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The dependence of GaN growth rate on electron temperature in an ECR plasma
Surface and Coatings Technology 131 Ž2000. 470᎐473
The dependence of GaN growth rate on electron temperature in an ECR plasma Yi-Kang Pu a,U , Yu-Feng Renb, Si-Ze Yang b, Daniel Dywer c , Xiao-Guang Zhang d, Xiou-Jun Jiad a
Department of Electrical Engineering, Tsinghua Uni¨ ersity, Beijing, PR China 100084 b Institute of Physics, Chinese Academy of Sciences, Beijing, PR China c Massachusetts Institute of Technology, Cambridge, MA 02139, USA d Beijing Institute of Technology, Beijing, PR China
Abstract Experiments on the deposition of GaN thin films were carried out in an ECR plasma reactor using nitrogen gas and trimethylgallium ŽTMG. as precursors. Electron temperature and nitrogen species adjacent to the substrate surface during deposition were measured by a CCD spectrometer. We observed an optimum electron temperature for the growth rate. The result suggests that tuning of electron energy Žor temperature. can be used to optimize the deposition and electron temperature near the substrate surface may be a candidate for one of the control parameters in plasma-assisted CVD. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: GaN; Plasma-assisted CVD; Electron temperature
1. Introduction Nitrogen plasmas have been widely used in nitride thin film deposition processes w1᎐15x. Electron cyclotron resonance ŽECR. is one of the popular discharges to generate such a plasma w1᎐5x. There have been many efforts to optimize the deposition processes by varying different parameters, such as substrate bias w3᎐5x, growth temperature w10,11x, group III᎐V flow rate ratio w9,11x, and nitridation procedure w12x. But the parameters that have been varied and monitored are external ones and the quantitative results from optimization experiments are generally reactor-dependent. In most cases, it is difficult to relate the optimization results to the in situ physical processes in the plasma that is ultimately responsible for the deposition process. In searching for the optimal growth condition and to U
Corresponding author.
identify the parameter most relevant to the growth process, many works examined the importance of atomic nitrogen by monitoring the intensity of optical emission at wavelengths in the vicinity of 745, 821, and 869 nm w7,11,14,16᎐19x. However, other experimental results suggested that ‘the presence of atomic nitrogen alone is not a sufficient condition for growth’ w15x. Furthermore, reasonable growth rate was obtained by Tokuda et al., in an RF plasma without strong atomic nitrogen emission in the visible wavelength range w8x. These experimental observations led some authors to conclude that ‘there is no consensus on the most desirable N species for GaN deposition’ w4x. A reason for these different conclusions may lie in the fact that, although line intensity at one of the wavelengths indicates the presence of atomic nitrogen, it does not accurately reflect its population. These emission lines, being atomic transitions from one excited state to another w20x, are proportional to the population density at the upper state. An increase of the line intensity does not necessarily indicate an in-
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 4 2 - 2
Y. Pu et al. r Surface and Coatings Technology 131 (2000) 470᎐473
crease of the total atomic nitrogen density Žin both excited and ground states . in the plasma. The transitions involving ground state are all in the vacuum UV region w20x, and are not detectable by an optical spectrometer. In the present work, using an intensity-calibrated optical spectrometer, we measured electron temperature Želectron mean energy. in the vicinity of the substrate surface during the GaN growth process in an ECR plasma by employing the line-ratio technique w21x. We found the GaN growth rate depends on electron temperature and the optimal electron temperature is ; 12 eV at which the growth rate is twice as that at 5 eV. We attribute this dependence on electron energy to the dependence of electron impact dissociation cross-section of nitrogen molecules, especially in its excited state. Our results thus suggest that, in a N2 plasma, electron temperature is the parameter that can be used to characterize the production of atomic nitrogen, independent of the reactor type, size and discharge mode.
2. Experiment Fig. 1 shows the schematic of the downstream-type ECR-CVD plasma reactor. A 2.45-GHz microwave is fed into the vacuum chamber through a ceramic window with a TM01 mode. The steady-state magnetic field is generated by two concentric coils, which produce a magnetic field strength of 875 Gauss Žor higher, depending on the coil current . in the center of the coils and a diverging magnetic field in the process chamber. High-purity Ž99.999%. nitrogen gas is fed into the chamber through a flow controller Žnot shown. and the neutral gas pressure is monitored in the process chamber by an ion gauge. Trimethylgallium ŽTMG. is used as the gallium source and its flow rate is also regulated by a flow controller. A small substrate heater ŽOmega Engineering SUB-750. is mounted on its underside on a stainless steel tubing with a thermocouple attached to monitor substrate temperature. This stainless steel tubing is connected to the vacuum chamber through a Wilson-sealed flange, so that substrate position can be adjusted along the axis. Ž0001. sapphire is used as the substrate. The substrate cleaning procedure, applied to all sapphire wafers before loading into the reactor, is similar to that used by most groups w2,13x. A 20-min nitridation procedure is used for the substrates before the growth. During the GaN growth, the TMG flow rate is held as 0.15 sccm and substrate temperature at 600⬚C. Neutral gas pressure is varied from 0.02 to 0.4 Pa by varying the nitrogen gas flow rate and the pumping speed. Under these conditions, several samples are grown with different neutral gas pressure and at different axial positions.
471
Fig. 1. A schematic of the experimental setup.
An optical port, 15 cm in diameter, is mounted on the side of the vacuum chamber. A fiber probe with a collimating tubing is placed just outside of this optical port and the position of this probe can be varied to measure spectra of plasma emission at a chosen position of the magnetic axis. Two different spectrometers are used to measure spectral line intensities from different nitrogen species during deposition. The first is a 0.25-m Czerny-Turner monochromator with a photomultiplier tube ŽHammamatsu R928. as a detector. With a 1200 groovesrmm grating, its special resolution ŽFWHM. is 0.35 nm in the wavelength range of 200᎐800 nm. The second is an Ocean Optics SQ2000 CCD spectrometer using fiber optic probes. Its wavelength range is between 200 and 1100 nm and its spectral resolution is 0.8 nm. A Labsphere lamp is used to calibrate the relative efficiency of the two spectrometers in the range of 300᎐1100 nm. Over a wide operating parameter regime with nitrogen ECR plasma, spectral line intensities from the two spectrometers agree to within 5% is obtained. The CCD spectrometer is preferred since it is much faster than the monochromator when time-dependent spectra are needed. This is often the case, since intensities from several lines needed to be collected simultaneously for line-ratio calculations.
3. Results and discussion A typical spectrum obtained from nitrogen ECR plasma is shown in Fig. 2. Most of the spectral lines come from the second positive series of nitrogen molecules ŽC 3 ⌸ u ª B 3 ⌸ g transitions ., first positive se. ries of nitrogen molecules ŽB 3 ⌸ g ª A3 ⌺q u transitions and the first negative series of nitrogen molecular ions 2 q ŽB 2 ⌺q . u ª X ⌺g transitions . We did not observe strong emissions from atomic nitrogen’s transitions from 3s 4 P ª 3p 4 S 0 . The electron temperature is obtained from the spectra by the line-ratio technique for an assumed Maxwellian distribution of electron energy and by using
472
Y. Pu et al. r Surface and Coatings Technology 131 (2000) 470᎐473
Fig. 2. A typical spectrum showing the nitrogen lines.
the ‘emission cross-section’ w22x. Several pairs of spectral line intensities from the second positive series of nitrogen molecules Ž337.1 nm., first positive series of nitrogen molecules Ž762.6 nm. and the first negative series of nitrogen molecular ions Ž391.4 nm. are used for the calculation. It should be noted that the electron temperature obtained this way are line-averaged Žor solid angle averaged.. Electron temperature determined from I᎐V curves from a Langmuir probe is compared with that from the spectroscopic line-ratio technique. Over a very wide range of neutral gas pressure Ž0.02᎐0.6 Pa. and on different locations on the magnetic axis, we have found satisfactory agreement between the two methods Žwith a maximum difference of 30%.. The comparison involves an independent determination of radial profile of electron temperature. A more detailed discussion about this will be presented in another paper. The measured electron temperature ranged from 5 to 25 eV. The high electron temperature is obtained at high microwave power Ž900 W. and low neutral gas pressure Ž0.02 Pa or 0.13 mtorr. in the plasma.
Transparent and single crystalline GaN films were obtained in our experiment, as confirmed by the XRD result. The thickness of each film is measured and thus the growth rate obtained. Fig. 3 shows the dependence of the growth rate on the electron temperature, in which each point represents one sample. Fig. 3 also shows that the growth rate has a peak around an electron temperature of 12 eV. We conjecture that this peak growth rate corresponds to peak production of atomic nitrogen from nitrogen molecules in its metastable state A3 ⌺q u , by electron impact dissociation. The importance of the metastable state A3 ⌺q u of molecular nitrogen, was first suggested by Vaudo et al. w17x, who cited the low threshold energy Ž3.9 eV. for dissociation of this species, even though no direct experimental evidence was shown. In electron impact processes, the cross-section will increase as the electron energy until it reaches a peak at an electron energy equal to approximately three times of its threshold energy. We postulate that this is the reason why the peak growth rate happens at 12 eV, which is about three times of the threshold energy Ž3.9 eV. of electron impact dissociation from the metastable state A3 ⌺q u. Since a sheath exists near the substrate surface and the sheath potential Žvoltage difference between the substrate and the bulk of the plasma. is proportional to electron temperature, impact by energetic ions in the sheath may contribute to the decrease of the growth rate.
4. Conclusion We have measured an optimal electron temperature Žapprox. 12 eV. near the substrate for the growth rate of GaN in an ECR plasma-CVD reactor. The result suggests electron impact dissociation of metastable state A3 ⌺q u of nitrogen molecules plays a major role in the production of atomic nitrogen and the growth of GaN. Since electron impact dissociation cross-section depends strongly on electron energy Žor electron temperature., the result indicates the importance of using electron temperature, rather than simple optical emission, as a direct and quantifiable technique in determining the abundance of atomic nitrogen near the substrate. The result of Fig. 3 also suggests that tuning the electron energy near the substrate surface as a method to enhance to enhance the growth rate.
Acknowledgements
Fig. 3. The dependence of growth rate of GaN thin film on electron temperature.
It is a pleasure to thank Dr Jim Terry, Prof. Miklos Porkolab, Dr Paul Woskov, and Dr Kamal Hadidi of MIT, Prof. Andre Bouchoule of University of Orleans in France, Prof. Kurt Behringer in Germany, Dr Dan
Y. Pu et al. r Surface and Coatings Technology 131 (2000) 470᎐473
Flamm, and Dr T.K. Chu for many enlightening discussions and valuable help during the course of this work. This research is partially supported by the National Science Foundation of China. References w1x S. Zembutsu, T. Sasaki, Appl. Phys. Lett. 48 Ž1986. 870. w2x D.B. Oberman, H. Lee, W.K. Gotz, J.S. Harris, Jr., J. Crystal Growth 150 Ž1995. 912. w3x K. Yasui, K. Iizuka, T. Akahane, J. Vac. Sci. Technol. A 16 Ž1998. 39. w4x I. Berishev, E. Kim, A. Bensaoula, J. Vac. Sci. Technol. A 16 Ž1998. 2791. w5x Y. Chiba, Y. Shimizu, T. Tominari, S. Hokuto, Y. Nanishi, J. Crystal Growth 189r190 Ž1998. 317. w6x M. Sato, J. Appl. Phys. 78 Ž1995. 2123. w7x J.M. Van Hove, G.J. Cosimini, E. Nelson, A.M. Wowchak, P.P. Chow, J. Crystal Growth 150 Ž1995. 908. w8x T. Tokuda, A. Wakahara, S. Noda, A. Sasaki, J. Crystal Growth 173 Ž1997. 237. w9x E.J. Tarsa, B. Heying, X.H. Wu, P. Fini, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 82 Ž1997. 5472. w10x J.M. Myoung, K.H. Shim, O. Gluschenkov et al., J. Crystal Growth 182 Ž1997. 241.
473
w11x M.A. Sanchez-Garcia, E. Calleja, E. Monroy et al., J. Crystal Growth 183 Ž1998. 23. w12x T. Tokuda, A. Wakahara, S. Noda, A. Sasaki, J. Crystal Growth 183 Ž1998. 62. w13x N. Fujita, M. Yoshizawa, K. Kushi, H. Sasamoto, A. Kikuchi, K. Kishino, J. Crystal Growth 189r190 Ž1998. 385. w14x H. Okumura, H. Hamaguchi, T. Koizumi et al., J. Crystal Growth 189r190 Ž1998. 390. w15x J.D. Mackenzie, L. Abbaschian, C.R. Abernathy et al., J. Electron. Mater. 26 Ž1997. 1266. w16x R.P. Vaudo, I.D. Goepfert, T.D. Moustakas, D.M. Beyea, T.J. Frey, K. Meehan, J. Appl. Phys. 79 Ž1996. 2799. w17x R.P. Vaudo, J.W. Cook Jr., J.F. Schetzina, J. Vac. Sci. Technol. B 12 Ž1994. 1232. w18x R.P. Vaudo, Z. Yu, J.W. Cook Jr., J.F. Schetzina, Opt. Lett. 18 Ž1843. 1994. w19x J. Wang, Z. Zhu, K. Park, K. Hiraga, T. Yao, J. Crystal Growth 177 Ž1997. 181. w20x A.R. Stringanow, N.S. Sventitskii, Tables of Spectral Lines of Neutral and Ionized Atoms, Plenum, New York, 1968. w21x I.H. Hutchinson, Principles of Plasma Diagnostics, Cambridge University Press, 1987. w22x Y. Itikawa, M. Hayashi, A. Ichimura et al., J. Phys. Chem. Ref. Data 15 Ž1986. 985.
The dependence of GaN growth rate on electron temperature in an ECR plasma Yi-Kang Pu a,U , Yu-Feng Renb, Si-Ze Yang b, Daniel Dywer c , Xiao-Guang Zhang d, Xiou-Jun Jiad a
Department of Electrical Engineering, Tsinghua Uni¨ ersity, Beijing, PR China 100084 b Institute of Physics, Chinese Academy of Sciences, Beijing, PR China c Massachusetts Institute of Technology, Cambridge, MA 02139, USA d Beijing Institute of Technology, Beijing, PR China
Abstract Experiments on the deposition of GaN thin films were carried out in an ECR plasma reactor using nitrogen gas and trimethylgallium ŽTMG. as precursors. Electron temperature and nitrogen species adjacent to the substrate surface during deposition were measured by a CCD spectrometer. We observed an optimum electron temperature for the growth rate. The result suggests that tuning of electron energy Žor temperature. can be used to optimize the deposition and electron temperature near the substrate surface may be a candidate for one of the control parameters in plasma-assisted CVD. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: GaN; Plasma-assisted CVD; Electron temperature
1. Introduction Nitrogen plasmas have been widely used in nitride thin film deposition processes w1᎐15x. Electron cyclotron resonance ŽECR. is one of the popular discharges to generate such a plasma w1᎐5x. There have been many efforts to optimize the deposition processes by varying different parameters, such as substrate bias w3᎐5x, growth temperature w10,11x, group III᎐V flow rate ratio w9,11x, and nitridation procedure w12x. But the parameters that have been varied and monitored are external ones and the quantitative results from optimization experiments are generally reactor-dependent. In most cases, it is difficult to relate the optimization results to the in situ physical processes in the plasma that is ultimately responsible for the deposition process. In searching for the optimal growth condition and to U
Corresponding author.
identify the parameter most relevant to the growth process, many works examined the importance of atomic nitrogen by monitoring the intensity of optical emission at wavelengths in the vicinity of 745, 821, and 869 nm w7,11,14,16᎐19x. However, other experimental results suggested that ‘the presence of atomic nitrogen alone is not a sufficient condition for growth’ w15x. Furthermore, reasonable growth rate was obtained by Tokuda et al., in an RF plasma without strong atomic nitrogen emission in the visible wavelength range w8x. These experimental observations led some authors to conclude that ‘there is no consensus on the most desirable N species for GaN deposition’ w4x. A reason for these different conclusions may lie in the fact that, although line intensity at one of the wavelengths indicates the presence of atomic nitrogen, it does not accurately reflect its population. These emission lines, being atomic transitions from one excited state to another w20x, are proportional to the population density at the upper state. An increase of the line intensity does not necessarily indicate an in-
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 4 2 - 2
Y. Pu et al. r Surface and Coatings Technology 131 (2000) 470᎐473
crease of the total atomic nitrogen density Žin both excited and ground states . in the plasma. The transitions involving ground state are all in the vacuum UV region w20x, and are not detectable by an optical spectrometer. In the present work, using an intensity-calibrated optical spectrometer, we measured electron temperature Želectron mean energy. in the vicinity of the substrate surface during the GaN growth process in an ECR plasma by employing the line-ratio technique w21x. We found the GaN growth rate depends on electron temperature and the optimal electron temperature is ; 12 eV at which the growth rate is twice as that at 5 eV. We attribute this dependence on electron energy to the dependence of electron impact dissociation cross-section of nitrogen molecules, especially in its excited state. Our results thus suggest that, in a N2 plasma, electron temperature is the parameter that can be used to characterize the production of atomic nitrogen, independent of the reactor type, size and discharge mode.
2. Experiment Fig. 1 shows the schematic of the downstream-type ECR-CVD plasma reactor. A 2.45-GHz microwave is fed into the vacuum chamber through a ceramic window with a TM01 mode. The steady-state magnetic field is generated by two concentric coils, which produce a magnetic field strength of 875 Gauss Žor higher, depending on the coil current . in the center of the coils and a diverging magnetic field in the process chamber. High-purity Ž99.999%. nitrogen gas is fed into the chamber through a flow controller Žnot shown. and the neutral gas pressure is monitored in the process chamber by an ion gauge. Trimethylgallium ŽTMG. is used as the gallium source and its flow rate is also regulated by a flow controller. A small substrate heater ŽOmega Engineering SUB-750. is mounted on its underside on a stainless steel tubing with a thermocouple attached to monitor substrate temperature. This stainless steel tubing is connected to the vacuum chamber through a Wilson-sealed flange, so that substrate position can be adjusted along the axis. Ž0001. sapphire is used as the substrate. The substrate cleaning procedure, applied to all sapphire wafers before loading into the reactor, is similar to that used by most groups w2,13x. A 20-min nitridation procedure is used for the substrates before the growth. During the GaN growth, the TMG flow rate is held as 0.15 sccm and substrate temperature at 600⬚C. Neutral gas pressure is varied from 0.02 to 0.4 Pa by varying the nitrogen gas flow rate and the pumping speed. Under these conditions, several samples are grown with different neutral gas pressure and at different axial positions.
471
Fig. 1. A schematic of the experimental setup.
An optical port, 15 cm in diameter, is mounted on the side of the vacuum chamber. A fiber probe with a collimating tubing is placed just outside of this optical port and the position of this probe can be varied to measure spectra of plasma emission at a chosen position of the magnetic axis. Two different spectrometers are used to measure spectral line intensities from different nitrogen species during deposition. The first is a 0.25-m Czerny-Turner monochromator with a photomultiplier tube ŽHammamatsu R928. as a detector. With a 1200 groovesrmm grating, its special resolution ŽFWHM. is 0.35 nm in the wavelength range of 200᎐800 nm. The second is an Ocean Optics SQ2000 CCD spectrometer using fiber optic probes. Its wavelength range is between 200 and 1100 nm and its spectral resolution is 0.8 nm. A Labsphere lamp is used to calibrate the relative efficiency of the two spectrometers in the range of 300᎐1100 nm. Over a wide operating parameter regime with nitrogen ECR plasma, spectral line intensities from the two spectrometers agree to within 5% is obtained. The CCD spectrometer is preferred since it is much faster than the monochromator when time-dependent spectra are needed. This is often the case, since intensities from several lines needed to be collected simultaneously for line-ratio calculations.
3. Results and discussion A typical spectrum obtained from nitrogen ECR plasma is shown in Fig. 2. Most of the spectral lines come from the second positive series of nitrogen molecules ŽC 3 ⌸ u ª B 3 ⌸ g transitions ., first positive se. ries of nitrogen molecules ŽB 3 ⌸ g ª A3 ⌺q u transitions and the first negative series of nitrogen molecular ions 2 q ŽB 2 ⌺q . u ª X ⌺g transitions . We did not observe strong emissions from atomic nitrogen’s transitions from 3s 4 P ª 3p 4 S 0 . The electron temperature is obtained from the spectra by the line-ratio technique for an assumed Maxwellian distribution of electron energy and by using
472
Y. Pu et al. r Surface and Coatings Technology 131 (2000) 470᎐473
Fig. 2. A typical spectrum showing the nitrogen lines.
the ‘emission cross-section’ w22x. Several pairs of spectral line intensities from the second positive series of nitrogen molecules Ž337.1 nm., first positive series of nitrogen molecules Ž762.6 nm. and the first negative series of nitrogen molecular ions Ž391.4 nm. are used for the calculation. It should be noted that the electron temperature obtained this way are line-averaged Žor solid angle averaged.. Electron temperature determined from I᎐V curves from a Langmuir probe is compared with that from the spectroscopic line-ratio technique. Over a very wide range of neutral gas pressure Ž0.02᎐0.6 Pa. and on different locations on the magnetic axis, we have found satisfactory agreement between the two methods Žwith a maximum difference of 30%.. The comparison involves an independent determination of radial profile of electron temperature. A more detailed discussion about this will be presented in another paper. The measured electron temperature ranged from 5 to 25 eV. The high electron temperature is obtained at high microwave power Ž900 W. and low neutral gas pressure Ž0.02 Pa or 0.13 mtorr. in the plasma.
Transparent and single crystalline GaN films were obtained in our experiment, as confirmed by the XRD result. The thickness of each film is measured and thus the growth rate obtained. Fig. 3 shows the dependence of the growth rate on the electron temperature, in which each point represents one sample. Fig. 3 also shows that the growth rate has a peak around an electron temperature of 12 eV. We conjecture that this peak growth rate corresponds to peak production of atomic nitrogen from nitrogen molecules in its metastable state A3 ⌺q u , by electron impact dissociation. The importance of the metastable state A3 ⌺q u of molecular nitrogen, was first suggested by Vaudo et al. w17x, who cited the low threshold energy Ž3.9 eV. for dissociation of this species, even though no direct experimental evidence was shown. In electron impact processes, the cross-section will increase as the electron energy until it reaches a peak at an electron energy equal to approximately three times of its threshold energy. We postulate that this is the reason why the peak growth rate happens at 12 eV, which is about three times of the threshold energy Ž3.9 eV. of electron impact dissociation from the metastable state A3 ⌺q u. Since a sheath exists near the substrate surface and the sheath potential Žvoltage difference between the substrate and the bulk of the plasma. is proportional to electron temperature, impact by energetic ions in the sheath may contribute to the decrease of the growth rate.
4. Conclusion We have measured an optimal electron temperature Žapprox. 12 eV. near the substrate for the growth rate of GaN in an ECR plasma-CVD reactor. The result suggests electron impact dissociation of metastable state A3 ⌺q u of nitrogen molecules plays a major role in the production of atomic nitrogen and the growth of GaN. Since electron impact dissociation cross-section depends strongly on electron energy Žor electron temperature., the result indicates the importance of using electron temperature, rather than simple optical emission, as a direct and quantifiable technique in determining the abundance of atomic nitrogen near the substrate. The result of Fig. 3 also suggests that tuning the electron energy near the substrate surface as a method to enhance to enhance the growth rate.
Acknowledgements
Fig. 3. The dependence of growth rate of GaN thin film on electron temperature.
It is a pleasure to thank Dr Jim Terry, Prof. Miklos Porkolab, Dr Paul Woskov, and Dr Kamal Hadidi of MIT, Prof. Andre Bouchoule of University of Orleans in France, Prof. Kurt Behringer in Germany, Dr Dan
Y. Pu et al. r Surface and Coatings Technology 131 (2000) 470᎐473
Flamm, and Dr T.K. Chu for many enlightening discussions and valuable help during the course of this work. This research is partially supported by the National Science Foundation of China. References w1x S. Zembutsu, T. Sasaki, Appl. Phys. Lett. 48 Ž1986. 870. w2x D.B. Oberman, H. Lee, W.K. Gotz, J.S. Harris, Jr., J. Crystal Growth 150 Ž1995. 912. w3x K. Yasui, K. Iizuka, T. Akahane, J. Vac. Sci. Technol. A 16 Ž1998. 39. w4x I. Berishev, E. Kim, A. Bensaoula, J. Vac. Sci. Technol. A 16 Ž1998. 2791. w5x Y. Chiba, Y. Shimizu, T. Tominari, S. Hokuto, Y. Nanishi, J. Crystal Growth 189r190 Ž1998. 317. w6x M. Sato, J. Appl. Phys. 78 Ž1995. 2123. w7x J.M. Van Hove, G.J. Cosimini, E. Nelson, A.M. Wowchak, P.P. Chow, J. Crystal Growth 150 Ž1995. 908. w8x T. Tokuda, A. Wakahara, S. Noda, A. Sasaki, J. Crystal Growth 173 Ž1997. 237. w9x E.J. Tarsa, B. Heying, X.H. Wu, P. Fini, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 82 Ž1997. 5472. w10x J.M. Myoung, K.H. Shim, O. Gluschenkov et al., J. Crystal Growth 182 Ž1997. 241.
473
w11x M.A. Sanchez-Garcia, E. Calleja, E. Monroy et al., J. Crystal Growth 183 Ž1998. 23. w12x T. Tokuda, A. Wakahara, S. Noda, A. Sasaki, J. Crystal Growth 183 Ž1998. 62. w13x N. Fujita, M. Yoshizawa, K. Kushi, H. Sasamoto, A. Kikuchi, K. Kishino, J. Crystal Growth 189r190 Ž1998. 385. w14x H. Okumura, H. Hamaguchi, T. Koizumi et al., J. Crystal Growth 189r190 Ž1998. 390. w15x J.D. Mackenzie, L. Abbaschian, C.R. Abernathy et al., J. Electron. Mater. 26 Ž1997. 1266. w16x R.P. Vaudo, I.D. Goepfert, T.D. Moustakas, D.M. Beyea, T.J. Frey, K. Meehan, J. Appl. Phys. 79 Ž1996. 2799. w17x R.P. Vaudo, J.W. Cook Jr., J.F. Schetzina, J. Vac. Sci. Technol. B 12 Ž1994. 1232. w18x R.P. Vaudo, Z. Yu, J.W. Cook Jr., J.F. Schetzina, Opt. Lett. 18 Ž1843. 1994. w19x J. Wang, Z. Zhu, K. Park, K. Hiraga, T. Yao, J. Crystal Growth 177 Ž1997. 181. w20x A.R. Stringanow, N.S. Sventitskii, Tables of Spectral Lines of Neutral and Ionized Atoms, Plenum, New York, 1968. w21x I.H. Hutchinson, Principles of Plasma Diagnostics, Cambridge University Press, 1987. w22x Y. Itikawa, M. Hayashi, A. Ichimura et al., J. Phys. Chem. Ref. Data 15 Ž1986. 985.