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GaN-MOVPE growth and its microscopic chemistry of gaseous phase by computational thermodynamic analysis
Journal of Crystal Growth 237–239 (2002) 931–935
GaN-MOVPE growth and its microscopic chemistry of gaseous phase by computational thermodynamic analysis A. Hirakoa, M. Yoshitania, M. Nishibayashia, Y. Nishikawab, K. Ohkawaa,b,* a b
Department of Applied Physics, Faculty of Science, Science University of Tokyo, Kagurazaka 1-3, Shinjuku, Tokyo 162-8601, Japan Gonokami Cooperative Excitation Project, Exploratory Research for Advanced Technology, Japan Science and Technology Corporation, Kagurazaka 1-3, Shinjuku, Tokyo 162-8601, Japan
Abstract We studied microscopic chemistry of gaseous phase in GaN growth by computational thermodynamic analysis of metalorganic vapor phase epitaxy with two- and three-flow methods. Correlations between quality of GaN layers and gaseous phase chemistry were found from the computational analysis. It was confirmed that laminar flow on a substrate during growth was necessary to obtain a high-quality GaN layer in spite of high growth temperature. Optimum decomposed-species V/III ratio (NH2/GaCH3) were considered in the range of 10002000 to achieve high electron mobility more than 200 cm2/V s. Two-flow method was easier to achieve the optimum condition than three-flow method both in experiments and in computational analysis. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea Keywords: A1. Computer simulation; A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction As the melting point of GaN and equilibrium vapor pressure of nitrogen are extremely high [1], epitaxial techniques such as molecular beam epitaxy (MBE) [2,3] and metalorganic vapor phase epitaxy (MOVPE) [4–6] are useful to grow GaN layers. Even by MOVPE, however, it is not easy to obtain high-quality GaN layers. Several groups have succeeded in achieving high-quality nitridesbased laser diodes [7], therefore, it implies obviously that an optimum growth condition *Corresponding author. Department of Applied Physics, Faculty of Science, Science University of Tokyo, Kagurazaka 13, Shinjuku, Tokyo 162-8601, Japan. Tel.: +81-3-3260-4280; fax: +81-3-3260-4280. E-mail address: [email protected] (K. Ohkawa).
exists in MOVPE growth for nitrides. Chemical and physical properties in gaseous phase during GaN-MOVPE growth are not clear yet. In this paper, we compare the quality of GaN layers grown by MOVPE with chemical property in gaseous phase from computational analysis in order to find out microscopic chemical and physical properties in gas phase just on a substrate under the optimum growth condition. We have introduced different types of MOVPE such as twoand three-flow methods to realize various chemical situations of gaseous phase during growth.
2. Experiment Undoped GaN layers were grown by atmospheric pressure MOVPE with two- and three-flow
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 9 9 9 - 6
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A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
methods. The two-flow method consists of a horizontal main-flow channel and a vertical subflow channel to suppress thermal convection [6]. These channels are made from quartz. The mainflow channel supplies trimethylgallium (TMG) and ammonia (NH3) which are carried by H2 and N2 gases. The sub-flow consists of H2 and N2. In the case of three-flow method, the main-flow channel has two lines, i.e. an upper TMG line and a lower NH3 line. GaN layers were grown with various conditions of main-flow velocity (0.51.5 m/s), growth temperature (100011501C), and V/III ratio (NH3/TMG=30005000). Layers were grown directly on (0 0 0 1)-sapphire substrates without low-temperature (LT) buffer layers to observe intrinsic surface morphology which reveals GaN quality. Carrier concentration and Hall mobility were measured by van der Pauw method. Chemical and physical properties in gas-phase were obtained numerically in steady-state by solving fluid equations using real parameters of our Nippon Sanso MOVPE system and growth conditions. The computational simulation was performed by using a commercial fluid dynamics code CFD-ACE+. The code solves basic fluid equations with molecular parameters, and the chemical reactions are considered. The fluid equations are mass, momentum and energy conservation laws. Molecular parameters used are molecular weight, viscosity coefficient, specific energy and collision coefficient. More details concerning the code are reported in Ref. [8]. We used the simulation model with real parameters from our MOVPE system and growth. The model as shown in Fig. 1 consists of main- and sub-flow channels, a susceptor. The gravity was 9.81 m/s2. The pressure of gas outlet was fixed at 100 kPa. The boundary condition at side walls of flow channels and a susceptor were set to be adiabatic and 0-m/s velocity just on surface due to gas viscosity. Susceptor temperature was fixed at each experimental value. Chemical reactions considered are summarized in Table 1. Using these conditions, we have obtained velocity, temperature and chemical species concentration distributions. We defined surface velocity as gas velocity at 0.1-mm height from the susceptor surface in our simulation.
Fig. 1. A simulation model of the two-flow method has a mainflow channel, a sub-flow channel and a susceptor threedimensionally. Each parameter for simulation is identical with the experimental one. Inset shows a picture magnified around susceptor surface. Surface velocity is defined at 0.1-mm height from substrate surface, and its positive direction is shown. Pressure of outlet is kept at 100 kPa. Susceptor temperature in model is each growth temperature around 10001C.
Table 1 Gas-phase chemical reactions and their parameters in Arrhenius expression k ¼ AT n expð E=RTÞ [9]. Notations k; T and R are rate constant, temperature and gas constant, respectively Reaction
A
n
E (J/mol)
Ga(CH3)3-Ga(CH3)2+CH3 Ga(CH3)2-GaCH3+CH3 Ga(CH3)3+H-Ga(CH3)2+CH4 Ga(CH3)2+H-GaCH3+CH4 NH3-NH2+H H+H-H2 CH3+H2-CH4+H CH3+CH3-C2H6 CH3+H-CH4
3.50E+15 8.70E+07 5.00E+13 5.00E+13 9.12E+15 1.60E+16 2.90E+02 2.00E+13 2.40E+22
0 0 0 0 0 0 3.1 0 1
2.49E+05 1.48E+05 4.19E+04 4.19E+04 3.53E+05 0 3.64E+04 0 0
3. Results and discussion Undoped GaN layers had thickness in the range of 25 mm for 1-h growth, and these were electrically n-type. These layers indicated electron concentrations of 8 10165 1018 cm3 and Hall mobility of 10300 cm2/V s at room temperature. High-quality GaN layers have shown good Hall mobility around 300 cm2/V s at room temperature in spite of no LT-buffer layers. These electrical
A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
properties are of the same level with those of GaN layers without LT-buffer reported by Nakamura et al. [6]. Fig. 2 indicates experimental Hall mobility mapping at its surface velocity and decomposedspecies V/III ratio calculated for two-flow method (a), and for three-flow method (b). For the twoflow method, high-mobility samples were obtained under conditions of forward surface velocity and decomposed-species V/III ratio in the range of
933
10002000, as shown in Fig. 2(a). Decomposedspecies V/III ratio was roughly 80% of the initial NH3/TMG ratio. Fig. 2(b) shows that the threeflow method also has a similar tendency. It was found that high-mobility samples were easily obtained by the two-flow method rather than the three-flow method, when we compared Figs. 2(a) and (b). It was difficult for the three-flow method to realize the optimum growth condition. The three-flow method caused backward surface flow
Fig. 2. Surface velocity versus decomposed-species V/III ratio. They are categorized by Hall mobility in the two-flow method (a) and in the three-flow method (b).
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A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
Fig. 3. (a) A cross-sectional view of the two-flow method during high-quality GaN growth. An arrow shows gas velocity at the point. Average velocities of the main- and sub-flows are 1 and 0.2 m/s, respectively. (b) A cross-sectional view of three-flow method during GaN growth. Average velocities of upper main-, lower main- and sub-flows are 1, 1.5 and 0.2 m/s, respectively.
easily as shown in Fig. 3(b). GaN layers grown with backward surface velocity condition in both methods indicated a low mobility of o100 cm2/ V s. Simulation has indicated that such backwardflow of material gases is caused by too strong a sub-flow pressure. It is noted that layers with high carrier concentration are also grown under the backward-flow condition. We think this mechanism as follows. Material gases in backward velocity are blocked and diluted by too strong a sub-flow consisted of N2 and H2, then flow backwards as shown in Fig. 3(b). NH3 and their decomposed-species concentrations become lower due to the dilution. It can be seen from Fig. 2 that decomposed-species V/III ratio in the backward velocity condition is o1000. Lower decomposed-
species V/III ratio will cause N-vacancies which act as donors in GaN layers. These vacancies would cause the high electron concentration measured [10]. Fig. 3(a) shows the two-flow method which realize the laminar flow and wellsuppressed thermal convection. The laminar flow was obtained in whole susceptor surface. Growth simulations of high-mobility samples have shown such laminar flow.
4. Summary We have compared the quality of GaN layers by MOVPE and gaseous phase chemistry by computational thermodynamic analysis under actual
A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
growth conditions. It is found that high-mobility GaN layers are grown under microscopic growth conditions such as realization of laminar flow and decomposed-species V/III ratio in the range of 10002000. The two-flow method would be superior to the three-flow method in order to grow highquality GaN layers. Further, microscopic chemical and physical properties of GaN-MOVPE will be understood by considering more effects such as adduct reactions [11] and thermal radiation and absorption of quartz used for flow channels [12].
References [1] J. Karpinski, J. Jum, S. Porowski, J. Crystal Growth 66 (1984) 1. [2] S. Strite, H. Morkoc-, J. Vac. Sci. Technol. B10 (1992) 1237.
935
[3] T.D. Moustakas, T. Lei, R.J. Molnar, Physica B 185 (1993) 36. [4] H. Manasevit, Appl. Phys. Lett. 12 (1968) 156. [5] H. Amano, N. Sawai, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [6] S. Nakamura, Y. Harada, M. Seno, Appl. Phys. Lett. 58 (1991) 2021. [7] I. Akasaki, H. Amano, Jpn. J. Appl. Phys. 36 (1997) 5393. [8] S. Mazumder, S.A. Lowry, J. Crystal Growth 224 (2001) 165. [9] M. Masi, H. Simka, K.F. Jensen, J. Crystal Growth 124 (1992) 483, in: Kagakubinran Kisohen II, Y. Iwasama (Ed.), Maruzen, 1993, p. 350 (in Japanese). [10] P. Boguslawski, E.L. Briggs, J. Bernhole, Phys. Rev. B 51 (1995) 17255. [11] O. Makino, K. Nakamura, A. Tachibana, H. Tokunaga, N. Akutsu, K. Matsumoto, Appl. Surf. Sci. 159–160 (2000) 374. [12] N. Nishibayashi, G. Masumoto, K. Ohkawa, Extended Abstracts of the 48th Spring Meeting of the Japan Society of Applied Physics and Related Societies, The Japan Society of Applied Physics, Tokyo, 2001, p. 385, 29p-M-4.
GaN-MOVPE growth and its microscopic chemistry of gaseous phase by computational thermodynamic analysis A. Hirakoa, M. Yoshitania, M. Nishibayashia, Y. Nishikawab, K. Ohkawaa,b,* a b
Department of Applied Physics, Faculty of Science, Science University of Tokyo, Kagurazaka 1-3, Shinjuku, Tokyo 162-8601, Japan Gonokami Cooperative Excitation Project, Exploratory Research for Advanced Technology, Japan Science and Technology Corporation, Kagurazaka 1-3, Shinjuku, Tokyo 162-8601, Japan
Abstract We studied microscopic chemistry of gaseous phase in GaN growth by computational thermodynamic analysis of metalorganic vapor phase epitaxy with two- and three-flow methods. Correlations between quality of GaN layers and gaseous phase chemistry were found from the computational analysis. It was confirmed that laminar flow on a substrate during growth was necessary to obtain a high-quality GaN layer in spite of high growth temperature. Optimum decomposed-species V/III ratio (NH2/GaCH3) were considered in the range of 10002000 to achieve high electron mobility more than 200 cm2/V s. Two-flow method was easier to achieve the optimum condition than three-flow method both in experiments and in computational analysis. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea Keywords: A1. Computer simulation; A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction As the melting point of GaN and equilibrium vapor pressure of nitrogen are extremely high [1], epitaxial techniques such as molecular beam epitaxy (MBE) [2,3] and metalorganic vapor phase epitaxy (MOVPE) [4–6] are useful to grow GaN layers. Even by MOVPE, however, it is not easy to obtain high-quality GaN layers. Several groups have succeeded in achieving high-quality nitridesbased laser diodes [7], therefore, it implies obviously that an optimum growth condition *Corresponding author. Department of Applied Physics, Faculty of Science, Science University of Tokyo, Kagurazaka 13, Shinjuku, Tokyo 162-8601, Japan. Tel.: +81-3-3260-4280; fax: +81-3-3260-4280. E-mail address: [email protected] (K. Ohkawa).
exists in MOVPE growth for nitrides. Chemical and physical properties in gaseous phase during GaN-MOVPE growth are not clear yet. In this paper, we compare the quality of GaN layers grown by MOVPE with chemical property in gaseous phase from computational analysis in order to find out microscopic chemical and physical properties in gas phase just on a substrate under the optimum growth condition. We have introduced different types of MOVPE such as twoand three-flow methods to realize various chemical situations of gaseous phase during growth.
2. Experiment Undoped GaN layers were grown by atmospheric pressure MOVPE with two- and three-flow
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 9 9 9 - 6
932
A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
methods. The two-flow method consists of a horizontal main-flow channel and a vertical subflow channel to suppress thermal convection [6]. These channels are made from quartz. The mainflow channel supplies trimethylgallium (TMG) and ammonia (NH3) which are carried by H2 and N2 gases. The sub-flow consists of H2 and N2. In the case of three-flow method, the main-flow channel has two lines, i.e. an upper TMG line and a lower NH3 line. GaN layers were grown with various conditions of main-flow velocity (0.51.5 m/s), growth temperature (100011501C), and V/III ratio (NH3/TMG=30005000). Layers were grown directly on (0 0 0 1)-sapphire substrates without low-temperature (LT) buffer layers to observe intrinsic surface morphology which reveals GaN quality. Carrier concentration and Hall mobility were measured by van der Pauw method. Chemical and physical properties in gas-phase were obtained numerically in steady-state by solving fluid equations using real parameters of our Nippon Sanso MOVPE system and growth conditions. The computational simulation was performed by using a commercial fluid dynamics code CFD-ACE+. The code solves basic fluid equations with molecular parameters, and the chemical reactions are considered. The fluid equations are mass, momentum and energy conservation laws. Molecular parameters used are molecular weight, viscosity coefficient, specific energy and collision coefficient. More details concerning the code are reported in Ref. [8]. We used the simulation model with real parameters from our MOVPE system and growth. The model as shown in Fig. 1 consists of main- and sub-flow channels, a susceptor. The gravity was 9.81 m/s2. The pressure of gas outlet was fixed at 100 kPa. The boundary condition at side walls of flow channels and a susceptor were set to be adiabatic and 0-m/s velocity just on surface due to gas viscosity. Susceptor temperature was fixed at each experimental value. Chemical reactions considered are summarized in Table 1. Using these conditions, we have obtained velocity, temperature and chemical species concentration distributions. We defined surface velocity as gas velocity at 0.1-mm height from the susceptor surface in our simulation.
Fig. 1. A simulation model of the two-flow method has a mainflow channel, a sub-flow channel and a susceptor threedimensionally. Each parameter for simulation is identical with the experimental one. Inset shows a picture magnified around susceptor surface. Surface velocity is defined at 0.1-mm height from substrate surface, and its positive direction is shown. Pressure of outlet is kept at 100 kPa. Susceptor temperature in model is each growth temperature around 10001C.
Table 1 Gas-phase chemical reactions and their parameters in Arrhenius expression k ¼ AT n expð E=RTÞ [9]. Notations k; T and R are rate constant, temperature and gas constant, respectively Reaction
A
n
E (J/mol)
Ga(CH3)3-Ga(CH3)2+CH3 Ga(CH3)2-GaCH3+CH3 Ga(CH3)3+H-Ga(CH3)2+CH4 Ga(CH3)2+H-GaCH3+CH4 NH3-NH2+H H+H-H2 CH3+H2-CH4+H CH3+CH3-C2H6 CH3+H-CH4
3.50E+15 8.70E+07 5.00E+13 5.00E+13 9.12E+15 1.60E+16 2.90E+02 2.00E+13 2.40E+22
0 0 0 0 0 0 3.1 0 1
2.49E+05 1.48E+05 4.19E+04 4.19E+04 3.53E+05 0 3.64E+04 0 0
3. Results and discussion Undoped GaN layers had thickness in the range of 25 mm for 1-h growth, and these were electrically n-type. These layers indicated electron concentrations of 8 10165 1018 cm3 and Hall mobility of 10300 cm2/V s at room temperature. High-quality GaN layers have shown good Hall mobility around 300 cm2/V s at room temperature in spite of no LT-buffer layers. These electrical
A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
properties are of the same level with those of GaN layers without LT-buffer reported by Nakamura et al. [6]. Fig. 2 indicates experimental Hall mobility mapping at its surface velocity and decomposedspecies V/III ratio calculated for two-flow method (a), and for three-flow method (b). For the twoflow method, high-mobility samples were obtained under conditions of forward surface velocity and decomposed-species V/III ratio in the range of
933
10002000, as shown in Fig. 2(a). Decomposedspecies V/III ratio was roughly 80% of the initial NH3/TMG ratio. Fig. 2(b) shows that the threeflow method also has a similar tendency. It was found that high-mobility samples were easily obtained by the two-flow method rather than the three-flow method, when we compared Figs. 2(a) and (b). It was difficult for the three-flow method to realize the optimum growth condition. The three-flow method caused backward surface flow
Fig. 2. Surface velocity versus decomposed-species V/III ratio. They are categorized by Hall mobility in the two-flow method (a) and in the three-flow method (b).
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A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
Fig. 3. (a) A cross-sectional view of the two-flow method during high-quality GaN growth. An arrow shows gas velocity at the point. Average velocities of the main- and sub-flows are 1 and 0.2 m/s, respectively. (b) A cross-sectional view of three-flow method during GaN growth. Average velocities of upper main-, lower main- and sub-flows are 1, 1.5 and 0.2 m/s, respectively.
easily as shown in Fig. 3(b). GaN layers grown with backward surface velocity condition in both methods indicated a low mobility of o100 cm2/ V s. Simulation has indicated that such backwardflow of material gases is caused by too strong a sub-flow pressure. It is noted that layers with high carrier concentration are also grown under the backward-flow condition. We think this mechanism as follows. Material gases in backward velocity are blocked and diluted by too strong a sub-flow consisted of N2 and H2, then flow backwards as shown in Fig. 3(b). NH3 and their decomposed-species concentrations become lower due to the dilution. It can be seen from Fig. 2 that decomposed-species V/III ratio in the backward velocity condition is o1000. Lower decomposed-
species V/III ratio will cause N-vacancies which act as donors in GaN layers. These vacancies would cause the high electron concentration measured [10]. Fig. 3(a) shows the two-flow method which realize the laminar flow and wellsuppressed thermal convection. The laminar flow was obtained in whole susceptor surface. Growth simulations of high-mobility samples have shown such laminar flow.
4. Summary We have compared the quality of GaN layers by MOVPE and gaseous phase chemistry by computational thermodynamic analysis under actual
A. Hirako et al. / Journal of Crystal Growth 237–239 (2002) 931–935
growth conditions. It is found that high-mobility GaN layers are grown under microscopic growth conditions such as realization of laminar flow and decomposed-species V/III ratio in the range of 10002000. The two-flow method would be superior to the three-flow method in order to grow highquality GaN layers. Further, microscopic chemical and physical properties of GaN-MOVPE will be understood by considering more effects such as adduct reactions [11] and thermal radiation and absorption of quartz used for flow channels [12].
References [1] J. Karpinski, J. Jum, S. Porowski, J. Crystal Growth 66 (1984) 1. [2] S. Strite, H. Morkoc-, J. Vac. Sci. Technol. B10 (1992) 1237.
935
[3] T.D. Moustakas, T. Lei, R.J. Molnar, Physica B 185 (1993) 36. [4] H. Manasevit, Appl. Phys. Lett. 12 (1968) 156. [5] H. Amano, N. Sawai, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [6] S. Nakamura, Y. Harada, M. Seno, Appl. Phys. Lett. 58 (1991) 2021. [7] I. Akasaki, H. Amano, Jpn. J. Appl. Phys. 36 (1997) 5393. [8] S. Mazumder, S.A. Lowry, J. Crystal Growth 224 (2001) 165. [9] M. Masi, H. Simka, K.F. Jensen, J. Crystal Growth 124 (1992) 483, in: Kagakubinran Kisohen II, Y. Iwasama (Ed.), Maruzen, 1993, p. 350 (in Japanese). [10] P. Boguslawski, E.L. Briggs, J. Bernhole, Phys. Rev. B 51 (1995) 17255. [11] O. Makino, K. Nakamura, A. Tachibana, H. Tokunaga, N. Akutsu, K. Matsumoto, Appl. Surf. Sci. 159–160 (2000) 374. [12] N. Nishibayashi, G. Masumoto, K. Ohkawa, Extended Abstracts of the 48th Spring Meeting of the Japan Society of Applied Physics and Related Societies, The Japan Society of Applied Physics, Tokyo, 2001, p. 385, 29p-M-4.