- Email: [email protected]
Growth of ZnO crystal on sapphire and nitridated sapphire substrates at 1000 °C by halide vapor phase epitaxy
Materials Letters 64 (2010) 25–27
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Growth of ZnO crystal on sapphire and nitridated sapphire substrates at 1000 °C by halide vapor phase epitaxy Naoki Yoshii a,⁎, Tetsuo Fujii b,c, Rui Masuda b, Shigetoshi Hosaka a, Akira Kamisawa c, Yoshinao Kumagai b, Akinori Koukitu b a b c
Technology Development Center, Tokyo Electron Ltd., 650 Mitsuzawa, Hosaka-cho, Nirasaki, Yamanashi 407-0192, Japan Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Research and Development Headquarters, ROHM Co., Ltd., 21 Saiin Mizosaki-cho, Ukyo-ku, Kyoto 615-8585, Japan
a r t i c l e
i n f o
Article history: Received 6 August 2009 Accepted 26 September 2009 Available online 6 October 2009 Keywords: ZnO Crystal growth Crystal structure Surfaces Vapor phase epitaxy
a b s t r a c t Using a halide vapor phase epitaxy (HVPE) technique in which the starting materials are ZnCl2 generated by the reaction between high purity Zn metal (7 N grade) and Cl2 gas, and H2O, ZnO crystals have been grown at a high temperature of 1000 °C on sapphire substrates with and without surface nitridation treatment. It was ̅ sapphire surface to a (0001) AlN found that the nitridation treatment resulted in a change of the (112 0) structure, leading to two possible sets of orientations for (0001) ZnO crystals. In addition, the nitridation treatment leads to a smaller average ZnO grain size and a higher density of nuclei. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Among compound semiconductors, zinc oxide (ZnO) is a promising material for optical devices operating in the blue and ultraviolet spectral region, owing to its wide band gap energy of 3.37 eV at room temperature and its large exciton binding energy of 60 meV. To date, ZnO crystals have been successfully grown on a variety of substrates, including Al2O3 [1,2], Si [3,4], ScAlMgO4 [5], and ZnO [6,7]. Of these, sapphire substrates are considered preferable for reasons related to thermal and chemical stability, and lower costs. In addition, a variety of methods, including molecular beam epitaxy (MBE) [8,9], pulse laser deposition (PLD) [5,7,10], sputter deposition [11,12], and metal organic chemical vapor deposition (MOCVD) [2,13] have been adopted to grow ZnO crystals on sapphire substrates. In this study, we employed the halide vapor phase epitaxy (HVPE) method to grow ZnO crystals because of the possibility of high temperature growth using chloride chemistry. In the field of nitride semiconductors, the HVPE method has been demonstrated to be an excellent technique for growing high quality crystals of GaN [14] and AlN [15] at high temperature. To date, HVPE growth of ZnO crystals has been reported by Takahashi et al. [16], using ZnCl2 powder and O2 as starting materials, and with growth temperatures in the range 500 to 950 °C. ZnCl2 powder, however, is known to suffer from the problems of low purity and high deliquescence, which makes it difficult to grow high
⁎ Corresponding author. Tel.: +81 551 23 4013; fax: +81 551 23 4454. E-mail address: [email protected] (N. Yoshii). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.060
quality ZnO crystals by HVPE. Furthermore, when ZnO crystals are grown on sapphire substrates at temperatures of around 1000 °C, the low adhesion of Zn to the sapphire becomes problematic. In the case of MBE, a surface nitridation treatment of (0001) sapphire substrates is known to be an effective technique for modifying the substrate surface [17], leading to improvements in adhesion, crystal orientation and crystal quality of ZnO. However, the effects of such a surface treatment have not yet been investigated in regard to the HVPE growth of ZnO on sapphire. Therefore, in this study, we examine a unique HVPE method for growing ZnO, in which ZnCl2 is generated by the reaction between high purity Zn metal and Cl2 gas on sapphire and nitridated sapphire substrates at a high temperature of 1000 °C. 2. Experimental procedure Growth of ZnO was carried out in a horizontal hot-wall quartz reactor, which was designed for this study. ZnCl2 was generated in the upstream region of the reactor, maintained at 375 °C, by the reaction of high purity Zn metal (7 N grade) and Cl2 gas, and transported to the downstream region with N2 carrier gas where a substrate was placed. H2O was introduced separately into the downstream region by a bubbling system using N2 carrier gas. All substrates were optical̅ orientation grade-polished sapphire (10 × 10 mm2) with a (112 0) (a-face). The substrates were first degreased by successive cleanings in acetone and deionized water, and then chemically etched in a hot solution (160 °C) of H3PO4 : H2SO4 = 1:3 for 10 min. After being loaded into the reactor, the sapphire substrate was heated at 1000 °C
26
N. Yoshii et al. / Materials Letters 64 (2010) 25–27
for 60 min in a NH3-containing H2 carrier gas prior to ZnO growth. The carrier gas was subsequently changed to N2 during growth of ZnO. ZnO crystal was grown at 1000 °C for 1 h with ZnCl2 and H2O input partial pressures of 2.2 × 10− 5 and 1.3 × 10− 2 atm, respectively. The surface morphology and the crystal quality of the samples were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). XRD rocking curves of the ZnO crystals were recorded using a double-crystal X-ray diffractometer. The surface condition of the nitridated sapphire substrates was assessed by X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). 3. Results and discussion Fig. 1 shows tilted SEM images of ZnO crystals grown on sapphire and nitridated sapphire substrates at a growth temperature of 1000 °C. In both cases, it can clearly be seen that the ZnO crystals exist in the form of columnar grains, leading to a discontinuous surface. In addition, it was found that the nitridation treatment leads to the presence of smaller ZnO grains in addition to a higher density of ZnO nuclei compared to the case without nitridation. This is thought to be due to a higher sticking coefficient of ZnCl2 on nitridated sapphire substrates compared to non-nitridated substrates. Fig. 2 shows XRD profiles of ZnO crystals grown at 1000 °C on the sapphire substrates without (a) and with (b) surface nitridation. In the XRD spectra of both samples, a strong and a much weak diffraction peaks appear at 34.4° and 36.3°, respectively, which are assigned to ̅ planes of ZnO with a diffraction peaks from the (0002) and (1011) wurtzite structure. This result implies that ZnO crystals can be grown almost epitaxially on sapphire and nitridated sapphire substrates at 1000 °C. The crystalline quality of the ZnO crystals was evaluated from the XRD rocking curves. The full-width at half-maximum (FWHM) ̅ ZnO peaks for crystals grown on values of the (0002) and (1011) sapphire substrates without nitridation treatment are 407 and 835 ″,
Fig. 2. XRD profiles of the ZnO crystals grown on sapphire substrates: (a) without nitridation treatment and (b) with nitridation treatment.
respectively. These values are superior to those for ZnO crystals grown by HVPE using ZnCl2 powder at a growth temperature of 950 °C [16], implying that high temperature growth at 1000 °C and the use of ZnCl2 generated by the reaction between Zn metal and Cl2 gas leads to a significant improvement in the crystallinity of ZnO. In contrast, the FWHM values of ZnO crystals formed on nitridated sapphire ̅ substrates (1091 ″ for (0002) and 1202 ″ for (1011)) are inferior to the case without nitridation. This is presumably due to modification of the sapphire surface by the nitridation treatment. In fact, it was confirmed by XPS and RHEED that the surface of the nitridated aplane sapphire substrate consists of a (0001) AlN structure. In the XPS spectrum of the nitridated a-plane sapphire substrate (not shown), the peak assigned to N (1 s) appears at a binding energy of 396.6 eV within 2 nm from the top surface. As shown in Fig. 3, the RHEED ̅ sapphire substrate is quite different pattern of the nitridated (112 0) ̅ sapphire substrate without the nitridation from that of the (112 0) treatment, while it agrees well with the RHEED pattern of a (0001) nitridated sapphire substrate (Fig. 3 (c)). These results confirm the ̅ formation of a (0001) AlN surface structure on the nitridated (112 0) sapphire substrate. ̅ planes for ZnO crystals on Fig. 4 shows XRD ϕ-scans of ZnO (1010) ̅ sapphire substrate with and without surface nitridation. It the (112 0) is seen that the ZnO crystals grown directly on sapphire substrates exhibit peaks at intervals of 60°, suggesting that the in-plane orientation of ZnO crystals grown by the HVPE method is well aligned ̅ sapphire substrate. In contrast, for the ZnO crystals to the (112 0) grown on nitridated a-plane sapphire substrates, two sets of 6-foldpeaks are observed 30° apart from each other, which implies that the crystals on the nitridated substrates have two possible arrangements. Fons et al. [18] reported that ZnO crystals grown on (0001) sapphire substrates have two possible sets of orientations. Therefore, it appears that the formation of the (0001) AlN structure on the nitridated sapphire substrates leads to a similar effect. This in turn causes a deterioration in the crystal quality of HVPE-ZnO grown on the nitridated surface, which suggests that the introduction of a suitable buffer layer such as physical vapor deposition (PVD)-ZnO may be required to produce high quality ZnO films by HVPE [19]. 4. Conclusions
Fig. 1. SEM images of ZnO crystals grown at 1000 °C on sapphire substrates: (a) without nitridation treatment and (b) with nitridation treatment.
In conclusion, a unique HVPE system employing ZnCl2 generated by the reaction between high purity Zn metal and Cl2 gas, and H2O as starting materials has been developed. ZnO crystals have been grown ̅ sapphire substrates with and at a temperature of 1000 °C on (112 0) without a nitridation treatment. The average grain size on nitridated substrates was found to be smaller than that without nitridation treatment. In addition, the density of ZnO nuclei was higher on the nitridated substrates. The nitridation treatment was found to change
N. Yoshii et al. / Materials Letters 64 (2010) 25–27
27
̅ sapphire substrate with nitridation treatment, (b) (112 0) ̅ sapphire substrate without nitridation treatment, and (c) (0001) sapphire substrate Fig. 3. RHEED patterns of (a) (112 0) ̅ sapphire. with nitridation treatment. Incident azimuth of electron beam was <1010>
̅ plane of the ZnO crystals on a-plane sapphire Fig. 4. The XRD ϕ-scan profiles of (1010) substrates: (a) without nitridation treatment and (b) with nitridation treatment.
̅ sapphire substrates into a (0001) AlN structure, the surface of (112 0) leading to poor crystalline quality of ZnO crystals grown by the HVPE method. References [1] Segawa Y, Ohtomo A, Kawasaki M, Koinuma H, Tang ZK, Yu P, et al. Phys Status Solidi B 1997;202:669.
[2] Maejima K, Ueda M, Fujita Sz, Fujita Sg. Jpn J Appl Phys 2003;42:2600. [3] Ogata K, Kawanishi T, Maejima K, Sakurai K, Fujita Sz, Fujita Sg. J Cryst Growth 2002;237–239:553. [4] Liu CY, Zhang BP, Binh NT, Wakatsuki K, Segawa Y. J Cryst Growth 2006;290:314. [5] Tamura K, Ohtomo A, Saikusa K, Osaka Y, Maikino T, Segawa Y, et al. J Cryst Growth 2000;214/215:59. [6] Heinze S, Krtshil A, Blasing J, Hempel T, Veit P, Dadgar A, et al. J Cryst Growth 2007;308:170. [7] Lorenz M, Wagner G, Rahm A, Schmidt H, Hochmuth H, Mader W, et al. Phys Status Solidi C 2008;5:3280. [8] Zu P, Tang ZK, Wong GKL, Kawasaki M, Ohtomo A, Koinuma H, et al. J Solid State Commun 1997;103:459. [9] Chen Y, Bagnall DM, Zhu Z, Sekiuchi T, Park K, Hiraga K, et al. J Cryst Growth 1997;181:165. [10] Fouchet A, Prellier W, Mercey B, Mechin L, Kulkarni VN, Venkatesan T. J Appl Phys 2004;96:3228. [11] Doh SJ, Park SI, Cho TS, Je JH. J Vac Sci Technol A 1999;17:3003. [12] Kim KK, Song JH, Jung HJ, Choi WK, Park SJ, Song JH. J Appl Phys 2000;87:3573. [13] Zhang BP, Manh LH, Wakatsuki K, Ohnishi T, Lippmaa M, Usami N, et al. Jpn J Appl Phys 2003;42:2291. [14] Kelly MK, Vaudo RP, Phanse VM, Görgens L, Ambacher O. Jpn J Appl Phys 1999;38: L217. [15] Kumagai Y, Nagashima T, Koukitu A. Jpn J Appl Phys 2007;46:L389. [16] Takahashi N, Kaiya K, Omichi K, Nakamura T, Okamoto S, Yamamoto H. J Cryst Growth 2000;209:822. [17] Wang X, Iwaki H, Murakami M, Du X, Ishitani Y, Yoshikawa A. Jpn J Appl Phys 2003;42:L99. [18] Fons P, Iwata K, Yamada A, Matsubara K, Niki S. Appl Phys Lett 2000;77:1801. [19] Fujii T, Yoshii N, Masuda R, Tanabe T, Kamisawa A, Hosaka S, et al. J Cryst Growth 2009;311:1056.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Growth of ZnO crystal on sapphire and nitridated sapphire substrates at 1000 °C by halide vapor phase epitaxy Naoki Yoshii a,⁎, Tetsuo Fujii b,c, Rui Masuda b, Shigetoshi Hosaka a, Akira Kamisawa c, Yoshinao Kumagai b, Akinori Koukitu b a b c
Technology Development Center, Tokyo Electron Ltd., 650 Mitsuzawa, Hosaka-cho, Nirasaki, Yamanashi 407-0192, Japan Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Research and Development Headquarters, ROHM Co., Ltd., 21 Saiin Mizosaki-cho, Ukyo-ku, Kyoto 615-8585, Japan
a r t i c l e
i n f o
Article history: Received 6 August 2009 Accepted 26 September 2009 Available online 6 October 2009 Keywords: ZnO Crystal growth Crystal structure Surfaces Vapor phase epitaxy
a b s t r a c t Using a halide vapor phase epitaxy (HVPE) technique in which the starting materials are ZnCl2 generated by the reaction between high purity Zn metal (7 N grade) and Cl2 gas, and H2O, ZnO crystals have been grown at a high temperature of 1000 °C on sapphire substrates with and without surface nitridation treatment. It was ̅ sapphire surface to a (0001) AlN found that the nitridation treatment resulted in a change of the (112 0) structure, leading to two possible sets of orientations for (0001) ZnO crystals. In addition, the nitridation treatment leads to a smaller average ZnO grain size and a higher density of nuclei. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Among compound semiconductors, zinc oxide (ZnO) is a promising material for optical devices operating in the blue and ultraviolet spectral region, owing to its wide band gap energy of 3.37 eV at room temperature and its large exciton binding energy of 60 meV. To date, ZnO crystals have been successfully grown on a variety of substrates, including Al2O3 [1,2], Si [3,4], ScAlMgO4 [5], and ZnO [6,7]. Of these, sapphire substrates are considered preferable for reasons related to thermal and chemical stability, and lower costs. In addition, a variety of methods, including molecular beam epitaxy (MBE) [8,9], pulse laser deposition (PLD) [5,7,10], sputter deposition [11,12], and metal organic chemical vapor deposition (MOCVD) [2,13] have been adopted to grow ZnO crystals on sapphire substrates. In this study, we employed the halide vapor phase epitaxy (HVPE) method to grow ZnO crystals because of the possibility of high temperature growth using chloride chemistry. In the field of nitride semiconductors, the HVPE method has been demonstrated to be an excellent technique for growing high quality crystals of GaN [14] and AlN [15] at high temperature. To date, HVPE growth of ZnO crystals has been reported by Takahashi et al. [16], using ZnCl2 powder and O2 as starting materials, and with growth temperatures in the range 500 to 950 °C. ZnCl2 powder, however, is known to suffer from the problems of low purity and high deliquescence, which makes it difficult to grow high
⁎ Corresponding author. Tel.: +81 551 23 4013; fax: +81 551 23 4454. E-mail address: [email protected] (N. Yoshii). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.060
quality ZnO crystals by HVPE. Furthermore, when ZnO crystals are grown on sapphire substrates at temperatures of around 1000 °C, the low adhesion of Zn to the sapphire becomes problematic. In the case of MBE, a surface nitridation treatment of (0001) sapphire substrates is known to be an effective technique for modifying the substrate surface [17], leading to improvements in adhesion, crystal orientation and crystal quality of ZnO. However, the effects of such a surface treatment have not yet been investigated in regard to the HVPE growth of ZnO on sapphire. Therefore, in this study, we examine a unique HVPE method for growing ZnO, in which ZnCl2 is generated by the reaction between high purity Zn metal and Cl2 gas on sapphire and nitridated sapphire substrates at a high temperature of 1000 °C. 2. Experimental procedure Growth of ZnO was carried out in a horizontal hot-wall quartz reactor, which was designed for this study. ZnCl2 was generated in the upstream region of the reactor, maintained at 375 °C, by the reaction of high purity Zn metal (7 N grade) and Cl2 gas, and transported to the downstream region with N2 carrier gas where a substrate was placed. H2O was introduced separately into the downstream region by a bubbling system using N2 carrier gas. All substrates were optical̅ orientation grade-polished sapphire (10 × 10 mm2) with a (112 0) (a-face). The substrates were first degreased by successive cleanings in acetone and deionized water, and then chemically etched in a hot solution (160 °C) of H3PO4 : H2SO4 = 1:3 for 10 min. After being loaded into the reactor, the sapphire substrate was heated at 1000 °C
26
N. Yoshii et al. / Materials Letters 64 (2010) 25–27
for 60 min in a NH3-containing H2 carrier gas prior to ZnO growth. The carrier gas was subsequently changed to N2 during growth of ZnO. ZnO crystal was grown at 1000 °C for 1 h with ZnCl2 and H2O input partial pressures of 2.2 × 10− 5 and 1.3 × 10− 2 atm, respectively. The surface morphology and the crystal quality of the samples were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). XRD rocking curves of the ZnO crystals were recorded using a double-crystal X-ray diffractometer. The surface condition of the nitridated sapphire substrates was assessed by X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). 3. Results and discussion Fig. 1 shows tilted SEM images of ZnO crystals grown on sapphire and nitridated sapphire substrates at a growth temperature of 1000 °C. In both cases, it can clearly be seen that the ZnO crystals exist in the form of columnar grains, leading to a discontinuous surface. In addition, it was found that the nitridation treatment leads to the presence of smaller ZnO grains in addition to a higher density of ZnO nuclei compared to the case without nitridation. This is thought to be due to a higher sticking coefficient of ZnCl2 on nitridated sapphire substrates compared to non-nitridated substrates. Fig. 2 shows XRD profiles of ZnO crystals grown at 1000 °C on the sapphire substrates without (a) and with (b) surface nitridation. In the XRD spectra of both samples, a strong and a much weak diffraction peaks appear at 34.4° and 36.3°, respectively, which are assigned to ̅ planes of ZnO with a diffraction peaks from the (0002) and (1011) wurtzite structure. This result implies that ZnO crystals can be grown almost epitaxially on sapphire and nitridated sapphire substrates at 1000 °C. The crystalline quality of the ZnO crystals was evaluated from the XRD rocking curves. The full-width at half-maximum (FWHM) ̅ ZnO peaks for crystals grown on values of the (0002) and (1011) sapphire substrates without nitridation treatment are 407 and 835 ″,
Fig. 2. XRD profiles of the ZnO crystals grown on sapphire substrates: (a) without nitridation treatment and (b) with nitridation treatment.
respectively. These values are superior to those for ZnO crystals grown by HVPE using ZnCl2 powder at a growth temperature of 950 °C [16], implying that high temperature growth at 1000 °C and the use of ZnCl2 generated by the reaction between Zn metal and Cl2 gas leads to a significant improvement in the crystallinity of ZnO. In contrast, the FWHM values of ZnO crystals formed on nitridated sapphire ̅ substrates (1091 ″ for (0002) and 1202 ″ for (1011)) are inferior to the case without nitridation. This is presumably due to modification of the sapphire surface by the nitridation treatment. In fact, it was confirmed by XPS and RHEED that the surface of the nitridated aplane sapphire substrate consists of a (0001) AlN structure. In the XPS spectrum of the nitridated a-plane sapphire substrate (not shown), the peak assigned to N (1 s) appears at a binding energy of 396.6 eV within 2 nm from the top surface. As shown in Fig. 3, the RHEED ̅ sapphire substrate is quite different pattern of the nitridated (112 0) ̅ sapphire substrate without the nitridation from that of the (112 0) treatment, while it agrees well with the RHEED pattern of a (0001) nitridated sapphire substrate (Fig. 3 (c)). These results confirm the ̅ formation of a (0001) AlN surface structure on the nitridated (112 0) sapphire substrate. ̅ planes for ZnO crystals on Fig. 4 shows XRD ϕ-scans of ZnO (1010) ̅ sapphire substrate with and without surface nitridation. It the (112 0) is seen that the ZnO crystals grown directly on sapphire substrates exhibit peaks at intervals of 60°, suggesting that the in-plane orientation of ZnO crystals grown by the HVPE method is well aligned ̅ sapphire substrate. In contrast, for the ZnO crystals to the (112 0) grown on nitridated a-plane sapphire substrates, two sets of 6-foldpeaks are observed 30° apart from each other, which implies that the crystals on the nitridated substrates have two possible arrangements. Fons et al. [18] reported that ZnO crystals grown on (0001) sapphire substrates have two possible sets of orientations. Therefore, it appears that the formation of the (0001) AlN structure on the nitridated sapphire substrates leads to a similar effect. This in turn causes a deterioration in the crystal quality of HVPE-ZnO grown on the nitridated surface, which suggests that the introduction of a suitable buffer layer such as physical vapor deposition (PVD)-ZnO may be required to produce high quality ZnO films by HVPE [19]. 4. Conclusions
Fig. 1. SEM images of ZnO crystals grown at 1000 °C on sapphire substrates: (a) without nitridation treatment and (b) with nitridation treatment.
In conclusion, a unique HVPE system employing ZnCl2 generated by the reaction between high purity Zn metal and Cl2 gas, and H2O as starting materials has been developed. ZnO crystals have been grown ̅ sapphire substrates with and at a temperature of 1000 °C on (112 0) without a nitridation treatment. The average grain size on nitridated substrates was found to be smaller than that without nitridation treatment. In addition, the density of ZnO nuclei was higher on the nitridated substrates. The nitridation treatment was found to change
N. Yoshii et al. / Materials Letters 64 (2010) 25–27
27
̅ sapphire substrate with nitridation treatment, (b) (112 0) ̅ sapphire substrate without nitridation treatment, and (c) (0001) sapphire substrate Fig. 3. RHEED patterns of (a) (112 0) ̅ sapphire. with nitridation treatment. Incident azimuth of electron beam was <1010>
̅ plane of the ZnO crystals on a-plane sapphire Fig. 4. The XRD ϕ-scan profiles of (1010) substrates: (a) without nitridation treatment and (b) with nitridation treatment.
̅ sapphire substrates into a (0001) AlN structure, the surface of (112 0) leading to poor crystalline quality of ZnO crystals grown by the HVPE method. References [1] Segawa Y, Ohtomo A, Kawasaki M, Koinuma H, Tang ZK, Yu P, et al. Phys Status Solidi B 1997;202:669.
[2] Maejima K, Ueda M, Fujita Sz, Fujita Sg. Jpn J Appl Phys 2003;42:2600. [3] Ogata K, Kawanishi T, Maejima K, Sakurai K, Fujita Sz, Fujita Sg. J Cryst Growth 2002;237–239:553. [4] Liu CY, Zhang BP, Binh NT, Wakatsuki K, Segawa Y. J Cryst Growth 2006;290:314. [5] Tamura K, Ohtomo A, Saikusa K, Osaka Y, Maikino T, Segawa Y, et al. J Cryst Growth 2000;214/215:59. [6] Heinze S, Krtshil A, Blasing J, Hempel T, Veit P, Dadgar A, et al. J Cryst Growth 2007;308:170. [7] Lorenz M, Wagner G, Rahm A, Schmidt H, Hochmuth H, Mader W, et al. Phys Status Solidi C 2008;5:3280. [8] Zu P, Tang ZK, Wong GKL, Kawasaki M, Ohtomo A, Koinuma H, et al. J Solid State Commun 1997;103:459. [9] Chen Y, Bagnall DM, Zhu Z, Sekiuchi T, Park K, Hiraga K, et al. J Cryst Growth 1997;181:165. [10] Fouchet A, Prellier W, Mercey B, Mechin L, Kulkarni VN, Venkatesan T. J Appl Phys 2004;96:3228. [11] Doh SJ, Park SI, Cho TS, Je JH. J Vac Sci Technol A 1999;17:3003. [12] Kim KK, Song JH, Jung HJ, Choi WK, Park SJ, Song JH. J Appl Phys 2000;87:3573. [13] Zhang BP, Manh LH, Wakatsuki K, Ohnishi T, Lippmaa M, Usami N, et al. Jpn J Appl Phys 2003;42:2291. [14] Kelly MK, Vaudo RP, Phanse VM, Görgens L, Ambacher O. Jpn J Appl Phys 1999;38: L217. [15] Kumagai Y, Nagashima T, Koukitu A. Jpn J Appl Phys 2007;46:L389. [16] Takahashi N, Kaiya K, Omichi K, Nakamura T, Okamoto S, Yamamoto H. J Cryst Growth 2000;209:822. [17] Wang X, Iwaki H, Murakami M, Du X, Ishitani Y, Yoshikawa A. Jpn J Appl Phys 2003;42:L99. [18] Fons P, Iwata K, Yamada A, Matsubara K, Niki S. Appl Phys Lett 2000;77:1801. [19] Fujii T, Yoshii N, Masuda R, Tanabe T, Kamisawa A, Hosaka S, et al. J Cryst Growth 2009;311:1056.