Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research B
journal homepage: www.elsevier.com/locate/nimb

Involvement of crystallinity in various luminescent bands in yttrium
aluminate
Takaaki Morimoto a,⇑, Masayuki Harima a, Yosuke Horii a, Yoshimichi Ohki a,b,⇑ab

Department of Electrical Engineering and Bioscience, Waseda University, Shinjuku, Tokyo 169-8555, Japan
Research Institute for Materials Science and Technology, Waseda University, Shinjuku, Tokyo 169-8555, Japana r t i c l e

i n f oArticle history:
Received 3 June 2015
Received in revised form 11 October 2015
Accepted 31 October 2015
Available online 18 November 2015
Keywords:
YAlO3
Ion implantation
Photoluminescence
Crystallinity

a b s t r a c t
When single crystal YAlO3 was implanted with P+ or B+ ions, optical absorption increases significantly atenergies slightly lower than the band gap energy, indicating that localized electronic states were induced.
Furthermore, the ion implantation decreases the intensity of an X-ray diffraction peak and changes its
position randomly, which indicates that the crystalline structure of the sample was deformed. The intensities of photoluminescence (PL) bands due to impurities of Cr3+ and Er3+ and those originating in selftrapped excitons and antisites become smaller or disappear after the ion implantation. On the other hand,
the intensity of the PL due to oxygen vacancies does not change. Such contrasting effects of the sample’s
crystallinity on the luminescence intensity are explained by the different manners of involvement of thecrystal structure in the luminescence mechanism among these PLs in YAlO3.
Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction
Various photoluminescence (PL) bands appear in yttrium aluminate (YAlO3) perovskite crystal. It is known that point defects
such as impurities and vacancies are deeply involved in thesePLs. It is also known that the existing forms of impurities or vacancies in inorganic crystals are closely related to the crystallinity ofthe mother crystal. Several PLs emitted from YAlO3 are used forlasers [1–3] and scintillators [4–6]. For example, the PL that emits
photons with the wavelength of 2.0 lm can be induced by doping
Tm3+ and is used for lasers [1]. Furthermore, the PL at 350 nm
induced by doping Ce3+ is used for scintillators [4–6]. Therefore,
it is important to investigate the effects of the crystallinity of YAlO3
on its luminescence.
On the other hand, YAlO3 is one of the promising candidates fora gate insulator in advanced metal oxide semiconductor fieldeffect transistors (MOSFETs) for large scale integration and power
conversion circuits, since it has a high relative permittivity (16)[7] and a wide band gap energy (Eg: 7.9 eV) [8]. However, YAlO3
is not used practically as gate insulators, since it contains point
defects abundantly. Therefore, investigation on the nature of thedefects is important. Regarding this, the authors have proved that

⇑ Corrsponding authors at: Department of Electrical Engineering and Bioscience,
Waseda University, Shinjuku, Tokyo 169-8555, Japan (T. Morimoto and Y. Ohki).
E-mail addresses: takaaki.morimoto@akane.waseda.jp (T. Morimoto), yohki@
waseda.jp (Y. Ohki).
http://dx.doi.org/10.1016/j.nimb.2015.10.072
0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

measurements of PL bands and analysis of their behavior can provide many pieces of valuable information on point defects in various inorganic insulating materials [9–24].
In other words, behavior of PL can provide important clues not
only to the study of luminescent phenomena themselves for lasers
or scintillators but also to the study of insulating properties of
YAlO3. From these backgrounds, in this research, the crystallinity
of YAlO3 was changed by ion implantation in order to investigate
its involvement in the properties of various PL bands. Furthermore,
PL characteristics observed in the present YAlO3 are compared withthose in LaAlO3 and YSZ reported in the authors’ preceding papers[23,24].
2. Experimental procedures
The samples examined are YAlO3(1 0 0) single crystals grown by
the Czochralski method in a German company, Crystal GmbH. They
were cut to rectangular plates with a thickness of 0.5 mm. The
plates were implanted with positive phosphorus (P+) or boron(B+) ions at a fluence of 1.0  1015 cm 2 with an accelerationenergy of 100 keV using an ion accelerator (Ulvac, UP-150) in
Waseda University.
Before and after the ion implantation, X-ray diffraction (XRD)
patterns were obtained in the in-plane mode, which is suitable
for analyzing the sample surface, with Cu Ka X-rays at room temperature using a Rigaku Rint-Ultima III. Furthermore, the presenceof impurities in the sample before the ion implantation was

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T. Morimoto et al. / Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

400

B

+

Range [nm]

300

200

P

400

600
I
400
II
200
III
0
0 10 20 30 40 50 60 70 80 90
2θχ [deg.]

I

II
III

200

0
30 31 32 33 34 35 36 37 38 39 40

2θχ [deg.]
Fig. 2. (Color online.) In-plane XRD patterns observed at angles from 30o to 40o,
before the ion implantation (curve I) and after the implantation of P+ ions (II) and B+
ions (III) at a fluence of 1015 cm 2. Inset: whole angle patterns.

3. Results
Fig. 1 shows the projected ranges of P+ and B+ ions in YAlO3 estimated using a simulation program called SRIM2011 [25]. The
range is estimated to be about 80 nm for the P+ ions and 200 nm
for the B+ ions with the energy of 100 keV. Since these projected
ranges are shallow, the in-plane mode was used to obtain XRD patterns. As indicated by the spectrum I shown in the inset in Fig. 2,
the unimplanted sample exhibits a peak at 2hv = 34.6o, which corresponds to the diffraction at (0 0 2) planes of crystalline YAlO3[26]. This is reasonable, since the sample had been set in a way thatthe (0 0 2) peak should be observed. In addition, the sample also
exhibits another peak at 2hv = 74.0o, the assignment of which is
unclear due to the lack of database. Here, the authors pay attention
to the clearer one at 34.6o. This peak appears at a different angle
and its height decreases if the sample was implanted with the ions.
To verify repeatability, XRD patterns were also observed for two
more P+-ion implanted, one more B+-ion implanted, and three
more unimplanted samples. The result is summarized in Fig. 3.
Here, each peak height is normalized by the average of the peakheights observed for the four unimplanted samples. The ion
implantation decreased the peak height by 30–95% and varied

600

Intensity[arb. unit]

Intensity [arb. unit]

800

XRD Peak Intensity [normalized]

measured by inductively coupled plasma mass spectrometry(ICP-MS) with an Agilent 7700x. In addition, electron spinresonance (ESR) spectra were obtained at the X-band frequencyof 9.20 GHz with a microwave power of 2.00 mW at room temperature using a JEOL JES-FA 300. Here, the magnetic field was applied
parallel to the principal axis of the sample plate.
In order to measure PL excitation and emission spectra andoptical absorption spectra, synchrotron radiation (SR, beamenergy: 750 MeV) at the BL3B line in UVSOR Facility at the Institutefor Molecular Science, Okazaki, Japan was used as a photon source.
The absorption was measured at 10 K using a 2.5 m off-plane
Eagle-type normal incidence monochromator and a photodiodeset before and after the sample plate, respectively. When measuring PLs, a PL excitation spectrum, namely a PL intensity spectrum
obtained by changing the energy of incident excitation photons,
was measured first using the incidence monochromator, a 300mm triple-grating rear monochromator (Acton, SpectraPro-300i),
and a charge-coupled device (CCD) camera (Princeton Instruments) at 10 K. Then, a PL emission spectrum, or an intensity spectrum of each PL as a function of photon energy, excited by photons
with the energy that can induce the strongest PL, was measured.
The spectral intensity was calibrated with the intensity of excitation SR at each photon energy for both the former PL excitation
spectrum and the latter PL emission spectrum.

1.1
1.0
four data

0.8
0.6
0.4
0.2
0
33

34

35

36

37

2θχ [deg.]
Fig. 3. (Color online.) Fluctuations in height and position of the XRD peak observedin the unimplanted (black solid circles), P+ ion-implanted (pink (light gray in b/w)
solid triangles), and B+ ion-implanted (blue (dark gray) open squares) samples. Notethat the peak intensities are normalized by their original intensities before theimplantation.

the peak position randomly. This is in contrast with the fact thatthe four unimplanted samples always exhibit the peak at the same
position.
Table 1 lists the averages and standard deviations of concentrations of impurities in YAlO3 analyzed by ICP-MS for five samples.
On average, the samples contain 20 ppm of Cr, 51 ppm of Er, and6 ppm of Nd. Note that contamination by Fe cannot be confirmed,
since the spectrometer is equipped with a Fe filament.
Fig. 4 shows PL emission spectra observed for YAlO3 crystal
plates at 10 K before the ion implantation in the whole photonenergy range measured in this research, namely from 1.5 to
6.0 eV. Here, the green (light gray in b/w) solid, brown (dark gray)dashed, black dashed, and violet (dark gray) solid curves represent+
Table 1
Impurities in YAlO3 analyzed for five samples by ICP-MS.

100

Elemental concentration (ppm)

0
0

50
100
150
Acceleration Energy [keV]200

Fig. 1. (Color online.) Projected ranges of P+ and B+ ions in YAlO3.

AverageStandard deviation
*

Cr

Er

Nd

Fe

20
6.5

51
2.1

6.1
0.74

<200*
–

Fe: below a method-dependent background level of 200 ppm.

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1000

200

PL Intensity [arb. unit]

A

800

150

600
100
400

B
C

200
0
1.5 2.0

3.0

D

4.0

E

5.0

50
0
6.0

Detected Photon Energy [eV]Fig. 4. (Color online.) PL emission spectra observed at 10 K before the ion
implantation in the whole photon-energy range measured in this research, namelyfrom 1.5 to 6.0 eV. Here, green (light gray in b/w) solid, brown (dark gray) dashed,black dashed, and violet (dark gray) solid curves represent PL emission spectra
induced by 5.2, 6.7, 7.7, and 8.0 eV photons, respectively.

PL emission spectra induced by 5.2, 6.7, 7.7, and 8.0 eV photons,
respectively. In total, five PL peaks or bands, which are called PL
A to E, are seen. The excitation and emission energies of each PL
are listed in the second and third columns in Table 2. Note thatan enlarged figure for each PL will be found in Fig. 5. First, the presence of multi-component PL A consisting of a dual component ataround 2.3 eV and two other sharp components at 2.6 and 3.1 eV
is confirmed. Secondly, sharp PL B with two components at 1.66
and 1.71 eV is seen. Thirdly, there is also a very broad PL band C
spreading from around 2.0 to around 3.4 eV with its maximum ataround 2.8 eV. Furthermore, two very broad PL bands D and E
ranging from around 3.2 to 5.0 eV with its maximum at around4.2 eV and from around 4.8 to 6.2 eV with its maximum at around
5.5 eV are also observable.
Next, Fig. 5 shows enlarged emission spectra for each PL
observed at 10 K before and after the sample had been implantedwith P+ or B+ ions at a fluence of 1015 cm 2. For all the spectra,
black curves (I) are for the unimplanted samples and pink (lightgray in b/w) ones (II) are for the P+-implanted samples, whereas
blue (dark gray) ones (III) are for the B+-implanted samples.
Fig. 5(a) shows that the multi-component PL A at around 2.3, 2.6
and 3.1 eV disappears completely, while the intensity of the verybroad PL C at around 2.0–3.4 eV is unchanged by the ion implantation. The PL B in Fig. 5(b) with two components at 1.66 and 1.71 eV
becomes weak by the ion implantation. As shown in Fig. 5(c), thevery broad PL D with its maximum at around 4.2 eV disappears
almost completely when ions were implanted. Complete disappearance of the sharp multi-component PL A can be reconfirmedin this figure. Furthermore, the broad PL E at around 5.5 eV also disappears completely by the ion implantation as shown in Fig. 5(d).

Table 2PL bands due to point defects in YAlO3.
PL

Excitationenergy (eV)

Emission energy (eV)

Effect ofions

Origin of PL

A
B
C

7.5
6.5–7.0
5.2

D

7.5–7.7

Disappear
DecreaseNochangeDisappear

Er3+ [48]Cr3+ [39]Oxygenvacancy [8]STE [43]E

8.0

2.3, 2.6, 3.1
1.66, 1.71
2.0–3.4 (maximum at
2.8 eV)
3.2–5.0 (maximum at
4.2 eV)
4.8–6.2 (maximum at
5.5 eV)

Disappear

Y3+/Al3+
antisite [44]Fig. 6 shows optical absorption spectra measured at 10 K before
and after the ion implantation. The value of Eg estimated for theunimplanted plate agrees with a literature value (=7.9 eV) [8]. It
is clearly shown that the implantation of either P+ or B+ ionsincreases significantly the optical absorption in an energy range
slightly lower than Eg.
Fig. 7 shows PL excitation spectra measured at 10 K. Since thespectra were taken by changing the excitation photon energy manually by a step of 0.1 or 0.2 eV using the incidence monochromator,
the spectra are represented by dots. Furthermore, the colors are inaccordance with those in Figs. 5 and 6. Namely, the black solid circles are for the unimplanted samples and the pink (light gray in b/w) solid triangles are for the samples implanted with P+ ions at the
fluence of 1015 cm 2, while the blue (dark gray) open squares arefor the B+-implanted samples.
According to Fig. 7(a), all the three sharp components of PL A at
2.3, 2.6 and 3.1 eV have similar or identical PL excitation spectrawith a broad but clear excitation maximum at around 7.5 eV,
which is slightly lower than Eg. On the other hand, in Fig. 7(b),the intensity of PL B becomes maximum at the excitation energy
around 6.5–7.0 eV, which is much smaller than Eg. As shown in
Fig. 7(c), PL C becomes maximum when it is excited at about
5.2 eV, which is also much smaller than Eg. Similarly to PL A, thebroad PL D with its maximum at 4.2 eV shown in Fig. 7(d) has a
PL excitation maximum at 7.5–7.7 eV, which is also slightly lower
than Eg. Lastly, broad PL E with its maximum at around 5.5 eV seen
only in the unimplanted sample exhibits a PL excitation maximum
at 8.0 eV, which is very close to Eg, as shown in Fig. 7(e).
Fig. 8 shows the ESR spectrum observed in the unimplanted
YAlO3 crystal sample at room temperature. A clear signal withthree features at 297, 320, and 352 mT is seen.

4. Discussion
4.1. Structural changes
First, the structural changes induced by the ion implantation are
analyzed from the results of optical absorption and XRD measurements. The increase in optical absorption at energies slightly lower
than Eg shown in Fig. 6, which was induced by the implantation
with P+ or B+ ions, indicates that new localized electronic states
were formed in the band gap by the ion implantation. On the other
hand, the decrease in the XRD peak height shown in Figs. 2 and 3,
which was also induced by the implantation, indicates the collapseof the crystal structure. Furthermore, the random shift of the XRD
peak position shown in Fig. 3 indicates that the lattice constants
were changed.
When ions are implanted in solids, the displacement of atoms
and the generation of atomic vacancies are induced [27,28], as aresult of the collapse of the crystal structure. Regarding this, numbers of vacancies of three different atoms Y, Al, and O, induced in
YAlO3 by the implantation with B+ or P+ ions, were calculated using
the software TRIM2013 [25] on the assumption that ions were
implanted at a fluence of 1.0  1015 cm 2 with an accelerationenergy of 100 keV. Fig. 9 shows the results. The atomic vacancies
are generated most abundantly at positions 170 and 50 nm deep
under the surface by the implantation with B+ and P+ ions, respectively. Fig. 9 also indicates that oxygen vacancies are induced moreabundantly than Y or Al vacancies, which is simply because YAlO3
contains more O atoms than Y or Al atoms. The distortion of latticeplanes, induced by such displacement of atoms and generation of
atomic vacancies, should be responsible for the shift of the XRD
peak [29,30]. The results mentioned above clearly show that thedegradation of crystallinity, the distortion of lattice planes, and201

T. Morimoto et al. / Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

200

1000
A

I

150

II and III(overlapping)

100

50

C

0
2.0

2.5
3.0
3.5
Detected Photon Energy [eV]

PL Intensity [arb. unit]

PL Intensity [arb. unit]

A
800

I

600
II and III(overlapping)

400
D

200
0
2.0

3.8

6.0

(c) PLs A and D(a) PLs A and C
500

500

B

400
300

PL Intensity [arb. unit]

PL Intensity [arb. unit]

3.0
4.0
5.0
Detected Photon Energy [eV]

I

200

II

100

III

0
1.5

1.6
1.7
Detected Photon Energy [eV]

1.8

400

I

300
200

E

II and III(overlapping)

100
0
4.0

4.5
5.0
5.5
6.0
6.5
Detected Photon Energy [eV]

7.0(d) PL E(b) PL B

Fig. 5. (Color online.) Enlarged emission spectra for each PL observed at 10 K before and after they had been implanted with P+ or B+ ions at a fluence of 1015 cm 2. For all the
spectra, black curves (I) are for the unimplanted samples and pink (light gray in b/w) ones (II) are for the P+-implanted samples, whereas blue (dark gray) ones (III) are for the
B+-implanted samples.

3.5
3.0

III

Absorbance

2.5

II
I

2.0
1.5
1.0
0.5
0
2

3

4

5

6

7

8

9

Photon Energy [eV]Fig. 6. (Color online.) Optical absorption spectra observed at 10 K. Curve I
represents the absorption observed before ion implantation, while II and III are
for the samples implanted with P+ and B+ ions at a fluence of 1015 cm 2,
respectively.

the generation of localized electronic states are induced by the ion
implantation.
As another possible reaction induced by the ion implantation,
point defects may change their charging states. If a certain PL isdue to such point defects, it is reasonably assumed that the change
in valence electrons should exert an unignorable influence on the
PL. Namely, the energy and spectral shape of the PL emission or excitation peak would be changed if the charging state of the responsi-

ble point defect is altered. However, at least as for PLs B and C
observed in the present research, their emission and excitation
spectra do not exhibit any meaningful change by the ion implantation as shown in Figs. 5(a) and (b) and 7(b) and (c). This indicatesthat the change in charging state is not a dominant process.
Among various ion-induced phenomena, the above-mentioned
change in charging state would be annealed at a low temperaturecompared to various structural changes such as lattice deformationand formation of atomic vacancies [28]. For example, it has beenreported that an electronic damage induced in SiO2 by ion implantation is recovered by annealing at 200 °C [28]. The authors also
revealed that the change in charging state of oxygen vacancies
induced in YSZ by the irradiation of ultraviolet photons is recovered by annealing at 260 °C [16]. Therefore, if the effects of annealing treatments at various temperatures are studied well, we will beable to find whether the dominant change induced by ions is electronic or structural. This will be studied in the near future. As a tentative examination, the sample was annealed at various
temperatures from 500 to 1000 °C in oxygen after it had been
implanted with P+ ions at a fluence of 1015 cm 2. As a result, it
has become clear that PL B, which was decreased once by the ion
implantation, only scarcely recovers its intensity by the annealing
at 900 °C as shown in Fig. 10. This result indicates that the reaction
influencing dominantly on the PL properties is structural.

4.2. PL excitation and emission spectra
Next, the effects of the above-mentioned structural changes of
YAlO3 on its PL characteristics are discussed. For that purpose,

T. Morimoto et al. / Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

60

PL Intensity [arb. unit]

1200
800
400
0
800
400
0
400

3.1 eV

2.6 eV

2.3 eV200
0
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0
Excitation Photon Energy [eV]

PL Intensity [arb. unit]

202

40

20

0
2.83.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Excitation Photon Energy [eV]

500
400
300
200
100
0
400
300
200
100
0
5.5

(c) PL C
200

1.71 eV

1.66 eV

6.0 6.5 7.0 7.5 8.0 8.5
Excitation Photon Energy [eV]9.0

PL Intensity [arb. unit]

PL Intensity [arb. unit](a) PL A

150
100
50
0
6.0

6.5
7.0
7.5
8.0
8.5
Excitation Photon Energy [eV]

9.0(d) PL D(b) PL B

PL Intensity [arb. unit]

200
150
100
50
0
6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0
Excitation Photon Energy [eV](e) PL E
Fig. 7. (Color online.) Excitation spectra of PL A detected at 2.3, 2.6, and 3.1 eV (a), PL B detected at 1.66 and 1.71 eV (b), PL C detected at 2.8 eV (c), PL D detected at 4.2 eV (d),and PL E detected at 5.5 eV (e). Black solid circles are for the unimplanted samples and the pink (light gray in b/w) solid triangles are for the samples implanted with P+ ions at
the fluence of 1015 cm 2, while the blue (dark gray) open squares are for the B+-implanted samples. All spectra were measured at 10 K.

the assignment of each PL has to be known. First, the very broad PL
C spreading very widely at around 2.8 eV seen in Fig. 5(a) is due tooxygen vacancies [8]. This type of PL due to oxygen vacancies isknown to appear in many oxides such as SiO2 [31–36], ZnO [37],Ta2O5 [38], LaAlO3 [10,24], and yttria-stabilized zirconia (YSZ)[23]. It has also been known that the PL due to oxygen vacancies
appears regardless of the sample’s crystallinity in all these materials with the exception of YSZ [23]. Namely, the PL appears in YSZ
only when its crystallinity is high. The reason for the exceptionof YSZ is that oxygen vacancies appear in YSZ according to the ratioof yttria to zirconia in YSZ [23]. Here, in the present YAlO3, the verybroad PL C due to oxygen vacancies appears almost similarly
before and after the ion implantation as shown in Fig. 5(a). That
is to say, the PL C is induced in YAlO3 by the transition process ofelectrons that is not affected by its crystallinity.

Although oxygen vacancies should have been induced by theion implantation as indicated in Fig. 9, the intensity of PL C due
to oxygen vacancies is not increased by the implantation as shownin Figs. 5(a) and 7(c). When Al2O3 is implanted with various ions,the intensity of optical absorption due to the F+ center, namely,
the oxygen vacancy that captures one electron, starts to increase
when the nuclear energy deposition reaches 1023 keV cm 3 [28].
Here, the project range of ions with an energy of 100 keV in a solid
substance is around 100 nm [25]. Therefore, if the whole energythat the ions possess is given to the substance, its energy density
becomes 1022 keV cm 3 at a fluence of 1015 cm 2. Taking accountof the fact that the nuclear energy deposition is a fraction of thetotal energy deposition, the energy of 100 keV and the fluence of1015 cm 2, used in the present research, are not high enough toincrease the density of oxygen vacancies to a level, which yields

203

7
6
5
4
3
2
1
0
-1
-2
-3
250

300

350

400

Magnetic Field [mT]Fig. 8. (Black and white.) ESR spectrum observed in unimplanted YAlO3 crystal atroom temperature. The magnetic field was applied parallel to the principal axis.

an appreciable increase in PL C. For the purpose of examining
whether oxygen vacancies are induced by the ion implantation in
the present YAlO3 samples, ESR measurements were carried out.
However, no obvious increase in the signal intensity due to oxygen
vacancies was detected, probably due to the fact that the ions
could reach only up to 100 nm from the surface of the 0.5-mmthick-sample.

0.15
cm ]

-2

0.1

[nm

-1

Number of Vacancies[×1016]

0.05

0

0

100

200

300

400

Depth [nm](a)

[nm-1 cm-2]

Number of Vacancies[×1016]

Secondly, as for the sharp PL B with two components at 1.66
and 1.71 eV seen in Fig. 5(b), its spectral shape is similar to thatof the R-line luminescence of Cr3+ in YAlO3 [39]. The ICP-MS results
listed in Table 1 indicate that the present YAlO3 contains Cr. Furthermore, the positions of the three ESR features at 297, 320 and
352 mT shown in Fig. 8 are similar to those of the features assigned
to Cr3+ in the literature [40]. Moreover, the presence of Cr3+ in single crystal LaAlO3 with a similar structure to YAlO3 was confirmed
by ESR and also by the presence of similar R-line luminescence[10,22]. It has been reported that Cr3+ occupies an Al3+ site in an
octahedral ligand field of O2 when the sample is crystalline andthat the ligand field resolves the degeneracy of the d-orbital of
Cr3+ due to the Stark effect [41,42]. By this resolution, the R-line
energy levels are formed and the PL appears. Therefore, PL B withtwo components at 1.66 and 1.71 eV is attributable to Cr3+ and it
appears only when the sample’s crystallinity is high. Furthermore,
PL B has its excitation maximum at around 6.5–7.0 eV as shown in
Fig. 7(b), which is much smaller than Eg. Therefore, the excitationof PL B is not related to an electronic transition across the bandgap.
Thirdly, the very broad PL D at 4.2 eV shown in Fig. 5(c) seems
to be due to self-trapped excitons (STEs) [43]. This type of excitons
were also assigned to the origin of a very similar PL in Y2O3 [43].
Here, STEs or excitons in general form localized electronic states
just below the conduction band edge. Therefore, PL D seems to
be induced by an electronic transition from such exciton states. If
additional localized electronic states are induced in the forbidden
band by the ion implantation as discussed in relation to Fig. 6, they
will mask the exciton states. Therefore, the disappearance of the PL
D at 4.2 eV after the ion implantation is reasonable. In other words,
this PL appears only when the sample’s crystallinity is high.
Fourthly, the PL E at 5.5 eV shown in Fig. 5(d) seems to be due toantisites where Y3+ substitutes for Al3+, because of a marked similarity between the PLs observed in this research and reported in
the literature [44]. The rising portion of the PL excitation spectrum
observed in the sample before the ion implantation shown by theblack circles in Fig. 7(e) is very close to that of the absorption curve
observed before the ion implantation shown in Fig. 6. This indicates that the excitation of PL E is directly or indirectly related tothe electronic excitation across the clear bandgap of the sample
before the ion implantation. Therefore, it is reasonable that the
PL E due to the antisites appears only when the sample’s crystallinity is high as in the case of PL D due to STEs.
Fifthly, as for the sharp PL A with components at 2.3, 2.6 and3.1 eV observable in Fig. 5(a) and (c), its spectral shape is similarly
sharp to that of the PL B with two components at 1.66 and 1.71 eV

0.8

50
0.6
0.4
0.2
0
0

100

200
300
Depth [nm]

400

(b)
Fig. 9. (Color online.) Numbers of atomic vacancies induced by the implantation
with B+ (a) and P+ (b) ions with an acceleration energy of 100 keV at a fluence of
1.0  1015 cm 2, calculated using software TRIM2013. Vacancies of O, Y, and Al aredenoted by pink (light gray in b/w) solid inverted triangles, blue (dark gray) open
diamonds, and green (dark gray) solid circles, respectively.

PL Intensity [arb. unit]

Intensity [arb. unit]

T. Morimoto et al. / Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

40
30
20
10
0

AsP+
received implanted 500

700

900

1000

Annealing temperature [oC]
Fig. 10. (Black and white.) Effects of thermal annealing, which followed theimplantation with P+ ions, on the intensity of PL B due to Cr3+.

4.3. Comparison with film samples
To confirm further whether the appearance of the sharp PL B
with two components at 1.66 and 1.71 eV shown in Fig. 5(b) is limited to the samples with high crystallinities, the following experiment was carried out. Namely, using an alkoxide solution
containing Y and Al (Kojundo Chemical Laboratory, AlY-05(1/1)),
thin films with a thickness of approximately 100 nm were spincoated on a Si(1 0 0) single crystal substrate. The films were thermally annealed at different temperatures between 600 and1000 °C for 120 min in air, and in-plane XRD patterns and PL emission spectra were taken. As shown in Fig. 11, the films annealed at600, 700 and 800 °C are amorphous, while those annealed at 900
and 1000 °C are polycrystalline containing YAlO3 and Y3Al5O12.
This is consistent with the fact that YAlO3 and Y3Al5O12 in a film
deposited by a sol–gel method start to crystallize at approximately
1000 and 900 °C, respectively [49].

(204)

(800)

(112)
(640)
(302)

(104)

(202) (040)

(521)

(100)

(002)

(101)

800

Intensity [arb. unit]

shown in Fig. 5(b). This indicates that the origins of two PLs A and B
are similar. Therefore, the origin of PL A with components at 2.3,
2.6 and 3.1 eV seems also to be a transition metal impurity like
Cr3+. One possible candidate would be Fe3+. Although the presenceof Fe could not be confirmed by ICP-MS due to the abundance of Fe
in the background noise as shown in Table 1, Fe has been confirmed to exist in Al2O3 [45] and several aluminates such as
Y3Al5O12 [46] and LaAlO3 [47]. In this regard, PL A could be due
to Fe3+. However, a more plausible candidate is a rare-earth ion.
As shown in Table 1, Er and Nd exist in the present YAlO3 samples.
It is well known that Er3+ has luminescent levels capable of emitting photons of 2.33, 2.61 and 3.07 eV [48], namely exactly thesame energies of the sharp components of PL A shown in Fig. 5(a) and (c). However, Nd3+ does not have such levels. In this regard,
Er3+ is the origin of the PL A.
Next, the intensities of the PLs A, B, D, and E, which are respectively due to Er3+, Cr3+, STEs, and antisites, disappear or become
smaller by the ion implantation. On the other hand, the intensityof the PL C due to oxygen vacancies does not change. This difference in the dependence of crystallinity on the disappearance of
PL is a reflection of the difference in the involvement of the sample’s crystallinity in each PL. Furthermore, according to Figs. 5–7,
although new localized electronic states were formed by the ion
implantation, no additional PL emission and excitation bands wereobserved at least in the energy range from 1.5 to 10 eV, indicatingthat the new states are non-radiative.
Lastly, if we compare Fig. 5(a) and (c) with Fig. 5(b), we find thatthe effect of ion implantation on the intensity of PL A due to Er3+ ismore obvious than that on the intensity of PL B due to Cr3+.
Namely, PL A due to Er3+ disappears completely by the ion implantation. The three components of PL A are induced by electronictransitions from an excited state (2P3/2) of 4f electrons to three
lower states (4I9/2, 4I11/2, and 4I13/2) [48]. Since 4f orbital electrons
are shielded by 5s orbital electrons, these transitions are hardly
influenced by the crystal field. On the other hand, the excitationof the 4f electrons to the excited 2P3/2 state is realized by being
assisted by STE, which exists stably only when the sample is crystalline. Therefore, the excitation of PL A is influenced by the crystal
field. On the other hand, as mentioned above, PL B is due to the Cr3+
ion that occupies the Al3+ site in the octahedral ligand field. Therefore, its intensity decreases if the ligand field collapses. For thisreason, the effect of ion implantation differs between PLs A and
B. The details of this will be reported elsewhere.
To summarize, the effects of ion implantation on all the PL
bands in YAlO3 are listed in the fourth column of Table 2. Furthermore, the origin of each PL, the certainty of which has been confirmed by the present research, is listed in the rightmost column.

(102)
(332)
(422)

T. Morimoto et al. / Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

600
1000 oC

400
o

900 C

200
0
10

800 oC
700 oC
600 oC20

30

40 50 602θχ [deg.]70

80

Fig. 11. (Color online.) Effects of thermal annealing on the in-plane XRD patterns offilm samples deposited on Si substrates. Black and pink (light gray in b/w) broken
vertical lines represent the characteristic diffraction positions of YAlO3 andY3Al5O12, respectively.

Curves (a)–(e) in Fig. 12 are the PL emission spectra induced inthese five film samples at 10 K. The PL emission spectrum inducedin the single crystal plate sample before ion implantation, namely
curve I in Fig. 5(b), is again shown for comparison as curve (f).
While the films annealed at 600, 700 and 800 °C exhibit no PL,
those annealed at 900 and 1000 °C have two or three PL components at about 1.70, 1.75 and 1.80 eV. Here, the peak positions
observed in the thermally annealed films differ slightly from thoseobserved in the single crystal plate sample. Regarding this, a very
similar phenomenon was observed in LaAlO3 [10]. Namely, theenergies of PL components are different between LaAlO3 samples
of a bulk shape and of a film shape deposited on a CaF2 substrate.
The authors have already revealed that this difference is due tomismatch in lattice constant between LaAlO3 and the CaF2 substrate [10]. Therefore, the slight difference in energy between spectrum (f) and spectra (a and b) seems to be due to the difference inlattice constant between YAlO3 and the Si substrate. It is also reasonable that the PL emission spectral curve (a) observed in the film
sample annealed at 1000 °C exhibits three components, since theR-line luminescence of Cr3+ is composed of two or three components [10,39,41]. Note that the authors could not judge whether
the PLs due to oxygen vacancies, Er3+, STEs, and antisites appeared250

PL Intensity [arb. unit]

204

200

(a)
(b)
(c)
(d)
(e)

150
100
50

(f)
0
1.6

1.7
1.8
Detected Photon Energy [eV]1.9

Fig. 12. (Color online.) Comparison among PL emission spectra obtained at 10 K forthe unimplanted film samples annealed at 1000 (a), 900 (b), 800 (c), 700 (d) and
600 °C (e) after having been deposited on Si substrates, and the spectrum obtained
for the unimplanted single crystal plate sample (f).

T. Morimoto et al. / Nuclear Instruments and Methods in Physics Research B 366 (2016) 198–205

or not, due to a very strong PL at around 2–4 eV from the CaF2
substrate.
The results shown in Figs. 11 and 12 clearly indicate that the PL
B due to Cr3+ appears only when the sample is crystalline. Namely,
the decrease in intensity of the PL B with the two components at
1.66 and 1.71 eV due to Cr3+ is not a phenomenon peculiar to theion implantation. A decrease in PL B should occur similarly to thecase shown in Fig. 5(b) when the sample’s crystallinity is loweredby any reason. As mentioned above, the PL D at 4.2 eV shown in
Fig. 5(c) due to STEs, the PL E at 5.5 eV shown in Fig. 5(d) due to
Y3+/Al3+ antisites, and the sharp PL A with components at 2.3, 2.6
and 3.1 eV shown in Fig. 5(a) and (c) due to Er3+ appear only when
the sample’s crystallinity is high. Therefore, these PLs disappear
when the crystalline structure collapses.
4.4. Comparison with other high permittivity materials
In the authors’ previous papers [23,24], the effects of implantation of P+ or B+ ions to LaAlO3 and YSZ were reported. While theeffect of ion implantation on LaAlO3 is basically very similar to thaton YAlO3 reported in the present paper [24], its effect on YSZ, especially on the PL due to oxygen vacancies is quite different [23].
Namely, the PL due to oxygen vacancies in LaAlO3 is scarcely changed by the ion implantation as in the case of PL C in YAlO3. However, the PL due to oxygen vacancies in YSZ decreases its
intensity with an increase in fluence of ion implantation. The reason for this is that oxygen vacancies in YSZ do not exist in the formof a structural point defect as in the case of YAlO3 or LaAlO3 [23].
The oxygen vacancies in YSZ exist as a part of its crystal structure
to compensate the deficiency of oxygen atoms induced by the stabilization of zirconia by yttria.
5. Conclusions
Single crystal YAlO3 plates were implanted with P+ or B+ ionsand the resultant changes were analyzed by measuring their optical absorption spectra, in-plane XRD patterns, and PL excitationand emission spectra. Important findings obtained in this work
are summarized as follows:(1) The ion implantation deforms the crystalline structure of
YAlO3 and induces localized electronic states in its forbidden
band. However, no new PLs are induced.
(2) The PL due to oxygen vacancies is independent of the crystallinity and its intensity is not changed by the ion implantation. On the other hand, the PL due to Cr3+ in the ligand
field becomes weak by the ion implantation.
(3) Both the PL due to STEs and the one due to Er3+ are diminished by the ion implantation. The reason for this is thatthe exciton states are masked by the localized electronic
states induced as a result of the degradation of crystallinity.
(4) The PL due to Y3+/Al3+ antisites is also diminished by the ion
implantation, since the PL appears only when the sample’s
crystallinity is high.

Acknowledgments
This research was partly conducted in the UVSOR Facility, Institute for Molecular Science, Okazaki, Japan. It was partly supported
by JSPS (Japan Society for the Promotion of Science) Grant 25 3090
for JSPS Fellows. It was also partly supported by the Early Bird
Grant, the ENEOS Research Encouragement Prize, and the205

Mitsubishi Materials Corporation Grant, all from Waseda ResearchInstitute for Science and Engineering.
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