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Cause of the appearance of oxygen vacancies in yttria-stabilized zirconia and its relation to
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2015 Jpn. J. Appl. Phys. 54 06GC03(http://iopscience.iop.org/1347-4065/54/6S2/06GC03)
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REGULAR PAPER

Japanese Journal of Applied Physics 54, 06GC03 (2015)
http://dx.doi.org/10.7567/JJAP.54.06GC03

Cause of the appearance of oxygen vacancies in yttria-stabilized zirconia
and its relation to 2.8 eV photoluminescence
Shoji Kaneko1*, Takaaki Morimoto1, and Yoshimichi Ohki1,2*
1

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, Japan
E-mail: yohki@waseda.jp; s.kaneko.4869@akane.waseda.jp
2

Received November 27, 2014; accepted March 17, 2015; published online June 1, 2015
When we implanted P+ or B+ ions into yttria-stabilized zirconia (YSZ), its crystallinity was degraded. Concurrently, the photoluminescence ataround 2.8 eV originating from two types of oxygen vacancies with one or two captured electrons became weak, indicating a decrease in thenumber of oxygen vacancies. Oxygen vacancies appear in YSZ as a result of the replacement of Zr4+ by Y3+ in ZrO2. Therefore, it seems that the
separation of YSZ into ZrO2 and Y2O3 induced by the ion implantation is responsible for the decrease in the number of oxygen vacancies.
Moreover, the intensity of the 2.8 eV photoluminescence returns to the value before the ion implantation if the sample is annealed thermally after
the implantation at temperatures higher than the crystallization temperature of YSZ. The reaction opposite to the above seems to be induced by thethermal annealing. © 2015 The Japan Society of Applied Physics

1.

Introduction

With the progress in the miniaturization of metal oxide
semiconductor ﬁeld-eﬀect transistors in large-scale integrated
circuits, the thickness of gate dielectrics become increasingly
smaller, causing SiO2 to become unsuitable for the gate
dielectric, since it would become too thin to maintain its
insulating property.1–3) One possible solution to this problem
is to use other dielectric materials that possess higher
dielectric constants (k).4)
Although hafnium-based oxides have been extensively
investigated,5–8) it is known that, in practical use, they form
silicates if they are on a Si substrate.9,10) Regarding thisproblem, yttria-stabilized zirconia (YSZ), which possesses ahigh k of around 27,11) is assumed to be promising for a
future gate insulator.12,13) It has been reported that high-k
dielectrics such as YSZ may have point defects.13–15) If thisis the case, they would be a potential risk for their use inelectronic devices.
Therefore, in this research, we examine what kinds ofdefects are present in YSZ, focusing particularly on oxygen
vacancies. Photoluminescence (PL) was used as the main
analytical tool in addition to the measurements of X-ray
diﬀraction (XRD), electron spin resonance (ESR), and opticalabsorption, since these methods have been very eﬀective
for examining the structural changes induced in various
inorganic insulating materials.6–8,16–36) We also use theimplantation of ions as a method of damaging the crystalline
structure and thermal annealing to recover the damaged
structure.
2.

Experimental methods

Commercially available YSZ (100) single-crystal samples,grown by the skull-melting method37) (Crystal Base), wereused in the experiments. They were cut to rectangular plateswith a thickness of 0.5 mm. The proportions of yttria andzirconia are 9.8 and 90.2 mol %, respectively.
Positive monovalent ions of phosphorus (P+) or boron+(B ) were implanted at room temperature, using the ion
accelerator (Ulvac UP-150) at Waseda University, into thesamples at an acceleration energy of 100 keV to various
doses ranging from 0.2 × 1015 to 1.0 × 1015 cm−2. After theion implantation to the dose of 1015 cm−2, the samples were

heated to various temperatures between 150 and 800 °C for
3 min in air and then kept at each annealing temperature fordesignated periods between 20 s and 20 min for annealing.
Furthermore, for comparison, ultraviolet (UV) photons were
irradiated to unimplanted samples for 30 min at room
temperature using a KrCl excimer lamp (Ushio UER 20222) that emits photons with an energy of 5.59 eV.
Before and after the ion implantation or thermal annealing,
in-plane XRD spectra were obtained with Cu Kα X-rays atroom temperature using Rigaku Rint-Ultima III, while ESR
spectra were obtained with JEOL JES-FA 300 at room temperature at 9.20 GHz (X band) using the microwave power of
5.00 mW. Optical absorption spectra in the visible-UV region
were measured with a double-beam spectrophotometer(Shimadzu UV-3100PC) in the transmission mode at room
temperature.
For PL measurements at 77 K, a ﬂuorescence spectrometer(JASCO FP-8500) was used. Furthermore, PL spectra werealso obtained at 10 K using synchrotron radiation (SR) to
excite PLs at the BL3B line with a beam energy of 750 MeV
equipped with a 2.5 m oﬀ-plane Eagle-type monochromatorin the UVSOR Facility (Institute for Molecular Science,
Okazaki, Japan). For the SR measurements, a 300 mm triplegrating monochromator (Acton SpectraPro-300i) was used as
an incidence monochromator and a CCD camera (Princeton
Instruments) was used to detect PLs. The intensity of PL wascalibrated by taking into account the excitation SR intensity
at each photon energy.
Moreover, the PL lifetime in the ms range was measured at
77 K using the FP-8500 spectrometer, whereas that in the ns
range was measured at 10 K by a time-correlated singlephoton counting technique using SR pulses. The SR pulse
under a single-bunch mode has an apparent duration of about
550 ps including the time response of the detection system,and the time interval between two consecutive SR pulses isabout 177.6 ns.38)
3.

Results

First, we estimated how deep ions can penetrate into YSZ
using a simulation program called SRIM2011.39) As shownin Fig. 1, the projected ranges of P+ and B+ ions with anenergy of 100 keV in YSZ are estimated to be about 80
and 200 nm, respectively. Compared with this, the depth

06GC03-1© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 06GC03 (2015)

S. Kaneko et al.

100

300
Ion Range [nm]250

ESR Intensity [arb. unit]

B+

200
150
P+

100
50
0
0

20

50

II

S1

S2

V

0
-50
-100
2.02

40 60 80 100 120 140
Acceleration Energy [keV]

S5
S4 IV

I & III(Overlapping)

2.00

1.98
1.96
g value

1.94

1.92

(a)

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

Intensity [arb. unit]

100
80

III
I

60

V

40
20
0
34.0

ESR Intensity [arb.unit]

2

II
IV

S3
1
0
-1
-2
1.92

1.91

1.90

1.89

g value34.5

35.0
2θχ [deg]

35.5

36.0

(b)

Fig. 2. (Color online) In-plane XRD patterns obtained for the following
YSZ samples: I) untreated, II) implanted with P+ ions to a dose of 1015 cm−2,
III) annealed at 300 °C for 10 min following the implantation of P+ ions,IV) implanted with B+ ions to a dose of 1015 cm−2, V) irradiated by photons
with an energy of 5.59 eV for 30 min.

Fig. 3. (Color online) (a) ESR spectra obtained for the following YSZ
samples: I) untreated, II) implanted with P+ ions to a dose of 1015 cm−2,
III) annealed at 300 °C for 10 min following the implantation of P+ ions,IV) implanted with B+ ions to a dose of 1015 cm−2, V) irradiated by photons
with an energy of 5.59 eV for 30 min. (b) ESR spectra observed before (blacksolid) and after the irradiation of 5.59 eV photons (orange dashed).

measurable using in-plane XRD is 2 to 8 nm.40) Here, we set
the incident angle of X-rays to be 0.32° so that the diﬀraction
intensity is maximum in YSZ. At this angle, the measurable
depth is about 5 nm. Therefore, the depth measurable using
XRD is much shallower than the projected ranges of ionswith the energy of 100 keV.
Figure 2 shows the in-plane XRD spectra observed forsamples treated by the following four procedures; implantation of P+ ions to a dose of 1015 cm−2, implantation of B+ ionsto the same dose, thermal annealing at 300 °C for 10 min after
the P+ implantation, and UV irradiation with a photon energyof 5.59 eV for 30 min, along with the spectrum observed forthe unimplanted sample. The unimplanted sample exhibits apeak at 2θχ = 34.9°, which corresponds to the diﬀraction by
crystalline YSZ at its (200) planes. The intensity of the XRD
peak is decreased by the implantation of P+ or B+ ions, as
shown by curve II or IV. The thermal annealing at 300 °C
that followed the P+ ion implantation returns the intensity tonearly the same strength as that of the unimplanted sample.
On the other hand, the UV irradiation given to the untreatedsample does not change the XRD intensity.
Figure 3 shows ESR spectra observed for the above ﬁvesamples. As indicated by curves II and IV in Fig. 3(a),two ESR signals appear at g = 2.00 and 1.97 after the
implantation of P+ or B+ ions to the dose of 1015 cm−2,

whereas they disappear after the following thermal annealing
at 300 °C for 10 min. In the present paper, the above two ESR
signals at g = 2.00 and 1.97 are referred to as S4 and S5,
respectively. On the other hand, as shown in Figs. 3(a) and3(b), three ESR signals appear after UV irradiation with aphoton energy of 5.59 eV for 30 min at g = 2.006, 1.97–1.95,
and 1.91–1.86, which were referred to as S1, S2, and S3,
respectively, in our previous paper.30)
Figure 4(a) shows optical absorption spectra observed forthe samples untreated and treated in the same manner as thosein the cases of Figs. 2 and 3, while Fig. 4(b) shows theirdiﬀerential spectra after each treatment. After the implantation of P+ or B+ ions to the dose of 1015 cm−2, an absorption
band appears at around 2.4 eV, which disappears after thefollowing thermal annealing at 300 °C. On the other hand,
UV irradiation of the untreated sample with a photon energyof 5.59 eV for 30 min yields a small and broad absorption
band at around 3.3 eV.30)
Figure 5(a) shows PL spectra observed for the samples
untreated and treated in the same manner as above. They
were excited by 5.2 eV photons and measured at 77 K using
the ﬂuorescence spectrometer FP-8500. As indicated byspectrum I, the unimplanted sample exhibits a peak at around
2.8 eV. The intensity of the 2.8 eV PL peak is decreased by
the implantation of P+ or B+ ions, as shown in spectrum II or

06GC03-2© 2015 The Japan Society of Applied Physics

S. Kaneko et al.

30

100
PL Intensity [arb. unit]

Absorption coefficient [cm -1]

Jpn. J. Appl. Phys. 54, 06GC03 (2015)20
II and IV(Overlapping)V

10
I and III(Overlapping)
0
1.5

2.0

2.5 3.0 3.5 4.0
Photon energy [eV]

4.5

80
60

II & IV(Overlapping)40
200
1.8 2.0

5.0

2.5
3.0
3.5
Detected Photon Energy [eV]100

3
PL Intensity [arb. unit]

ΔAbsorption coefficient [cm -1]

4.0

(a)

(a)

2
IV'
II'
1
V'
III'
0
1.5

I

III & V(Overlapping)

2.0

2.5
3.0
3.5
Photon energy [eV]

80
60
40
20
0
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8
Detected Photon Energy [eV]

4.0

(b)

(b)

IV, whereas it returns to a similar strength to its original value
upon the following annealing at 300 °C. On the other hand,
UV irradiation does not yield any change, as shown by
spectrum V. This dependence of the 2.8 eV PL on thetreatment given to the sample is similar to that of the XRD
intensity shown in Fig. 2. Figure 5(b) shows the temperaturedependence of the intensity of the 2.8 eV PL excited by
5.2 eV photons, measured at 10, 77, and 300 K using SR. The
PL intensity decreases with increasing temperature.
Figure 6 shows the eﬀects of the dose of implanted P+ ionson the intensities of the above-mentioned various physicalfeatures. With the increase in dose, two ESR signals, S4 andS5, and optical absorption at 2.4 eV increase monotonically,
while the XRD and the broad PL at 2.8 eV show a monotonic
decrease.
We conducted a similar study on the eﬀects of ion
implantation using two high-k dielectrics, LaAlO3 andYAlO3.35,36) Therefore, PL spectra of YSZ observed at
10 K before and after the implantation of P+ ions to the doseof 1015 cm−2 are shown in Fig. 7 with those of LaAlO3 andYAlO3. Here, the PLs were excited by photons with energies
of 5.2 eV for YSZ, 5.0 eV for LaAlO3, and 5.6 eV for YAlO3,
so that the intensity of the 2.8 eV PL became maximum. Avery broad PL is seen in all the samples at around 2.8 eV.

Fig. 5. (Color online) (a) 2.8 eV PL spectra at 77 K excited by 5.2 eV
photons obtained for the following YSZ samples: I) untreated, II) implantedwith P+ ions to a dose of 1015 cm−2, III) annealed at 300 °C for 10 min
following the implantation of P+ ions, IV) implanted with B+ ions to a doseof 1015 cm−2, V) irradiated by photons with an energy of 5.59 eV for 30 min.
(b) PL spectra observed in untreated YSZ at 10 (black solid), 77 (reddashed), and 300 K (gray solid).

1.2
Intensity [normalized]

Fig. 4. (Color online) (a) Optical absorption spectra obtained for thefollowing YSZ samples: I) untreated, II) implanted with P+ ions to a dose of1015 cm−2, III) annealed at 300 °C for 10 min following the implantation of
P+ ions, IV) implanted with B+ ions to a dose of 1015 cm−2, V) irradiated byphotons with an energy of 5.59 eV for 30 min. (b) Curves IIA, IIIA, IVA, and VA
represent the increments in absorption coeﬃcient obtained by subtracting
curve I from curves II, III, IV, and V, respectively.

1.0
0.8
0.6
0.4
0.2
0

0

0.2

0.4

0.6
0.8
dose [cm-2]

1.0

1.2 15×10

Fig. 6. (Color online) Eﬀects of the implanted dose of P+ ions on theintensities of the two ESR signals S4 at g = 2.00 (red closed triangles) and S5
at g = 1.97 (orange inverted closed triangles) and the absorption band at
2.4 eV (black closed circles), normalized by respective intensities observed
right after the implantation to a dose of 1015 cm−2 into YSZ, together with thedose-dependent decrease in XRD peak intensity (blue open squares) and thatof the 2.8 eV PL intensity excited by 5.2 eV photons (green open circles),normalized by their intensities before the implantation.

One important thing here is that the ordinate diﬀers among(a), (b), and (c) and that the PL intensity is very strong in
YSZ compared with those in LaAlO3 and YAlO3. Another

06GC03-3© 2015 The Japan Society of Applied Physics

S. Kaneko et al.

100

100
80 (a)
60
40
20
0
12 (b)
8
4
0
2

XRD Intensity [arb. unit]

PL Intensity [arb. unit]

Jpn. J. Appl. Phys. 54, 06GC03 (2015)

(c)

2.5
3.0
3.5
Detected Photon Energy [eV]

(b)

(c)

60
40
20
0
34

1
0
1.8 2.0

(a)

80

35

3.8

23 24 32 34 36 38
2θ [deg]

Fig. 9. (Color online) In-plane XRD patterns observed in (a) YSZ,(b) LaAlO3, and (c) YAlO3 before (black solid) and after the P+ ion
implantation (red dashed).

Fig. 7. (Color online) PL spectra at 10 K observed in (a) YSZ,(b) LaAlO3, and (c) YAlO3 before (black solid) and after the P+ ion
implantation (red dashed).

PL intensity [arb. unit]

100
80
60
40
20
0

5
Time [ns]

10

0.6
0.4
0.2

1
10-1
10-2

2
3
Time [ms]

10-1

100
Annealing time [min]101Fig. 10. (Color online) Time-dependent decay proﬁles of the intensitiesof the ion-induced ESR signals S4 at g = 2.00 (red closed triangles) and S5 at
g = 1.97 (orange inverted closed triangles) and the absorption band at 2.4 eV(black closed circles) during annealing at 300 °C, normalized by theirintensities right after the implantation of P+ ions to a dose of 1015 cm−2,
together with the recovery of the XRD peak intensity (blue open squares) andthat of the 2.8 eV PL intensity (green open circles), normalized by theirintensities before the implantation.

10

1

0

15

(a)

Intensity [normalized]

0.8

0

0
-5

10-3
0

1.0

~
~

Intensity [normalized]

1.2

4

5

(b)
Fig. 8. (Color online) (a) 2.8-eV PL decay proﬁle in the ns range
observed in untreated YSZ. (b) 2.8-eV PL decay proﬁle in the ms range
observed in untreated YSZ (black open circles), and the decay of excitation
photons with an energy of 5.2 eV (red open triangles), normalized by theirrespective initial values.

important thing is that the PL intensity decreases substantially in YSZ upon ion implantation, although it negligiblychanges in LaAlO3 and YAlO3. Note that several sharp peaks
appearing in YAlO3 will be discussed later.
Figure 8(a) shows the decay of the 2.8 eV PL observed in
the unimplanted YSZ when it was excited by repeated pulsesof 5.2 eV SR photons in the single-bunch mode. From thisdecay, the decay constant of the 2.8 eV PL can be calculated
to be 7 ns. Figure 8(b) shows the decay of the 2.8 eV PL in
the unimplanted YSZ when it was excited at 5.2 eV using theﬂuorescence spectrometer FP-8500. The weakening of theintensity of the excitation photons as a function of timeowing to the movement of a mechanical shutter is also
shown. From this decay, it can be estimated that the 2.8 eV
PL also has a slow component with a decay constant ofaround 0.3 ms.
Figure 9 shows in-plane XRD spectra observed before andafter the implantation of P+ ions to a dose of 1015 cm−2 for
YSZ (a), LaAlO3 (b), and YAlO3 (c).35,36) The intensities of
the XRD spectral peaks are decreased upon ion implantationin the three high-k dielectrics.
Figure 10 shows the eﬀects of the thermal annealing period
at 300 °C for designated periods ranging from 20 s to 20 min
after the implantation of P+ ions to the dose of 1015 cm−2.
The two ESR signals, S4 and S5, and the absorption at
2.4 eV, which were induced by the ion implantation, decrease
promptly and they disappear in 5 min. Furthermore, theintensities of the XRD peak and the 2.8 eV PL, which became

06GC03-4© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 06GC03 (2015)

S. Kaneko et al.

irradiation, decrease to less than 40% at 150 °C and disappear
at 200 °C. Therefore, Fig. 11 clearly shows that reactions
exactly opposite to those induced by ion implantation or by
UV irradiation are induced by thermal annealing for all theabove-mentioned phenomena. Note that annealing experiments could not be performed for the two photoinduced ESR
signals S1 and S3, because S1 decreases immediately, even atroom temperature, and S3 is too weak.

1.0
0.8
0.6
0.4
0.2

4.

~
~

0

~
~

Intensity [normalized]

1.2

room
150 200 250 300 350
temperature

800

Temperature [°C](a)Intensity [normalized]

1.2
1.0
0.8
0.6
0.4
0.2
~
~

0
RT

100

150 200 250
Temperature [°C]

300

350

(b)
Fig. 11. (Color online) (a) Decay proﬁles of the intensities of the ioninduced ESR signals S4 at g = 2.00 (red closed triangles) and S5 at g = 1.97
(orange inverted closed triangles) and the absorption band at 2.4 eV (black
closed circles) after annealing at various temperatures for 10 min, normalized
by their intensities right after the implantation of P+ ions to a dose of1015 cm−2, together with the recovery of the XRD peak intensity (blue open
squares) and that of the 2.8 eV PL intensity (green open circles), normalized
by their intensities before the implantation. (b) Similar changes in the photoninduced ESR signal S2 at g = 1.97–1.95 (black open triangles) andabsorption band at 3.3 eV (red closed circles), normalized by their intensities
right after the irradiation of photons with an energy of 5.59 eV for 30 min.

smaller after ion implantation, recover completely in 5 min to
the values before ion implantation.
Figure 11(a) shows the eﬀects of the temperature ofannealing for 10 min between 150 and 800 °C after the
implantation of P+ ions to a dose of 1015 cm−2. With theincrease in annealing temperature, the two ESR signals andthe optical absorption at 2.4 eV, which were induced by theion implantation, show a monotonic decrease and they
disappear at 300 °C. In contrast, the XRD peak and the 2.8 eV
PL, which became weaker after the implantation, increaseback to their original values. Although the data are not shown
here, the two ESR signals, S4 and S5, the absorption at
2.4 eV, the XRD peak, and the 2.8 eV PL exhibit essentiallythe same recovery behavior after annealing at 300 °C for
10 min following the implantation of B+ ions to the samedose.
A similar annealing was conducted for 10 min on theunimplanted samples after they had been irradiated by UVphotons with an energy of 5.59 eV for 30 min. Figure 11(b)shows the eﬀects of the annealing temperature on the
ESR signal S2 and the absorption at 3.3 eV. Both the ESR
signal and the absorption, which were induced by the UVDiscussion

In YSZ, an oxygen vacancy that captures one electron (F+center) and an oxygen vacancy that captures no electrons (F2+center) are known to form a defect pair, namely, the F+–F2+
pair.41) Judging from their anisotropic nature and g values,41)
the two ESR signals S4 and S5 shown in Fig. 3(a), which are
induced by ion implantation, seem to be due to the F+–F2+
pair. Note that ESR cannot detect the single presence of F2+
centers or F centers (oxygen vacancies with two captured
electrons). As for the absorption at 2.4 eV induced by the ion
implantation (Fig. 4), a very similar absorption has been
attributed to F+ centers,41) although it was still uncertain in
Ref. 41 whether the F+ centers were present alone or in theform of the above-mentioned F+–F2+ pair. As shown in
Fig. 6, the two ESR signals, S4 and S5, and the absorption at
2.4 eV show a monotonic increase with increasing dose of
the implanted P+ ions. This means that the ion implantationinduces ionization or the generation of electron–hole pairs tochange the charging state of oxygen vacancies. On the other
hand, the monotonic decrease in the intensity of the XRD
peak with increasing dose of the implanted ions shown in
Fig. 6 indicates the collapse of the crystalline structure. Bytaking this into account, it can be summarized that the ion
implantation induces the generation of electron–hole pairs
and the deformation of the crystalline structure.
As for the 2.8 eV PL, PL similar to the one shown in Fig. 5
has been ascribed to oxygen vacancies without mentioning
their details42) or to the F+ centers.43) The intensity of this PL
decreases signiﬁcantly with the increase in temperature, as
shown in Fig. 5(b). Furthermore, the 2.8 eV PL has a decay
constant of about 7 ns, as shown in Fig. 8(a), indicating that it
is ﬂuorescence. In addition to this fast component, the PL has
a slow component with a decay constant of about 0.3 ms, as
shown in Fig. 8(b).
In alumina,44) MgO,45) and YAlO3,46) it has been reported
that the F centers produce phosphorescence with a time
constant of the order of ms, while the F+ centers have been
assigned to the luminophore of the ﬂuorescence with a time
constant of the order of ns. Taking these facts into account,the 2.8 eV PL in YSZ would be ascribable to both the F+ andF centers. However, it is not plausible that both the F+ andF centers with diﬀerent charging states emit photons withthe same energy. Regarding this, we assume the following
luminescent mechanism to be a plausible model. Namely, the
2.8 eV PL in YSZ is induced by the F+ centers with the decay
constant of 7 ns. One important fact is that the F+ centers canbe generated if one electron is released from the F centers in
addition to those originally present in YSZ. If this electron
release requires some time, the 2.8 eV PL must have a large
decay constant such as 0.3 ms if it originates from the Fcenters. Note that a similar PL due to F centers in alumina has
a large decay constant of about 30 ms.44)

06GC03-5© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 06GC03 (2015)

S. Kaneko et al.

The PL that is ascribed to oxygen vacancies has been observed in many oxides such as YSZ,42,43) silica glass,19,21,25)
Ge-doped silica glass,24) buried SiO2 in Si,22,23) andalumina,44,46,47) as well as in silicon nitride26) and several
aluminates such as LaAlO331,32,35,36) and YAlO3.35,36,46,47)
Except for YSZ, namely, the target material of the present
research, the PL due to the oxygen vacancies appears
regardless of the sample being crystalline or amorphous.
For example, the PL is clearly observed in completely
amorphous SiO2 deposited by a soot remelting method21) orby thermal oxidation.22,48) Moreover, although many types of
PL appearing in LaAlO3 and YAlO3 become weak if thecrystallinity of the sample becomes poor, only the intensity of
the PL due to the oxygen vacancies does not decrease,35,36)
which is also shown in Fig. 7. Therefore, the strong
dependence of the 2.8 eV PL in YSZ on the crystallinity ofthe sample, which can be seen in Figs. 5, 6, 7(a), 10, and11(a), is exceptional. Namely, its intensity becomes weak
upon the implantation of P+ ions in a similar manner to thedecrease in crystallinity. Compared with YSZ, the intensityof the corresponding PL does not decrease in LaAlO3 orYAlO3 after the implantation, although the crystallinity of thetwo aluminates degrades similarly to that of YSZ. Furthermore, the PL intensity of YSZ is more than ten times as
intense as the corresponding PL in LaAlO3 and YAlO3, asshown in Fig. 7. In addition, the ESR signals S1, S4, and S5,
which indicate the existence of the F+ centers, appear in YSZ
after the ion implantation, as shown in Fig. 3. In more detail,signal S1 is due to F+ centers30) and the two signals S4 andS5 are both due to the F+–F2+ pair.41) In contrast to this, F+centers were not observed in LaAlO3 or YAlO3 when we
performed similar experiments under the same conditions
using the same spectrometer.
Oxygen vacancies appear in oxides such as silica or inaluminates such as LaAlO3 and YAlO3 only when theircrystals have defects. However, YSZ is a kind of ZrO2 inwhich Zr4+ is replaced by Y3+, as shown by the following
chemical formula:49) x
Y2 O3 ! Zr1x Yx O2x=2 ðYSZÞ: ð1Þ
ð1  xÞZrO2 þ
2This means that YSZ has the same number of oxygen
vacancies as that of the incorporated Y2O3.41) Therefore,
many more oxygen vacancies exist in YSZ than in LaAlO3
and YAlO3.
It has been shown that the temperature of a path in adielectric substance along which an ion passes increases toabout 3000 °C, taking SiO2 as an example.50) If ZrO2 andY2O3 are locally formed in YSZ through the opposite
reaction of Eq. (1) upon the implantation of ions as a result ofsuch a high temperature, the crystallinity should become poorand the number of oxygen vacancies should decrease. This is
the reason for the decrease in the intensity of the 2.8 eV PL
upon the degradation of crystallinity.
On the other hand, as shown in Fig. 4, the absorption band
appears after the implantation of P+ or B+ ions at around
2.4 eV, which is near the energy of the 2.8 eV PL. There is apossibility that the 2.8 eV PL is absorbed by this absorption
band and thus its intensity is measured as being lower than its
true intensity. Regarding this, the absorption of photons at
2.8 eV can be calculated to be 2.3% on the assumption that the

2.8 eV PL was emitted at a depth of 100 µm from the sample
surface. This means that the eﬀect of the increase inabsorption at 2.4 eV after the ion implantation on the intensityof the 2.8 eV PL is negligible. Therefore, the decrease in theintensity of the 2.8 eV PL induced after the ion implantation
is ascribable to the decrease in the number of the oxygen
vacancies induced by the opposite reaction of Eq. (1).
Namely, the PL intensity decreases as a result of the partial
separation of YSZ to ZrO2 and Y2O3. Although this separation
of YSZ induces the degradation of the crystallinity of YSZ, it
is not the direct reason for the decrease in the PL intensity.
As shown by curve V in Fig. 3, three ESR signals, S1, S2,
and S3, are induced in YSZ upon the irradiation of UV
photons with an energy of 5.59 eV. Here, S1 is due to the F+
center, while S3 is due to the T center or two oxygen
vacancies on two diagonal sites adjacent to Zr3+.30) The
origin of S2 is unknown. The F+ centers induced by theUV irradiation exist alone without forming a defect pair likethe above-mentioned F+–F2+ center, since it does not show
anisotropy. On the other hand, a broad absorption increase
with a peak at around 3.3 eV, indicated by curve V or VA in
Fig. 4, induced by the irradiation of UV photons, isconsidered to be due to the T center.30,41) This assignment
indicates that the charging state of oxygen vacancies ischanged upon the irradiation of the UV photons.
Neither the XRD peak shown in Fig. 2 nor the 2.8 eV PL
due to both the F+ and F centers shown in Fig. 5(a) exhibits ameaningful change upon the irradiation of UV photons. As
mentioned above, the 2.8 eV PL appears bright if the sample
contains oxygen vacancies in the form of O2−x=2 in the righthand term of Eq. (1). From these results, shown in Figs. 2–5,
the irradiation of UV photons seems to induce the generationof electron–hole pairs, but not the opposite reaction of
Eq. (1) or the deformation of the crystallinity of YSZ. This isin strong contrast to the changes induced in YSZ upon theimplantation of ions.
As shown in Figs. 2–5, 10, and 11, reactions opposite tothose caused by the implantation of ions are induced by thethermal annealing performed successively after the implantation. Namely, the intensities of the XRD peak and the 2.8 eV
PL, which were decreased by the ion implantation, increaseback to their original values after the thermal annealing.
Furthermore, two ESR signals, S4 and S5, and the absorption
at 2.4 eV, which were induced by the ion implantation,
become weak. In addition, the ﬁve physical quantities shownin Fig. 10 mutually correlate very well with respect to thedependence on the annealing time at 300 °C. Moreover, thesequantities return to the values before the ion implantation
when the sample is annealed at 300 °C for 5 min. Note thatthey also mutually correlate very well regarding the dose
dependence of the implantation of P+ ions shown in Fig. 6.
As shown in Fig. 11(a), the annealing at temperatures higher
than 300 °C, which is the crystallization temperature of
YSZ,51) is needed to return the ﬁve physical quantities backto their values before the ion implantation when the annealing
period is 10 min.
Although the data are not shown, the ESR signal S1 at
g = 2.006 shown in Fig. 3(a), which is due to the single F+
centers induced by the irradiation of photons with an energyof 5.59 eV for 30 min, decreases rapidly even at room
temperature.30) Furthermore, as mentioned above, the appear-

06GC03-6© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 06GC03 (2015)

S. Kaneko et al.

ance of the ESR signal S2 and the absorption at 3.3 eV owing
to the T centers indicate that the charging states of defects
in YSZ are changed by the irradiation of photons at 5.59 eV
for 30 min. As shown in Fig. 11(b), the two quantities begin
to decrease at 100 °C and disappear at 200 °C in 10 min.
Namely, the signals S1 and S2 and the absorption at 3.3 eV,
induced by the UV irradiation, disappear more easily than
the signals S4 and S5 and the absorption at 2.4 eV induced by
the implantation of ions. The reason for this is simply thatthe change in the charging state of oxygen vacancies induced
by UV irradiation is caused by photoionization or photogenerated electrons and holes, whereas the implantation ofions causes the degradation of crystallinity or the reaction
opposite to Eq. (1) and the resultant disappearance of oxygen
vacancies. Therefore, the changes induced by the UV
irradiation are relaxed at low annealing temperatures,
whereas those induced by ions need high temperatures. It
has also been reported for crystalline silica that the recovery
of crystalline damage needs temperatures as high as 1000 °C,
while the change in the charging states can be recovered at
200 °C.52,53)
Note that sharp PL peaks appearing at 2.3, 2.6, and 3.1 eV
in YAlO3 [Fig. 7(c)] are ascribable to Er 3+,35,36) judging
from their energies.54) Since Er is a rare-earth element
similarly to Y, it would be easily included in YAlO3 as animpurity. Moreover, the shift in the angle of the XRD peak
appearing only in YAlO3, shown in Fig. 9(c), is attributableto elemental displacement caused by the implantation ofions.35,36) The reason for this shift being observed only in
YAlO3 is unclear, but it may indicate that YAlO3 is moreeasily subject to ion implantation.
In addition, as mentioned above, the presence of oxygen
vacancies in LaAlO3 and YAlO3 can be detected by PL, asshown in Fig. 7, while it is not detectable by ESR. This
indicates that the sensitivity of PL measurements is fairly
high. Although only the F+ center with one captured electron
is detectable by ESR, which can detect only radicals, both theF+ and F centers contribute to the 2.8 eV PL. This seems to
make the PL measurements more suitable to the detection ofoxygen vacancies than ESR.
5.

Conclusions

The eﬀects of ion implantation and thermal annealing on theformation of point defects in YSZ were analyzed experimentally by obtaining spectra of in-plane XRD, ESR, opticalabsorption in the visible-UV region, and PL. Importantresults obtained are summarized as follows.
1) The presence of oxygen vacancies was conﬁrmed as
point defects in YSZ.
2) After the ion implantation, the charging state of oxygen
vacancies is changed. This generates two ESR signals
at g = 2.00 and 1.97 and optical absorption at 2.4 eV,
which are ascribable to the F+–F2+ pair and the F+
center, respectively. Concurrently, the crystalline structure of YSZ collapses and the intensity of the 2.8 eV PL
due to both the F+ and F centers decreases.
3) The intensity of the 2.8 eV PL is very strong, since theoxygen vacancies are present in YSZ as a result of thereplacement of Zr4+ by Y3+. The intensity is decreased
signiﬁcantly after the ion implantation, concurrently
with the degradation of crystallinity. This suggests thatthe number of oxygen vacancies is decreased as a resultof the separation of ZrO2 and Y2O3 upon the ion
implantation.
4) When YSZ is exposed to UV photons at 5.59 eV, two
ESR signals at g = 2.006 ascribable to the F+ centerand at 1.91–1.86 ascribable to the T center increase.
An unknown signal at 1.97–1.95 also increases. The
strength of the optical absorption at 3.3 eV due to the T
center also shows an increase. However, neither the
crystallinity nor the 2.8 eV PL exhibits a meaningfulchange. These results indicate that only the charging
state of oxygen vacancies is changed.
5) Because of the above-mentioned diﬀerence between theeﬀects of ion implantation and UV irradiation, a higher
temperature is required to recover the damage inducedby ion implantation than the temperature needed to
repair the changes induced by UV irradiation.
Acknowledgments

This study was partly conducted in the UVSOR Facility,
Institute for Molecular Science, Okazaki, Japan. It was partly
supported by a Mitsubishi Materials Corporation Grant fromWaseda Research Institute for Science and Engineering.

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