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Initial stages of InN thin film growth onto MgAl2O4(1 1 1) and α-Al2O3(0 0·1) substrates
Journal of Crystal Growth 220 (2000) 191}196
Initial stages of InN thin "lm growth onto MgAl O (1 1 1) and a-Al O (0 0 ) 1) substrates Tohru Tsuchiya*, Masato Ohnishi, Akihiro Wakahara, Akira Yoshida Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan Received 19 January 2000; accepted 16 May 2000 Communicated by G.B. Stringfellow
Abstract The initial growth stage of InN on both a-Al O (0 0 ) 1) and MgAl O (1 1 1) substrates has been investigated. Both the re#ection high-energy electron di!raction (RHEED) and the atomic force microscope (AFM) measurements indicated that the growth of InN on a-Al O (0 0 ) 1) was three-dimensional, but was two-dimensional on MgAl O (1 1 1). 2000 Published by Elsevier Science B.V. Keywords: InN; a-Al O (0 0 ) 1); MgAl O (1 1 1); Initial stage
1. Introduction Group-III nitride alloy semiconductors have received much attention for use in optoelectronic and electronic devices, because of their wide bandgaps [1]. Though blue light-emitting diodes and blue lasers are realized using GaInN [2}4], the quality of the InN layers is not su$ciently high to obtain red-light emission due to the extremely low dissociation temperature of InN. Moreover, the lattice mismatch between InN and a-Al O (0 0 ) 1) sub strates, which are widely used, is about 25% [5}13] and this large mismatch results in an extremely high density of structural defects [11,12]. We have been investigating the epitaxial growth of InN by using microwave-excited metalorganic vapor-phase epitaxy (MEMOVPE), in which active nitrogen species are supplied by the remote-plasma technique, and have reported that the InN epitaxial
* Corresponding author.
layer can be obtained on various substrates, such as a-Al O , GaAs, GaP, and MgAl O [11}16]. On the basis of our preliminary results, the crystalline quality of InN on a MgAl O (1 1 1) substrate is considered comparable to that of InN grown on a-Al O (0 0 ) 1) [16], but the details of the initial stage of the growth have not been thoroughly investigated. In this study, we have investigated the initial stages of InN growth on both MgAl O (1 1 1) and a-Al O (0 0 ) 1) substrates, and compared the re sults in order to obtain high-quality InN layers.
2. Experiments InN was grown by microwave-excited metalorganic vapor-phase epitaxy, in which nitrogen remote plasma excited by 2.45 GHz of microwave radiation and trimethylindium (TMIn) were used as the nitrogen and In sources, respectively. Details of the growth system have been described in
0022-0248/00/$ - see front matter 2000 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 4 8 - 0
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previous publications [11}13]. a-Al O (0 0 ) 1) and MgAl O (1 1 1) wafers were used as the substrates. Prior to the growth, the a-Al O (0 0 ) 1) substrates were ultrasonically cleaned in organic solution and then etched in a hot H PO #H SO solution, but no etching processes were adopted for the MgAl O (1 1 1) substrates. The substrates were thermally cleaned in a pure hydrogen atmosphere at 7503C for 30 min and then nitrided downstream of the nitrogen remote plasma. During the adjustment of the growth conditions, the substrates were kept in a nitrogen atmosphere to avoid further nitridation. The experimental conditions are listed in Table 1. The conditions of nitridation were the optimum conditions for a-Al O substrates [13]. The epitaxial growth was carried out at the temperature of 4503C with the microwave power of 150 W. The crystalline quality of the InN layer was examined by re#ection high-energy electron di!raction (RHEED), recorded using an imaging plate (IP) [17}19]. The recorded images were numerically read out using a BAS2500 system (FUJI PHOTO FILM Co. Ltd.) for the detailed analysis of the RHEED. The surface morphology of InN was observed using an atomic force microscope (AFM).
3. Results and discussion Fig. 1 shows RHEED patterns of InN layers grown on a-Al O (0 0 ) 1) and MgAl O (1 1 1) sub Table 1 Experimental conditions Thermal cleaning
Nitridation
Epitaxial growth
Temperature Atmosphere Cleaning time Pressure Temperature Nitridation time Microwave power Excited N #ow rate Pressure Temperature Microwave power Excited N #ow rate TMI #ow rate Pressure
7003C Hydrogen 30 min 133 Pa 7003C 10 min 150 W 4.47 mmol/min 133 Pa 4503C 150 W 8.93 mmol/min 1.18 lmol/min 133 Pa
strates with various layer thicknesses. Here, the layer thickness indicates the amount of the grown InN. In the case of InN/a-Al O (0 0 ) 1) with the layer thickness below 2 nm, the RHEED pattern consists of short arcs and Debye}Scherrer rings, indicating that the InN layer is polycrystalline. With increasing thickness, the RHEED pattern becomes a spot pattern with weak spots observed between the main bright spots. This means that the InN layer is single crystalline, but the layer contains a domain which has a di!erent epitaxial arrangement, as reported previously [11,12]. These results indicate that InN nuclei with speci"c orientation grow preferentially, and coalesce to form the InN layer. On the other hand, in the case of InN/MgAl O (1 1 1), the RHEED pattern is a streak pattern even though the layer thickness is 1 nm. The results indicate that the InN growth is epitaxial in the initial stage. Fig. 2 shows AFM images of the initial stage of InN growth on both a-Al O (0 0 ) 1) and MgAl O (1 1 1) substrates. In the case of InN/ a-Al O (0 0 ) 1), relatively large three-dimensional (3D) islands of InN are clearly seen on the surface. For a layer thickness of 1 nm, the root-mean-square (RMS) value of the surface roughness is 1.5 nm, indicating the sparse nucleation of InN islands. The island size increases with increasing layer thickness. For a layer thickness greater than 10 nm, the RMS value decreases with increasing layer thickness, which indicates that the islands coalesce and form a layer structure (Fig. 3). On the other hand, for InN/MgAl O (1 1 1), it appears to be a layered structure, because the RMS value is less than the layer thickness and the value is almost constant. The results indicate that InN can be two-dimensionally grown on MgAl O . In order to clarify the mechanism of two-dimensional (2D) growth of InN on MgAl O (1 1 1), the di!erence in lattice mismatch between the InN layer and the substrates is considered. The crystallographic orientation relationships between the InN and the a-Al O (0 0 ) 1) substrate are InN(0 0 ) 1)#a-Al O (0 0 ) 1) and InN[1 1 ) 0]#a Al O [1 0 ) 0] [11,12] as shown in Fig. 4(a). The a-axis of InN rotates 303 from that of the hexagonal surface lattice of the a-Al O (0 0 ) 1) substrate. In this case, the lattice mismatch is slightly reduced
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
193
Fig. 1. RHEED patterns of InN initial layers grown on (a) a-Al O (0 0 ) 1) and (b) MgAl O (1 1 1) substrates.
from 29%, which is for InN/a-Al O , because the substrate surface was converted into AlON by the nitridation before the growth [20]. On the other hand, the crystallographic orientation relation-
ships between the InN and the MgAl O (1 1 1) substrate are InN(0 0 ) 1)#MgAl O (1 1 1) and InN[1 1 ) 0]#MgAl O [1 0 0] [16], as shown in Fig. 4(b). In this case, the a-axis of InN also rotates
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T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
Fig. 2. AFM images of InN initial layers grown on (a) a-Al O (0 0 ) 1) and (b) MgAl O (1 1 1) substrates.
Fig. 3. Root-mean-square (RMS) value of the surface roughness as a function of the InN layer thickness. Filled circles and triangles indicate InN/a-Al O (0 0 ) 1) and InN/MgAl O (1 1 1), respectively.
303 from that of the quasi-hexagonal surface lattice of the MgAl O (1 1 1) substrates and the lattice mismatch is 15%. Moreover, if the Mg atoms under the oxygen top layer act as an e!ective positioning sites as well as oxygen sites, the lattice mismatch becomes 7%. The value is much smaller than that for a-Al O (0 0 ) 1). In addition to the smaller lat tice mismatch, the lattice point of the InN is better attached to the surface lattice of the MgAl O (1 1 1) than in the case of a-Al O (0 0 ) 1). These results indicate that In atoms adsorbed on the MgAl O (1 1 1) substrate can "nd the nuclea tion sites more easily than those on the a-Al O (0 0 ) 1) substrate. In the case of GaN/a Al O (0 0 ) 1), it has been reported that the wetta bility of Ga is poor on a-Al O (0 0 ) 1), and thus, the nucleation sites decrease and large islands are formed in the initial stage of growth [21]. The identical situation may occur in the case of InN/aAl O (0 0 ) 1). On the basis of these results and previous research, we consider the potential of both substrates
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
195
InN layers will be obtained if the MgAl O (1 1 1) substrate is nitrided under optimum conditions. Accordingly, for InN epitaxial growth, it is believed that a MgAl O (1 1 1) substrate is more suitable than an a-Al O (0 0 ) 1) substrate.
4. Conclusions The initial growth stage of InN on both a-Al O (0 0 ) 1) and MgAl O (1 1 1) substrates has been investigated. Both RHEED and AFM measurements indicated that the growth of InN on a-Al O (0 0 ) 1) was three-dimensional, but was two-dimensional on MgAl O (1 1 1).
Acknowledgements The authors acknowledge the support of FUJI PHOTO FILM Co. Ltd., for providing the IP and analyzing systems. This work was supported in part by a Scienti"c Research Grant-in-Aid (C10450128) from the Ministry of Education, Science, Sports and Culture of Japan, and the Yazaki Memorial Foundation for Science and Technology. Fig. 4. Crystallographic relations between the InN epitaxial layer and (a) a-Al O (0 0 ) 1), and (b) MgAl O (1 1 1) substrates.
References
in terms of InN epitaxial layer growth. In the case of InN/a-Al O (0 0 ) 1), the single-crystalline InN was not obtained without substrate nitridation [13]. However, in the case of InN/MgAl O (1 1 1), single-crystalline InN was obtained without nitridation [22]. Because the single-crystalline InN is obtained without nitridation, the direction of nucleation can be controlled in the epitaxial growth of InN. In addition, in the case of InN/a-Al O , the InN layer was polycrystalline with substrate nitridation in the initial stage. On the other hand, in the case of InN/MgAl O , although the sub strates were nitrided with the assumption of a-Al O (0 0 ) 1), the InN layer was single-crystal line in the initial stage. Therefore, higher quality
[1] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. [2] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [3] S. Nakamura, T. Matsushita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 69 (1996) 3034. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 2014. [5] D. Feiler, R.S. Williams, A.A. Talin, H. Yoon, M.S. Goorsky, J. Crystal Growth 171 (1997) 12. [6] Y. Sato, S. Sato, J. Crystal Growth 144 (1994) 15. [7] N. Takahashi, J. Ogasawara, A. Koukitu, J. Crystal Growth 172 (1997) 298. [8] M. Sato, Jpn. J. Appl. Phys. 36 (1997) L595. [9] T.J. Kistenmacher, W.A. Bryden, J.S. Morgan, T.O. Poehler, J. Appl. Phys. 68 (1990) 1541. [10] N. Takahashi, R. Matsumoto, A. Koukiku, H. Seki, Jpn. J. Appl. Phys. 36 (1997) L743. [11] A. Wakahara, A. Yoshida, Appl. Phys. Lett. 54 (1989) 709.
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T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
[12] A. Wakahara, T. Tsuchiya, A. Yoshida, J. Crystal Growth 99 (1990) 385. [13] T. Tsuchiya, H. Yamano, O. Miki, A. Wakahara, A. Yoshida, Jpn. J. Appl. Phys. 38 (1999) 1884. [14] Q.X. Guo, T. Yamamura, A. Yoshida, N. Itoh, J. Appl. Phys. 75 (1994) 4927. [15] Q.X. Guo, H. Ogawa, A. Yoshida, Appl. Phys. Lett. 66 (1995) 715. [16] T. Tsuchiya, O. Miki, K. Shimada, M. Ohnishi, A. Wakahara, A. Yoshida, J. Crystal Growth 220 (2000) 185.
[17] Y. Amemiya, J. Miyahara, Nature 336 (1988) 89. [18] Y. Iwabuchi, N. Mori, K. Takahashi, T. Matsuda, S. Shionoya, Jpn. J. Appl. Phys. 33 (1994) 178. [19] H. Ayato, N. Mori, J. Miyahara, T. Oikawa, J. Electron Microsc. 39 (1990) 444. [20] K. Uchida, A. Watanabe, F. Yano, M. Kouguchi, T. Tanaka, S. Minagawa, J. Appl. Phys. 79 (1996) 3487. [21] Ju.V. Naidich, Ju.N. Chuvashov, J. Mater. Sci. 18 (1993) 2071. [22] T. Tsuchiya, A. Wakahara, A. Yoshida, in preparation for publication.
Initial stages of InN thin "lm growth onto MgAl O (1 1 1) and a-Al O (0 0 ) 1) substrates Tohru Tsuchiya*, Masato Ohnishi, Akihiro Wakahara, Akira Yoshida Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan Received 19 January 2000; accepted 16 May 2000 Communicated by G.B. Stringfellow
Abstract The initial growth stage of InN on both a-Al O (0 0 ) 1) and MgAl O (1 1 1) substrates has been investigated. Both the re#ection high-energy electron di!raction (RHEED) and the atomic force microscope (AFM) measurements indicated that the growth of InN on a-Al O (0 0 ) 1) was three-dimensional, but was two-dimensional on MgAl O (1 1 1). 2000 Published by Elsevier Science B.V. Keywords: InN; a-Al O (0 0 ) 1); MgAl O (1 1 1); Initial stage
1. Introduction Group-III nitride alloy semiconductors have received much attention for use in optoelectronic and electronic devices, because of their wide bandgaps [1]. Though blue light-emitting diodes and blue lasers are realized using GaInN [2}4], the quality of the InN layers is not su$ciently high to obtain red-light emission due to the extremely low dissociation temperature of InN. Moreover, the lattice mismatch between InN and a-Al O (0 0 ) 1) sub strates, which are widely used, is about 25% [5}13] and this large mismatch results in an extremely high density of structural defects [11,12]. We have been investigating the epitaxial growth of InN by using microwave-excited metalorganic vapor-phase epitaxy (MEMOVPE), in which active nitrogen species are supplied by the remote-plasma technique, and have reported that the InN epitaxial
* Corresponding author.
layer can be obtained on various substrates, such as a-Al O , GaAs, GaP, and MgAl O [11}16]. On the basis of our preliminary results, the crystalline quality of InN on a MgAl O (1 1 1) substrate is considered comparable to that of InN grown on a-Al O (0 0 ) 1) [16], but the details of the initial stage of the growth have not been thoroughly investigated. In this study, we have investigated the initial stages of InN growth on both MgAl O (1 1 1) and a-Al O (0 0 ) 1) substrates, and compared the re sults in order to obtain high-quality InN layers.
2. Experiments InN was grown by microwave-excited metalorganic vapor-phase epitaxy, in which nitrogen remote plasma excited by 2.45 GHz of microwave radiation and trimethylindium (TMIn) were used as the nitrogen and In sources, respectively. Details of the growth system have been described in
0022-0248/00/$ - see front matter 2000 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 4 8 - 0
192
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
previous publications [11}13]. a-Al O (0 0 ) 1) and MgAl O (1 1 1) wafers were used as the substrates. Prior to the growth, the a-Al O (0 0 ) 1) substrates were ultrasonically cleaned in organic solution and then etched in a hot H PO #H SO solution, but no etching processes were adopted for the MgAl O (1 1 1) substrates. The substrates were thermally cleaned in a pure hydrogen atmosphere at 7503C for 30 min and then nitrided downstream of the nitrogen remote plasma. During the adjustment of the growth conditions, the substrates were kept in a nitrogen atmosphere to avoid further nitridation. The experimental conditions are listed in Table 1. The conditions of nitridation were the optimum conditions for a-Al O substrates [13]. The epitaxial growth was carried out at the temperature of 4503C with the microwave power of 150 W. The crystalline quality of the InN layer was examined by re#ection high-energy electron di!raction (RHEED), recorded using an imaging plate (IP) [17}19]. The recorded images were numerically read out using a BAS2500 system (FUJI PHOTO FILM Co. Ltd.) for the detailed analysis of the RHEED. The surface morphology of InN was observed using an atomic force microscope (AFM).
3. Results and discussion Fig. 1 shows RHEED patterns of InN layers grown on a-Al O (0 0 ) 1) and MgAl O (1 1 1) sub Table 1 Experimental conditions Thermal cleaning
Nitridation
Epitaxial growth
Temperature Atmosphere Cleaning time Pressure Temperature Nitridation time Microwave power Excited N #ow rate Pressure Temperature Microwave power Excited N #ow rate TMI #ow rate Pressure
7003C Hydrogen 30 min 133 Pa 7003C 10 min 150 W 4.47 mmol/min 133 Pa 4503C 150 W 8.93 mmol/min 1.18 lmol/min 133 Pa
strates with various layer thicknesses. Here, the layer thickness indicates the amount of the grown InN. In the case of InN/a-Al O (0 0 ) 1) with the layer thickness below 2 nm, the RHEED pattern consists of short arcs and Debye}Scherrer rings, indicating that the InN layer is polycrystalline. With increasing thickness, the RHEED pattern becomes a spot pattern with weak spots observed between the main bright spots. This means that the InN layer is single crystalline, but the layer contains a domain which has a di!erent epitaxial arrangement, as reported previously [11,12]. These results indicate that InN nuclei with speci"c orientation grow preferentially, and coalesce to form the InN layer. On the other hand, in the case of InN/MgAl O (1 1 1), the RHEED pattern is a streak pattern even though the layer thickness is 1 nm. The results indicate that the InN growth is epitaxial in the initial stage. Fig. 2 shows AFM images of the initial stage of InN growth on both a-Al O (0 0 ) 1) and MgAl O (1 1 1) substrates. In the case of InN/ a-Al O (0 0 ) 1), relatively large three-dimensional (3D) islands of InN are clearly seen on the surface. For a layer thickness of 1 nm, the root-mean-square (RMS) value of the surface roughness is 1.5 nm, indicating the sparse nucleation of InN islands. The island size increases with increasing layer thickness. For a layer thickness greater than 10 nm, the RMS value decreases with increasing layer thickness, which indicates that the islands coalesce and form a layer structure (Fig. 3). On the other hand, for InN/MgAl O (1 1 1), it appears to be a layered structure, because the RMS value is less than the layer thickness and the value is almost constant. The results indicate that InN can be two-dimensionally grown on MgAl O . In order to clarify the mechanism of two-dimensional (2D) growth of InN on MgAl O (1 1 1), the di!erence in lattice mismatch between the InN layer and the substrates is considered. The crystallographic orientation relationships between the InN and the a-Al O (0 0 ) 1) substrate are InN(0 0 ) 1)#a-Al O (0 0 ) 1) and InN[1 1 ) 0]#a Al O [1 0 ) 0] [11,12] as shown in Fig. 4(a). The a-axis of InN rotates 303 from that of the hexagonal surface lattice of the a-Al O (0 0 ) 1) substrate. In this case, the lattice mismatch is slightly reduced
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
193
Fig. 1. RHEED patterns of InN initial layers grown on (a) a-Al O (0 0 ) 1) and (b) MgAl O (1 1 1) substrates.
from 29%, which is for InN/a-Al O , because the substrate surface was converted into AlON by the nitridation before the growth [20]. On the other hand, the crystallographic orientation relation-
ships between the InN and the MgAl O (1 1 1) substrate are InN(0 0 ) 1)#MgAl O (1 1 1) and InN[1 1 ) 0]#MgAl O [1 0 0] [16], as shown in Fig. 4(b). In this case, the a-axis of InN also rotates
194
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
Fig. 2. AFM images of InN initial layers grown on (a) a-Al O (0 0 ) 1) and (b) MgAl O (1 1 1) substrates.
Fig. 3. Root-mean-square (RMS) value of the surface roughness as a function of the InN layer thickness. Filled circles and triangles indicate InN/a-Al O (0 0 ) 1) and InN/MgAl O (1 1 1), respectively.
303 from that of the quasi-hexagonal surface lattice of the MgAl O (1 1 1) substrates and the lattice mismatch is 15%. Moreover, if the Mg atoms under the oxygen top layer act as an e!ective positioning sites as well as oxygen sites, the lattice mismatch becomes 7%. The value is much smaller than that for a-Al O (0 0 ) 1). In addition to the smaller lat tice mismatch, the lattice point of the InN is better attached to the surface lattice of the MgAl O (1 1 1) than in the case of a-Al O (0 0 ) 1). These results indicate that In atoms adsorbed on the MgAl O (1 1 1) substrate can "nd the nuclea tion sites more easily than those on the a-Al O (0 0 ) 1) substrate. In the case of GaN/a Al O (0 0 ) 1), it has been reported that the wetta bility of Ga is poor on a-Al O (0 0 ) 1), and thus, the nucleation sites decrease and large islands are formed in the initial stage of growth [21]. The identical situation may occur in the case of InN/aAl O (0 0 ) 1). On the basis of these results and previous research, we consider the potential of both substrates
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
195
InN layers will be obtained if the MgAl O (1 1 1) substrate is nitrided under optimum conditions. Accordingly, for InN epitaxial growth, it is believed that a MgAl O (1 1 1) substrate is more suitable than an a-Al O (0 0 ) 1) substrate.
4. Conclusions The initial growth stage of InN on both a-Al O (0 0 ) 1) and MgAl O (1 1 1) substrates has been investigated. Both RHEED and AFM measurements indicated that the growth of InN on a-Al O (0 0 ) 1) was three-dimensional, but was two-dimensional on MgAl O (1 1 1).
Acknowledgements The authors acknowledge the support of FUJI PHOTO FILM Co. Ltd., for providing the IP and analyzing systems. This work was supported in part by a Scienti"c Research Grant-in-Aid (C10450128) from the Ministry of Education, Science, Sports and Culture of Japan, and the Yazaki Memorial Foundation for Science and Technology. Fig. 4. Crystallographic relations between the InN epitaxial layer and (a) a-Al O (0 0 ) 1), and (b) MgAl O (1 1 1) substrates.
References
in terms of InN epitaxial layer growth. In the case of InN/a-Al O (0 0 ) 1), the single-crystalline InN was not obtained without substrate nitridation [13]. However, in the case of InN/MgAl O (1 1 1), single-crystalline InN was obtained without nitridation [22]. Because the single-crystalline InN is obtained without nitridation, the direction of nucleation can be controlled in the epitaxial growth of InN. In addition, in the case of InN/a-Al O , the InN layer was polycrystalline with substrate nitridation in the initial stage. On the other hand, in the case of InN/MgAl O , although the sub strates were nitrided with the assumption of a-Al O (0 0 ) 1), the InN layer was single-crystal line in the initial stage. Therefore, higher quality
[1] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. [2] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [3] S. Nakamura, T. Matsushita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 69 (1996) 3034. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 2014. [5] D. Feiler, R.S. Williams, A.A. Talin, H. Yoon, M.S. Goorsky, J. Crystal Growth 171 (1997) 12. [6] Y. Sato, S. Sato, J. Crystal Growth 144 (1994) 15. [7] N. Takahashi, J. Ogasawara, A. Koukitu, J. Crystal Growth 172 (1997) 298. [8] M. Sato, Jpn. J. Appl. Phys. 36 (1997) L595. [9] T.J. Kistenmacher, W.A. Bryden, J.S. Morgan, T.O. Poehler, J. Appl. Phys. 68 (1990) 1541. [10] N. Takahashi, R. Matsumoto, A. Koukiku, H. Seki, Jpn. J. Appl. Phys. 36 (1997) L743. [11] A. Wakahara, A. Yoshida, Appl. Phys. Lett. 54 (1989) 709.
196
T. Tsuchiya et al. / Journal of Crystal Growth 220 (2000) 191}196
[12] A. Wakahara, T. Tsuchiya, A. Yoshida, J. Crystal Growth 99 (1990) 385. [13] T. Tsuchiya, H. Yamano, O. Miki, A. Wakahara, A. Yoshida, Jpn. J. Appl. Phys. 38 (1999) 1884. [14] Q.X. Guo, T. Yamamura, A. Yoshida, N. Itoh, J. Appl. Phys. 75 (1994) 4927. [15] Q.X. Guo, H. Ogawa, A. Yoshida, Appl. Phys. Lett. 66 (1995) 715. [16] T. Tsuchiya, O. Miki, K. Shimada, M. Ohnishi, A. Wakahara, A. Yoshida, J. Crystal Growth 220 (2000) 185.
[17] Y. Amemiya, J. Miyahara, Nature 336 (1988) 89. [18] Y. Iwabuchi, N. Mori, K. Takahashi, T. Matsuda, S. Shionoya, Jpn. J. Appl. Phys. 33 (1994) 178. [19] H. Ayato, N. Mori, J. Miyahara, T. Oikawa, J. Electron Microsc. 39 (1990) 444. [20] K. Uchida, A. Watanabe, F. Yano, M. Kouguchi, T. Tanaka, S. Minagawa, J. Appl. Phys. 79 (1996) 3487. [21] Ju.V. Naidich, Ju.N. Chuvashov, J. Mater. Sci. 18 (1993) 2071. [22] T. Tsuchiya, A. Wakahara, A. Yoshida, in preparation for publication.