JOURNAL OF APPLIED PHYSICS 109, 033512 共2011兲
Characterization of AlInN/GaN structures on AlN templates for high-performance ultraviolet photodiodes Yusuke Sakai,a兲 Pum Chian Khai, Junki Ichikawa, Takashi Egawa, and Takashi Jimbo Research Center for Nano-Device and System, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
共Received 6 August 2010; accepted 11 December 2010; published online 3 February 2011兲 The authors characterize AlInN/GaN structures on AlN templates for high-performance ultraviolet photodiodes. AlInN/GaN structures were grown with various growth parameters by metal organic chemical vapor deposition. In the case of nearly lattice-matched to GaN underlying layers, AlInN/ GaN structures are found to have smooth interface. AlInN layers grown at the low pressure are confirmed to have high crystal quality from x-ray diffraction measurements and good surface morphology from atomic force microscope images. The noble AlInN-based photodiodes were fabricated. Their performances show the leakage current of 48 nA at a reverse voltage of 5 V and the cutoff wavelength around 260 nm. A cutoff-wavelength responsivity of 21.84 mA/W is obtained, corresponding to quantum efficiency of 10.6%. It may be possible to realize high-performance ultraviolet photodiodes by further optimizing AlInN/GaN structures. © 2011 American Institute of Physics. 关doi:10.1063/1.3544425兴 I. INTRODUCTION
In recent years, GaN and III-nitride-based semiconductors are a wonderful research objective for many industrial and research groups because it is promising to provide numerous commercial applications for electronic and optical devices. One of the attractive applications is III-nitride-based photodiode,1 due to the selection of a desirable wavelength using ternary or quaternary alloys as well as its potentials for high temperature operation, low cost, and long lifetime. Solar-blind ultraviolet 共UV兲 sensors have been developed,2,3 which have features of no response to sunlight and room lamp and the detection of only UV light inside flame. Therefore, this kind of sensors can be used as flame detectors, such as fire alarms, flame monitors of combustion equipments, and so on. There are two ternary III-nitride alloys, AlGaN and AlInN, at the wavelength of UV region. Especially, AlInN alloys are attractive materials not only because of the wide range of bandgap energy from 6.2 eV 共AlN兲 to 0.7 eV 共InN兲 but also of lattice-matching to GaN layer at the indium composition of 17%–18%.4,5 They are highly promising for achieving the high-performance GaN-based devices, instead of AlGaN or InGaN alloys.6 Recently, AlInN alloys have been studied enthusiastically for the applications in various semiconductor devices.7–14 High-quality AlInN films are considered to play an important role in improving the device performance. However, it is significantly difficult to obtain high-quality AlInN layers because of the large differences, such as covalent bonds and growth temperature, between AlN and InN. This problem can be solved by using AlN template15 as a substrate because a high-quality GaN layer was obtained earlier.16 Actually, we demonstrated the growth of the high-quality AlInN layers using AlN templates and the good device characteristics for AlInN/GaN high-electrona兲
Electronic mail:
[email protected].
0021-8979/2011/109共3兲/033512/6/$30.00
mobility transistors.17,18 In addition, other high-performance GaN-based devices on AlN templates have been reported.3,19–21 Therefore, AlN templates are quite useful for the growth of III-nitride materials. Recently, our groups have also reported AlInN-based Schottky photodiodes on sapphire substrates.13,14 For realizing the high-quality UV photodiodes, the optimization of AlInN growth on AlN template can be indispensable. In this paper, we report on the characterization of AlInN/ GaN structures on AlN templates to optimize the growth parameters of AlInN layers, and investigate the potentials for high-performance UV photodiodes. II. EXPERIMENTS A. Epitaxial growth of AlInN/GaN on AlN templates
AlInN/GaN structures were grown on AlN templates served as underlying substrates by metal organic chemical vapor deposition 共MOCVD兲, Taiyo Nippon Sanso SR-2000 system. Trimethylgallium, trimethylaluminum, trimethylindium, and ammonia 共NH3兲 were used as the sources of gallium, aluminum, indium, and nitrogen, respectively. Both hydrogen and nitrogen were used as the carrier gas for GaN growth, and nitrogen was used for AlInN growth. A 2-m-thick high-resistive GaN layers were directly grown on AlN templates at 1160 ° C. Subsequently, the temperature was reduced and undoped ternary AlInN layers was grown with various growth parameters, such as the reactor pressure of 100, 300 and 760 Torr and the growth temperature of 785, 800, 810, and 840 ° C. The V/III ratio and the growth time are kept constant at the value of approximately 8100 and 65 min in these experiments, respectively. AlInN/GaN structures thus prepared were characterized by various methods. The thickness of AlInN layer was confirmed by cross-sectional scanning electron microscope 共SEM兲, HITACHI S-5000. The indium composition in AlInN layers was determined using
109, 033512-1
© 2011 American Institute of Physics
033512-2
J. Appl. Phys. 109, 033512 共2011兲
Sakai et al. Light illumination
GaN
(a) Schottky
AlInN
600 m
Ohmic
500 m
AlInN
Intensity [a.u.]
Ohmic
100 m
100 Torr
300 Torr
i-GaN
760 Torr
16.5
17
17.5
250
18.5
Top View
FIG. 1. Dimensional layout of fabricated photodiode in cross-sectional and top view.
high-resolution x-ray diffraction 共XRD兲, a Philips X’Pert MRD system. The crystal quality of AlInN layer was also investigated by measuring x-ray rocking curves 共XRCs兲. The surface morphology was observed by atomic force microscope 共AFM兲, SII SPA300. The photoluminescence 共PL兲 measurement was performed at room temperature. A Nd:YAG laser was used as the excitation source with the wavelength of 266 nm, and the spectra were obtained using a Jobin Yvon’s Symphony UV-enhanced liquid nitrogencooled charge-coupled-device detector.
(c) Indium composition [%]
Schottky
Ohmic
30
(b) XRC FWHMs [arcsec.]
Cross-sectional View
18
Omega/2theta [º]
AlN template sapphire
AlN
200
150
100 100 Torr 300 Torr
50
100 Torr
25
300 Torr 760 Torr
20 15 10 5
760 Torr
0
0 760
780
800
820
840
860
760
Growth temperature [ºC]
780
800
820
840
860
Growth temperature [ºC]
FIG. 2. 共a兲 XRD -2 scan profiles in 关0002兴 reflection for AlInN/GaN structures with various reactor pressures. 共b兲 FWHMs of XRC in 关00 002兴 reflection as a function of growth temperature. 共c兲 Indium composition in AlInN layers as a function of growth temperature.
III. RESULTS AND DISCUSSION
B. Device fabrication of AlInN-based photodiodes
A. Structural characteristics of AlInN/GaN on AlN templates
The circular photodiodes with 500-m-diameter were fabricated using conventional photolithographic liftoff technique. Ohmic contacts were formed using electron beam deposition of Ti/Al/Ni/Au 共15/80/12/40 nm兲, with subsequent annealing at 850 ° C for 30 s in nitrogen atmosphere using a rapid thermal annealing system. Schottky contacts were formed using electron-beam deposition of Pd 共100 nm兲. To simplify the device processing, mesa-type structures were not employed. The dimensional layout of fabricated photodiode in the cross-sectional and top view was shown schematically in Fig. 1. To characterize the fabricated devices, current-voltage 共I-V兲 characteristics were measured by a semiconductor parameter analyzer, Agilent 4156C system. Capacitance-voltage 共C-V兲 characteristics were measured by a HP4284 LCR meter at 1 MHz. The spectral response of the photodiodes was also measured in the wavelength range of 200–500 nm using several light sources, such as a deuterium lamp, a xenon lamp, and halogen lamp. The wavelength of the light coming from the source lamps was selected by 25 cm monocrometer with 600 grooves/mm grating blazed at 200 nm. The optical system was calibrated using a calibrated UV-enhanced Si detector. A programmable electrometer, Keithley 6517A system, was employed to record the photocurrent. The light was illuminated from the top of photodiode, and then the optical transmission analysis was performed for Pd-based Schottky contact showing a spectral transmittance of ranging from 0.04% to 0.05%.
Figure 2共a兲 shows XRD -2 scan profiles in 关0002兴 reflection for AlInN/GaN structures with various reactor pressures of 100, 300, and 760 Torr. These data are obtained for the samples grown at the growth temperature of 800 ° C. For clarity, the intensity was normalized and shifted in vertical direction. In this figure, some fringe peaks are seen clearly around the AlInN peaks for 100-Torr-grown and 300Torr-grown samples. It indicates that these two samples have the smooth interface between GaN and AlInN layers. Some fringe peaks are also observed for both samples grown at the growth temperature of 810 ° C. However, at the growth temperatures of 785 and 840 ° C, there are no fringe peaks. These results may indicate that AlInN layers grown at the growth temperature of 800 or 810 ° C is nearly latticematched to underlying GaN layers because of the smooth interface between GaN and AlInN layers. On the other hand, 760-Torr-grown samples have no fringe peaks for the whole growth temperature range of 785– 840 ° C. This may be because these samples have poor crystal qualities, compared to low-pressure-grown samples. Moreover, AlInN peak intensity of 760-Torr-grown samples is weaker than that of the other two samples. Figure 2共b兲 shows full-width at halfmaximum 共FWHM兲 values of XRCs in 关0002兴 reflection as a function of the growth temperature. From this figure, it is found that FWHMs are almost kept constant in the whole growth temperature range. FWHM values for 100-Torrgrown, 300-Torr-grown, and 760-Torr-grown samples are ap-
033512-3 (a)
J. Appl. Phys. 109, 033512 共2011兲
Sakai et al. (b)
(c)
2.5
(a)
100 Torr 300 Torr
2
FIG. 3. 共Color online兲 AFM images for AlInN/GaN structures with the different reactor pressure of 共a兲 100 Torr, 共b兲 300 Torr, and 共c兲 760 Torr.
RMS [nm]
760 Torr
1.5
1
0.5
0 760
780
800
820
840
860
Growth temperature [ºC] 20
(b) 100 Torr
Pit density [108/cm2]
proximately 180 arcsec, 180 arcsec, and 230 arcsec, respectively. From these results, we confirm the higher crystal quality is obtained at low-pressure-grown AlInN layers. In order to determine the indium composition in AlInN layers, we analyze the AlInN peak position of XRD -2 scan profiles using the Vegard’s rule. Figure 2共c兲 shows the indium composition in AlInN layers as a function of the growth temperature. For 100-Torr-grown and 300-Torrgrown samples, the indium composition decreases linearly with increasing the growth temperature. This result is in good agreement with previous reports.13,22 For 760-Torrgrown samples below the growth temperature of 800 ° C, the indium composition is changed as the same with lowpressure-grown samples. However, above the growth temperature of 810 ° C, its slope becomes gentle and then the indium composition is saturated to a certain value. Therefore, the indium composition depends not only on the growth temperature but also on the reactor pressure. At the growth temperature around 800– 810 ° C, AlInN layers for lowpressure-grown samples are nearly lattice-matched to underlying GaN layers due to the indium composition of ⬃17% or lower. We believe this is the reason that some fringe peaks appeared in Fig. 2共a兲. Consequently, nearly lattice-matched AlInN/GaN structures are confirmed to have smooth interface. Figures 3共a兲–3共c兲 show AFM images for AlInN/GaN structures with various reactor pressures of 100 Torr, 300 Torr, and 760 Torr, respectively. These images are obtained for the samples grown at the growth temperature of 800 ° C and the scanning area of 3 ⫻ 3 m2. As seen clearly in these images, surface morphology is drastically affected by the reactor pressure. Some pits exist for 100-Torr-grown samples and pit sizes are relatively small. With the increase in the reactor pressure to 300 Torr, the numbers and the size of pits increase, and then a few indium-droplets begin to appear on the surface. With further increase in the reactor pressure to 760 Torr, a large number of indium-droplets cover throughout the surface, and no pit is observed. These kinds of surface morphology for 760-Torr samples are observed in the whole growth temperature range, which may be the reason to saturate the indium composition as shown in Fig. 2共c兲. To investigate AlInN surface morphology, we analyzed the AFM images. Figures 4共a兲 and 4共b兲 show the root-mean-square 共rms兲 surface roughness and the pit density as a function of the growth temperature, respectively. From these figures, it is obvious that both the rms and the pit density decrease with increasing the growth temperature. This indicates there is the correlation between the rms and the pit density. We also ob-
15
300 Torr
10
5
0 760
780
800
820
840
860
Growth temperature [ºC] FIG. 4. 共a兲 rms and 共b兲 pit density as a function of growth temperature.
tained the flat surface around 1 nm at the high growth temperature, however thick AlInN layer of more than 200 nm have reduced the surface flatness. Therefore, better surface morphology is obtained for AlInN layers grown under the condition of both higher growth temperature and lower reactor pressure. Figure 5共a兲 shows the typical cross-sectional SEM image for AlInN/GaN structure with the reactor pressure of 100 Torr. From this image, the boundary between AlInN and GaN layers is distinguishable, and the AlInN layer thickness is easily estimated. Figure 5共b兲 shows the growth rate of AlInN layers as a function of the growth temperature. As seen clearly in this figure, the growth rate is found to be independent of the growth temperature because it is constant for all samples grown at various temperatures. The growth rate was calculated at the same around 110 nm/h for both 100-Torr-grown and 300-Torr-grown samples. On the other hand, 760-Torr-grown samples have slower growth rate of approximately 25 nm/h, which indicate the AlInN layer is thin. This may result in the weak AlInN-peak intensity of XRD scan profiles shown in Fig. 2共a兲. Figure 6 shows PL spectra for AlInN/GaN structures with various reactor pressures of 100, 300, and 760 Torr.
033512-4
J. Appl. Phys. 109, 033512 共2011兲
Sakai et al.
(a)
(a)
10-20
AlInN
100 Torr
10-30
300 Torr
10-40
Current [A]
GaN 1 m
10-50 10-60 10-70 10-80
AlN
10-90 10-10 0 10-11 0
-10
-8
-6
-4
-2
0
2
4
Voltage [V] (b)
(b)
120
1E+19 1019 100 Torr
80
100 Torr 300 Torr
60
300 Torr
Carrier concentration [cm-3]
Growth rate [nm/h]
100
1018 1E+18
760 Torr
40 20 0 760
780
800
820
840
Growth temperature [ºC]
860
40
FIG. 5. 共a兲 Typical cross-sectional SEM image of AlInN/GaN structure. 共b兲 Growth rate of AlInN layers as a function of growth temperature.
These spectra were obtained for the samples grown at the growth temperature of 800 ° C. Due to thin AlInN layer, 760Torr-grown sample was measured for a longer time. From these spectra, we can find the AlInN-related PL peaks. As indium composition in AlInN layers is different, these peaks are slightly shifted toward a short wavelength. By analyzing the AlInN-related PL spectra using Gaussian-fitting method, the peak wavelength and its FWHM were calculated at 330.3
RT 300 Torr
PL intensity [a.u.]
100 Torr
760 Torr
280
290
300
310
320
330
1017 1E+17
340
350
Wavelength [nm]
FIG. 6. PL spectra for AlInN/GaN structures with the different reactor pressure.
50
60
70
Depth [nm] FIG. 7. 共a兲 I-V characteristics of fabricated photodiodes. 共b兲 Depth profiles of net carrier concentration for fabricated photodiodes.
nm and 43.3 nm for 100-Torr-grown sample, 323.4 nm and 41.5 nm for 300-Torr-grown sample, and 321.4 nm and 48.1 nm for 760-Torr-grown sample, respectively. We cannot find any remarkable difference among optical properties of these three samples, except the peak shifts. B. Device Performances of AlInN-based photodiodes
The photodiodes were fabricated for lattice-matched AlInN/GaN structures with the reactor pressure of 100 and 300 Torr. All the photodiodes fabricated in this study have the same thickness of approximately 110 nm. Figure 7共a兲 shows the I-V characteristics of fabricated photodiodes. We checked the specific contact resistivity of 100-Torr-grown and 300-Torr-grown sample from transmission line model measurements, corresponding to 7 ⫻ 10−4 ⍀ cm2 and 4 ⫻ 10−3 ⍀ cm2, respectively. The reverse I-V curves represent the leakage current at a reverse bias of 5 V is 48 nA for 100-Torr-grown sample and 1.3 A for 300-Torr-grown sample, corresponding to current density of 6.15 ⫻ 10−4 A / cm2 and 1.64⫻ 10−2 A / cm2, respectively. Ideality factor and series resistance were estimated from the forward I-V curves by linear fitting based on the thermionic emission model. The calculated values of ideality factor were
033512-5
J. Appl. Phys. 109, 033512 共2011兲
Sakai et al. 1001
respectively. This could be due to poor I-V characteristics for 300-Torr-grown sample. It is considered that ideality factor obtained from forward I-V curves have relation to the QE. The large ideality factor may be attributed to low QE. For another possibility, the QE is considered to be sensitive for surface morphology because all the contacts are formed at AlInN surface. The AFM images shown in Fig. 2 reveal the surface morphology is quite changed between 100 and 300Torr-grown sample, resulting in different surface states. In views of theses points, there is the possibility of reducing the QE for 300-Torr-grown sample. From these results, we confirm 100-Torr-grown AlInN/GaN structures have good photodiode performances. Further optimized AlInN/GaN structures can facilitate high-performance photodiodes.
QE=100 %
Responsivity [A/W]
100-1
100 Torr 300 Torr
10-2 0
100-3
100-4
100-5
-6 100
200
250
300
350
400
450
500
Wavelength [nm]
FIG. 8. Spectral responses of fabricated photodiodes.
2.4 for 100-Torr-grown sample and 3.1 for 300-Torr-grown sample. Also, the calculated values of series resistance were 650 ⍀ and 420 ⍀, respectively. From these results, we confirm 100-Torr-grown sample has good I-V characteristics, compared to 300-Torr-grown sample. This may be attributed to some pits observed on AlInN surface.23,24 However, I-V characteristics of these photodiodes can be considered to have some room to improve for realizing high-performance photodiodes. Figure 7共b兲 show the depth profiles of net carrier concentration for fabricated photodiodes. These data were obtained from C-V measurements. The carrier concentration in AlInN layer remains constant at the region deeper than approximately 45 nm, and the average value is 6.8 ⫻ 1017 cm−3 and 5.5⫻ 1017 cm−3 on for 100-Torr-grown and 300-Torr-grown sample, respectively. Due to relatively high background doping level, grown AlInN layers were not intentionally doped. The space-charge thickness at zero-bias was estimated to the range of 60–70 nm for both samples, indicating AlInN layer thickness of 110 nm is sufficient as a detective layer for photodiodes. From the view point of space-charge region, the doping level of approximately 2 ⫻ 1017 cm−3 may be optimal because the space-charge thickness is the almost same thickness of AlInN layer. We believe controlling the background doping level is one of the important factors in further improving the device performance. Figure 8 shows the spectral responses of fabricated photodiodes. This measurement was performed at zero-bias under the uniform light illumination of 1 W / cm2. As seen clearly in this figure, the spectral response is cutoff around 260 nm for both photodiodes. However, the responsivity for 100-Torr-grown sample is much higher than that for 300Torr-grown sample. It is found that a cutoff-wavelength responsivity is 21.84 mA/W for 100-Torr-grown sample and 0.134 mA/W for 300-Torr-grown sample. This indicates 100Torr-grown sample exhibits the higher signal/noise 共S/N兲 ratio due to the same noise signals. The S/N for 100-Torrgrown sample was estimated to be higher in two order magnitudes than that for 300-Torr-grown sample. Also, the quantum efficiency 共QE兲 estimated from the responsivity is 10.6% and 0.066% for 100-Torr and 300-Torr-grown sample,
IV. CONCLUSIONS
We have evaluated AlInN/GaN structures on AlN templates for high-performance UV photodiodes. AlInN/GaN structures with various growth parameters are characterized. We confirm the good crystal quality of AlInN layers by the low pressure growth. At the nearly lattice-matched to GaN underlying layers, AlInN/GaN structures have the smooth interface. As for surface morphology, both flat surface and low pit density are obtained at the high-temperature growth. On the other hand, indium-droplets morphology was observed for 760-Torr-grown samples. The low-pressure growth is optimum for obtaining high-quality AlInN layers. The performances of noble AlInN-based photodiodes were also investigated. It is found to have a leakage current of 48 nA at a reverse voltage of 5 V and ideality factor of 2.4 from I-V characteristics. The spectral response is cutoff at the wavelength around 260 nm and the responsivity at 260 nm is 21.84 mA/W, corresponding to QE of 10.6%. We believe further optimized AlInN/GaN structures have better photodiode performances. ACKNOWLEDGMENTS
This work was partly supported by the Ministry of Education, Culture, Sports, Science and Technology, the Knowledge Cluster Initiative 共the Second Stage兲, Tokai Region Nanotechnology Manufacturing Cluster. The authors are very grateful to Mr. T. Morimoto for his significant contributions to this research. One of the authors 共Y.S.兲 would like to thank Specialized Professor O. Oda for variable comments, Mr. M. Masuda for device fabrication, and Dr. S. L. Selvaraj and Ms. N. Imura for their assistance to make this paper. 1
H. MorKoç, Handbook of Nitride Semiconductors and Devices: Electronic and Optical Process in Nitrides 共Wiley-VCH, Weinheim, 2008兲. 2 H. Jiang, T. Egawa, and H. Ishikawa, IEEE Photonics Technol. Lett. 18, 1353 共2006兲. 3 H. Jiang and T. Egawa, Appl. Phys. Lett. 90, 121121 共2007兲. 4 J. F. Carlin and M. Ilegems, Appl. Phys. Lett. 83, 668 共2003兲. 5 K. Lorenz, N. Franco, E. Alves, I. M. Watson, R. W. Martin, and K. P. O’Donnell, Phys. Rev. Lett. 97, 085501 共2006兲. 6 J. Kuzmik, IEEE Electron Device Lett. 22, 510 共2001兲. 7 J. Xie, X. Ni, M. Wu, J. H. Leach, Ü. Özgür, and H. Morkoç, Appl. Phys. Lett. 91, 132116 共2007兲. 8 S. Choi, H. J. Kim, Z. Lochner, Y. Zhang, Y. C. Lee, S. C. Shen, J. H. Ryou, and R. D. Dupulis, Appl. Phys. Lett. 96, 243506 共2010兲. 9 H. P. D. Schenk, M. Nemoz, M. Korytov, P. Vennéguès, A. D. Dräger, and
033512-6
A. Hangleiter, Appl. Phys. Lett. 93, 081116 共2008兲. D. Simeonov, E. Feltin, H. J. Bühlmann, T. Zhu, A. Castiglia, M. Mosca, J. F. Carlin, R. Butté, and N. Grandjean, Appl. Phys. Lett. 90, 061106 共2007兲. 11 A. Castiglia, E. Feltin, G. Cosendey, A. Altoukhov, J. F. Carlin, R. Butté, and N. Grandjean, Appl. Phys. Lett. 94, 193506 共2009兲. 12 J. Dorsaz, J. F. Carlin, S. Gradecak, and M. Ilegems, J. Appl. Phys. 97, 084505 共2005兲. 13 S. Senda, H. Jiang, and T. Egawa, Appl. Phys. Lett. 92, 203507 共2008兲. 14 Z. T. Chen, S. X. Tan, Y. Sakai, and T. Egawa, Appl. Phys. Lett. 94, 213504 共2009兲. 15 T. Shibata, K. Asai, S. Sumiya, M. Mouri, M. Tanaka, O. Oda, H. Katsukawa, H. Miyake, and K. Hiramatsu, Phys. Status Solidi C 0, 2023 共2003兲. 16 M. Sakai, H. Ishikawa, T. Egawa, T. Jimbo, M. Umeno, T. Shibata, K. Asai, S. Sumiya, Y. Kuraoka, and O. Oda, J. Cryst. Growth 244, 6 共2002兲. 10
J. Appl. Phys. 109, 033512 共2011兲
Sakai et al. 17
M. Miyoshi, Y. Kuraoka, M. Tanaka, and T. Egawa, Appl. Phys. Express 1, 081102 共2008兲. 18 J. Selvaraj, S. J. Selvaraj, M. Miyoshi, Y. Kuraoka, M. Tanaka, and T. Egawa, Jpn. J. Appl. Phys., Part 1 48, 04C102 共2009兲. 19 S. Arulkumaran, M. Sakai, T. Egawa, H. Ishikawa, T. Jimbo, T. Shibata, K. Asai, S. Sumiya, Y. Kuraoka, M. Tanaka, and O. Oda, Appl. Phys. Lett. 81, 1131 共2002兲. 20 T. Egawa, H. Ohmura, H. Ishikawa, and T. Jimbo, Appl. Phys. Lett. 81, 292 共2002兲. 21 S. Sumiya, Y. Zhu, J. Zhang, K. Kosaka, M. Miyoshi, T. Shibata, M. Tanaka, and T. Egawa, Jpn. J. Appl. Phys., Part 1 47, 43 共2008兲. 22 T. C. Sadler, M. J. Kappers, and R. A. Oliver, J. Cryst. Growth 311, 3380 共2009兲. 23 E. J. Miller, X. D. Dang, and E. T. Yu, J. Appl. Phys. 88, 5951 共2000兲. 24 E. J. Miller, D. M. Schaadt, E. T. Yu, C. Poblenz, C. Elsass, and J. S. Speck, J. Appl. Phys. 91, 9821 共2002兲.
