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GaN films deposited by middle-frequency magnetron sputtering
Applied Surface Science 253 (2007) 9077–9080 www.elsevier.com/locate/apsusc
GaN films deposited by middle-frequency magnetron sputtering C.W. Zou a,b,*, M.L. Yin a,b, M. Li a,b, L.P. Guo a,b, D.J. Fu a,b b
a Accelerator Laboratory, Department of Physics, Wuhan University, Wuhan 430072, China Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China
Received 26 March 2007; received in revised form 11 May 2007; accepted 12 May 2007 Available online 21 May 2007
Abstract GaN films were deposited on Si (111) substrates by middle-frequency magnetron sputtering. X-ray diffraction revealed preferential GaN (0 0 0 2) orientation normal to the substrate surface for all the films deposited. The diffraction intensity and N contents were found to depend strongly on the total gas pressure. Good quality films were only obtained at pressures in the range of 0.4–1.0 Pa. Little diffraction of GaN (0 0 0 2) could be observed either at total pressures below 0.4 Pa or above 1.0 Pa. The GaN films produced under the optimized conditions have an N:Ga ratio of 1:1 as determined by energy-dispersive X-ray spectroscopy. # 2007 Published by Elsevier B.V. Keywords: GaN; Middle-frequency; Magnetron sputtering; Structure
1. Introduction Much attention has been paid to III-nitride semiconductors recently for their huge application potential in electronic and optoelectronic devices, such as electron field emitter and lightemitting diodes in the blue, violet, and near-ultraviolet spectral ranges [1–5]. Among them, GaN, with a band gap of 3.4 eV, is the most widely studied wide gap semiconductor material. The conventional growth methods for GaN films are metalorganic chemical vapor deposition, molecular beam epitaxy, and hydride vapor phase epitaxy. However, efforts have recently been made on the magnetron sputtering technique owing to its lowtemperature and low-cost nature of process, without harmful exhaust gas to the environment. High quality GaN films with an X-ray rocking curve width of 240 arcsec has been prepared by ultrahigh-rate RF magnetron sputter epitaxy [6], and DC reactive magnetron sputter deposition of GaN films have also been studied [7]. However, to our best knowledge, no work has reported on production of GaN films by middle-frequency (MF) magnetron sputtering, a technique developed and industrialized in late 1990s. MF magnetron sputtering is able to effectively eliminate poisoning of targets and, therefore, is possible to produce films with very high growth rate. Previously, we have * Corresponding author. E-mail addresses: [email protected] (C.W. Zou), [email protected] (L.P. Guo), [email protected] (D.J. Fu). 0169-4332/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.apsusc.2007.05.037
deposited Ti-containing amorphous carbon nanocomposite coatings by a home-made MF magnetron sputter machine [8]. In the present letter, we for the first time report on the deposition of GaN films by using a modified MF magnetron system. 2. Experimental details The schematic diagram of the middle-frequency magnetron sputtering system has been shown in Ref. [9]. GaN films were deposited using a MF sputter system operating at 40 kHz, with dish shape twin targets specially designed for liquid Ga. The target dish was made of stainless steel which was water cooled at 10 8C and back supported by a Cu plate. The dish is 80 mm in diameter and 3 mm in depth and horizontally placed in the deposition chamber. The target for sputtering was 99.999% purity Ga. Si (111) substrates were ultrasonically cleaned in sequence in TCE, acetone, and methanol and rinsed in deionized water before being mounted onto the substrate holder. The substrate temperature during the deposition was monitored using a thermocouple. The substrate was negatively biased and can be heated to 500 8C. N2 and Ar were let into the chamber through two 100 sccm mass-flow controllers. The chamber was evacuated to a base pressure of 5 10 3 Pa before deposition. The substrates were cleaned for 10 min by glow discharge in an Ar atmosphere at 4 Pa. Then presputtering of the target was performed for 10 min with a shutter located between the target and substrate. The influence of total
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pressure, N2:Ar ratio, substrate temperature, and target-tosubstrate distance on the properties of the deposited films were investigated. The structure of the deposited GaN film was characterized by X-ray diffraction (XRD, Bruker axs D8ADVACE) with a Cu Ka radiation. The surface morphology and the cross-section images were measured using an FEI SIRION IMP SEM system and the composition of the GaN films was determined by using an EDAX genesis 7000 energy dispersive spectroscopy (EDS) system operated at 12 kV. The surface topography was analyzed using an atomic force microscope (AFM) (SHIMAD ZU SPM-9500J3) operating in the tapping mode. The surface atomic composition and the chemical bonding states were investigated by X-ray photoelectron spectroscopy (Kratos Ltd. XSAM800) with Mg Ka excitation. 3. Results and discussion
Fig. 2. Nitrogen contents of GaN films deposited on Si substrate at various total gas pressure. The inset shows a cross-section SEM graph of the GaN film deposited under the standard conditions.
Fig. 1 shows the total pressure dependence of XRD patterns of the GaN films deposited at a constant input power of 1600 W and cathode target current of 2.5 A, with N2/(N2 + Ar) = 70%, substrate temperature of 300 8C, negative bias of 50 V and target–substrate distance of 10 cm. This is designated as the ‘‘standard condition’’ of the machine. The GaN (0 0 2) peak at 34.58 is clearly seen in the films deposited at 0.67, 0.8, 1.0, and 1.5 Pa, but not at 0.4 Pa. Full width half maximum (FWHM) of the GaN (0 0 2) peaks are 650, 1200, 1300, 1500 arcsec for 0.67, 0.8, 1.0, and 1.5 Pa, respectively. The crystal quality is believed to be determined by two competing processes. First, the mean free path of the sputtered atoms could be reduced by the increase of the total pressure, which results in a lower kinetic energy and leads to a poor mobility when the atoms arrive at the substrate surface. This hinders nucleation and crystallization of the film. On the other hand, when the total pressure is too low, the higher energy of the depositing atoms results in re-sputtering of the deposited film. Therefore, an optimal pressure exists, e.g., 0.65–0.8 Pa in the present experiment. The observed peaks corresponding to gallium oxide in the XRD spectra can be attributed to oxidation in the deposition process or during exposure to the air before measurement.
The total pressure also influences N content of the deposited films, as can be seen in Fig. 2. When the total pressure is 0.5 Pa, the N content of the film is 36 at%, and then the N contents increase with the increase of the total pressure. However, it starts to decrease when the pressure exceeds 0.8 Pa. When the pressure is low, re-sputtering of the films by the high-energy impinging atoms results in film deficient in N. At high pressure, the mean free path is short and N2 is less ionized, leading to a reduction of N concentration in the film. The inset of Fig. 2 shows the cross-section SEM graph of a GaN film deposited under the standard condition. The growth is columnar and the deposition rate evaluated is 60 nm/min. The XPS measurement revealed core level peaks of Ga3d at 19.8 eV and N1s at 396.8 eV (not shown here), which are in good agreement with the published values of compound GaN [10], and the positive from element Ga3d peak centered at 18.5 eV indicates that there is no metallic Ga in the films produced under the standard conditions. The films are light yellow and highly transparent. Fig. 3 shows the XRD patterns for GaN films deposited on substrates at various target-to-substrate distances, and the other conditions the same as those of Fig. 1. The intensity of GaN
Fig. 1. X-ray diffraction patterns of GaN films deposited on Si substrate at various total gas pressures.
Fig. 3. X-ray diffraction patterns of GaN films deposited on Si substrate at various distance between the target and substrate.
C.W. Zou et al. / Applied Surface Science 253 (2007) 9077–9080
Fig. 4. X-ray diffraction patterns of GaN films deposited on Si substrate at various substrate temperatures.
(0 0 0 2) diffraction at 2u = 34.58 is highest when d = 10 cm, while it disappears when d < 5 cm. FWHM of (0 0 0 2) peaks are 1300, 1300, 1200, 1500, 1900 arcsec for target-to-substrate distance of 15, 12, 10, and 8 cm, respectively. GaN films produced by magnetron sputtering are known to be subject to damaging by plasma exposure [11]. When the target–substrate distance is too small, the ionic N may not react sufficiently with the sputtered Ga and ion bombardment to the substrate makes the film deficient in N and with poor crystallity. Indeed, the films are black colored and amorphous, apparently resulting from a large quantity of Ga clusters. At the other extreme, at the
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largest target-to-substrate distance, reduced kinetic energies of Ga and N atoms due to collision with N2 and Ar also makes the films poor in crystallization. Fig. 4 shows the diffraction patterns for GaN films deposited on Si (111) at various temperatures between 80 and 400 8C. All the samples exhibit a strong (0 0 0 2) peak and the FWHM of (0 0 0 2) peaks are 1050, 1200, 1500, 1500, 2000 arcsec for substrate temperature of 400, 300, 200, 100, and 80 8C, respectively. The crystallization quality is constantly improved with increasing substrate temperature. To further investigate the impact of temperature on the properties of films, we measured SEM and AFM images of two GaN samples both 3.13 mm in thickness prepared under standard conditions (300 8C) and at a substrate temperature of 100 8C with other parameters the same. As shown in Fig. 5, the surface of the GaN film is composed of crystallites with various diameters, some of which contact and form an island (Fig. 5(a and b)). In contrary, when deposited at 100 8C, the GaN film consists of uniform nanocrystalline grains with diameters of 5–6 nm (Fig. 5(c and d)). This phenomenon has been observed by a few authors and can be interpreted by the growth mechanism proposed by Kusaka et al. [12]. At low substrate temperatures, large amount of atoms deposited on the substrate are not able to move freely on the substrate surface, resulting in the formation of many crystal nuclei. These nuclei grow without surface diffusion, leading to the formation of crystallites in huge amount. On the other hand, at higher temperatures, atoms deposited on the substrate have higher mobility due to thermal energy imparted by the substrate. This results in the formation of larger crystallites. Liu et al. [13] put forward a three or multiple-
Fig. 5. SEM (a and c) and AFM (b and d) images of GaN films deposited under standard conditions (a and b) and at a substrate temperature of 100 8C (c and d).
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process mechanism: at lower temperature, the island can conserve its original orientation and the result turn to be polycrystalline, while at higher temperature, all the islands can reorient to adopt the most thermodynamically stable single orientation, leading to an epitaxial film growth. 4. Conclusion In summary, middle-frequency magnetron sputtering was adopted to deposit GaN films on Si (111) substrates at a deposition rate of 60 nm/min under optimized conditions. The GaN samples have a N:Ga = 1:1 and without elemental Ga. The key parameters influencing the crystal quality of GaN film are the total pressure, substrate temperature, and target-substrate distance. By tuning these conditions, GaN film with preferential (0 0 0 2) orientation could be grown. Acknowledgements This work was supported by national Natural Science Foundation of China under contract 10435060 and 10675095 and by SRF for ROCS, State Education Ministry.
References [1] E. Nogales, I. Joachimsthaler, R. Heiderhoff, J. piqueras, L.J. Balk, J. Appl. Phys. 92 (2002) 976. [2] F. Ye, E.Q. Xie, X.J. Pan, H. Li, H.G. Duan, C.W. Jia, J. Vac. Sci. Technol. 24 (2006) 1358. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [4] S. Nakamura, M. Senoh, N. Iwasa, S.I. Nagahama, T. Yamada, T. Mukail, Jpn. J. Appl. Phys. 34 (1995) L1332. [5] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [6] M. Park, J.P. Maria, J.J. Cuomo, Y.C. Chang, J.F. Muth, R.M. Kolbas, R.J. Nemanich, E. Carlson, J. Bumgarner, Appl. Phys. Lett. 81 (2002) 1797. [7] P.K. Song, E. Yoshida, Y. Sato, K.H. Kim, Y. Shigesato, Jpn. J. Appl. Phys. 43 (2004) L164. [8] B. Yang, Z.H. Huang, C.S. Liu, Z.Y. Zeng, X.J. Fan, D.J. Fu, Jpn. J. Appl. Phys. 44 (2005) L1022. [9] M. L. Yin, C. W. Zou, M. Li, D. J. Fu, Nucl. Inst. Meth. B, in press. [10] J.S. Paek, K.K. Kim, J.M. Lee, D.J. Kim, M.S. Yi, D.Y. Noh, J. Cryst. Growth 200 (1999) 55. [11] K.K. Tominaga, K. Kusaka, M. Chong, T. Hanabusa, Y. Shintani, Jpn. J. Appl. Phys. 33 (1994) 5235. [12] K. Kusaka, T. Hanabusa, K. Tominaga, N. Yamauchi, J. Vac. Sci. Technol. 22 (2004) 1587. [13] Q.L. Liu, Y. Bando, F.F. Xu, C.C. Tang, Appl. Phys. Lett. 85 (2004) 4890.
GaN films deposited by middle-frequency magnetron sputtering C.W. Zou a,b,*, M.L. Yin a,b, M. Li a,b, L.P. Guo a,b, D.J. Fu a,b b
a Accelerator Laboratory, Department of Physics, Wuhan University, Wuhan 430072, China Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China
Received 26 March 2007; received in revised form 11 May 2007; accepted 12 May 2007 Available online 21 May 2007
Abstract GaN films were deposited on Si (111) substrates by middle-frequency magnetron sputtering. X-ray diffraction revealed preferential GaN (0 0 0 2) orientation normal to the substrate surface for all the films deposited. The diffraction intensity and N contents were found to depend strongly on the total gas pressure. Good quality films were only obtained at pressures in the range of 0.4–1.0 Pa. Little diffraction of GaN (0 0 0 2) could be observed either at total pressures below 0.4 Pa or above 1.0 Pa. The GaN films produced under the optimized conditions have an N:Ga ratio of 1:1 as determined by energy-dispersive X-ray spectroscopy. # 2007 Published by Elsevier B.V. Keywords: GaN; Middle-frequency; Magnetron sputtering; Structure
1. Introduction Much attention has been paid to III-nitride semiconductors recently for their huge application potential in electronic and optoelectronic devices, such as electron field emitter and lightemitting diodes in the blue, violet, and near-ultraviolet spectral ranges [1–5]. Among them, GaN, with a band gap of 3.4 eV, is the most widely studied wide gap semiconductor material. The conventional growth methods for GaN films are metalorganic chemical vapor deposition, molecular beam epitaxy, and hydride vapor phase epitaxy. However, efforts have recently been made on the magnetron sputtering technique owing to its lowtemperature and low-cost nature of process, without harmful exhaust gas to the environment. High quality GaN films with an X-ray rocking curve width of 240 arcsec has been prepared by ultrahigh-rate RF magnetron sputter epitaxy [6], and DC reactive magnetron sputter deposition of GaN films have also been studied [7]. However, to our best knowledge, no work has reported on production of GaN films by middle-frequency (MF) magnetron sputtering, a technique developed and industrialized in late 1990s. MF magnetron sputtering is able to effectively eliminate poisoning of targets and, therefore, is possible to produce films with very high growth rate. Previously, we have * Corresponding author. E-mail addresses: [email protected] (C.W. Zou), [email protected] (L.P. Guo), [email protected] (D.J. Fu). 0169-4332/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.apsusc.2007.05.037
deposited Ti-containing amorphous carbon nanocomposite coatings by a home-made MF magnetron sputter machine [8]. In the present letter, we for the first time report on the deposition of GaN films by using a modified MF magnetron system. 2. Experimental details The schematic diagram of the middle-frequency magnetron sputtering system has been shown in Ref. [9]. GaN films were deposited using a MF sputter system operating at 40 kHz, with dish shape twin targets specially designed for liquid Ga. The target dish was made of stainless steel which was water cooled at 10 8C and back supported by a Cu plate. The dish is 80 mm in diameter and 3 mm in depth and horizontally placed in the deposition chamber. The target for sputtering was 99.999% purity Ga. Si (111) substrates were ultrasonically cleaned in sequence in TCE, acetone, and methanol and rinsed in deionized water before being mounted onto the substrate holder. The substrate temperature during the deposition was monitored using a thermocouple. The substrate was negatively biased and can be heated to 500 8C. N2 and Ar were let into the chamber through two 100 sccm mass-flow controllers. The chamber was evacuated to a base pressure of 5 10 3 Pa before deposition. The substrates were cleaned for 10 min by glow discharge in an Ar atmosphere at 4 Pa. Then presputtering of the target was performed for 10 min with a shutter located between the target and substrate. The influence of total
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pressure, N2:Ar ratio, substrate temperature, and target-tosubstrate distance on the properties of the deposited films were investigated. The structure of the deposited GaN film was characterized by X-ray diffraction (XRD, Bruker axs D8ADVACE) with a Cu Ka radiation. The surface morphology and the cross-section images were measured using an FEI SIRION IMP SEM system and the composition of the GaN films was determined by using an EDAX genesis 7000 energy dispersive spectroscopy (EDS) system operated at 12 kV. The surface topography was analyzed using an atomic force microscope (AFM) (SHIMAD ZU SPM-9500J3) operating in the tapping mode. The surface atomic composition and the chemical bonding states were investigated by X-ray photoelectron spectroscopy (Kratos Ltd. XSAM800) with Mg Ka excitation. 3. Results and discussion
Fig. 2. Nitrogen contents of GaN films deposited on Si substrate at various total gas pressure. The inset shows a cross-section SEM graph of the GaN film deposited under the standard conditions.
Fig. 1 shows the total pressure dependence of XRD patterns of the GaN films deposited at a constant input power of 1600 W and cathode target current of 2.5 A, with N2/(N2 + Ar) = 70%, substrate temperature of 300 8C, negative bias of 50 V and target–substrate distance of 10 cm. This is designated as the ‘‘standard condition’’ of the machine. The GaN (0 0 2) peak at 34.58 is clearly seen in the films deposited at 0.67, 0.8, 1.0, and 1.5 Pa, but not at 0.4 Pa. Full width half maximum (FWHM) of the GaN (0 0 2) peaks are 650, 1200, 1300, 1500 arcsec for 0.67, 0.8, 1.0, and 1.5 Pa, respectively. The crystal quality is believed to be determined by two competing processes. First, the mean free path of the sputtered atoms could be reduced by the increase of the total pressure, which results in a lower kinetic energy and leads to a poor mobility when the atoms arrive at the substrate surface. This hinders nucleation and crystallization of the film. On the other hand, when the total pressure is too low, the higher energy of the depositing atoms results in re-sputtering of the deposited film. Therefore, an optimal pressure exists, e.g., 0.65–0.8 Pa in the present experiment. The observed peaks corresponding to gallium oxide in the XRD spectra can be attributed to oxidation in the deposition process or during exposure to the air before measurement.
The total pressure also influences N content of the deposited films, as can be seen in Fig. 2. When the total pressure is 0.5 Pa, the N content of the film is 36 at%, and then the N contents increase with the increase of the total pressure. However, it starts to decrease when the pressure exceeds 0.8 Pa. When the pressure is low, re-sputtering of the films by the high-energy impinging atoms results in film deficient in N. At high pressure, the mean free path is short and N2 is less ionized, leading to a reduction of N concentration in the film. The inset of Fig. 2 shows the cross-section SEM graph of a GaN film deposited under the standard condition. The growth is columnar and the deposition rate evaluated is 60 nm/min. The XPS measurement revealed core level peaks of Ga3d at 19.8 eV and N1s at 396.8 eV (not shown here), which are in good agreement with the published values of compound GaN [10], and the positive from element Ga3d peak centered at 18.5 eV indicates that there is no metallic Ga in the films produced under the standard conditions. The films are light yellow and highly transparent. Fig. 3 shows the XRD patterns for GaN films deposited on substrates at various target-to-substrate distances, and the other conditions the same as those of Fig. 1. The intensity of GaN
Fig. 1. X-ray diffraction patterns of GaN films deposited on Si substrate at various total gas pressures.
Fig. 3. X-ray diffraction patterns of GaN films deposited on Si substrate at various distance between the target and substrate.
C.W. Zou et al. / Applied Surface Science 253 (2007) 9077–9080
Fig. 4. X-ray diffraction patterns of GaN films deposited on Si substrate at various substrate temperatures.
(0 0 0 2) diffraction at 2u = 34.58 is highest when d = 10 cm, while it disappears when d < 5 cm. FWHM of (0 0 0 2) peaks are 1300, 1300, 1200, 1500, 1900 arcsec for target-to-substrate distance of 15, 12, 10, and 8 cm, respectively. GaN films produced by magnetron sputtering are known to be subject to damaging by plasma exposure [11]. When the target–substrate distance is too small, the ionic N may not react sufficiently with the sputtered Ga and ion bombardment to the substrate makes the film deficient in N and with poor crystallity. Indeed, the films are black colored and amorphous, apparently resulting from a large quantity of Ga clusters. At the other extreme, at the
9079
largest target-to-substrate distance, reduced kinetic energies of Ga and N atoms due to collision with N2 and Ar also makes the films poor in crystallization. Fig. 4 shows the diffraction patterns for GaN films deposited on Si (111) at various temperatures between 80 and 400 8C. All the samples exhibit a strong (0 0 0 2) peak and the FWHM of (0 0 0 2) peaks are 1050, 1200, 1500, 1500, 2000 arcsec for substrate temperature of 400, 300, 200, 100, and 80 8C, respectively. The crystallization quality is constantly improved with increasing substrate temperature. To further investigate the impact of temperature on the properties of films, we measured SEM and AFM images of two GaN samples both 3.13 mm in thickness prepared under standard conditions (300 8C) and at a substrate temperature of 100 8C with other parameters the same. As shown in Fig. 5, the surface of the GaN film is composed of crystallites with various diameters, some of which contact and form an island (Fig. 5(a and b)). In contrary, when deposited at 100 8C, the GaN film consists of uniform nanocrystalline grains with diameters of 5–6 nm (Fig. 5(c and d)). This phenomenon has been observed by a few authors and can be interpreted by the growth mechanism proposed by Kusaka et al. [12]. At low substrate temperatures, large amount of atoms deposited on the substrate are not able to move freely on the substrate surface, resulting in the formation of many crystal nuclei. These nuclei grow without surface diffusion, leading to the formation of crystallites in huge amount. On the other hand, at higher temperatures, atoms deposited on the substrate have higher mobility due to thermal energy imparted by the substrate. This results in the formation of larger crystallites. Liu et al. [13] put forward a three or multiple-
Fig. 5. SEM (a and c) and AFM (b and d) images of GaN films deposited under standard conditions (a and b) and at a substrate temperature of 100 8C (c and d).
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process mechanism: at lower temperature, the island can conserve its original orientation and the result turn to be polycrystalline, while at higher temperature, all the islands can reorient to adopt the most thermodynamically stable single orientation, leading to an epitaxial film growth. 4. Conclusion In summary, middle-frequency magnetron sputtering was adopted to deposit GaN films on Si (111) substrates at a deposition rate of 60 nm/min under optimized conditions. The GaN samples have a N:Ga = 1:1 and without elemental Ga. The key parameters influencing the crystal quality of GaN film are the total pressure, substrate temperature, and target-substrate distance. By tuning these conditions, GaN film with preferential (0 0 0 2) orientation could be grown. Acknowledgements This work was supported by national Natural Science Foundation of China under contract 10435060 and 10675095 and by SRF for ROCS, State Education Ministry.
References [1] E. Nogales, I. Joachimsthaler, R. Heiderhoff, J. piqueras, L.J. Balk, J. Appl. Phys. 92 (2002) 976. [2] F. Ye, E.Q. Xie, X.J. Pan, H. Li, H.G. Duan, C.W. Jia, J. Vac. Sci. Technol. 24 (2006) 1358. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [4] S. Nakamura, M. Senoh, N. Iwasa, S.I. Nagahama, T. Yamada, T. Mukail, Jpn. J. Appl. Phys. 34 (1995) L1332. [5] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [6] M. Park, J.P. Maria, J.J. Cuomo, Y.C. Chang, J.F. Muth, R.M. Kolbas, R.J. Nemanich, E. Carlson, J. Bumgarner, Appl. Phys. Lett. 81 (2002) 1797. [7] P.K. Song, E. Yoshida, Y. Sato, K.H. Kim, Y. Shigesato, Jpn. J. Appl. Phys. 43 (2004) L164. [8] B. Yang, Z.H. Huang, C.S. Liu, Z.Y. Zeng, X.J. Fan, D.J. Fu, Jpn. J. Appl. Phys. 44 (2005) L1022. [9] M. L. Yin, C. W. Zou, M. Li, D. J. Fu, Nucl. Inst. Meth. B, in press. [10] J.S. Paek, K.K. Kim, J.M. Lee, D.J. Kim, M.S. Yi, D.Y. Noh, J. Cryst. Growth 200 (1999) 55. [11] K.K. Tominaga, K. Kusaka, M. Chong, T. Hanabusa, Y. Shintani, Jpn. J. Appl. Phys. 33 (1994) 5235. [12] K. Kusaka, T. Hanabusa, K. Tominaga, N. Yamauchi, J. Vac. Sci. Technol. 22 (2004) 1587. [13] Q.L. Liu, Y. Bando, F.F. Xu, C.C. Tang, Appl. Phys. Lett. 85 (2004) 4890.