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Low-temperature deposition of Si and SiO2 thin-film layers in an ultrahigh vacuum system
Journal of Crystal Growth 209 (2000) 331}334
Low-temperature deposition of Si and SiO thin-"lm layers 2 in an ultrahigh vacuum system K. Ohtsuka*, T. Yoshida, T. Oizumi, A. Murai, T. Kurabayashi, J. Nishizawa Telecommunications Advancement Organization of Japan, Sendai Research Center, Koeji 19, Nagamachi, Aoba-ku, Sendai 980-0868, Japan
Abstract Low-temperature chemical beam epitaxy of Si using hydride sources was investigated for fabricating thin-"lm layers. In the case of a Si H source, the temperature dependence of the growth rate is relatively small in the region of 2 6 300}6003C where the growth rate is dominated by desorption and/or decomposition at the surface. A SiO thin "lm was 2 also fabricated by a simultaneous supply of Si H and activated oxygen from a helicon plasma source. The temperature 2 6 dependence of the deposition rate is relatively small, which may correspond to the relatively small temperature dependence in the case of chemical beam epitaxy of Si in the same temperature region. Relatively low deposition rates, 5 nm/h for Si and 0.2}0.3 nm/min for SiO , were achieved. Atomic and molecular level controllability is at2 tained. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 52.75.Rx; 73.40.Qv; 81.05.Cy; 81.15.Hi Keywords: Silicon; Silicon dioxide; Chemical beam epitaxy; Active oxygen; Helicon plasma
1. Introduction Recent progress in Si ultralarge-scale-integrated circuits has necessitated smaller-scale fabrication techniques. The development of a low-temperature and low-damage processing technique is essential for the fabrication of microstructures because the change of the doping pro"le and electrical properties due to impurity di!usion and/or charged species should be avoided. Atomic or molecular level
* Corresponding author. Present address: Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan. Tel.: #81-6-6497-7086; fax: #81-6-6497-7295. E-mail address: [email protected] (K. Ohtsuka)
controllability is required for the deposition technique. Molecular layer epitaxy (MLE) of Si was performed by alternating the supply of SiH Cl 2 2 and H at a growth temperature over 8003C [1,2]. 2 Lowering the growth temperature to 540}6503C was achieved by using atomic hydrogen instead of H [3]. However, these temperatures are still too 2 high for fabricating microstructures. Research on atomic layer epitaxy (ALE) and MLE of SiO is 2 limited to complicated gas sources such as Si(NCO) and N(C H ) and gas species which 4 2 53 damage growth systems such as SiCl and H O 4 2 [4,5]. Special care should be taken when introducing these gas species into an ultrahigh vacuum deposition system. Taking this into account, the use of a deposition technique with atomic and molecular level controllability using the well-known
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hydrides, SiH and Si H , is important. Although 4 2 6 SiH and Si H were also used for ALE and MLE 4 2 6 of Si by alternating the supply of hydrides and desorption of hydrogen by heating, self-limiting one monolayer growth per growth cycle was not attained except for desorption using synchrotron radiation or a supply of thermally cracked silane [6}9]. Moreover, the deposition of Si-based material with atomic and molecular level controllability by continuous or simultaneous supply has not been thoroughly investigated. Lowering the deposition temperature of Si-based material is also required. Thus far, the investigation of SiO has been focused 2 on techniques using active species from remote plasma [10}12]. However, most of the research is limited to deposition at pressures higher than 1 Pa, which corresponds to deposition pressures under chemical vapor deposition (CVD) and results in a high deposition rate. The deposition of SiO with 2 atomic and molecular level controllability has not been thoroughly investigated. In this study, the fabrication of Si and SiO 2 thin-"lm layers in an ultrahigh vacuum system where deposition rates are dominated by surface reaction is examined. Si thin-"lm layers were grown by low-temperature chemical beam epitaxy. SiO 2 thin-"lm layers were fabricated using activated oxygen from a helicon plasma source. Helicon plasma is more useful for deposition under low supplied pressure and low in#uence of charged species compared to normal RF plasma and electron cyclotron resonance (ECR) assisted plasma.
2. Experimental procedure The deposition system employed is depicted in Fig. 1. It consists of a growth chamber and a loadlock chamber. The system was evacuated using a turbomolecular pump (480 l/s for growth chamber and 300 l/s for vent line) with a dry pump (1300 l/min). The base pressure of the growth chamber was less than 10~7 Pa. Substrates on a quartz holder were heated by a lamp heater. The substrate temperature was controlled using thermocouples at the susceptor. The temperatures described in this work are values measured at the substrate position when the Ta plate with ther-
Fig. 1. Schematic illustration of growth system.
mocouples was set on the quartz holder. The temperature at the substrate position was approximately 1003C higher than that at the susceptor. The gas #ow was controlled using a mass-#ow controller. SiH (99.9995%) and Si H (99.99%) were 4 2 6 introduced from a gas nozzle and O (99.99%) 2 from a gas nozzle or a helicon plasma source (ULVAC). Si(1 0 0) B-doped p-type wafers with the resistivity of &10 ) cm were used as substrates. For the measurements of deposited Si layer thickness, wafers with SiO selective masks were fabricated by 2 thermal oxidation and a photolithographic technique. Surface treatment was carried out by H SO : H O , NH OH : H O : H O and HCl : 2 4 2 2 4 2 2 2 H O : H O. A hydrogen-terminated Si surface 2 2 2 was obtained by diluted HF treatment prior to loading into the chamber [13]. After loading, the wafers were heated to 7503C to obtain a clean surface [14]. Deposition was performed at temperatures of 180}7503C and at pressures of 10~2}10~1 Pa. The thickness of SiO was evalu2 ated by spectroscopic ellipsometry measurements.
3. Results and discussion SiO masks enable the selective growth of Si, 2 except for depositions at 7503C, where polycrystalline Si is deposited on SiO . With the supply of 2 SiH and Si H , epitaxial growth is con"rmed 4 2 6 by re#ection high-energy electron di!raction
K. Ohtsuka et al. / Journal of Crystal Growth 209 (2000) 331}334
333
Fig. 3. In#uence of oxygen supply on the growth rate of Si. Fig. 2. Growth temperature dependence of the growth rate of Si for the SiH and Si H gas sources. 4 2 6
(RHEED) measurements. Fig. 2 shows the growth temperature dependence of the deposition rate of Si. In the higher temperature region, the deposition rate depends on the supplied pressures of SiH and 4 Si H . On the other hand, the deposition rate is 2 6 independent of the gas supply for the lower growth temperatures. Hence, deposition rates are dominated by gas supply and reaction at Si surfaces for high and low growth temperatures, respectively [15,16]. In the low-temperature region, a relatively small temperature dependence is observed for the Si H supply. Because of the large time constant of 2 6 decomposition of the Si}H bond, desorption and decomposition of hydrogen from Si H absorbed 2 6 on the Si surface are considered to dominate the growth rate. Based on the saturation of hydrogen coverage at the surface in the employed growth temperature and supplied Si H pressure [17], re2 6 actions such as desorption of hydrogen and decomposition from Si H adsorbed on Si surface are 2 6 considered to have little dependence on temperature. The in#uence of the O supply was investigated 2 in the case of using a Si H supply. Deposition of 2 6 SiO was not observed even at 7503C where poly2 crystalline Si is deposited on SiO . The RHEED 2 measurement reveals that the deposited layer on the Si substrate is single crystalline. The deposited Si thickness was measured by the same method as that used in the case of the Si layer deposited by the
supply of only silane. Fig. 3 shows the deposition rate of Si for a Si H supply, a simultaneous supply 2 6 of Si H and O , and an alternating supply of 2 6 2 Si H and O . In the alternating supply, the supply 2 6 2 time and evacuation time of each gas was 10 s, and the deposition rate was obtained from the thickness after 360 growth cycles, corresponding to a total Si H supply time of 1 h. The Si thickness was 2 6 decreased by both simultaneous and alternating O supply. The oxygen concentration in the layers 2 was measured by secondary ion mass spectrometry. The incorporation of oxygen is low, because the measured oxygen signal in the layer deposited by the supply of Si H and O is the same as in the Si 2 6 2 layer deposited by the supply of SiH or Si H . 4 2 6 These results indicate that an O supply prevents 2 adsorption of Si H and/or desorption of hydro2 6 gen from the surface and that an active oxygen species is required for deposition of SiO "lms. 2 Hence, the fabrication of SiO thin-"lm layers us2 ing activated oxygen from a helicon plasma source (13.56 MHz RF power of 200 W) was investigated. Oxidation of Si by activated oxygen from a helicon plasma source was also investigated. Fig. 4 shows the temperature dependence of the oxidation rate and the deposition rate of SiO . For 2 comparison, those by ECR assisted plasma are also shown in the "gure. The deposition rate below 1 nm/min is attained. Although the oxidation rate by helicon plasma is of the same order as ECRassisted plasma [18], the deposition rate by helicon
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K. Ohtsuka et al. / Journal of Crystal Growth 209 (2000) 331}334
4. Conclusions Si and SiO thin "lms were fabricated by 2 a Si H supply and a simultaneous supply of 2 6 Si H and activated oxygen from a helicon plasma 2 6 source, respectively. Fairly low deposition rates, 5 nm/h for Si and 0.2}0.3 nm/min for SiO , were 2 attained. Small temperature and pressure dependence of the deposition rate in the temperature region of 300}6003C is useful for atomic and molecular level controlled process. Fig. 4. Temperature dependence of the oxidation rate and the deposition rate of SiO . 2
References plasma is two orders of magnitude lower than that by ECR plasma [10]. It is suggested that activated oxygen is the species which limits the oxidation and deposition rate, except for the deposition by ECRassisted plasma where charged species may bring about the high deposition rate. The dependence on the temperature and the supplied pressures is fairly small. The deposition rate increased slightly with the lowering of deposition temperature. In remote plasma CVD, hydroxyls are reported to play an important role [12]. Hence, the incomplete decomposition of Si H on the Si surface at lower 2 6 temperatures is considered to generate hydroxyls and bring about the slight increase in the deposition rate. This small temperature dependence may be related to the temperature dependence of the Si thin-"lm deposition discussed above. For atomic and molecular level controlled growth, this temperature region is useful because the precise control of the pressure and growth temperature is not required. Electrical characterization was performed by a metal}oxide}semiconductor structure with Al electrodes for samples deposited at 480}5803C. Interface state densities were evaluated to be 3.5]1011/cm2 eV based on capacitance}voltage measurements at 1 MHz and the breakdown electric "eld was 10 MV/cm based on current}voltage characteristics.
[1] J. Nishizawa, K. Aoki, S. Suzuki, K. Kikuchi, J. Electrochem. Soc. 137 (1990) 1898. [2] J. Nishizawa, K. Aoki, S. Suzuki, K. Kikuchi, J. Cryst. Growth 99 (1990) 502. [3] S. Imai, T. Iizuka, O. Sugiura, M. Matsumura, Thin Solid Films 225 (1993) 168. [4] S.M. George, O. Sneh, A.C. Dillon, M.L. Wise, A.W. Ott, L.A. Okada, Appl. Surf. Sci. 82/83 (1994) 460. [5] W. Gasser, Y. Uchida, M. Matsumura, Thin Solid Films 250 (1994) 213. [6] J. Murota, M. Sakuraba, S. Ono, Appl. Phys. Lett. 62 (1993) 2353. [7] Y. Suda, M. Ishida, M. Yamashita, H. Ikeda, Appl. Surf. Sci. 82/83 (1994) 332. [8] H. Akazawa, Y. Utsumi, J. Appl. Phys. 78 (1995) 2724. [9] Y. Suda, Y. Misato, D. Shiratori, Jpn. J. Appl. Phys. 38 (1999) 2390. [10] S. Matsuo, M. Kiuchi, Jpn. J. Appl. Phys. 22 (1983) L210. [11] G. Lucovsky, Y. Ma, T. Yasuda, C. Silvestre, J.R. Hauser, Jpn. J. Appl. Phys. 31 (1992) 4387. [12] T. Fuyuki, T. Oka, H. Matsunami, Jpn. J. Appl. Phys. 33 (1994) 4417. [13] T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, Y. Nagasawa, J. Appl. Phys. 64 (1988) 3516. [14] C.M. Greenlief, M. Armstrong, J. Vac. Sci. Technol. B 13 (1995) 1810. [15] H. Hirayama, T. Tatsumi, N. Aizaki, Appl. Phys. Lett. 52 (1988) 1484. [16] F. Hirose, M. Suemitsu, N. Miyamoto, Jpn. J. Appl. Phys. 28 (1989) L2003. [17] Y. Takakuwa, T. Yamaguchi, N. Miyamoto, J. Cryst. Growth 136 (1994) 328. [18] T. Fuyuki, S. Muranaka, H. Matsunami, Appl. Surf. Sci. 117/118 (1997) 123.
Low-temperature deposition of Si and SiO thin-"lm layers 2 in an ultrahigh vacuum system K. Ohtsuka*, T. Yoshida, T. Oizumi, A. Murai, T. Kurabayashi, J. Nishizawa Telecommunications Advancement Organization of Japan, Sendai Research Center, Koeji 19, Nagamachi, Aoba-ku, Sendai 980-0868, Japan
Abstract Low-temperature chemical beam epitaxy of Si using hydride sources was investigated for fabricating thin-"lm layers. In the case of a Si H source, the temperature dependence of the growth rate is relatively small in the region of 2 6 300}6003C where the growth rate is dominated by desorption and/or decomposition at the surface. A SiO thin "lm was 2 also fabricated by a simultaneous supply of Si H and activated oxygen from a helicon plasma source. The temperature 2 6 dependence of the deposition rate is relatively small, which may correspond to the relatively small temperature dependence in the case of chemical beam epitaxy of Si in the same temperature region. Relatively low deposition rates, 5 nm/h for Si and 0.2}0.3 nm/min for SiO , were achieved. Atomic and molecular level controllability is at2 tained. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 52.75.Rx; 73.40.Qv; 81.05.Cy; 81.15.Hi Keywords: Silicon; Silicon dioxide; Chemical beam epitaxy; Active oxygen; Helicon plasma
1. Introduction Recent progress in Si ultralarge-scale-integrated circuits has necessitated smaller-scale fabrication techniques. The development of a low-temperature and low-damage processing technique is essential for the fabrication of microstructures because the change of the doping pro"le and electrical properties due to impurity di!usion and/or charged species should be avoided. Atomic or molecular level
* Corresponding author. Present address: Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan. Tel.: #81-6-6497-7086; fax: #81-6-6497-7295. E-mail address: [email protected] (K. Ohtsuka)
controllability is required for the deposition technique. Molecular layer epitaxy (MLE) of Si was performed by alternating the supply of SiH Cl 2 2 and H at a growth temperature over 8003C [1,2]. 2 Lowering the growth temperature to 540}6503C was achieved by using atomic hydrogen instead of H [3]. However, these temperatures are still too 2 high for fabricating microstructures. Research on atomic layer epitaxy (ALE) and MLE of SiO is 2 limited to complicated gas sources such as Si(NCO) and N(C H ) and gas species which 4 2 53 damage growth systems such as SiCl and H O 4 2 [4,5]. Special care should be taken when introducing these gas species into an ultrahigh vacuum deposition system. Taking this into account, the use of a deposition technique with atomic and molecular level controllability using the well-known
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 6 5 - 5
332
K. Ohtsuka et al. / Journal of Crystal Growth 209 (2000) 331}334
hydrides, SiH and Si H , is important. Although 4 2 6 SiH and Si H were also used for ALE and MLE 4 2 6 of Si by alternating the supply of hydrides and desorption of hydrogen by heating, self-limiting one monolayer growth per growth cycle was not attained except for desorption using synchrotron radiation or a supply of thermally cracked silane [6}9]. Moreover, the deposition of Si-based material with atomic and molecular level controllability by continuous or simultaneous supply has not been thoroughly investigated. Lowering the deposition temperature of Si-based material is also required. Thus far, the investigation of SiO has been focused 2 on techniques using active species from remote plasma [10}12]. However, most of the research is limited to deposition at pressures higher than 1 Pa, which corresponds to deposition pressures under chemical vapor deposition (CVD) and results in a high deposition rate. The deposition of SiO with 2 atomic and molecular level controllability has not been thoroughly investigated. In this study, the fabrication of Si and SiO 2 thin-"lm layers in an ultrahigh vacuum system where deposition rates are dominated by surface reaction is examined. Si thin-"lm layers were grown by low-temperature chemical beam epitaxy. SiO 2 thin-"lm layers were fabricated using activated oxygen from a helicon plasma source. Helicon plasma is more useful for deposition under low supplied pressure and low in#uence of charged species compared to normal RF plasma and electron cyclotron resonance (ECR) assisted plasma.
2. Experimental procedure The deposition system employed is depicted in Fig. 1. It consists of a growth chamber and a loadlock chamber. The system was evacuated using a turbomolecular pump (480 l/s for growth chamber and 300 l/s for vent line) with a dry pump (1300 l/min). The base pressure of the growth chamber was less than 10~7 Pa. Substrates on a quartz holder were heated by a lamp heater. The substrate temperature was controlled using thermocouples at the susceptor. The temperatures described in this work are values measured at the substrate position when the Ta plate with ther-
Fig. 1. Schematic illustration of growth system.
mocouples was set on the quartz holder. The temperature at the substrate position was approximately 1003C higher than that at the susceptor. The gas #ow was controlled using a mass-#ow controller. SiH (99.9995%) and Si H (99.99%) were 4 2 6 introduced from a gas nozzle and O (99.99%) 2 from a gas nozzle or a helicon plasma source (ULVAC). Si(1 0 0) B-doped p-type wafers with the resistivity of &10 ) cm were used as substrates. For the measurements of deposited Si layer thickness, wafers with SiO selective masks were fabricated by 2 thermal oxidation and a photolithographic technique. Surface treatment was carried out by H SO : H O , NH OH : H O : H O and HCl : 2 4 2 2 4 2 2 2 H O : H O. A hydrogen-terminated Si surface 2 2 2 was obtained by diluted HF treatment prior to loading into the chamber [13]. After loading, the wafers were heated to 7503C to obtain a clean surface [14]. Deposition was performed at temperatures of 180}7503C and at pressures of 10~2}10~1 Pa. The thickness of SiO was evalu2 ated by spectroscopic ellipsometry measurements.
3. Results and discussion SiO masks enable the selective growth of Si, 2 except for depositions at 7503C, where polycrystalline Si is deposited on SiO . With the supply of 2 SiH and Si H , epitaxial growth is con"rmed 4 2 6 by re#ection high-energy electron di!raction
K. Ohtsuka et al. / Journal of Crystal Growth 209 (2000) 331}334
333
Fig. 3. In#uence of oxygen supply on the growth rate of Si. Fig. 2. Growth temperature dependence of the growth rate of Si for the SiH and Si H gas sources. 4 2 6
(RHEED) measurements. Fig. 2 shows the growth temperature dependence of the deposition rate of Si. In the higher temperature region, the deposition rate depends on the supplied pressures of SiH and 4 Si H . On the other hand, the deposition rate is 2 6 independent of the gas supply for the lower growth temperatures. Hence, deposition rates are dominated by gas supply and reaction at Si surfaces for high and low growth temperatures, respectively [15,16]. In the low-temperature region, a relatively small temperature dependence is observed for the Si H supply. Because of the large time constant of 2 6 decomposition of the Si}H bond, desorption and decomposition of hydrogen from Si H absorbed 2 6 on the Si surface are considered to dominate the growth rate. Based on the saturation of hydrogen coverage at the surface in the employed growth temperature and supplied Si H pressure [17], re2 6 actions such as desorption of hydrogen and decomposition from Si H adsorbed on Si surface are 2 6 considered to have little dependence on temperature. The in#uence of the O supply was investigated 2 in the case of using a Si H supply. Deposition of 2 6 SiO was not observed even at 7503C where poly2 crystalline Si is deposited on SiO . The RHEED 2 measurement reveals that the deposited layer on the Si substrate is single crystalline. The deposited Si thickness was measured by the same method as that used in the case of the Si layer deposited by the
supply of only silane. Fig. 3 shows the deposition rate of Si for a Si H supply, a simultaneous supply 2 6 of Si H and O , and an alternating supply of 2 6 2 Si H and O . In the alternating supply, the supply 2 6 2 time and evacuation time of each gas was 10 s, and the deposition rate was obtained from the thickness after 360 growth cycles, corresponding to a total Si H supply time of 1 h. The Si thickness was 2 6 decreased by both simultaneous and alternating O supply. The oxygen concentration in the layers 2 was measured by secondary ion mass spectrometry. The incorporation of oxygen is low, because the measured oxygen signal in the layer deposited by the supply of Si H and O is the same as in the Si 2 6 2 layer deposited by the supply of SiH or Si H . 4 2 6 These results indicate that an O supply prevents 2 adsorption of Si H and/or desorption of hydro2 6 gen from the surface and that an active oxygen species is required for deposition of SiO "lms. 2 Hence, the fabrication of SiO thin-"lm layers us2 ing activated oxygen from a helicon plasma source (13.56 MHz RF power of 200 W) was investigated. Oxidation of Si by activated oxygen from a helicon plasma source was also investigated. Fig. 4 shows the temperature dependence of the oxidation rate and the deposition rate of SiO . For 2 comparison, those by ECR assisted plasma are also shown in the "gure. The deposition rate below 1 nm/min is attained. Although the oxidation rate by helicon plasma is of the same order as ECRassisted plasma [18], the deposition rate by helicon
334
K. Ohtsuka et al. / Journal of Crystal Growth 209 (2000) 331}334
4. Conclusions Si and SiO thin "lms were fabricated by 2 a Si H supply and a simultaneous supply of 2 6 Si H and activated oxygen from a helicon plasma 2 6 source, respectively. Fairly low deposition rates, 5 nm/h for Si and 0.2}0.3 nm/min for SiO , were 2 attained. Small temperature and pressure dependence of the deposition rate in the temperature region of 300}6003C is useful for atomic and molecular level controlled process. Fig. 4. Temperature dependence of the oxidation rate and the deposition rate of SiO . 2
References plasma is two orders of magnitude lower than that by ECR plasma [10]. It is suggested that activated oxygen is the species which limits the oxidation and deposition rate, except for the deposition by ECRassisted plasma where charged species may bring about the high deposition rate. The dependence on the temperature and the supplied pressures is fairly small. The deposition rate increased slightly with the lowering of deposition temperature. In remote plasma CVD, hydroxyls are reported to play an important role [12]. Hence, the incomplete decomposition of Si H on the Si surface at lower 2 6 temperatures is considered to generate hydroxyls and bring about the slight increase in the deposition rate. This small temperature dependence may be related to the temperature dependence of the Si thin-"lm deposition discussed above. For atomic and molecular level controlled growth, this temperature region is useful because the precise control of the pressure and growth temperature is not required. Electrical characterization was performed by a metal}oxide}semiconductor structure with Al electrodes for samples deposited at 480}5803C. Interface state densities were evaluated to be 3.5]1011/cm2 eV based on capacitance}voltage measurements at 1 MHz and the breakdown electric "eld was 10 MV/cm based on current}voltage characteristics.
[1] J. Nishizawa, K. Aoki, S. Suzuki, K. Kikuchi, J. Electrochem. Soc. 137 (1990) 1898. [2] J. Nishizawa, K. Aoki, S. Suzuki, K. Kikuchi, J. Cryst. Growth 99 (1990) 502. [3] S. Imai, T. Iizuka, O. Sugiura, M. Matsumura, Thin Solid Films 225 (1993) 168. [4] S.M. George, O. Sneh, A.C. Dillon, M.L. Wise, A.W. Ott, L.A. Okada, Appl. Surf. Sci. 82/83 (1994) 460. [5] W. Gasser, Y. Uchida, M. Matsumura, Thin Solid Films 250 (1994) 213. [6] J. Murota, M. Sakuraba, S. Ono, Appl. Phys. Lett. 62 (1993) 2353. [7] Y. Suda, M. Ishida, M. Yamashita, H. Ikeda, Appl. Surf. Sci. 82/83 (1994) 332. [8] H. Akazawa, Y. Utsumi, J. Appl. Phys. 78 (1995) 2724. [9] Y. Suda, Y. Misato, D. Shiratori, Jpn. J. Appl. Phys. 38 (1999) 2390. [10] S. Matsuo, M. Kiuchi, Jpn. J. Appl. Phys. 22 (1983) L210. [11] G. Lucovsky, Y. Ma, T. Yasuda, C. Silvestre, J.R. Hauser, Jpn. J. Appl. Phys. 31 (1992) 4387. [12] T. Fuyuki, T. Oka, H. Matsunami, Jpn. J. Appl. Phys. 33 (1994) 4417. [13] T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, Y. Nagasawa, J. Appl. Phys. 64 (1988) 3516. [14] C.M. Greenlief, M. Armstrong, J. Vac. Sci. Technol. B 13 (1995) 1810. [15] H. Hirayama, T. Tatsumi, N. Aizaki, Appl. Phys. Lett. 52 (1988) 1484. [16] F. Hirose, M. Suemitsu, N. Miyamoto, Jpn. J. Appl. Phys. 28 (1989) L2003. [17] Y. Takakuwa, T. Yamaguchi, N. Miyamoto, J. Cryst. Growth 136 (1994) 328. [18] T. Fuyuki, S. Muranaka, H. Matsunami, Appl. Surf. Sci. 117/118 (1997) 123.