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Employment of a GaN buffer in the OMVPE growth of InN on sapphire substrates
ARTICLE IN PRESS
Journal of Crystal Growth 261 (2004) 271–274
Employment of a GaN buffer in the OMVPE growth of InN on sapphire substrates A. Yamamoto*, N. Imai, K. Sugita, A. Hashimoto Department of Electrical and Electronic Engineering, Faculty of Engineering, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan
Abstract This paper reports the effects of a GaN buffer layer in the OMVPE growth of InN on sapphire substrates. The GaN buffer is grown at 550 C and then annealed at different temperatures. The effects of GaN buffer are found to be dependent on the position of the substrate on the 18 cm-long susceptor. Such a dependence on substrate position is caused by a different effective V/III ratio; a lower V/III ratio near the upstream end and a higher V/III ratio near the downstream end. Uniformity of morphology for grown InN film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. The crystalline quality of the GaN buffer is improved with increasing annealing temperature. However, the crystalline quality of an InN film is almost insensitive to that of the underlying layer when the film is grown under the high effective V/III ratio condition. By employing a GaN buffer, electrical property of InN is improved; a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1 are obtained as the best data. It is believed that the mobility of 870 cm2 V 1 s 1 is the highest ever reported for OMVPE InN. r 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 68.35.Bs Keywords: A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction Although indium nitride (InN) is still a less studied material than other III-nitride semiconductors, it is expected to have the smallest effective mass, 0:07m0 [1] to 0:14m0 [2], and the highest electron drift velocity, 4.2 107 cm s 1 [3], in IIInitrides. Therefore, InN is promising for a channel material in high-speed and high-frequency electron *Corresponding author. Tel.: +81-776-27-8566; fax: +81776-27-8749. E-mail address: [email protected] (A. Yamamoto).
devices. Recently, there has been a rapid increase in the scientific and technological attention paid to InN, because InN films with a relatively low carrier density ( 1018 cm 3) and a high electron mobility ( 1200 cm2 V 1 s 1) have been obtained [4–6], and the band gap of InN was found to be much smaller than the widely reported value of 2 eV [7]. Compared with MBE-grown InN, OMVPE grown InN still has inferior electrical properties. In the case of MBE InN, the quality of grown InN has been markedly improved by employing an InN buffer [8] or a combination of GaN and InN buffers [9]. In the case of OMVPE InN, on the other hand, there are very few reports
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.040
ARTICLE IN PRESS 272
A. Yamamoto et al. / Journal of Crystal Growth 261 (2004) 271–274
on the employment of a buffer layer. It can be argued that the employment of a buffer layer is more important in the OMVPE of InN because the growth temperature is relatively high ( 600 C) compared with that used for MBE (550 C). In this paper, we report effects resulting from the use of a GaN buffer in the OMVPE growth of InN. An InN film is grown at 600 C on a GaN buffer. The buffer layer is grown at 550 C and annealed at various temperatures. The uniformity of the morphology of the InN film is markedly improved by employing a GaN buffer layer. The electrical properties of InN are also improved; a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1 are obtained as the best data. The effects of the GaN buffer are found to be dependent on the position of substrate on the 18 cm-long susceptor.
2. Experimental procedure Using an OMVPE system with a horizontal reactor, single crystalline InN films are grown on a-Al2O3 (0 0 0 1) substrates. Just before the InN growth, the substrate surface is nitrided at 900 C for 30 min. After growth of a GaN buffer layer at 550 C for 6 min (the thickness is about 30 nm) in a reduced pressure of 76 Torr, the substrate temperature is increased up to 900–1100 C and kept for 10 s–10 min in NH3 flow. Then, InN is grown at 600 C for 120 min (the thickness is about 0.3 mm) in a pressure of 760 Torr. As sources TMI, TEG and NH3 are used, and N2 as a carrier gas. Sapphire substrates are placed at a distance of 3, 9 and 15 cm from the upstream end of the 18 cmlong carbon susceptor. The crystalline quality of the GaN buffer and epitaxial InN layers is evaluated with X-ray diffraction. The electrical properties of InN are evaluated by Hall measurement.
the flowing NH3 gas temperature rises and, therefore, the decomposition rate of NH3 is increased while the TMI concentration is decreased due to the consumption, effective V/III ratio is believed to increase with increasing distance along the susceptor. As a result, the effects of the GaN buffer are found to be dependent on the position of the substrate on the susceptor. Fig. 1 shows the surface morphologies of InN films (substrate position 9 cm) grown without (a) and with a GaN buffer (b). One can see that the morphology of the grown film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. In the growth of InN directly on the sapphire substrate, nucleation of InN seems not to be easy because of the enhanced migration of the depositing species. This is attributed to the large lattice mismatch and/or a weak interaction between InN and the substrate surface. The introduction of a lowtemperature GaN buffer can lead to uniform nucleation for InN as can be seen in Fig. 1(b). Peeling of a grown film from the substrate is found frequently for an InN film grown near the upstream end (substrate position 3 cm), which is grown under a lower effective V/III ratio. The introduction of a GaN buffer is also found to suppress such a peeling. Fig. 2 shows the surface morphologies of GaN buffer layers after annealing at different temperatures. With increasing annealing temperature, the roughness of the buffer layer is increased. At the same time, its crystalline quality is improved. Fig. 3 shows FWHMs of X-ray rocking curve (XRC) for GaN buffers annealed under different
3. Results and discussion As mentioned above, a long (18 cm) susceptor is used in the present study. Since, with increasing distance from the upstream end on the susceptor,
Fig. 1. Surface morphologies (SEM images) of InN films (substrate position 9 cm) grown without (a) and with a GaN buffer (b).
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Crystal Growth 261 (2004) 271–274
273
A: Without GaN buffer B: No annealing With C: 900, 10 sec GaN D: 1000, 10 sec buffer E: 1100, 10 sec
6000 Buffer
4000 InN
F: 1000, 10 min
2000 : 3 cm,
0
A
B
: 9 cm,
C
D
: 15 cm
E
F
Fig. 3. FWHM of XRC for GaN buffers annealed under different conditions. Data for InN films grown on the buffer are also shown in the figure.
conditions. Data for InN films grown on the buffer are also shown in the figure. For GaN buffers, the XRC-FWHM decreases with increasing annealing temperature (as shown by B through E). This trend is pronounced for the substrate positions 3 and 9 cm and less evident for the position 15 cm where the effective V/III ratio is high. Compared with the GaN buffer, the XRC-FWHMs of InN films show a relatively small dependence on the annealing condition. Especially, the XRC-FWHM for films grown at the position 15 cm is independent on the annealing condition. This means that the crystalline quality of the InN film is almost insensitive to that of the underlying layer when the films are grown under a high effective V/III ratio. Metallic In is sometimes detected by X-ray diffraction in OMVPE-grown InN. It was reported that the amount of metallic In is markedly dependent on the sort of underlying layer [10]. Fig. 4 shows the In content in InN, as deduced from the X-ray diffraction spectra, as a function of the XRC-FWHM of the GaN buffer. A marked dependence of the In content on the XRC-FWHM of the GaN buffer is found when InN films are
101 In (101)/In N (0002) [%]
XRC-FWHM
(arcsec)
Fig. 2. Surface morphologies (AFM images) of GaN buffer layers after annealing at different temperatures.
Substrate position (cm)
;3
;9
; 15
100
10-1 3000
6000 4000 5000 XRC-FWHM (arcsec)
Fig. 4. Metallic In content in InN, as detected by X-ray diffraction, as a function of XRC-FWHM of GaN buffer.
grown under the low effective V/III ratio condition (substrate position 3 cm). The metallic indium content, for films grown in this position, was found to be greater in samples where the XRCFWHM of the underlying GaN buffer layer was small. InN films grown under the higher effective V/III ratio (substrate positions 9 and 15 cm), on the other hand show no such dependence on the XRC-FWHM of the GaN buffer. Chemical etching of the GaN buffer with a KOH solution [11] was used to check the polarity of the GaN buffer. It was revealed that Ga-polarity is dominant for all the GaN buffers shown here. From the results shown in Fig. 4, therefore, one can see that the concentration of metallic In is increased with increasing crystalline quality of the underlying layer when an InN film is grown on the Gapolarity GaN under low V/III ratio condition, while no dependence is observed when the InN
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Crystal Growth 261 (2004) 271–274
Carrier density (cm-3)
Hall mobility (cm2/Vs)
274 1000 800 600 400
Substrate position (cm) :3 :9 : 15
200 0
1020
A: Without GaN buffer B: No annealing With C: 900°C, 10 sec D: 1000°C, 10 sec GaN E: 1100°C, 10 sec buffer F: 1000°C, 10 min
1019
1018
A
B
C
D
E
F
Fig. 5. Carrier concentration and Hall mobility for InN films grown on GaN buffer layers prepared under different conditions.
film is grown under high V/III ratio condition even on the GaN buffer layers exhibiting Ga-polarity. Fig. 5 shows the carrier concentration and Hall mobility for InN films grown on GaN buffer layers prepared under different conditions. One can see that a GaN buffer improves the electrical property of InN. However, the transport properties of the InN layers are not strongly influenced by the annealing temperature of the GaN buffer, for temperatures up to 1000 C. Moreover, the annealing at the highest temperature ( 1100 C) and for the longest time seems cause the properties of the InN films to deteriorate. The best data, a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1, are obtained with the GaN buffer annealed at 1000 C for 10 s. It is believed that the mobility of 870 cm2 V 1 s 1 is the highest ever reported for OMVPE-grown InN. Such a high mobility is due to the improved uniformity of nucleation and morphology of InN.
4. Conclusion The effects of a GaN buffer layer in the OMVPE growth of InN have been studied. The InN films were grown at 600 C on GaN buffer layers grown at 550 C and annealed at different temperatures. The effects of the GaN buffer are found to be dependent on the position of substrate
on the 18 cm-long susceptor. Such a dependence on substrate position is due to different effective V/III ratio; a lower V/III ratio near the upstream end and a higher V/III ratio near the downstream end. The surface morphology of the InN film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. The crystalline quality of the GaN buffer, as measured by XRC-FWHM, is improved as the annealing temperature is increased. However, the crystalline quality of InN is insensitive to that of the underlying layer when the films are grown under high effective V/III ratio. By employing a GaN buffer, the electrical properties of InN are improved. However, the electrical properties of the InN film are not markedly affected by annealing the buffer at temperatures up to 1000 C. Annealing at a higher temperature ( 1100 C) and for a longer time seems to deteriorate the electrical properties of InN films. The best data, a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1 are obtained with the GaN buffer annealed at 1000 C for 10 s.
References [1] J. Wu, W. Walukiewicz, W. Shan, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Phys. Rev. B 66 (2002) 201403. [2] A. Kasic, M. Schubert, Y. Saito, Y. Nanishi, G. Wagner, Phys. Rev. B 65 (2002) 115206. [3] B.E. Fortz, S.K. O’Leary, M.S. Shur, L.F. Eastman, J. Appl. Phys. 85 (1999) 7727. [4] J. Wu, W. Walukiewicz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 80 (2002) 3967. [5] M. Higashiwaki, T. Matsui, J. Crystal Growth 252 (2003) 128. [6] A. Yamamoto, T. Tanaka, K. Koide, A. Hashimoto, Phys. Stat. Sol. a 194 (2002) 510. [7] Motlan, E.M. Goldys, T.L. Tansley, J. Crystal Growth 241 (2002) 165. [8] Y. Saito, N. Terauchi, A. Suzuki, T. Araki, Y. Nanishi, Jpn. J. Appl. Phys. 40 (2001) L91. [9] M. Higashiwaki, T. Matsui, Jpn. J. Appl. Phys. 41 (2002) L540. [10] M. Adachi, A. Seki, A. Hashimoto, A. Yamamoto, IWN2000, IPAP Conference Series 1, 2000, p. 347. [11] A. Yamamoto, Y. Murakami, K. Ikuta, T. Yamauchi, A. Hashimoto, Y. Ito, Phys. Stat. Sol. c 0 (2002) 152.
Journal of Crystal Growth 261 (2004) 271–274
Employment of a GaN buffer in the OMVPE growth of InN on sapphire substrates A. Yamamoto*, N. Imai, K. Sugita, A. Hashimoto Department of Electrical and Electronic Engineering, Faculty of Engineering, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan
Abstract This paper reports the effects of a GaN buffer layer in the OMVPE growth of InN on sapphire substrates. The GaN buffer is grown at 550 C and then annealed at different temperatures. The effects of GaN buffer are found to be dependent on the position of the substrate on the 18 cm-long susceptor. Such a dependence on substrate position is caused by a different effective V/III ratio; a lower V/III ratio near the upstream end and a higher V/III ratio near the downstream end. Uniformity of morphology for grown InN film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. The crystalline quality of the GaN buffer is improved with increasing annealing temperature. However, the crystalline quality of an InN film is almost insensitive to that of the underlying layer when the film is grown under the high effective V/III ratio condition. By employing a GaN buffer, electrical property of InN is improved; a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1 are obtained as the best data. It is believed that the mobility of 870 cm2 V 1 s 1 is the highest ever reported for OMVPE InN. r 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 68.35.Bs Keywords: A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction Although indium nitride (InN) is still a less studied material than other III-nitride semiconductors, it is expected to have the smallest effective mass, 0:07m0 [1] to 0:14m0 [2], and the highest electron drift velocity, 4.2 107 cm s 1 [3], in IIInitrides. Therefore, InN is promising for a channel material in high-speed and high-frequency electron *Corresponding author. Tel.: +81-776-27-8566; fax: +81776-27-8749. E-mail address: [email protected] (A. Yamamoto).
devices. Recently, there has been a rapid increase in the scientific and technological attention paid to InN, because InN films with a relatively low carrier density ( 1018 cm 3) and a high electron mobility ( 1200 cm2 V 1 s 1) have been obtained [4–6], and the band gap of InN was found to be much smaller than the widely reported value of 2 eV [7]. Compared with MBE-grown InN, OMVPE grown InN still has inferior electrical properties. In the case of MBE InN, the quality of grown InN has been markedly improved by employing an InN buffer [8] or a combination of GaN and InN buffers [9]. In the case of OMVPE InN, on the other hand, there are very few reports
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.040
ARTICLE IN PRESS 272
A. Yamamoto et al. / Journal of Crystal Growth 261 (2004) 271–274
on the employment of a buffer layer. It can be argued that the employment of a buffer layer is more important in the OMVPE of InN because the growth temperature is relatively high ( 600 C) compared with that used for MBE (550 C). In this paper, we report effects resulting from the use of a GaN buffer in the OMVPE growth of InN. An InN film is grown at 600 C on a GaN buffer. The buffer layer is grown at 550 C and annealed at various temperatures. The uniformity of the morphology of the InN film is markedly improved by employing a GaN buffer layer. The electrical properties of InN are also improved; a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1 are obtained as the best data. The effects of the GaN buffer are found to be dependent on the position of substrate on the 18 cm-long susceptor.
2. Experimental procedure Using an OMVPE system with a horizontal reactor, single crystalline InN films are grown on a-Al2O3 (0 0 0 1) substrates. Just before the InN growth, the substrate surface is nitrided at 900 C for 30 min. After growth of a GaN buffer layer at 550 C for 6 min (the thickness is about 30 nm) in a reduced pressure of 76 Torr, the substrate temperature is increased up to 900–1100 C and kept for 10 s–10 min in NH3 flow. Then, InN is grown at 600 C for 120 min (the thickness is about 0.3 mm) in a pressure of 760 Torr. As sources TMI, TEG and NH3 are used, and N2 as a carrier gas. Sapphire substrates are placed at a distance of 3, 9 and 15 cm from the upstream end of the 18 cmlong carbon susceptor. The crystalline quality of the GaN buffer and epitaxial InN layers is evaluated with X-ray diffraction. The electrical properties of InN are evaluated by Hall measurement.
the flowing NH3 gas temperature rises and, therefore, the decomposition rate of NH3 is increased while the TMI concentration is decreased due to the consumption, effective V/III ratio is believed to increase with increasing distance along the susceptor. As a result, the effects of the GaN buffer are found to be dependent on the position of the substrate on the susceptor. Fig. 1 shows the surface morphologies of InN films (substrate position 9 cm) grown without (a) and with a GaN buffer (b). One can see that the morphology of the grown film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. In the growth of InN directly on the sapphire substrate, nucleation of InN seems not to be easy because of the enhanced migration of the depositing species. This is attributed to the large lattice mismatch and/or a weak interaction between InN and the substrate surface. The introduction of a lowtemperature GaN buffer can lead to uniform nucleation for InN as can be seen in Fig. 1(b). Peeling of a grown film from the substrate is found frequently for an InN film grown near the upstream end (substrate position 3 cm), which is grown under a lower effective V/III ratio. The introduction of a GaN buffer is also found to suppress such a peeling. Fig. 2 shows the surface morphologies of GaN buffer layers after annealing at different temperatures. With increasing annealing temperature, the roughness of the buffer layer is increased. At the same time, its crystalline quality is improved. Fig. 3 shows FWHMs of X-ray rocking curve (XRC) for GaN buffers annealed under different
3. Results and discussion As mentioned above, a long (18 cm) susceptor is used in the present study. Since, with increasing distance from the upstream end on the susceptor,
Fig. 1. Surface morphologies (SEM images) of InN films (substrate position 9 cm) grown without (a) and with a GaN buffer (b).
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Crystal Growth 261 (2004) 271–274
273
A: Without GaN buffer B: No annealing With C: 900, 10 sec GaN D: 1000, 10 sec buffer E: 1100, 10 sec
6000 Buffer
4000 InN
F: 1000, 10 min
2000 : 3 cm,
0
A
B
: 9 cm,
C
D
: 15 cm
E
F
Fig. 3. FWHM of XRC for GaN buffers annealed under different conditions. Data for InN films grown on the buffer are also shown in the figure.
conditions. Data for InN films grown on the buffer are also shown in the figure. For GaN buffers, the XRC-FWHM decreases with increasing annealing temperature (as shown by B through E). This trend is pronounced for the substrate positions 3 and 9 cm and less evident for the position 15 cm where the effective V/III ratio is high. Compared with the GaN buffer, the XRC-FWHMs of InN films show a relatively small dependence on the annealing condition. Especially, the XRC-FWHM for films grown at the position 15 cm is independent on the annealing condition. This means that the crystalline quality of the InN film is almost insensitive to that of the underlying layer when the films are grown under a high effective V/III ratio. Metallic In is sometimes detected by X-ray diffraction in OMVPE-grown InN. It was reported that the amount of metallic In is markedly dependent on the sort of underlying layer [10]. Fig. 4 shows the In content in InN, as deduced from the X-ray diffraction spectra, as a function of the XRC-FWHM of the GaN buffer. A marked dependence of the In content on the XRC-FWHM of the GaN buffer is found when InN films are
101 In (101)/In N (0002) [%]
XRC-FWHM
(arcsec)
Fig. 2. Surface morphologies (AFM images) of GaN buffer layers after annealing at different temperatures.
Substrate position (cm)
;3
;9
; 15
100
10-1 3000
6000 4000 5000 XRC-FWHM (arcsec)
Fig. 4. Metallic In content in InN, as detected by X-ray diffraction, as a function of XRC-FWHM of GaN buffer.
grown under the low effective V/III ratio condition (substrate position 3 cm). The metallic indium content, for films grown in this position, was found to be greater in samples where the XRCFWHM of the underlying GaN buffer layer was small. InN films grown under the higher effective V/III ratio (substrate positions 9 and 15 cm), on the other hand show no such dependence on the XRC-FWHM of the GaN buffer. Chemical etching of the GaN buffer with a KOH solution [11] was used to check the polarity of the GaN buffer. It was revealed that Ga-polarity is dominant for all the GaN buffers shown here. From the results shown in Fig. 4, therefore, one can see that the concentration of metallic In is increased with increasing crystalline quality of the underlying layer when an InN film is grown on the Gapolarity GaN under low V/III ratio condition, while no dependence is observed when the InN
ARTICLE IN PRESS A. Yamamoto et al. / Journal of Crystal Growth 261 (2004) 271–274
Carrier density (cm-3)
Hall mobility (cm2/Vs)
274 1000 800 600 400
Substrate position (cm) :3 :9 : 15
200 0
1020
A: Without GaN buffer B: No annealing With C: 900°C, 10 sec D: 1000°C, 10 sec GaN E: 1100°C, 10 sec buffer F: 1000°C, 10 min
1019
1018
A
B
C
D
E
F
Fig. 5. Carrier concentration and Hall mobility for InN films grown on GaN buffer layers prepared under different conditions.
film is grown under high V/III ratio condition even on the GaN buffer layers exhibiting Ga-polarity. Fig. 5 shows the carrier concentration and Hall mobility for InN films grown on GaN buffer layers prepared under different conditions. One can see that a GaN buffer improves the electrical property of InN. However, the transport properties of the InN layers are not strongly influenced by the annealing temperature of the GaN buffer, for temperatures up to 1000 C. Moreover, the annealing at the highest temperature ( 1100 C) and for the longest time seems cause the properties of the InN films to deteriorate. The best data, a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1, are obtained with the GaN buffer annealed at 1000 C for 10 s. It is believed that the mobility of 870 cm2 V 1 s 1 is the highest ever reported for OMVPE-grown InN. Such a high mobility is due to the improved uniformity of nucleation and morphology of InN.
4. Conclusion The effects of a GaN buffer layer in the OMVPE growth of InN have been studied. The InN films were grown at 600 C on GaN buffer layers grown at 550 C and annealed at different temperatures. The effects of the GaN buffer are found to be dependent on the position of substrate
on the 18 cm-long susceptor. Such a dependence on substrate position is due to different effective V/III ratio; a lower V/III ratio near the upstream end and a higher V/III ratio near the downstream end. The surface morphology of the InN film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. The crystalline quality of the GaN buffer, as measured by XRC-FWHM, is improved as the annealing temperature is increased. However, the crystalline quality of InN is insensitive to that of the underlying layer when the films are grown under high effective V/III ratio. By employing a GaN buffer, the electrical properties of InN are improved. However, the electrical properties of the InN film are not markedly affected by annealing the buffer at temperatures up to 1000 C. Annealing at a higher temperature ( 1100 C) and for a longer time seems to deteriorate the electrical properties of InN films. The best data, a carrier concentration of 1.1 1019 cm 3 and a Hall mobility of 870 cm2 V 1 s 1 are obtained with the GaN buffer annealed at 1000 C for 10 s.
References [1] J. Wu, W. Walukiewicz, W. Shan, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Phys. Rev. B 66 (2002) 201403. [2] A. Kasic, M. Schubert, Y. Saito, Y. Nanishi, G. Wagner, Phys. Rev. B 65 (2002) 115206. [3] B.E. Fortz, S.K. O’Leary, M.S. Shur, L.F. Eastman, J. Appl. Phys. 85 (1999) 7727. [4] J. Wu, W. Walukiewicz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 80 (2002) 3967. [5] M. Higashiwaki, T. Matsui, J. Crystal Growth 252 (2003) 128. [6] A. Yamamoto, T. Tanaka, K. Koide, A. Hashimoto, Phys. Stat. Sol. a 194 (2002) 510. [7] Motlan, E.M. Goldys, T.L. Tansley, J. Crystal Growth 241 (2002) 165. [8] Y. Saito, N. Terauchi, A. Suzuki, T. Araki, Y. Nanishi, Jpn. J. Appl. Phys. 40 (2001) L91. [9] M. Higashiwaki, T. Matsui, Jpn. J. Appl. Phys. 41 (2002) L540. [10] M. Adachi, A. Seki, A. Hashimoto, A. Yamamoto, IWN2000, IPAP Conference Series 1, 2000, p. 347. [11] A. Yamamoto, Y. Murakami, K. Ikuta, T. Yamauchi, A. Hashimoto, Y. Ito, Phys. Stat. Sol. c 0 (2002) 152.