Appl. Phys. A (2015) 119:1423–1429
DOI 10.1007/s00339-015-9115-2

Experimental observations on the crystalline structures of YAlO3
single crystal at high temperatures
Takahiro Inoue1 • Takaaki Morimoto1 • Yoshimichi Ohki1,2Received: 3 September 2014 / Accepted: 18 March 2015 / Published online: 31 March 2015
 Springer-Verlag Berlin Heidelberg 2015

Abstract To understand the effects of thermal annealing
on a high-permittivity gate insulating material YAlO3, its
single crystal was annealed at various high temperatures
ranging from 900 to 1300 C and was examined by various
instrumental analyses such as X-ray diffraction, infraredabsorption spectroscopy, laser confocal microscopy, andatomic force microscopy. As a result, it was clarified thatthe crystalline YAlO3 takes at least three structures in atemperature range up to 1300 C. Namely, it is perovskite
below 1160 C, at which the garnet structure starts to appear, while an unknown structure appears at 1250 C. On
the other hand, the sample surface also shows drasticchanges if the temperature exceeds 1160 C. Furthermore,
many elliptical spots are induced after the annealing at
1200 or 1250 C. Moreover, hydroxyl groups are generated
at 1300 C. It is desirable to take these structural changes
into account in determining the annealing temperature of
YAlO3 for various industrial purposes such as the
manufacture of semiconductor devices.

1 Introduction
Nowadays, power semiconductors are used more widely invarious aspects of our daily life for various purposes. For
example, power semiconductors used in power inverters,& Takahiro Inouet.kiwoku@fuji.waseda.jp
Yoshimichi Ohki
yohki@waseda.jp
1

Department of Electrical Engineering and Bioscience,
Waseda University, Tokyo 169-8555, Japan

2

Research Institute for Materials Science and Technology,
Waseda University, Tokyo 169-8555, Japan

which are powerful tools for saving energy and electricity,
are one of the key components in power electronic devices.
In order to accelerate the use of power electronic devices
more widely in various power apparatus, their operating
voltages must be increased. In this regard, silicon carbide(SiC) is considered to be a promising candidate as a nextgeneration semiconducting material suitable for power
semiconductors [1–3], because it has a wider band gap andcan withstand much higher voltages than Si [1, 2, 4].
Furthermore, it has been reported that SiC power semiconductors are able to decrease losses in power substations[5]. Therefore, they can minimize energy consumption for
power conversion significantly [1, 3, 5, 6]. As a typicalrecent example of their industrial applications, it has beenreported that the use of SiC power semiconductors for
power regression in an urban electric railway could reduce
the energy consumption by about 30 % [7].
For gate insulating layers of Si-based semiconductor
devices, SiO2 has been used more widely. However, SiO2
is not suitable for SiC-based semiconductor devices, since
it cannot withstand high electric fields. The replacement of
SiO2 by a high-permittivity (high-k) dielectric can be apossible solution to this problem, since the layer thickness
can be thickened [8, 9]. For this purpose, yttrium aluminate(YAlO3) with a high-k (*15) [10] and a wide band-gap
energy (Eg: 7.9 eV) [11] is a promising candidate. Anotherconcern for SiC-based semiconductor devices is that highcarrier mobility may not be attained due to the presence ofa considerable amount of defects at the interfaces between
the SiC and its dielectric [12]. Therefore, the high-k dielectric for SiC-based semiconductors must be free fromdefects. In this regard, a study on inherent or process-induced defects in high-k dielectrics is important.
Relating to the above, various annealing processes ortreatments are necessary to manufacture semiconductor

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1424

(a) 1500
1200 oC
1300 oC

before1250 oCYAlO3
Perovskite
(002)

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1000

500

Intensity [arb.unit]

Commercially available YAlO3 (100) single crystals,
grown by the Czochralski method by a German company
Crystal GmbH, were examined in this study. The samples,in the shape of a plate with a thickness of 0.5 mm, wereannealed in air at 1 atm using an electric furnace (KDF1700, Denken) by raising the sample temperature fromroom temperature to various designated high temperatures
between 900 and 1300 C in 2 h. At each annealing temperature, the temperature was kept constant for 12 h.
After the sample was cooled naturally to room temperature from each annealing temperature, in-plane X-ray
diffraction (XRD) patterns were obtained with Cu Ka
X-rays using a Rigaku Rint-Ultima III, while Fouriertransform infrared (FT-IR) spectra were obtained with boththe transmission and attenuated total reflection (ATR)
modes using a JASCO FT/IR-4200. Furthermore, the surface of the sample that had been used for the above-mentioned instrumental analyses was observed by the following
four instrumental analyses. For two-dimensional observation in a square of several hundred micrometers, an optical
microscope (VHX-2000, Keyence) and a laser confocal
microscope (OLS4100, Shimadzu), which can weaken theeffects of light scattering by focusing the laser beam, wereused. In addition, an atomic force microscope (AFM, SPM9700, Shimadzu), which can observe the surface in asquare of several micrometers with a height resolution of
nanometers, and a surface profilometer (Dektak 6M, Horiba), which measures the surface roughness along a straightline in a range of several hundred micrometers, were used.
The surface profilometer has a height resolution of around
10 nm.

Intensity [arb.unit]

2 Samples and experimental methods

shown to make the differences clear, although the annealing had also been done at 900, 1100, and 1160 C.
The XRD peak at 2h = 34.6 appearing in the untreatedsample is due to the perovskite YAlO3 [20]. After the annealing at 1160 or 1200 C, the intensity of the peak at
34.6 becomes weak, and it disappears at 1250 C. In accordance with this decrease in the 34.6 peak, three new
peaks appear at 33.6, 32.6, and 25.9. While the peak at
33.6 is due to YAG (=Y3Al5O12) [20], the one at 25.9 isattributable to Al2O3 [21]. However, the origin of the peakat 32.6 is unknown.
Figure 1b shows the annealing temperature dependenceof each peak, normalized by the intensity of the peak at
34.6 before the annealing. Here, the intensities of thepeaks after the annealing at 900, 1100, and 1160 C thatwere omitted in Fig. 1a are also shown. While one of thethree new peaks at 33.6, which is rather strong, appears at

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YAG
Garnet
(220)

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2 θ [deg.]

26.5

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Al 2O3

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2θ [deg.](b)

1

Intensity (Normalized)

devices. Such treatments often induce electronic localizedstates and structural changes to dielectrics, which could
degrade their insulating properties. From this viewpoint,
we have been studying the effects of various treatments
such as ion implantation, ultraviolet photon irradiation, andthermal annealing on various high-k dielectrics such as
yttria-stabilized zirconia, lanthanum aluminate, and YAlO3[13–19]. In the present research, structural changes induced
in YAlO3 single-crystal samples by thermal annealing
given in air at various high temperatures are examined.

T. Inoue et al.

0.8
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3 Results
In-plane XRD patterns measured before and after the annealing at various temperatures are shown in Fig. 1a. Notethat only the patterns measured for the untreated sample
and the samples annealed at 1200, 1250, and 1300 C are123

Fig. 1 a Effects of the thermal annealing at various temperatures on
the in-plane XRD pattern of YAlO3. The patterns at around 25.9 areenlarged in the inset. b Changes in the diffraction peak heights at
34.6 (open square), 33.6 (filled triangle), 32.6 (open circle), and25.9 (filled diamond), normalized by the height of the 34.6 peak
before the annealing. The symbols on the abscissa that cannot be seen
are hidden by some others

Experimental observations on the crystalline structures of YAlO3 single crystal at high…

(a) 1.0
0.9

Absorbance

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Absorbance1160 C and becomes the maximum at 1200 C, it becomes smaller at 1250 C and disappears at 1300 C.
Concurrently with the behavior of the 33.6 peak, the peakat 34.6 that disappeared at 1250 C re-appears again at
1300 C. The two other peaks at 32.6 and 25.9 are very
weak, and they appear only after the annealing at 1250 and1300 C, respectively.
The FT-IR spectra measured after the annealing at different temperatures are shown in Figs. 2 and 3. The spectra
taken in the transmission mode shown in Fig. 2 do notprovide meaningful information, apparently due to thethick sample thicknesses. Therefore, we mainly mention
the ATR spectra shown in Fig. 3. Note that a clear peakseen in some samples in a range from 2300 to 2400 cm-1
in Fig. 3a is due to CO2 gas remaining in the spectrometer.
Besides this peak, an absorption peak is seen at around655 cm-1 as shown in Fig. 3b. It has been reported thatseveral absorption peaks, which are attributable to vibrations of bonds between metal and oxygen, namely either
between Y and O or between Al and O, appear in YAlO3[22] and YAG [20] in a range from 300 to 800 cm-1. The
Ref. [20] assigned the vibrations due to Al–O in YAG topeaks at 791 and 692 cm-1, and those due to Y–O to 727,
568, and 463 cm-1. It is true that the wavenumber range of
the FT-IR spectrum reported in [22] for YAlO3 is too wide
to judge whether the present absorption is the same as the
one reported. Moreover, [22] did not mention the precise
peak positions. However, these reports mentioned above
together with the fact that the absorption at 655 cm-1 appears clearly before the thermal annealing seem to beenough for the assignment of the metal–oxygen vibration
in YAlO3 as the origin of the 655-cm-1 absorption.
After the annealing at 1200 C, an IR-ATR peak appears
at around 635 cm-1. This peak may not be new, but a branchof the above-mentioned metal–oxygen vibration. Whether itis the case or not, it indicates the appearance of a crystal
structure different from YAlO3. Although the 635-cm-1
peak is also observable after the annealing at 1250 C (notshown), it disappears after the annealing at 1300 C. Thistemperature dependence of appearance and disappearanceof the 635-cm-1 peak is quite similar to that of the 33.6
diffraction shown in Fig. 1b, which is attributed to YAG.
Therefore, the 635-cm-1 peak originates in YAG.
Furthermore, only after the annealing at 1300 C, three
sharp ATR peaks appear in a range from 1000 to1130 cm-1. It has been reported that absorption appears in
Al2O3 at 1080 cm-1 due to vibrations of Al–O–H [23].
Furthermore, it has also been reported that Al(OH)3 shows
absorption at around 1100 cm-1 [24]. Therefore, the threepeaks are attributable to vibrations of bonds in a new
structure induced by the thermal annealing at 1300 C,which presumably contains OH. Since the three peaks are
not observable in the IR spectra measured in the

1425

0.7

0.6

0.5
1200

1100

1000

900

Wavenumber [cm-1]
Fig. 2 Temperature-dependent changes in FT-IR spectra measured in
the transmission mode: in the whole range of wave numbers(500–400 cm-1; a), in the 600–700 cm-1 range (b), and in the
900–1200 cm-1 range (c). Black solid lines: before, blue dashedlines: 1100 C, red dotted lines: 1200 C, and green solid lines:
1300 C

transmission mode as shown in Fig. 2c, the new structure
was induced only in the surface layer of the sample.
After the annealing at temperatures above 1250 C, theapparent color and transparency of the sample changed

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1426

T. Inoue et al.

Absorbance(a) 0.3

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Wavenumber [cm ]Absorbance(b) 0.3

In order to show the above-mentioned changes clearly,
the surface structure was observed by the laser confocal
microscope, and the images obtained are shown in Fig. 4. It
is evident that the surface structure starts to change significantly at 1160 C as shown in Fig. 4c. Complex multiform spots that first appeared on the sample surface at1160 C become elliptical, showing the average size of
about 20 9 50 lm2 at 1200 C (Fig. 4d), and their size andnumber increase at 1250 C (e). Furthermore, they seem togather, converge, and become much larger at 1300 C (f).
The appearance and growth of elliptical spots quite
similar to the cases shown in Fig. 4 can also be confirmedby the ordinary optical microscope as shown in Fig. 5,
although the appearance of ellipsoids is difficult to confirm
for the annealing at 1160 C. Interestingly, all the ellipticalspots shown in Figs. 4d, e and 5b, c have very similar sizes,

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Wavenumber [cm-1]

Absorbance(c) 0.03

0.02

0.01

0
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1100

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900

Wavenumber [cm-1]
Fig. 3 Temperature-dependent changes in FT-IR spectra measured in
the ATR mode: in the whole range of wave numbers (500–400 cm-1;a), in the 600–700 cm-1 range (b), and in the 900–1200 cm-1 range(c). Black solid lines: before, blue dashed lines: 1100 C, red dottedlines: 1200 C, and green solid lines: 1300 Cdrastically. That is to say, the surface appeared very dim
like ground glass. Furthermore, if the annealing temperature exceeded 1300 C, the sample became too brittle
and easily broken.

123

Fig. 4 Temperature-dependent changes in the surface images observed by a laser confocal microscope. a before, b 1100 C,
c 1160 C, d 1200 C, e 1250 C, f 1300 CExperimental observations on the crystalline structures of YAlO3 single crystal at high…

(a) 2520

Height [nm]and they are aligned along the \011[ axis. This factindicates that the growth of a certain crystalline structure isrelated to these spots.
To examine the roughness induced by the thermal annealing, the surface of each sample was observed by thesurface profilometer. The change in surface height measured along a straight line with a length of 300 lm is shownin Fig. 6 for the samples before and after the annealing.
Note that the ordinate differs in each figure. Furthermore,
the average height along each line is shown in Fig. 7 as afunction of annealing temperature. In addition, the averageand standard deviation of roughness observed by AFM are
shown in Fig. 8. It is clearly shown that the surface becomesrough at temperatures above 1160 C. The roughness isundoubtedly due to the appearance of elliptical spots.

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4 Discussion
It has been reported that compounds consisting of Y, Al,
and O exhibit the following three crystalline structures.
Namely, YAlO3, which is called YAP with a perovskite
structure, is the first structure. The second structure,
Y3Al5O12, which is called YAG, takes a garnet structure.
The third one is Y4Al2O9, which takes a monoclinic
structure and is called YAM [25, 26].

1427

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Length [ μm](c) 450400
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Height [nm]

300
250
200
150
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Length [ μm]Fig. 6 Position- and temperature-dependent changes in surface
roughness observed by a surface profilometer. a before (black) andat 1100 C (gray), b at 1160 C (black) and 1200 C (gray), c at
1250 C (black) and 1300 C (gray)Fig. 5 Surface images of the samples annealed at a 1160 C,
b 1200 C, c 1250 C, and d 1300 C observed by an optical
microscope. Note that apparently different colors of images are
simply due to different back lights used for observation

To our knowledge, there have been no papers clarifying the structural changes induced on the surface of single-crystal YAlO3 by thermal annealing. However, as for
YAG or Y3Al5O12, it has been reported that the crystalline YAlO3 and crystalline Al2O3 are segregated if
YAG is cooled within 5 s to 1375 C after it was

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T. Inoue et al.

Y3 Al5 O12 ! 3YAlO3 þ Al2 O3

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Height [nm]

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Fig. 7 Average height of the surface roughness observed by thesurface profilometer as a function of annealing temperature

Height [nm]

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ð3Þ

Although the detailed synthesis process of the present
samples is unknown, YAlO3 can be synthesized through achemical reaction of Y2O3 and AlOOH [28]. This meansthat there is a possibility of existence of OH and H in thepresent YAlO3 samples. This can be a reason for the appearance of OH bonds at 1300 C. This is consistent withthe fact that the intensities of the three IR absorption peaks
from 1000 to 1130 cm-1 shown in Fig. 3c are small.
The reason for the appearance of the elliptical spots on
the sample surface at 1200 and 1250 C is unknown.
However, the spots would have a certain relation with theappearance of the YAG structure, since the two appear anddisappear at the respective same temperatures.
All the results obtained in the present work clearly
indicate the appearance of YAG at 1160 C as well as thesegregation of YAG and the appearance of OH bonds on
the surface at 1300 C. The findings obtained in the present
research are not only crystallographically interesting, butalso valuable for semiconductor industries. If thermal annealing is needed in manufacturing semiconductor devices
after YAlO3 was deposited, its temperature should be
carefully decided by taking the above-mentioned structuralchanges into account.

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50

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RT

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1000

1100
o

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1300

Temperature [ C]

Fig. 8 Changes in the average (open triangle) and the standard
deviation (I) of the surface height as a function of annealing
temperature observed by AFM

annealed at temperatures above its melting point(*1970 C) [27]. It has also been reported that Al2O3
reacts with water vapor at 80 C to form Al(OH)3 on its
surface via AlOOH as an intermediate if it was heated bya microwave oven [21].
Referring to these papers, the above-mentioned results
obtained by the present research indicate that the hightemperature annealing induces the following serial structural changes to YAlO3. First, at around 1160 C, thestructure of YAlO3 begins to collapse, forming the structure of YAG via the following two reactions.
2YAlO3 ! Y2 O3 þ Al2 O3

ð1Þ

3Y2 O3 þ 5Al2 O3 ! 2Y3 Al5 O12

ð2Þ

While the YAG structure becomes stable at 1200 C, it
then segregates to Al2O3 and YAlO3 at 1300 C via thefollowing reaction [27].

123

Perovskite YAlO3 single crystals were thermally annealed
at various high temperatures, and the resultant structuralchanges were examined. As a result, the following serialchanges have been clarified.
1.

2.

3.

After the annealing at 1160 C, the crystalline YAlO3
collapses to induce YAG, which coexists with YAlO3
up to 1250 C.
After the annealing at 1200 C, the YAlO3 structure
collapses further to stabilize the YAG structure, andmany elliptical spots with a size of about 20 9 50 lm2
are also induced on the surface.
After the annealing at 1300 C, the YAG structure
segregates to Al2O3 and YAlO3 and OH bonds appear
on the surface.

In situ high-temperature instrumental analyses, such as
XRD, FT-IR, and Raman in particular, would be needed for
future study.
Acknowledgments Part of this research was supported by Power
Academy and JSPS Grant 25-3090 for JSPS Fellows. It was also
supported by Research Institute for Science and Engineering, Waseda
University, through the ENEOS Research Encouragement Prize andgrants from Mitsubishi Materials Corporation and ‘‘Early Bird’’ foryoung researchers.

Experimental observations on the crystalline structures of YAlO3 single crystal at high…

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