitial) Sh , if the exciton is formed in the bulk.
On surfaces, the hole centre is emitted from the surface, thus leaving only the electron centre.
Therefore, such exciton instability may result in oxygen desorption at SrTiO3 surfaces invacuum and the oxygen vacancies thus produced give rise to B luminescence. Similar
desorption has already been discussed for photo-induced breaking of the Si–O bond in asilica (Shluger and Stefanovich 1990). By a quantum chemical calculation, Eglitis et al(2004) has demonstrated that the chemical bonding of the perovskite structure contains aconsiderable covalent component, like the case for silica. It is noted that CW 325 nm laser
light is indispensable for spectral change due to the photo-induced oxidation and reductionin SrTiO3 . The CW laser fluence in the present experiments was at most 0.8 W cm−2 , andtherefore any bond-breaking mechanism based on the instability under high density excitation
may be excluded from the present discussion. Unlike the case for alkali halides, a single
interband excitation never induces the desorption of surface atoms in semiconductors, since
the binding energy of atoms is larger than the energy gap. In such photoexcited semiconductor

Photoluminescence and reversible photo-induced spectral change of SrTiO3

941

cases, only atoms around crystal defects at the surfaces can be released (Puchin et al 1993, Itohet al 1995, Itoh 1995, Itoh and Stoneham 2000, Song and Williams 1993). Astala and Bristowe(2001) calculated the oxygen vacancy formation energy of about 7 eV, which is approximately
twice the bulk energy gap and slightly smaller than twice the CW 325 nm He–Cd laser photon
energy, at the TiO2 -terminated (100) surface of SrTiO3 . Both the oxygen defects (for example,
Ti3+ –OV –Ti3+ and OV ) inherent to SrTiO3 surfaces and the infinite continuity of the laser light
may enable the 325 nm laser photons to release O2 molecules from the surface. Incidentally,
pulsed 355 nm laser light (the pulse width is 4–5 ns), although the photon energy is close to thatof the CW 325 nm laser light, never induced spectral change even at 14 mJ cm −2 , as a resultof (3). Besides the surface defects, a small amount of OH− ions introduced inevitably during
crystal growth by the Verneuil method should also be taken into account for the photo-induced
spectral change. Incidentally, Ohmukai et al (1999) have observed a 126 nm laser-induced
bond breaking and some polycrystalline precipitates on a SiO2 glass. We could not detect any
light scattering due to precipitates on the 325 nm laser light irradiated SrTiO3 crystal.
It has been said that the desorption via such an excited state occurs within 10−13 s afterlaser pulse incidence. However, it is found that a relatively long time (more than 90 min) istaken to complete the spectral change in SrTiO3 , as a result of (2). This indicates that thephoto-induced oxygen desorption probability is very low at the SrTiO3 surface. The observed
saturation tendency with further increasing tir suggests that photo-induced oxygen defects
increase the potential energy of the light irradiated surface. In such a case, equilibrium may
be realized in a system composed of matter (SrTiO3 crystal) and a radiation field. It can beassumed that the intensity, coherency and continuity of laser light determine the degree offluctuation in the system and the dynamics of photo-induced spectral change. Unlike the casefor a pulsed laser light, the possibility of successive electronic excitation is not ignored for
CW laser light. More detailed experiments with varying frequency, pulse width and duty ratioof a periodic laser light pulse are in progress in our laboratory.
In relation to the UV laser light-induced oxygen defects and white luminescence, it
should be noted that the crystals (for example, LaAlO3 ) grown by light heating methods(for example, the floating zone methods and the arc furnace method) display intense white
luminescence. During the growth, the crystals are exposed to intense ultraviolet radiation
and, therefore, oxygen defects may be produced, exhibiting oxygen defect-related white
luminescence between 3 and 2 eV (Kawabe et al 2000).
4.2. The B luminescence band
The present experimental results on the B luminescence of SrTiO3 single crystal may besummarized as having the following features.
(1) The broad B luminescence band is observed well under band-to-band excitation(>3.27 eV) in SrTiO3 with a large energy shift (0.80 eV). Although the quantum yieldis small, the B luminescence is also observed under a 442 nm (=2.80 eV) laser light
excitation.
(2) The B luminescence intensity is considerably increased by photo-induced reduction with325 nm (=3.81 eV) laser light irradiation in vacuum.
(3) The PL intensity increases with progressive annealing in a reducing atmosphere. The
intensity peak energy and spectral width of the B luminescence depend on annealing
history.
(4) The B luminescence decay curve consists of at least four components.
The
phosphorescence component lasts for at least several seconds. It is hard to fit the decay
curve uniquely to a power law time (t −2 ) dependence.

942

S Mochizuki et al(5) With increasing temperature, the B luminescence intensity decreases, increasing the NBE
and A luminescence bands. The B luminescence of both the single crystal and the powder
compact disappeared above about 40 K.
(6) With increasing excitation intensity, the B luminescence intensity increases. On further
increasing the excitation intensity, the B luminescence intensity attains a maximum atabout 4 mJ cm−2 and then decreases, increasing the NBE and A luminescence bands. The
SrTiO3 power compacts display the B luminescence intensity maximum at approximately
0.5 mJ cm−2 .
Feature (1) indicates that the B luminescence arises from several kinds of PL mechanisms.
Both feature (2) and the feature (3) indicate that oxygen deficiency is a main cause of B
luminescence. Feature (4) indicates that the deviation from stoichiometry creates different
types of defect which give different luminescence centres and traps. Most workers have been
studying SrTiO3 crystals produced by the Verneuil method. Unfortunately, the compositional
deviation of the crystals produced by the Verneuil method is known to be between 10−2 and10−4 even for simple transition metal oxides (MnO, CoO, NiO and TiO2 ). The chemical
purities and isotope separation of the constituent elements (especially strontium and titanium)
are not necessarily satisfactory and therefore most of the available SrTiO3 crystals are at most
99.99% pure. These are problems common to many transition metal oxides. Moreover, since
a flame consisting of H2 gas and O2 gas is used for the Verneuil crystal growth process, somecontamination due to OH− ions may be inevitable. Feature (6) indicates that most of the
B luminescence centres are at the specimen surface. Ignoring the above-described chemical
impurity effects and noting only the effects due to the deviation from SrTiO3 stoichiometry,
we will discuss the results of the PL measurements.
We first discuss the excited states created in SrTiO3 crystal by the third harmonics (355 nm)of the pulsed Nd3+ :YAG laser. Unlike CW 325 nm laser light, this 355 nm laser light neverinduced the spectral changes discussed in the previous subsection. The valence and conduction
bands of SrTiO3 mainly consist of the 2p orbits of O2− ions and the 3d orbits of Ti4+ ions,
respectively. Therefore, the fundamental absorption transfers the electron from the O2− ionto the Ti4+ one. After such photoexcitation, some of the photo-produced electrons and holes
recombine directly, displaying luminescence via some processes with short lifetime. However,perfect SrTiO3 crystal is an indirect gap semiconductor and, therefore, the PL intensity is so
weak that it cannot be measured. Another portion of electrons and holes may be trapped byintrinsic crystal defects at the surface and by the defects related to the deviation from thestoichiometry. In such a case, the PL property differs with the type of defect and also with thesites of defects, as follows.
4.2.1. Oxygen defects at the surface and in the bulk. Stashans and Vargas (2001) havecalculated structural and electronic properties of the F centres (two electrons trapped by anoxygen vacancy) in SrTiO3 , as have Astala and Bristowe (2001). The results indicate thatthe wavefunctions of the two extra electrons extend over the titanium atoms closest to thetwo vacancies and over other nearby atoms, which is the same as the Ti3+ –oxygen vacancy
complex model proposed by Henrich et al (1978), Cord and Courths (1985) and Kimura et al(1995). In other words, most of the oxygen vacancies are Ti3+ –oxygen vacancy complexes
in real SrTiO3 crystal. In this complex, Ti3+ ion is one of the candidate hole-trapping centres
and the oxygen vacancy is one of the candidate electron-trapping centres. Since electricalconduction due to holes is not observed, most of the photo-generated holes may be deeply
trapped at Ti3+ sites of the complexes. Unlike such holes, photo-generated electrons may
be itinerant, experiencing repeatedly collisions with phonons and trapping by oxygen defects.

Photoluminescence and reversible photo-induced spectral change of SrTiO3

943

When some portions of photo-generated holes and electrons are trapped at distant complexes, aluminescence arises from accidental recombination of shallowly trapped electrons with distantholes localized around the defects, exhibiting a long lasting luminescence. Taking account of
the electronic conductivity, both the absorption tail in the OD spectrum shown in the inset (a)of figure 2 and the energy difference of 0.80 eV may guarantee the existence of such deeply
trapped hole states and shallowly trapped electron states. The broadness of the OD tail shownin the inset (a) of figure 2 suggests that such hole trap levels and electron trap ones arise
from many slightly different defects. Using ultraviolet photoemission spectroscopy (UPS),
Henrich et al (1978) found that the Fermi level of the fractured SrTiO3 lies about 3 eV above
the top of the valence band, which places it within 0.1 eV of the bottom of the conduction
band. Also, they observed surface sensitive band gap emission band at energies between the
top of the valence band and the Fermi level. These UPS results confirm also the existence ofband gap states of the SrTiO3 surface. Thus, the energy difference of 0.8 eV may correspondto the average energy difference between the shallow electron trap levels and the deep hole
trap levels in the band gap. Since the observed luminescence decay curve was expressed asa multi-component exponential curve (at most four components), the overlapping between
electron and hole wavefunctions is thought to be almost time independent.
With increasing laser fluence, the numbers of photo-generated electrons and holes increase
and then the PL intensity may increase. With further increasing laser fluence, photo-generated
electrons and holes become diffused more rapidly inside the crystal (bulk), and therefore the
PL intensity decreases, as shown in figures 4(a) and (b). This is why the PL intensity maximum
appears at 4 mJ cm−2 . In other words, this means that most of the defect luminescence centres
are at the specimen surface. Since a powder specimen has a large specific surface area, a
SrTiO3 powder specimen may exhibit a PL intensity maximum at the laser fluence smaller thanthat of the single-crystal specimen. This suggestion was certainly confirmed by the present
experimental result that the PL intensity maximum of the B luminescence for the powder
compacts appeared at laser fluences (approximately 0.5 mJ cm−2 ) lower than that (4 mJ cm−2 )observed for the single crystal. On the other hand, the luminescence arising from the defects inthe bulk is thought to saturate at higher laser fluence. In addition to these defect effects,it should
be taken into consideration that, at higher laser fluence, the recombination of photo-generated
electrons and holes is enhanced and therefore it also reduces the luminescence intensity.
4.2.2. Chemical heterogeneity in the surface region. As stated in the previous sections, acolourless transparent SrTiO3 single crystal is obtained by thermally annealing dark blue asgrown crystal at high temperature under an appropriate reducing atmosphere. Although it iscolourless and transparent, the crystal is thought to be affected to some extent by such annealing
treatment. In particular, it has been reported that the surfaces of reduced and oxidized SrTiO3
contain not only the above-described oxygen defects but also Ruddlesden–Popper phases(Ruddlesden and Popper 1957, 1958). The combination of x-ray diffraction analysis withsurface sensitive techniques has revealed a chemical inhomogeneity in the surface region of
single crystals of SrTiO3 prepared under low and high partial pressures of oxygen at elevatedtemperatures (Szot et al 1997). A solid state reaction leads to the formation of a multilayertype structure. For oxidized crystals, they observed SrO-rich Ruddlesden–Popper phases atthe surface and Magnelli phases of Ti in deeper layers of the surface region. The order ofthe layered structure is reversed for the reduced crystals, with Ti oxides of different oxidationlevels at the surface and Ruddlesden–Popper phases in lower parts of the surface region.
Measurements by atomic force microscopy are also reported for the (100) and (110) surfacesof SrTiO3 single crystals prepared with different oxidizing and reducing conditions at elevatedtemperatures (1073–1273 K) (Szot and Speier 1999). The morphology of the surfaces turns

944

S Mochizuki et alout to be drastically altered for both oxidized and reduced crystals in comparison with theoriginal stoichiometric surfaces. The observed changes on the surface of SrTiO3 due to theapplied extensive thermal treatment cannot be explained by the formation of point defects,
relaxation of the uppermost surface layer, rumpling or reconstruction due to vacancy ordering.
Instead, the results have to be interpreted in terms of segregation processes and solid state
reactions at elevated temperatures, which cause the formation of new chemical phases on thesurface and in the region underneath. On the surface of oxygen-annealed SrTiO3 , this leads to
the growth of steps perpendicular to the surface with step heights larger than the unit cell of
the perovskite structure. Crystals prepared above 1173 K are shown to exhibit a step height of
1.18 nm which is attributed to the formation of a Ruddlesden–Popper phase SrO∗ (SrTiO3 )nwith n = 1 on the surface. In the case of reduced crystals, the topographic changes on thesurface are caused by the formation of Ti-rich phases such as TiO and Ti2 O on the surface
above 1173 K. The complex interplay of the processes at the surface for different temperatures,in particular its dependence on the details of the heat treatment, is discussed. The induced
chemical heterogeneity of the surface and in the near-surface region is interpreted in terms ofa kinetic demixing.
Certainly, as shown in figures 11(a) and (b), we observed complicated spectral change
under CW 325 nm laser light irradiation in oxygen gas and vacuum for the as-grown SrTiO3
crystal, which cannot be explained entirely by the simple oxygen point defect model. Since
the as-grown crystal has a higher light absorption coefficient than the colourless annealed
crystal, the observed spectral change may be more affected by the surface crystal defects,
including the chemical heterogeneity (for example, Ruddlesden–Popper phases) in the surface
region.
Bearing these chemical heterogeneities in mind, we have measured the PL propertiesof the as-grown crystal at different stages of annealing in a reducing atmosphere and alsomeasured those of a colourless transparent stepped TiO2 -terminated SrTiO3 crystal whose
step height is 0.4 nm. The detailed results will be reported in a separate paper (Mochizuki
and Fujishiro 2005). The results are summarized as follows. The low temperature PL spectral
structures of these crystals are almost the same as those of the colourless transparent (100) faced
crystal, apart from the PL intensity. However, with progressive annealing, the dark blue colour
becomes weak, the intensities of the NBE, A and B luminescence bands decrease and the lower
energy tail of the PLE for the B luminescence becomes weak. Moreover, the excitation laser
fluence giving the B luminescence intensity maximum (we call this fluence Fmax ) increases
with progressive annealing. Like that for the as-grown crystals, the Fmax for the stepped
TiO2 -terminated SrTiO3 crystal decreases considerably in comparison with the Fmax for thecolourless transparent SrTiO3 crystal. The observed change in Fmax may be due to the change inthe probability of electron–hole pairs with surface defects arising from chemical heterogeneity.
We have observed also at least five broad optical absorption bands at about 2.9 eV, about 2.4 eV,
about 2.1, 1.6 eV and about 0.9 eV for the as-grown crystal at different annealing stages. The
dark blue colour of the as-grown crystal is attributed to this spectral structure in which the blue
colour wavelength range is a spectral window. With progressive annealing, these absorptions
decrease in intensity. These optical absorption bands may be assigned to oxygen defects andsome chemical heterogeneity (Szot et al 2002). The broad OD tail observed for the colourlesstransparent crystal is thought to be a trace from these absorption bands; see the inset (a) offigure 2 in the present paper and the figure 4 in the paper reported by Szot et al (2002). In
order to clarify the optical excitation in the Ruddlesden–Popper phases, more detailed surface
sensitive spectroscopic measurements (for example, reflectivity and PL measurements under
total reflection conditions) and morphological and structural measurements for the as-grown
crystals at different stages of annealing are now in progress in our laboratory.

Photoluminescence and reversible photo-induced spectral change of SrTiO3945

Taking account of these studies on different kinds of defects in the real SrTiO3 crystal
grown by the Verneuil method, we thus assign the B luminescence band to the radiative decayof the crystal defect-related excited electronic states. Alternatively, it is theoretically assumed
that a long lasting luminescence arises also from some forbidden transition, and therefore wecannot ignore the contribution from de-excitation of some triplet excited states. Previously,
many workers have discussed the long lasting B luminescence on the basis of intrinsic selftrapped electronic (exciton) states, ignoring the effects of crystal defects. As discussed above,
it should now be apparent that defect effects are important for such self-trapping, and more
careful consideration of them is necessary for deducing the intrinsic nature of SrTiO3 fromexperimental data.
4.3. The NBE and A luminescence bands
Both the NBE luminescence and the A band luminescence have the following features:(α) Both luminescences become prominent under higher excitation intensity and do notsaturate under intense excitation (14 mJ cm−2 ). They grow to the detriment of the B
luminescence with increasing excitation intensity.
(β) With increasing temperature, both luminescences become prominent to the detriment of
the B luminescence and survive even at high temperatures. The spectral structures of thetwo luminescences at room temperature are hardly changed by changing the excitation
laser wavelength from 325 to 355 nm.
(γ ) Both luminescences are considerably enhanced in the as-grown crystal.
(δ) Both luminescences appear near the band edge energy region and have very short decay
times (50 ns).
Feature (α) indicates that both the NBE luminescence and the A luminescence arise not froma small number of impurities and crystal defects, but from numerous luminescent species.
Feature (β) indicates also that the NBE and A luminescence centres are stable at hightemperatures, without any thermal exhaustion. This feature indicates that both luminescences
are assisted by phonons. Feature (γ ) indicates that the luminescence is emission assisted by
crystal defects. Together with the features (α), (β) and (γ ), feature (δ) suggests that the twoluminescences arise from the same origin and are assignable to some kind of electron–holerecombination emission assisted by crystal defects and phonons.
We discuss these luminescence bands from two different viewpoints as follows. The
frequency shift of the NBE luminescence band from incident 355 nm laser light is about
2207 cm −1 . This is too large for assigning this band to non-resonant Raman scattering (Sirenko
et al 1999). Non-resonant Raman scattering in a cubic centrally symmetric crystal is forbiddenby the odd parity of the optical phonons of F1u symmetry, and therefore cubic SrTiO3 crystal
displays weak broad Raman spectrum due to second-order Raman scattering in which two
phonons are involved in the scattering process. In the tetragonal phase below 105 K, thephonon mode splits into two Raman-inactive modes of A2u and Eu symmetry. When crystal
defects (impurities and vacancies) are introduced (Kleemann et al 1997) or when uniaxial
external fields (electric field and stress) are applied (Akimov et al 2000, Worlock and Fleury1967), these first-order Raman-inactive modes can be observed only below 1000 cm −1 . They
cannot explain the observed large frequency shift (about 2207 cm−1 ) of the NBE luminescence.
On the other hand, since the photon energy of the incident 355 nm (=3.49 eV) laser light isclose to the reported direct gap energy (=3.47 eV; Capizzi and Frova 1970) and the observed
PLE edge energy (=3.26 eV), we have to take account of the resonant Raman scattering effects.
In this case, the Raman spectrum may display several resonance lines due to the scattering bydifferent phonons near the direct and indirect gap energies, together with other critical points of946

S Mochizuki et al

the interband transitions (Cardona 1982). As is well known theoretically, the resonant Ramanprocess involves effects due to different intraband and interband scatterings, which display abroad resonance. If we regard the NBE and A luminescences as a result of such resonanceRaman scattering, the phonon energy may be estimated from the difference between the indirect
band edge energy (PLE edge energy = 3.26 eV) and the NBE luminescence peak energy (=3.23 eV): the estimated value is 30 meV (=242 cm−1 ), which is comparable to the transverse
optical phonon frequency. Further discussion on the basis of such resonant Raman scattering
requires measurements at different incident wavelengths and information about the chemical
heterogeneity and defect effects on the Raman scattering.
Next, we discuss the NBE and A luminescence bands as follows. The NBE and the A
luminescences were seen even at high temperatures, and they do not exhibit any PL intensity
saturation under higher excitation intensity. Although an explanation with so-called donor–
acceptor pair (D–A pair) luminescence due to unwanted impurities is frequently given for
the intense broad luminescence band with intensity peaks for semiconductors, such D–Apair luminescence related to inevitable impurities may saturate under intense photoexcitation
because of the limited numbers of donors and acceptors. Moreover, the luminescence intensity
decreases remarkably with increasing temperature. Moreover, such a luminescence spectrum
for D–A pairs related to unwanted impurities is thought to be time resolved, since separations
between donors and acceptors differ. Therefore, we can ignore the contribution from theimpurity-related D–A pair luminescence in the present discussion.
Now, we note that, since ideal SrTiO3 crystal has an indirect gap, the crystal does not exhibit
directly any intense intrinsic edge emission. When crystal defects giving shallow levels are
introduced at the surface, a PL process becomes possible. We note moreover that the NBE
and A luminescence bands are observed even at high temperatures under intense excitationand that these luminescence bands are considerably enhanced for the as-grown crystal, as seenin figure 12. Therefore, we regard the observed NBE and A luminescence bands as defectinduced edge luminescence (emission) ones, in which processes different phonons are created.
The NBE luminescence band having a shift of 242 cm−1 may be assigned to one of the opticalphonon lines. A zero-phonon line anticipated at 3.26 eV is thought to be buried in the riseof the A luminescence band. The full width at half-maximum (FWHM) of the NBE bandis about 30 meV. Such a large FWHM is thought be due to some structural inhomogeneityand damping. It is noted that the A luminescence displays an almost smooth and continuousband. This suggests that, besides optical phonons, many longitudinal and transverse acoustic
phonons are created during electron–hole recombination.
5. Conclusion and remarks
In summary, we have investigated the reversible PL spectral change and the basic PL propertiesof as-grown and annealed SrTiO3 single crystals, together with sintered SrTiO3 powder
compacts, at different temperatures from 12 K to room temperature. Three luminescence bands,
NBE, A and B, are observed around 3.2, 2.9 and 2.4 eV, respectively. The B luminescenceintensity changes with CW 325 nm laser light irradiation in oxygen gas and a vacuum. This
result indicates that the B luminescence is not intrinsic luminescence but results from radiative
decays of the oxygen defect-related photoexcited states and the chemical heterogeneity inthe surface region. The NBE and A luminescences remain at high temperatures and they do
not saturate under high density photoexcitation (at least 14 mJ cm−2 ). Their PL properties(temperature dependence, excitation intensity dependence and time dependence) suggest that
the NBE luminescence and the A luminescence arise from the same origin. Taking accountof crystal defects, the NBE and the A luminescence are discussed qualitatively from two

Photoluminescence and reversible photo-induced spectral change of SrTiO3

947

Figure 13. Some of the possible photoluminescence processes in real SrTiO3 crystal. The solid
arrow and dotted arrow indicate the optical absorption and radiative transition, respectively. The
hollow arrow and double arrow indicate the luminescence and phonon creation, respectively. E gid
and eslt denote the indirect gap energy and the shallow electron trap level, respectively. For theaccidental recombination model, the short dotted arrow and the broken arrow indicate the electron
trapping and hole trapping, respectively.

different viewpoints: defect-induced resonant Raman scattering and defect-induced electron–
hole recombination luminescence accompanying creation of phonons. At present, we do not
exclude the possibility of other explanations. Although it constitutes a difficult problem, thesurface defect structure and the defect-induced effects on the resonant Raman scattering andthe edge luminescence (edge emission) should be considered in more detail.
A tentative energy diagram showing some of the possible PL processes in real SrTiO3
crystal containing crystal defects is given in figure 13. The solid arrow and dotted arrow
indicate the optical absorption and radiative transition, respectively. The hollow arrow anddouble arrow indicate the luminescence and phonon creation, respectively. eslt indicates ashallow electron trap. For the accidental recombination model, the short dotted arrow and thebroken arrow indicate the electron trapping and hole trapping, respectively.
Incidentally, Pontes et al (2002) found intense PL in amorphous SrTiO3 at room
temperature under 488 nm (=2.539 eV) radiation from an Ar + ion laser. They ascribed the PL
excited by low energy photons to non-bridging oxygen defects, for example, TiO5 . Study of
how the non-bridging oxygen defects enhance the PL in amorphous SrTiO3 is also helpful in
discussing the PL properties of real SrTiO3 crystal surfaces.
Finally, more detailed PL measurements, with excitation laser wavelengths varying aroundthe band gap energies, on different annealing stages of SrTiO3 crystals are now progress in ourlaboratory. Experiments with chopped laser light with different duty ratios on the reversible
photo-induced spectral change are also now in progress. The results will be reported in a
separate paper.
Acknowledgments
This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministryof Education, Science, Sports, Culture and Technology, Japan. This work was supported by
an Interdisciplinary General Joint Research Grant for Nihon University. This work was also
partially supported by a Project Research Grant from The Institute of Information Sciences of
the College of Humanities and Sciences (Nihon University) and by a Cooperative Research
Grant from The Institute of Natural Sciences (Nihon University).

948

S Mochizuki et al

References
Akimov I A, Sirenko A A, Clark A M, Hao J-H and Xi X X 2000 Phys. Rev. Lett. 84 4625
Astala R and Bristowe P D 2001 Modelling Simul. Mater. Sci. Eng. 9 415
Blazey K W 1971 Phys. Rev. Lett. 27 146
Capizzi M and Frova A 1970 Phys. Rev. Lett. 25 1298
Capizzi M, Frova A and Dunn D 1972 Solid State Commun. 10 1165
Cardona M 1965 Phys. Rev. 140 A651
Cardona M 1982 Light Scattering in Solids II (Berlin: Springer)
Cohen M I and Blunt R F 1968 Phys. Rev. 168 929
Cord B and Courths R 1985 Surf. Sci. 162 34
Eglitis R I, Kotomin E A and Borstel G 2004 Comput. Mater. Sci. 30 376
Fujishiro F and Mochizuki S 2005 in preparation
Grabner L 1969 Phys. Rev. 177 1315
Hasegawa T, Mouri S, Yamada Y and Tanaka K 2003 J. Phys. Soc. Japan 72 41
Hasegawa T, Shirai M and Tanaka K 2000 J. Lumin. 87–89 1217
Henrich V E, Dresselhaus G and Zeiger H J 1978 Phys. Rev. B 17 4908
Itoh N 1995 Butsuri 50 704 (in Japanese)
Itoh N, Kansaki J, Okano A and Nakai Y 1995 Annu. Rev. Mater. Sci. 25 97
Itoh N and Stoneham A M 2000 Material Modification by Electronic Excitation (Cambridge: Cambridge University
Press)
Jourdan M and Adrian H 2003 Physica C 388/389 509
Katsu H, Tanaka H and Kawai T 2001 J. Appl. Phys. 90 4578
Kawabe Y, Yamanaka A, Hanamura E, Kimura T, Takiguchi Y, Kan H and Tokura Y 2000 J. Appl. Phys. 88 1175
Kimura S, Yamauchi J and Tsukada M 1995 Phys. Rev. B 51 11049
Kleemann W, Albertini A, Kuss M and Linder R 1997 Ferroelectrics 203 57
Kröger F A and Vink H J 1956 Solid State Physics vol 3 (New York: Academic) p 307
Mochizuki S 2003 Physica B 340–342 944
Mochizuki S and Araki H 2003a Physica B 340–342 913
Mochizuki S and Araki H 2003b Physica B 340–342 969
Mochizuki S, Nakanishi T, Suzuki Y and Ishi K 2001 Appl. Phys. Lett. 79 3785
Mochizuki S, Shimizu T and Fujishiro F 2003 Physica B 340–342 956
Mochizuki S and Fujishiro F 2005 in preparationMüller K A, Berlinger W and Tosatti E 1991 Z. Phys. B 84 277
Müller K A and Burkard H 1979 Phys. Rev. B 19 3593
Nasu K 2004 Rep. Prog. Phys. 67 1607
Ohmukai M, Takigawa Y and Kurosawa K 1999 Appl. Surf. Sci. 137 78
Pontes F M, Longo E, Leite E R, Lee E J H, Varela J A, Pizani P S, Campos C E M, Lanciotti F, Mastellaro V and
Pinheiro C D 2002 Mater. Chem. Phys. 77 598
Puchin V E, Shluger A L and Itoh N 1993 Phys. Rev. B 47 10760
Ruddlesden S N and Popper P 1957 Acta Crystallogr. 10 538
Ruddlesden S N and Popper P 1958 Acta Crystallogr. 11 54
Shluger A and Stefanovich E 1990 Phys. Rev. B 42 9664
Sirenko A A, Akimov I A, Fox J R, Clark A M, Li H C, Si W and Xi X X 1999 Phys. Rev. Lett. 82 4500
Song K S and Williams R T 1993 Self-Trapped Excitons (Berlin: Springer)
Stashans A and Vargas F 2001 Mater. Lett. 50 145
Szot K and Speier W 1999 Phys. Rev. B 60 5909
Szot K, Speier W, Carius R, Zastrow U and Beyer W 2002 Phys. Rev. Lett. 88 075508-1
Szot K, Speier W, Herion J and Freiburg C 1997 Appl. Phys. A 64 55
Takesada M, Yagi T, Itoh M and Koshihara S 2003 J. Phys. Soc. Japan 72 37
Toyozawa Y 1983 Physica B 112 & 118 23
Tsang C and Street R A 1979 Phys. Rev. B 19 3027
Worlock J M and Fleury P A 1967 Phys. Rev. Lett. 19 1176
Zollner S, Demkov A A, Liu R, Fejes P L, Gregory R B, Alluri P, Curless J A, Yu Z, Ramdani J, Droopad R, Tiwald T
E, Hilfiker J N and Woollam J A 2000 J. Vac. Sci. Technol. B 18 2242

ARTICLE IN PRESS

Journal of Luminescence 112 (2005) 71–74
www.elsevier.com/locate/jlumin

Photoluminescence studies on AgI–ZrO2 composites
Fumito Fujishiro, Shosuke Mochizuki
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
AgI–ZrO2 ﬁne particle composites were fabricated over a wide composition range of AgI. We have measured the Xray diffractograms and photoluminescence properties of the composites, together with those of pristine AgI and pristine
ZrO2 ﬁne particles. The results give the information about the atomic structure of AgI/ZrO2 interfaces, which may
provide the pathway for the high ionic conduction.
r 2004 Elsevier B.V. All rights reserved.
PACS: 78.55.m; 61.72.y; 66.10.Ed
Keywords: Exciton; Photoluminescence; Ionic conductor; Interface; Oxygen defect

The ionic conductivity of AgI is considerablyenhanced by adding oxide ﬁne particles [1,2]. It isfrequently assumed that highly conductive layers
are created at AgI/oxide ﬁne particle interfaces.
However, it is difﬁcult to clarify experimentally theatomic structure of the interfaces. In order toinvestigate such interface structure, the photoluminescence (PL) study is effective. Besides AgI [3],most oxides also show structure-sensitive PL. Veryrecently, we have fabricated AgI–ZrO2 composites
and observed considerable conductivity enhancement [4]. In the present paper, we report thestructural and PL studies on AgI–ZrO2 compoCorresponding author. Tel./fax: +81 3 5317 9771.

E-mail address: fumito@phys.chs.nihon-u.ac.jp(F. Fujishiro).

sites, and we discuss the structure of AgI/ZrO2 ﬁneparticle interfaces.
AgI–ZrO2 composites were prepared by mixing
AgI powder (99.999%, Alfa) and ZrO2 powder(498%, Aldrich) with an average diameter of
26 nm, compressing uniaxially the mixture under
0.2 GPa for 30 min into pellets and then heating
the pellets in the lidded crucible at 673 K in air for
12 h. The composite pellets were characterized by
the X-ray diffraction analysis with Cu Ka1 radiation. The third harmonics, l ¼ 355 nm (=3.49 eV), of a pulsed Nd3+–YAG laser light wasused to excite PL.
Fig. 1 shows the X-ray diffractograms ofdifferent (x)AgI–(1–x)ZrO2 composites. All of
the diffraction lines observed for a pristine ZrO2(x=0) specimen are assigned to the monoclinic

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jlumin.2004.09.086

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Journal of Luminescence 112 (2005) 71–74

72

phase, except for the tetragonal phase line at 30.21.
Since the diffraction line widths are hardly
changed with adding AgI, the average size ofZrO2 particles in the composites is thought to bealmost the same as that of the starting material
powder. On further increasing x, the AgI diffraction lines become pronounced. The diffraction
intensities of the bAgI(100) line at 22.31 andbAgI(200) line at 45.61 are considerably enhanced(x)AgI−(1−x)ZrO2
x = 1 (AgI)XRD intensity (arb. scale)x = 0.8
x = 0.6

in comparison with other AgI-based composites[1], which may be due to preferred-orientations.
The PL spectra for different (x)AgI–(1x)ZrO2
composites at 12 K are shown in Fig. 2. The
excitation laser ﬂuence Iex was 0.066 mJ/cm2.
Pristine ZrO2 shows a broad PL band centered
at about 2.9 eV. The PL peak energy depends onthe excitation photon energy Eex, as shown in
Figs. 3(a) and (b). Adding small amount(0.1p x p0.2) of AgI, the intensity of the broad
PL band decreases extremely, and a new PL band
appears at about 2.80 eV. At x ¼ 0:4; two PL
bands appear at 2.89 and 2.83 eV. On further
adding AgI (0.6p x p0.8), another new sharp PL
band appears at 2.94 eV, accompanying by someweak PL bands at its lower energy side. The
2.94 eV band is assigned to the radiative decay of
free excitons (FE) in AgI domain [3]. At x=1,

x = 0.4
x = 0.2

x = 0 (ZrO2)
20

30

40
2θ (degree)

50

Fig.
1. TheX-ray
diffractograms(x)AgI–(1x)ZrO2 composites at 297 K.

60

for

PL intensity (arb. scale)x = 0.1

2:Eex= 4.59 eV

1:Eex= 4.96 eV

x = 0.6

PL peak energy (eV)(x)AgI−(1−x)ZrO2

x = 0.4(b)x = 1 (AgI)x = 0.8

3.2

2.8
2.6
2.4
Photon energy (eV)

2

2.8

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

Fig. 2. The PL spectra for different (x)AgI–(1x)ZrO2
composites at 12 K.

(c)

2.0

4
3

2.7

5

1

2.6

4.5
4.0
3.5
Excitation photon energy Eex (eV)
PLE

α

β

x = 0 (ZrO2)
3.2

2.2

2.9

PL intensity

x = 0.1

3.0

3.0

5.0

x = 0.2

4:Eex= 3.44 eV

3:Eex= 4.13 eV

different(a)

PL intensity (arb. scale)

ZrO2
5:Eex= 3.10 eV654Photon energy (eV)

3.0PL
Eex= 4.13 eV

Eex= 3.49 eV

32Fig. 3. (a) The PL spectra of pristine ZrO2 excited with various
Eex at 13 K; (b) the PL peak energy plotted against Eex; (c) the
PL and PLE spectra for pristine ZrO2 at 13 K.

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F. Fujishiro, S. Mochizuki / Journal of Luminescence 112 (2005) 71–74(x)AgI−(1−x)ZrO2

12 K

288 K

x = 1 (AgI)

12 K
288 K

PL intensity (arb. scale)x = 0.8
12 K
287 K
x = 0.6
13 K
288 K
12 K

99 K

99 K

x = 0.4
x = 0.2
x = 0.1

12 K

3.0

2.9

(a)
PL intensity

namely, pristine AgI, besides the free exciton
emission, more intense PL bands grow at 2.89
and 2.86 eV. These intense PL bands are assigned
to radiative decays of excitons trapped arounddefects and grain boundaries in AgI specimen.
In pristine ZrO2 (x=0), we have found that the
PL intensity peak energy depends on Eex, as shownin Fig. 3(a). The PL intensity peak energy isplotted against Eex in Fig. 3(b). It is noted thateach PL spectrum is considerably Stokes-shifted,
which indicates that there are different localized
electronic states in the energy gap (5.8 eV) of ZrO2.
The PL and PL excitation (PLE) spectra are also
shown in Fig. 3(c). The PLE curve a is monitored
at 2.99 eV, which corresponds to the PL intensity
peak energy for the excitation Eex=3.49 eV, while
the PLE curve b is monitored at 2.53 eV whichcorresponds to the PL intensity peak energy forthe excitation Eex=4.13 eV. The PLE spectra
ascertain the existence of the luminescent localized
electronic states in the energy gap. Since the PL
intensity of ZrO2 was decreased remarkably byadding AgI, as shown in Fig. 2, such luminescencecenters may be at ZrO2 particle surfaces.
The temperature dependence of PL spectra fordifferent (x)AgI–(1x)ZrO2 composites was measured. The results are shown in Figs. 4(a) and (b).
Each spectrum is normalized in intensity at eachmaximum. In small x range (0.1p x p0.2), withincreasing temperature, the 2.80 eV band is considerably weakened and almost disappears above
about 100 K. The broad PL band observed at 99 K
may arise from ZrO2. However, the PL spectral
shape above 2.85 eV is affected by the opticalabsorptions due to AgI [3]. At x ¼ 0:4; the PL
intensity of the 2.83 eV band decreases, while the
PL intensity of the 2.89 eV band increases incomparison with that of the 2.83 eV band, withincreasing temperature. It suggests that phononassisted reverse transition from the 2.83 eV luminescence center to the 2.89 eV luminescence centeroccurs. In large x range (0.6p x p0.8), withincreasing temperature, free exciton emission
decreases in intensity, while another PL band
appears at 2.90 eV. This indicates that the 2.90 eVband arises from radiative decay of excitons
trapped around thermally ionized defects orthermally ionized impurities.

73

2.7

12 K
288 K

3.2

(b)

2.8
Photon energy (eV)

3.0

2.8
2.6
2.4
Photon energy (eV)2.6

x = 0 (ZrO2)

2.2

2.0

Fig. 4. The temperature dependence of the PL spectra: (a) for
different (x)AgI–(1x)ZrO2 composites; (b) for pristine ZrO2.

Figs. 5(a) and (b) show the Iex dependence of the
PL spectra for different (x)AgI–(1x)ZrO2 composites at 12 K. Each spectrum is normalized inintensity at each maximum. Labels, I and W,
indicated by the curves denote intense (X1.0 mJ/cm2) and weak (p0.1 mJ/cm2) excitations, respectively. For small x region (p0.2), the PLcomponent below 2.80 eV saturates at intense
excitation, which indicates that the luminescencecenter giving this PL component is limited in
number, though it has high quantum yield. We
note the x dependence of the spectra for weakexcitation. With adding AgI, the 2.80 eV band
shifts to 2.78 eV at x ¼ 0:2 and, with further
adding AgI, it shifts to 2.81 eV at x ¼ 0:4: At bothx ¼ 0:6 and 0.8, the band remains as a shoulder of
AgI-related PL bands. Since adding AgI to ZrO2
induces the oxygen defects, Zr1–xAgxO2–3x/2(Vö)3x/2 [5], the shift may be connected with thechange in the electronic states at AgI/ZrO2
interfaces. On further adding AgI, the shoulders
at 2.94 and 2.86 eV become prominent.

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F. Fujishiro, S. Mochizuki / Journal of Luminescence 112 (2005) 71–74

W (x 72)

PL intensity (arb. scale)I (x 1)

x = 1 (AgI)

W (x 67)
x = 0.8

I (x 1)W (x 1361 )4(0.1)AgI-(0.9)ZrO2

I (x 1)W (x 120)x = 0.4

I (x 1)

2
1
0
3.2

3.0

2.9

PL intensity

2.8
Photon energy (eV)I (x 1)

3.0

2.8
2.6
2.4
Photon energy (eV)

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

Fig. 6. Reversible photo-induced PL spectral change of(0.1)AgI–(0.9)ZrO2 composite at 295 K.

x = 0.2
W (x 48)2.7

2.6

x = 0 (ZrO2)
W (x 65)

3.2

in oxygen

W (x 69)x = 0.1(a)in vacuum3

x = 0.6

I (x 1)(b)5(x)AgI-(1-x)ZrO2

I (x 1)

PL intensity

74

2.2

2.0

Fig. 5. The Iex dependence of the PL spectra: (a) for different(x)AgI–(1x)ZrO2 composites; (b) for pristine ZrO2 at 12 K.

The spectral transition with adding AgI is
explained as follows. For small x region, the free
excitons generated in AgI domain collides frequently with AgI surfaces, and they are trapped at
AgI/ZrO2 interfaces, creating bound excitons.
With increasing x, the size of AgI domainincreases and free excitons are able to propagate
in a long distance. Therefore, before trapping at
AgI/ZrO2 interfaces, a part of free exciton decays
spontaneously with generating the 2.94 eV luminescence, and other part of free exciton is trappedat crystal defects and impurities within AgI
domain. The radiative decay of these trapped
excitons generates the 2.93 and 2.86 eV luminescence.
Besides the above PL properties, we have foundalso a reversible photo-induced PL spectralchange in the present AgI–ZrO2 composites atroom temperature. One of the results is shown in
Fig. 6, as a typical example. After fully irradiatingwith CW 325 nm laser light in an evacuatedspecimen chamber for 10 min, the specimen showsa broad PL band at 2.57 eV. Oxygen gas is thenintroduced into the specimen chamber and, withincreasing irradiating time, the PL component
below 2.45 eV decreases in intensity. Throughmany successive experiments, it is found that such
spectral changes appear repeatedly. The observed
spectral changes may originate from excitationenergy exchange between the photo-induced oxygen vacancies on ZrO2 particle and AgI. The
spectral transitions yield materials for opticalsensor devices.
In summary, we have studied the PL propertiesof (x)AgI–(1x)ZrO2 (0p x p1) composites. The
PL spectra have shown well the behavior of free
and trapped excitons at AgI/ZrO2 interfaces whereAg+ ions may move with high-ionic conductivity.
The specimen fabrications using different startingmaterial ZrO2 powders and more detailed studies
are now in progress.
This work is supported by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education, Science, Sports, Culture and Technology,
Japan. This work is partially supported byInterdisciplinary General Joint Research Grant
for Nihon University.

References[1] S. Mochizuki, F. Fujishiro, J. Phys.: Condens. Matter 15(2003) 5057.
[2] M.C.R. Shastry, K.J. Rao, Solid State Ionics 51 (1992)
311.
[3] S. Mochizuki, F. Fujishiro, J. Phys.: Condens. Matter 16(2004) 3239.
[4] F. Fujishiro, S. Mochizuki, in preparation.
[5] F.A. Kröger, H.J. Vink, Solid State Phys. 3 (1956) 307.

ARTICLE IN PRESS

Journal of Luminescence 112 (2005) 267–270
www.elsevier.com/locate/jlumin

The reversible UV-laser-light-induced spectral change andorigin of the 2.4 eV luminescence band in SrTiO3
Shosuke Mochizuki, Seiko Minami, Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Available online 24 November 2004

Abstract
In order to clarify the origin of the 2.4 eV luminescence band in SrTiO3, as-grown and annealed SrTiO3 single crystals
are irradiated with a continuous-wave ultraviolet laser light (l ¼ 325 nm) in different atmospheres at room temperature.
Under the laser light irradiation in a vacuum, the 2.4 eV luminescence band grows with increasing irradiation time,
while it returns to the original weak-luminescence state under the same laser light irradiation in oxygen gas. The
excitation-intensity, temperature and time dependences of the photoluminescence spectra are also measured for the
crystals. All the results indicate that the 2.4 eV luminescence band arises not from intrinsic self-trapped excitons but
from extrinsic excitons trapped around oxygen defects. Near-band-edge emissions have been observed, for the ﬁrst time,at 3.2 and 2.9 eV under intense excitation.
r 2004 Elsevier B.V. All rights reserved.
PACS: 71.35.y; 78.55.m; 82.50.m; 61.72.yKeywords: Quantum paraelectric state; Photo-induced defects

A large number of the optical studies have been
devoted to clarify the quantum paraelectric state instrontium titanate (SrTiO3). Grabner [1] measured
the photoluminescence (PL) properties of doped
and undoped SrTiO3 crystals, and observed anear-infrared luminescence band around approximately 1.6 eV and a visible luminescence bandCorresponding author. Tel.: +81 3 5317 9771;+81 3 5317 9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

fax:

around 2.4 eV.These PL bands were Stokes-shifted
considerably. He assigned them to intrinsic defects
or intrinsic excitons. Recently, Hasegawa et al. [2]measured the optical absorption and PL properties
of SrTiO3 crystal. They assigned the Stokes-shiftedlong-lasting luminescence around 2.4 eV to theradiative decay of intrinsic self-trapped excitons.
However, many metal oxides display the similar
visible PL, which are related to oxygen defects[3–6]. Unfortunately, such oxygen defect effect has
been accorded too low evaluation in the optical
study of SrTiO3 and it has led many researchers

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jlumin.2004.09.050

ARTICLE IN PRESS

PL intensity (arb. units)

15x10

as-grown SrTiO3

3

60 min.
50 min.
40 min.

10

5

2 min. 10 min.
0 min. 4 min.
30 min.
20 min.

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 1. The CW 325 nm laser light-induced spectral change ofan as-grown SrTiO3 crystal at room temperature in vacuum.
The tir is given in min.

15x10

as-grown SrTiO3

3

tir = 60 min.
10

tir = 0 min.
(rescaled: x 2.5)5

Difference

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 2. The CW 325 nm laser light-induced PL component ofan as-grown SrTiO3 crystal at room temperature.

PL intensity (arb. units)

astray. Very recently, we have observed that the
2.4 eV luminescence band of SrTiO3 grows under acontinuous wave (CW) 325 nm-laser-light irradiation in a vacuum, while it disappears under thesame laser light irradiation in oxygen gas [7]. It
was found that the observed phenomenon relates
closely to the creation and annihilation of oxygen
defects under the laser light. Taking account of theoxygen defects in real SrTiO3 crystals [8–10], thePL properties of SrTiO3 should be now restudied
in detail. In the present paper, we report the PL
properties and reversible photo-induced spectralchange of as-grown and annealed SrTiO3 single
crystals.
SrTiO3 single crystals were grown by the Vernuil
method. As-grown crystals are dark blue and they
become transparent by annealing in an appropriate atmosphere. During CW 325 nm laser-lightirradiation, we have measured the PL spectra ofthese as-grown and annealed crystals as a functionof irradiation time tir. Fig. 1 shows the photoinduced PL spectral change of the as-grown
SrTiO3 crystal at room temperature in a vacuum
as a function of tir (in min). As seen in this ﬁgure,the initial PL spectrum (tir ¼ 0) is very broad, andit has an intensity peak at about 2.8 eV and two
faint kinks at about 2.4 and 2.15 eV. With
increasing tir ; the intensity increases, growing the
PL component below 2.8 eV. With tir increasing
further, the rate of the spectral change becomesdecreased. The spectrum at tir ¼ 0 min is rescaled,

PL intensity (arb. units)

S. Mochizuki et al. / Journal of Luminescence 112 (2005) 267–270

268

1400

annealed SrTiO 3

1200

60 min.

1000

50 min.
40 min.

800
600
400

0 min.
10 min.
20 min.
30 min.

200
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 3. The CW 325 nm laser light-induced spectral change ofan annealed SrTiO3 crystal at room temperature in vacuum.
The tir is given in min.

and then a difference between the spectrum at tir ¼
60 min and the rescaled spectrum is calculated,which is shown in Fig. 2. This ﬁgure shows clearly
that a broad PL band grew at 2.4 eV under 325 nm
laser light irradiation in a vacuum. Fig. 3 shows
the photo-induced PL spectral change of theannealed SrTiO3 crystal at room temperature ina vacuum. It is noted that the annealing treatment
reduces the PL intensity to about one-tenth. The
PL component below 2.8 eV grows with increasingtir. With tir increasing further, the rate of thespectral change becomes decreased. As done forthe results of the as-grown crystal, the difference
spectrum is also calculated, which is given in
Fig. 4. It indicates clearly that a broad PL band of
the annealed crystal grew at 2.4 eV under 325 nm
laser light irradiation in a vacuum. The same
spectral change in the 2.4 eV band was observed

ARTICLE IN PRESS

1600
tir = 0 min. (rescaled: x 1.34)

annealed SrTiO 3

1200

50x10

269

3

as-grown SrTiO3

tir = 60 min.

12 K

800
400
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 4. The CW 325 nm laser light-induced PL component ofan annealed SrTiO3 crystal at room temperature.

also at 12 K in our previous study [7]. As well as
the as-grown crystal, it is found, under the samelaser light irradiation in oxygen gas, the 2.4 eVband disappears, returning to the original weak PL
state. Since the 325 nm laser light never dissociates
directly free O2 molecule, the observed spectralchanges arise from the photo-induced associative
detachment and photo-induced dissociative adsorption of O2 molecules near the SrTiO3 surfaces[6].
We have studied the PL spectra of the as-grown
and annealed SrTiO3 crystals at 13 K as a functionof excitation laser ﬂuence of the third harmonics(l ¼ 355 nm) of a Nd3+-YAG laser. The results
are shown in Figs. 5 and 6. We have conﬁrmed
that, unlike CW 325 nm laser light, the pulsed
355 nm laser light cannot induce any reversible
spectral change even at 14 mJ/cm2. At the lowest
laser ﬂuence, both crystals exhibit only the intense
2.4 eV band. However, the center energy for the asgrown crystal is slightly smaller than that for theannealed crystal. We compare the PL spectra
observed at 4 mJ/cm2, before and after annealing.
The annealed crystal displays more intense 2.4 eV
luminescence than that for the as-grown crystal.
This may be due to decrease in the number of nonradiative trap by the annealing treatment. With
increasing laser ﬂuence, two new PL bands appear
at 3.2 eV, which is near indirect gap of SrTiO3 andat 2.9 eV. On the contrary to the 2.4 eV band, the
3.2 and 2.9 eV bands for the as-grown crystal are
more intense than those of the annealed crystal.
This indicates that the 3.2 eV luminescence and2.9 eV one are enhanced by oxygen defects. As
seen from Fig. 5, with increasing laser ﬂuence, the2.4 eV band for the as-grown crystal grows and

PL intensity (arb. units)

40

Difference

120 K
30

2

14 mJ/cm

297 K
3.2 2.8 2.4 2.0
Photon energy (eV)22 mJ/cm

20
2

12 mJ/cm

2

0.8 mJ/cm

2

10 mJ/cm

108 mJ/cm
4 mJ/cm

0.16 mJ/cm

2

2

2

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 5. The excitation laser intensity dependence of the PL
spectra of an as-grown SrTiO3 crystal at 12 K. The temperaturedependence of the PL spectrum is shown in the inset.
3

35x10

annealed SrTiO3
12 K

30

PL intensity (arb. units)

PL intensity (arb. units)

S. Mochizuki et al. / Journal of Luminescence 112 (2005) 267–270

39 K

4.0 mJ/cm

25

2

297 K
20

2

3.2 2.8 2.4 2.0
Photon energy (eV)

10 mJ/cm

152

14 mJ/cm
105-2

8.1x10 mJ/cm

2

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 6. The excitation laser intensity dependence of the PL
spectra of an annealed SrTiO3 crystal at 12 K. The temperaturedependence of the PL spectrum is shown in the inset.

then becomes saturated at 8 mJ/cm2, while the 3.2
and 2.9 eV bands continue to grow without any
saturation. As seen in Fig. 6, the PL intensity of
the 2.4 eV band for the annealed crystal increases

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270

S. Mochizuki et al. / Journal of Luminescence 112 (2005) 267–270

also with increasing laser ﬂuence, and then it
reaches a maximum at 4 mJ/cm2, while the 3.2 and2.9 eV bands continue to grow without displaying
any maximum. Besides these measurements, thePL spectra were measured at different temperatures between 12 and 297 K. Of them, the PL
spectra at 12, 120 and 297 K for the as-grown
crystal are given in the inset of Fig. 5, and the PL
spectra at 12, 39 and 297 K for the annealed crystal
are given in the inset of Fig. 6. As seen from these
insets, the 3.2 and 2.9 eV bands grow deteriorating
the 2.4 eV band with increasing temperature. The
2.4 eV band observed for the as-grown andannealed crystals disappeared at 120 and 39 K.
The 3.2 eV band becomes weak and broad withincreasing temperature. The 2.9 eV band remains
even at room temperature. We have also measured
the time-resolved spectra and found that the 2.4 eV
luminescence has long lifetime (Z1 ms), while boththe 3.2 luminescence and the 2.9 eV one have short
lifetime (550 ns), at 12 K [7].
Finally, the experimental results obtained in thepresent study are summarized and discussed asfollows. Since the PL intensity of the 2.4 eV band isaffected considerably by oxygen defects and it
saturates under intense excitation, the 2.4 eV bandis assigned to the radiative decay of extrinsic
excitons trapped around oxygen defects. The
excitation intensity dependences of the PL shownin Figs. 5 and 6 indicate that, with increasing laser
ﬂuence, the 3.2 and 2.9 eV bands grow deteriorating the 2.4 eV band. This intensity relation among
these PL bands at 2.4, 3.2 and 2.9 eV withincreasing excitation intensity may arise fromtwo causes: (1) ﬁnite number of the 2.4 eV
luminescence centers, and (2) electron–hole recombination enhanced with higher laser ﬂuence.
Since the recombination rate increases withincreasing carrier density, the near-band edge
emissions at 3.2 and 2.9 eV increase linearly withincreasing laser ﬂuence, while the rate of electron(including hole) migration to the 2.4 eV luminescence centers decreases with increasing laser
ﬂuence. The excitation-intensity and temperaturedependences of the 3.2 and 2.9 eV bands also
indicate that two PL bands occur from the same
origin. We measured also the PL excitation (PLE)
spectrum for the 2.4 eV band and found a sharp

PLE edge at 3.26 eV.This edge corresponds well tothe indirect gap of SrTiO3. Generally, an indirectgap-type semiconductor crystal does not exhibit
intense PL. However, some impurities and crystal
defects activate the PL of indirect excitons,
exhibiting intense PL band near band edge.
Therefore, the intense PL at 3.2 and 2.9 eV may
be assigned to the radiative decay of indirect
excitons affected by oxygen defects, creating
phonons. The energy difference (271 cm1) between the PLE edge and the peak energy of the
3.2 eV band is close to the transverse optic (TO)phonon frequency [11]. Thus, the 3.2 eV band isassigned to the one TO phonon sideband, while
the broad 2.9 eV band is also assigned to acontinuum consisting of other phonon sidebands.

Acknowledgements
This work is supported by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education, Science, Sports, Culture and Technology,
Japan. This work is partially supported byInterdisciplinary General Joint Research Grant
for Nihon University.

References[1] L. Grabner, Phys. Rev. 177 (1969) 1315.
[2] T. Hasegawa, M. Shirai, K. Tanaka, J. Lumin. 87–89(2000) 1217.
[3] S. Mochizuki, T. Shimizu, F. Fujishiro, Physica B 340-342(2003) 956.
[4] Y. Kawabe, A. Yamanaka, E. Hanamura, T. Kimura, Y.
Takiguchi, H. Kan, Y. Tokura, J. Appl. Phys. 88 (2000)
1175.
[5] S. Mochizuki, H. Araki, Physica B 340–342 (2003) 969.
[6] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl.
Phys. Lett. 79 (2001) 3785.
[7] S. Mochizuki, S. Minami, F. Fujishiro, J. Phys.: Condens.
Matter., submitted for publication.
[8] S. Kimura, J. Yamauchi, M. Tsukada M, Phys. Rev. B 51
(1995) 11049.
[9] V.E. Henrich, G. Dresselhous, H.J. Zeiger, Phys. Rev. B 17
(1978) 4908.
[10] R. Astala, P.D. Bristowe, Modell. Simul. Mater. Sci. Eng.
9 (2001) 415.
[11] R.A. Cowley, Phys. Rev. 134 (1964) A981.