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Nucleation control in MOVPE of group III-nitrides on SiC substrate
Journal of Crystal Growth 221 (2000) 297}300
Nucleation control in MOVPE of group III-nitrides on SiC substrate T. Nishida*, N. Kobayashi Vapor Phase Epitaxy Research Group, Physical Science Research Laboratory, NTT Basic Research Laboratories, 3-1, Morinosato-wakamiya, Atsugi-shi, Kanagawa Pref., 243-0198, Japan
Abstract We characterized nitride growth on SiC substrates by using surface sensitive in situ monitoring of shallow angle re#ectance (SAR). The growth initiation of the AlN wetting layer on SiC substrate, and that of GaN on the AlN wetting layer are studied. Flat growth of the AlN wetting layer on SiC substrate is achieved by excess source supply at the start, and the growth evolution of GaN on the AlN wetting layer depends not only on the source #ow rate but also on the species of metalorganic source. Flat GaN and AlN wetting layer growth is achieved by intentional nucleation. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh Keywords: MOVPE; GaN; SiC; AlN; Nucleation
1. Introduction Nitride applications to optical devices and electronic devices, such as LEDs [1,2], LDs [3], and FETs [4] have been intensively investigated in the last decade. Quality improvement of nitride crystal utilizing the enhancement of coalescence has been clari"ed to be the key technique for device applications [5}7]. For purposes of practical use, crystal uniformity is another key issue. For example, we achieved uniform nitride layers for FET applications which provide low leak current and perfect pinch-o! characteristics even at high operating temperature of 4003C [8]. Usually, imperfections
* Corresponding author. Tel.: #81-462-40-3174; fax: #81462-40-4729. E-mail address: [email protected] (T. Nishida).
such as leak current of p}n junctions and/or transistor gates due to the micron size or submicron size defects restrict the product yield. Well-controlled nucleation of su$cient density, which, of course, depends on coalescence control, is indispensable to achieve uniform epitaxial "lms. Shallow angle re#ectance (SAR) is a roughness sensitive in situ monitoring method [9] under practical growth conditions, and SiC is one of the most promising substrate materials for nitride growth [8,10]. In this report, nucleation control of nitrides in metalorganic vapor-phase epitaxy (MOVPE) on SiC substrates is studied by SAR. We found that #at growth of the AlN wetting layer on SiC substrate is achieved by excess source supply at the start, and that the growth evolution of GaN on the AlN wetting layer depends not only on the source #ow but also on the species of metalorganic source.
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 0 3 - X
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T. Nishida, N. Kobayashi / Journal of Crystal Growth 221 (2000) 297}300
2. Experiments Epitaxial growth was performed by a verticaltype MOVPE reactor installed with an SAR equipment. Before the growth, on-axis SiC substrates are cleaned by organic solvent and hydro#uoric acid, followed by annealing under hydrogen environment at a high temperature of 10703C. The working pressure was 76 Torr. The growth evolution of the AlN wetting layer [11] and GaN was monitored by re#ectance of He}Cd laser (325 nm) with a shallow incident angle of 753 [9].
3. Inhomogeneous nucleation of GaN without nucleation control Fig. 1 shows typical examples of the surface morphologies of the GaN grown on on-axis SiC substrates: (a) the GaN directly grown on SiC, (b) the GaN morphology grown at 9603C on the AlN wetting layer, and (c) the GaN morphology grown at 10603C on the AlN wetting layer. TMGa supply was set at 2.5 sccm which corresponds to the growth rate of 1 lm/h at 9603C. As shown in Fig. 1(a), the GaN stickiness to SiC surface is so poor that only a small amount of microcrystals are
formed, and almost all of SiC surface is bare. Even when using the AlN wetting layer, GaN nucleation is insu$cient so that hexagonal pits are found on planar GaN surface as shown in Fig. 1(b). Further, poor nucleation at higher growth temperature of 10603C results in microcrystals which nucleate only at the SiC defect sites.
4. Nucleation control of AlN wetting layer and of GaN on AlN To improve nucleation homogeneity, we investigated how nucleation depends on the source #ux. Fig. 2 shows the growth evolution monitored by re#ectance where we grew AlN wetting layers, (a) without nucleation procedure, and (b) with nucleation procedure. In the nucleation procedure, 200 sccm of triethylaluminium (TEA) and 4 SLM of NH are supplied for 10 s, while 90 sccm of TEA (corresponding to 2 lmol/min) and 0.25 SLM of NH are supplied in the following growth. As shown in Fig. 2(a), the re#ectance is signi"cantly damped. The band gap wavelength of AlN is much shorter than the SAR probe wavelength of 325 nm. Therefore, #at AlN growth should result in the non-damped re#ectance oscillation due to the
Fig. 1. Morphologies of GaN. (a) GaN directly grown on SiC; (b) GaN grown on AlN at 9603C; and (c) GaN grown on AlN at 10603C.
T. Nishida, N. Kobayashi / Journal of Crystal Growth 221 (2000) 297}300
299
Fig. 2. SAR transients of AlN growth on SiC (0 0 0 1) substrates: (a) without nucleation procedure; and (b) with nucleation procedure. 1
Fig. 3. SAR transient of GaN growth on the AlN wetting layers at 10003C: (a) and (b) are GaN grown by TMGa, and (c) by TEGa.
optical interference e!ect, and damped oscillation means the increase of surface roughness. By comparing Fig. 2(a) and (b), it is seen that the supply increment at the start of growth process drastically improves the #atness of the AlN wetting layer, resulting in continuous oscillation in the re#ectance transient, which means a much #atter growth condition, as shown in Fig. 2(b). Fig. 3 shows results of the re#ectance measurement of the GaN growth on the AlN wetting layers which are about 200 nm thick. Without nucleation procedure, pit-free growth of GaN was di$cult as shown in Fig. 1. We enhanced nucleation by increasing source supply at the beginning of GaN growth. The gallium source and NH supply at the beginning of GaN growth is shown in Fig. 3. In Fig. 3(a) and (b), supply of trimethylgallium (TMG) was set 4 times that of the continuous growth condition. By comparing re#ectance transients of these "gures, the increase of NH supply enhances the #at growth of GaN. Further #at growth was achieved by using triethylgallium (TEG) as Ga source, as shown in Fig. 3(c). Although the growth rate corresponding to 10 sccm of TEG is almost the same as that of 2.5 sccm of TMG (corresponding to 6 lmol/min), four periods of re#ectance oscillation
was observed and re#ectance intensity changes little. In case of TMG, nucleation is insu$cient for the following coalescence so that the re#ectance oscillation is small and the re#ectance intensity gradually decreases due to the surface roughness [9], as shown in Fig. 3(a) and (b). But, su$cient nucleation is achieved in the case where 3.53-tilted SiC substrate is used. These results imply the di!erence of the stickiness of these Ga sources on on-axis surface. The chemical bond between Ga atom and alkyl-base is stronger for the methylorganics than for the ethylorganics. Therefore, TEGa easily decomposes and adsorbs to the AlN wetting layer. We also found the carbon incorporation in the GaN "lm is higher when TMGa is used as the Ga source, than that in the GaN grown by using TEGa, which is indicative of the di!erence of the alkyl decomposition. As shown above, nucleation of GaN on AlN wetting layer and that of AlN on SiC substrate easily occur as the source #ux increases at the beginning of the growth of each layer, and pit-free nitride layer is achieved. This result shows the importance of intentional nucleation control and of its monitoring for homogeneous nitride layer growth.
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T. Nishida, N. Kobayashi / Journal of Crystal Growth 221 (2000) 297}300
5. Conclusion In conclusion, we characterized nitride growth on SiC substrates by using surface sensitive in situ monitoring of SAR. The #at growth condition of the AlN wetting layer on SiC substrate, and that of GaN on the AlN wetting layer are con"rmed. We have also shown the importance of intentional nucleation control in nitride layer growth.
References [1] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Inst. Phys. Conf. Ser. 106 (1989) 725. [2] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (1991) L1998.
[3] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [4] M.A. Kahn, Q. Chen, M.S. Shur, B.T. McDermott, J.A. Higgins, IEEE Electron Device Lett. 17 (1996) 325. [5] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Crystal Growth 98 (1989) 209. [6] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) 1620. [7] J. Han, T.-G. Ng, R.M. Biefeld, M.H. Crawford, D.M. Follstaedt, Appl. Phys. Lett. 71 (1997) 3114. [8] N. Maeda, T. Saito, K. Tsubaki, T. Nishida, N. Kobayashi, Jpn. J. Appl. Phys. 38 (1999) L987. [9] Y. Kobayashi, T. Akasaka, N. Kobayashi, J. Crystal Growth 195 (1998) 187. [10] A. Kuramata, K. Domen, R. Soejima, K. Horino, S. Kubota, T. Tanahashi, Jpn. J. Appl. Phys. 36 (1970) L1130. [11] T.W. Weeks, M.D. Bremser, K.S. Ailey, E. Carlson, W.G. Perry, R.F. Davis, Appl. Phys. Lett. 67 (1995) 401.
Nucleation control in MOVPE of group III-nitrides on SiC substrate T. Nishida*, N. Kobayashi Vapor Phase Epitaxy Research Group, Physical Science Research Laboratory, NTT Basic Research Laboratories, 3-1, Morinosato-wakamiya, Atsugi-shi, Kanagawa Pref., 243-0198, Japan
Abstract We characterized nitride growth on SiC substrates by using surface sensitive in situ monitoring of shallow angle re#ectance (SAR). The growth initiation of the AlN wetting layer on SiC substrate, and that of GaN on the AlN wetting layer are studied. Flat growth of the AlN wetting layer on SiC substrate is achieved by excess source supply at the start, and the growth evolution of GaN on the AlN wetting layer depends not only on the source #ow rate but also on the species of metalorganic source. Flat GaN and AlN wetting layer growth is achieved by intentional nucleation. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh Keywords: MOVPE; GaN; SiC; AlN; Nucleation
1. Introduction Nitride applications to optical devices and electronic devices, such as LEDs [1,2], LDs [3], and FETs [4] have been intensively investigated in the last decade. Quality improvement of nitride crystal utilizing the enhancement of coalescence has been clari"ed to be the key technique for device applications [5}7]. For purposes of practical use, crystal uniformity is another key issue. For example, we achieved uniform nitride layers for FET applications which provide low leak current and perfect pinch-o! characteristics even at high operating temperature of 4003C [8]. Usually, imperfections
* Corresponding author. Tel.: #81-462-40-3174; fax: #81462-40-4729. E-mail address: [email protected] (T. Nishida).
such as leak current of p}n junctions and/or transistor gates due to the micron size or submicron size defects restrict the product yield. Well-controlled nucleation of su$cient density, which, of course, depends on coalescence control, is indispensable to achieve uniform epitaxial "lms. Shallow angle re#ectance (SAR) is a roughness sensitive in situ monitoring method [9] under practical growth conditions, and SiC is one of the most promising substrate materials for nitride growth [8,10]. In this report, nucleation control of nitrides in metalorganic vapor-phase epitaxy (MOVPE) on SiC substrates is studied by SAR. We found that #at growth of the AlN wetting layer on SiC substrate is achieved by excess source supply at the start, and that the growth evolution of GaN on the AlN wetting layer depends not only on the source #ow but also on the species of metalorganic source.
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 0 3 - X
298
T. Nishida, N. Kobayashi / Journal of Crystal Growth 221 (2000) 297}300
2. Experiments Epitaxial growth was performed by a verticaltype MOVPE reactor installed with an SAR equipment. Before the growth, on-axis SiC substrates are cleaned by organic solvent and hydro#uoric acid, followed by annealing under hydrogen environment at a high temperature of 10703C. The working pressure was 76 Torr. The growth evolution of the AlN wetting layer [11] and GaN was monitored by re#ectance of He}Cd laser (325 nm) with a shallow incident angle of 753 [9].
3. Inhomogeneous nucleation of GaN without nucleation control Fig. 1 shows typical examples of the surface morphologies of the GaN grown on on-axis SiC substrates: (a) the GaN directly grown on SiC, (b) the GaN morphology grown at 9603C on the AlN wetting layer, and (c) the GaN morphology grown at 10603C on the AlN wetting layer. TMGa supply was set at 2.5 sccm which corresponds to the growth rate of 1 lm/h at 9603C. As shown in Fig. 1(a), the GaN stickiness to SiC surface is so poor that only a small amount of microcrystals are
formed, and almost all of SiC surface is bare. Even when using the AlN wetting layer, GaN nucleation is insu$cient so that hexagonal pits are found on planar GaN surface as shown in Fig. 1(b). Further, poor nucleation at higher growth temperature of 10603C results in microcrystals which nucleate only at the SiC defect sites.
4. Nucleation control of AlN wetting layer and of GaN on AlN To improve nucleation homogeneity, we investigated how nucleation depends on the source #ux. Fig. 2 shows the growth evolution monitored by re#ectance where we grew AlN wetting layers, (a) without nucleation procedure, and (b) with nucleation procedure. In the nucleation procedure, 200 sccm of triethylaluminium (TEA) and 4 SLM of NH are supplied for 10 s, while 90 sccm of TEA (corresponding to 2 lmol/min) and 0.25 SLM of NH are supplied in the following growth. As shown in Fig. 2(a), the re#ectance is signi"cantly damped. The band gap wavelength of AlN is much shorter than the SAR probe wavelength of 325 nm. Therefore, #at AlN growth should result in the non-damped re#ectance oscillation due to the
Fig. 1. Morphologies of GaN. (a) GaN directly grown on SiC; (b) GaN grown on AlN at 9603C; and (c) GaN grown on AlN at 10603C.
T. Nishida, N. Kobayashi / Journal of Crystal Growth 221 (2000) 297}300
299
Fig. 2. SAR transients of AlN growth on SiC (0 0 0 1) substrates: (a) without nucleation procedure; and (b) with nucleation procedure. 1
Fig. 3. SAR transient of GaN growth on the AlN wetting layers at 10003C: (a) and (b) are GaN grown by TMGa, and (c) by TEGa.
optical interference e!ect, and damped oscillation means the increase of surface roughness. By comparing Fig. 2(a) and (b), it is seen that the supply increment at the start of growth process drastically improves the #atness of the AlN wetting layer, resulting in continuous oscillation in the re#ectance transient, which means a much #atter growth condition, as shown in Fig. 2(b). Fig. 3 shows results of the re#ectance measurement of the GaN growth on the AlN wetting layers which are about 200 nm thick. Without nucleation procedure, pit-free growth of GaN was di$cult as shown in Fig. 1. We enhanced nucleation by increasing source supply at the beginning of GaN growth. The gallium source and NH supply at the beginning of GaN growth is shown in Fig. 3. In Fig. 3(a) and (b), supply of trimethylgallium (TMG) was set 4 times that of the continuous growth condition. By comparing re#ectance transients of these "gures, the increase of NH supply enhances the #at growth of GaN. Further #at growth was achieved by using triethylgallium (TEG) as Ga source, as shown in Fig. 3(c). Although the growth rate corresponding to 10 sccm of TEG is almost the same as that of 2.5 sccm of TMG (corresponding to 6 lmol/min), four periods of re#ectance oscillation
was observed and re#ectance intensity changes little. In case of TMG, nucleation is insu$cient for the following coalescence so that the re#ectance oscillation is small and the re#ectance intensity gradually decreases due to the surface roughness [9], as shown in Fig. 3(a) and (b). But, su$cient nucleation is achieved in the case where 3.53-tilted SiC substrate is used. These results imply the di!erence of the stickiness of these Ga sources on on-axis surface. The chemical bond between Ga atom and alkyl-base is stronger for the methylorganics than for the ethylorganics. Therefore, TEGa easily decomposes and adsorbs to the AlN wetting layer. We also found the carbon incorporation in the GaN "lm is higher when TMGa is used as the Ga source, than that in the GaN grown by using TEGa, which is indicative of the di!erence of the alkyl decomposition. As shown above, nucleation of GaN on AlN wetting layer and that of AlN on SiC substrate easily occur as the source #ux increases at the beginning of the growth of each layer, and pit-free nitride layer is achieved. This result shows the importance of intentional nucleation control and of its monitoring for homogeneous nitride layer growth.
300
T. Nishida, N. Kobayashi / Journal of Crystal Growth 221 (2000) 297}300
5. Conclusion In conclusion, we characterized nitride growth on SiC substrates by using surface sensitive in situ monitoring of SAR. The #at growth condition of the AlN wetting layer on SiC substrate, and that of GaN on the AlN wetting layer are con"rmed. We have also shown the importance of intentional nucleation control in nitride layer growth.
References [1] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Inst. Phys. Conf. Ser. 106 (1989) 725. [2] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (1991) L1998.
[3] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [4] M.A. Kahn, Q. Chen, M.S. Shur, B.T. McDermott, J.A. Higgins, IEEE Electron Device Lett. 17 (1996) 325. [5] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Crystal Growth 98 (1989) 209. [6] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) 1620. [7] J. Han, T.-G. Ng, R.M. Biefeld, M.H. Crawford, D.M. Follstaedt, Appl. Phys. Lett. 71 (1997) 3114. [8] N. Maeda, T. Saito, K. Tsubaki, T. Nishida, N. Kobayashi, Jpn. J. Appl. Phys. 38 (1999) L987. [9] Y. Kobayashi, T. Akasaka, N. Kobayashi, J. Crystal Growth 195 (1998) 187. [10] A. Kuramata, K. Domen, R. Soejima, K. Horino, S. Kubota, T. Tanahashi, Jpn. J. Appl. Phys. 36 (1970) L1130. [11] T.W. Weeks, M.D. Bremser, K.S. Ailey, E. Carlson, W.G. Perry, R.F. Davis, Appl. Phys. Lett. 67 (1995) 401.