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Influence of N2 carrier gas on surface stoichiometry in GaN MOVPE studied by surface photoabsorption
Journal of Crystal Growth 189/190 (1998) 301—304
Influence of N carrier gas on surface stoichiometry in GaN 2 MOVPE studied by surface photoabsorption Yasuyuki Kobayashi*, Naoki Kobayashi NTT Basic Research Laboratories, 3-1 Morinosato, Wakamiya, Atsugi-shi, Kanagawa 243-01, Japan
Abstract The surface stoichiometry of GaN grown by metal-organic vapor-phase epitaxy (MOVPE) on (0 0 0 1) sapphire substrate at temperatures up to about 1000°C in N and H carrier gases was monitored in situ by surface photoabsorp2 2 tion (SPA). In the N carrier gas with NH supply, a stable N-rich surface was formed at temperatures up to 1030°C. In 2 3 contrast, the surface in H carrier gas was N-rich at temperatures below 850—900°C. Above these temperatures the 2 surface became Ga-rich. These results indicate that GaN MOVPE growth at temperatures around 1000°C proceeds under N-rich and Ga-rich surface stoichiometry in N and H , respectively. The N desorption rate in N was lower than 2 2 2 the rate in H , indicating that the N carrier gas suppresses the N desorption from the GaN MOVPE surface compared 2 2 with H . ( 1998 Elsevier Science B.V. All rights reserved. 2 PACS: 81.05.Ea; 81.15.Gh; 82.30.Lp Keywords: GaN; MOVPE; Surface photo-absorption; Carrier gas; H ; N 2 2
1. Introduction In GaN and AlGaN metal-organic vapor-phase epitaxy (MOVPE), H carrier gas is normally used 2 to ensure high-quality growth. However, a highquality InGaN MOVPE layer can be grown by using N carrier gas instead of H [1]. Koukitu et 2 2 al.’s thermodynamic calculations suggested that In content increases when the H partial pressure is 2 * Corresponding author. Tel.: #81 462 40 3408; fax: 4729; e-mail: [email protected].
reduced during InGaN growth [2]. Scholz et al. reported that the In content could be increased by reducing the H /N flow ratio in the main carrier 2 2 gas [3]. These results indicate that surface stoichiometry during MOVPE varies in growths using H and N carrier gases. The surface 2 2 stoichiometry during GaN MOVPE, however, is an open question. To analyze this, in situ monitoring of the surface during growth is needed. Optical reflectance is often used to control thickness in GaN MOVPE. From the oscillation period observed, the thickness of the GaN and AlGaN layers and their growth rates can be monitored [4]. There
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are, however, few reports of in situ monitoring by surface sensitive methods like reflectance difference spectroscopy [5] and surface photoabsorption (SPA) [6]. One big problem in monitoring the GaN surface at high temperatures is how to minimize the contribution of black-body radiation from the substrate holder heated at above 1000°C. Since the radiation contains light in the visible and nearinfrared regions, it is difficult to obtain a high signal-to-noise ratio with these surface-sensitive methods. In this paper, we succeeded in monitoring the surface by SPA during GaN MOVPE by using a 325 nm light to minimize the effect of the radiation. As a result, we clarified the surface stoichiometry changes with the substrate temperature and the carrier gas.
2. Experimental procedure The GaN layer was grown in a vertical, lowpressure (76 Torr) MOVPE reactor. The (0 0 0 1) sapphire substrates we used were cleaned with organic solvents. They were then heated to 1100°C for 5 min in a H flow. The sapphire surfaces were next 2 exposed to NH at 1050°C for sapphire nitridation 3 for 5 min. A GaN buffer layer was deposited on the substrates at a substrate temperature of 600°C, and then the substrate temperature was increased to 1000°C for GaN epitaxial growth. The growth rate was 0.6 lm/h and the growth time was 2 h. Triethylgallium was used to grow the GaN buffer layer and trimethylgallium (TMG) was used to grow a GaN epitaxial layer at 1000°C. We used NH as the N source. The flow rate for N or 3 2 H was 5.7 slm. For SPA monitoring, linearly p2 polarized light from a He—Cd laser with a 325 nm line was irradiated on the GaN epitaxial surfaces at an incidence angle of 75°. By using this light, the reflectivity becomes highly surface-sensitive as a result of minimizing the contribution of black-body radiation from the heated substrate holder and minimizing the reflection from the bulk crystal. For a typical GaN layer, the FWHM of the double crystal X-ray rocking curve for the (0 0 0 2) diffraction was 310 arcsec and the FWHM of the photoluminescence at 10 K was about 8 meV.
3. Results and discussions The reflectivity observed in the SPA optical configuration includes both the surface-sensitive part and the reflection part from the bulk. The surfacesensitive part is related to the photoabsorption of surface chemical bond [7]. This part changes with the surface stoichiometry. In fact, the reflectivity from the growing GaN surface in SPA changes considerably with the substrate temperature, NH 3 flow rate, and the carrier gas. Fig. 1 compares the changes in reflectivity at different substrate temperatures for N and H carrier gases. In our experi2 2 ment, NH is supplied to the GaN surfaces. The 3 NH flow rate (0.25 slm) was the same as that used 3 for GaN growth at 1000°C. The substrate temperature was varied from 650 to 1030°C. In H carrier 2 gas, the reflectivity stays constant up to 720°C (region A), then gradually decreases (transition region) and becomes constant again at a lower value above 850°C (region B). This reflectivity change was observed with good reproducibility. No surface degradation such as Ga droplet formation was observed. Therefore, we conclude that this reflectivity change is caused by the change in surface stoichiometry, i.e., the surface coverage change of N atoms. This result indicates that two distinct surface stoichiometry states exist in this substrate temperature range. In contrast, in N carrier gas, 2 the reflectivity stays almost constant from 650 to
Fig. 1. Dependence of reflectivity on substrate temperature under NH supply to the GaN surface in N and H . 3 2 2
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1030°C (region A). This result indicates that the surface stoichiometry in N carrier gas is controlled 2 by only one surface phase up to around 1030°C. To clarify the surface phase, we monitored the desorption process in H and N carrier gases by 2 2 SPA. Fig. 2 shows a typical reflectivity change in N carrier gas at 1000°C when NH supply was 2 3 turned on and off. The NH flow rate was 0.25 slm 3 and TMG supply was stopped. The reflectivity remained constant during the NH supply. When the 3 supply was stopped, the reflectivity decreased and saturated at a certain level. This level is almost the same as that observed in H carrier gas above 2 850°C. A reflectivity change was also observed in H carrier gas in region A when NH supply was 2 3 stopped. This reflectivity change is considered to be caused by the desorption of N-related surface adsorbed species by the following reason. The desorption rate constants were measured from the reflectivity change. When NH supply was stopped, 3 the reflectivity decreased. The time period of decay in the reflectivity change due to the desorption can be defined as the duration needed for e~1 decay. Here, an inverse of the time period of decay corresponds to the desorption rate constant (k ). Fig. 3 $%4 shows Arrhenius plots of the rate constants for desorption from (0 0 0 1) GaN surface in N and 2 H carrier gases. We also plotted the desorption 2 rate constants of P from (0 0 1) GaP and As from (0 0 1) GaAs for references [8]. The rates of desorption from the (0 0 0 1) GaN surface in N and 2 H are lower than the rates of P desorption from 2 GaP and As desorption from GaAs surfaces, indicating that the adsorbed species on (0 0 0 1) GaN have a stronger bond than P on GaP and As on GaAs. The measured activation energies are 1.88 eV (in N ) and 0.91 eV (in H ); these energies 2 2 are much smaller than that reported for the evaporation of Ga (2.8 eV) [9]. In addition, the vapor pressure of N is much higher than that of Ga, and N atoms desorb more easily than Ga atoms. Therefore, we conclude that the reflectivity drop observed corresponds to the N atom desorption from the GaN surface when the NH supply is stopped. 3 When NH was supplied to the Ga-rich surface, the 3 reflectivity recovered in the N-rich surface because of the decomposition of NH . In the N carrier gas, 3 2 a reflectivity change resulting from the N desorp-
303
Fig. 2. Typical reflectivity change when NH supply is turned 3 on and off at 1000°C in N . 2
Fig. 3. Arrhenius plots of desorption rate constants from GaN in N and H , GaP, and GaAs. 2 2
tion could be observed at temperatures up to 1030°C with good reproducibility, indicating that in this gas the N-rich surface (region A) was stable at up to 1030°C under the NH flow. In the H carrier 3 2 gas, even when the NH supply to the GaN surface 3 was stopped, the same reflectivity change could be observed in region A, indicating that in this region the GaN surface was N-rich during the NH sup3 ply. In contrast, the amplitude of the reflectivity change in the transition region decreased as the substrate temperature increased, and in region B no reflectivity change was observed. These results indicate that the GaN surfaces in H carrier gas 2 changed from N-rich to Ga-rich surfaces in the transition region and a stable Ga-rich surface was
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formed in region B. In this region the residence lifetime of adsorbed N atoms on the Ga-rich surfaces is too small to form N-rich surface and therefore no reflectivity change was observed. Thus, in H carrier gas, the surface below 720°C is N-rich 2 and the surface above 850°C is Ga-rich. To summarize these results, at the growth temperature used for GaN MOVPE, the surface is N-rich in N carrier gas and is Ga-rich in H . These results 2 2 also suggest that N desorption from the GaN surface in N carrier gas is remarkably suppressed 2 compared with that in H (the lifetime of an 2 N atom on the GaN surface is 4 s at 1000°C). The activation energy (1.88 eV) of N desorption in N was larger than that (0.98 eV) in H . One pos2 2 sible explanation for this difference is that H car2 rier gas may react with N atoms on the surface and enhance the N desorption by etching from the GaN surface, whereas the reaction may not occur in N carrier gas. These results also support the use of 2 N carrier gas in InGaN MOVPE because N 2 2 carrier gas should suppress the N desorption from the surface, and, as a result, suppress In droplet formation on the surface or In desorption from the surface.
4. Conclusions We monitored the N desorption process on a GaN MOVPE surface in N carrier gas in situ 2 using SPA and compared it with that in H . Under 2 a NH flow, a Ga-rich surface was formed at tem3
peratures above 850°C in H carrier gas, whereas 2 a stable N-rich surface is formed at temperatures up to 1030°C in N carrier gas. The N desorption 2 measurement indicates that the N carrier gas sup2 presses N desorption compared with H carrier 2 gas.
Acknowledgements We would like to thank Naosi Uesugi and Tatsuo Izawa for their encouragement throughout this study.
References [1] S. Nakamura, T. Mukai, Jpn. J. Appl. Phys. 31 (1992) L1457. [2] A. Koukitu, N. Takahashi, T. Taki, H. Seki, J. Crystal Growth 170 (1997) 306. [3] F. Scholz, V. Harle, F. Steuber, H. Bolay, A. Dornen, B. Kaufmann, V. Syganow, A. Hangleiter, J. Crystal Growth 170 (1997) 321. [4] B. Beaumont, M. Vaille, T. Boufaden, B. el Jani, P. Gibart, J. Crystal Growth 170 (1997) 316. [5] D.E. Aspnes, J.P. Harbison, A.A. Studna, L.T. Florez, Phys. Rev. Lett. 59 (1987) 1687. [6] N. Kobayshi, Y. Horikoshi, Jpn. J. Appl. Phys. 28 (1989) L1880. [7] N. Kobayashi, Y. Kobayashi, K. Uwai, J. Crystal Growth 170 (1997) 225. [8] N. Kobayashi, Y. Kobayashi, Jpn. J. Appl. Phys. 30 (1991) L1699. [9] Determined from R.E. Honig, D.A. Kramer, RCA Rev. 30 (1969) 285.
Influence of N carrier gas on surface stoichiometry in GaN 2 MOVPE studied by surface photoabsorption Yasuyuki Kobayashi*, Naoki Kobayashi NTT Basic Research Laboratories, 3-1 Morinosato, Wakamiya, Atsugi-shi, Kanagawa 243-01, Japan
Abstract The surface stoichiometry of GaN grown by metal-organic vapor-phase epitaxy (MOVPE) on (0 0 0 1) sapphire substrate at temperatures up to about 1000°C in N and H carrier gases was monitored in situ by surface photoabsorp2 2 tion (SPA). In the N carrier gas with NH supply, a stable N-rich surface was formed at temperatures up to 1030°C. In 2 3 contrast, the surface in H carrier gas was N-rich at temperatures below 850—900°C. Above these temperatures the 2 surface became Ga-rich. These results indicate that GaN MOVPE growth at temperatures around 1000°C proceeds under N-rich and Ga-rich surface stoichiometry in N and H , respectively. The N desorption rate in N was lower than 2 2 2 the rate in H , indicating that the N carrier gas suppresses the N desorption from the GaN MOVPE surface compared 2 2 with H . ( 1998 Elsevier Science B.V. All rights reserved. 2 PACS: 81.05.Ea; 81.15.Gh; 82.30.Lp Keywords: GaN; MOVPE; Surface photo-absorption; Carrier gas; H ; N 2 2
1. Introduction In GaN and AlGaN metal-organic vapor-phase epitaxy (MOVPE), H carrier gas is normally used 2 to ensure high-quality growth. However, a highquality InGaN MOVPE layer can be grown by using N carrier gas instead of H [1]. Koukitu et 2 2 al.’s thermodynamic calculations suggested that In content increases when the H partial pressure is 2 * Corresponding author. Tel.: #81 462 40 3408; fax: 4729; e-mail: [email protected].
reduced during InGaN growth [2]. Scholz et al. reported that the In content could be increased by reducing the H /N flow ratio in the main carrier 2 2 gas [3]. These results indicate that surface stoichiometry during MOVPE varies in growths using H and N carrier gases. The surface 2 2 stoichiometry during GaN MOVPE, however, is an open question. To analyze this, in situ monitoring of the surface during growth is needed. Optical reflectance is often used to control thickness in GaN MOVPE. From the oscillation period observed, the thickness of the GaN and AlGaN layers and their growth rates can be monitored [4]. There
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are, however, few reports of in situ monitoring by surface sensitive methods like reflectance difference spectroscopy [5] and surface photoabsorption (SPA) [6]. One big problem in monitoring the GaN surface at high temperatures is how to minimize the contribution of black-body radiation from the substrate holder heated at above 1000°C. Since the radiation contains light in the visible and nearinfrared regions, it is difficult to obtain a high signal-to-noise ratio with these surface-sensitive methods. In this paper, we succeeded in monitoring the surface by SPA during GaN MOVPE by using a 325 nm light to minimize the effect of the radiation. As a result, we clarified the surface stoichiometry changes with the substrate temperature and the carrier gas.
2. Experimental procedure The GaN layer was grown in a vertical, lowpressure (76 Torr) MOVPE reactor. The (0 0 0 1) sapphire substrates we used were cleaned with organic solvents. They were then heated to 1100°C for 5 min in a H flow. The sapphire surfaces were next 2 exposed to NH at 1050°C for sapphire nitridation 3 for 5 min. A GaN buffer layer was deposited on the substrates at a substrate temperature of 600°C, and then the substrate temperature was increased to 1000°C for GaN epitaxial growth. The growth rate was 0.6 lm/h and the growth time was 2 h. Triethylgallium was used to grow the GaN buffer layer and trimethylgallium (TMG) was used to grow a GaN epitaxial layer at 1000°C. We used NH as the N source. The flow rate for N or 3 2 H was 5.7 slm. For SPA monitoring, linearly p2 polarized light from a He—Cd laser with a 325 nm line was irradiated on the GaN epitaxial surfaces at an incidence angle of 75°. By using this light, the reflectivity becomes highly surface-sensitive as a result of minimizing the contribution of black-body radiation from the heated substrate holder and minimizing the reflection from the bulk crystal. For a typical GaN layer, the FWHM of the double crystal X-ray rocking curve for the (0 0 0 2) diffraction was 310 arcsec and the FWHM of the photoluminescence at 10 K was about 8 meV.
3. Results and discussions The reflectivity observed in the SPA optical configuration includes both the surface-sensitive part and the reflection part from the bulk. The surfacesensitive part is related to the photoabsorption of surface chemical bond [7]. This part changes with the surface stoichiometry. In fact, the reflectivity from the growing GaN surface in SPA changes considerably with the substrate temperature, NH 3 flow rate, and the carrier gas. Fig. 1 compares the changes in reflectivity at different substrate temperatures for N and H carrier gases. In our experi2 2 ment, NH is supplied to the GaN surfaces. The 3 NH flow rate (0.25 slm) was the same as that used 3 for GaN growth at 1000°C. The substrate temperature was varied from 650 to 1030°C. In H carrier 2 gas, the reflectivity stays constant up to 720°C (region A), then gradually decreases (transition region) and becomes constant again at a lower value above 850°C (region B). This reflectivity change was observed with good reproducibility. No surface degradation such as Ga droplet formation was observed. Therefore, we conclude that this reflectivity change is caused by the change in surface stoichiometry, i.e., the surface coverage change of N atoms. This result indicates that two distinct surface stoichiometry states exist in this substrate temperature range. In contrast, in N carrier gas, 2 the reflectivity stays almost constant from 650 to
Fig. 1. Dependence of reflectivity on substrate temperature under NH supply to the GaN surface in N and H . 3 2 2
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1030°C (region A). This result indicates that the surface stoichiometry in N carrier gas is controlled 2 by only one surface phase up to around 1030°C. To clarify the surface phase, we monitored the desorption process in H and N carrier gases by 2 2 SPA. Fig. 2 shows a typical reflectivity change in N carrier gas at 1000°C when NH supply was 2 3 turned on and off. The NH flow rate was 0.25 slm 3 and TMG supply was stopped. The reflectivity remained constant during the NH supply. When the 3 supply was stopped, the reflectivity decreased and saturated at a certain level. This level is almost the same as that observed in H carrier gas above 2 850°C. A reflectivity change was also observed in H carrier gas in region A when NH supply was 2 3 stopped. This reflectivity change is considered to be caused by the desorption of N-related surface adsorbed species by the following reason. The desorption rate constants were measured from the reflectivity change. When NH supply was stopped, 3 the reflectivity decreased. The time period of decay in the reflectivity change due to the desorption can be defined as the duration needed for e~1 decay. Here, an inverse of the time period of decay corresponds to the desorption rate constant (k ). Fig. 3 $%4 shows Arrhenius plots of the rate constants for desorption from (0 0 0 1) GaN surface in N and 2 H carrier gases. We also plotted the desorption 2 rate constants of P from (0 0 1) GaP and As from (0 0 1) GaAs for references [8]. The rates of desorption from the (0 0 0 1) GaN surface in N and 2 H are lower than the rates of P desorption from 2 GaP and As desorption from GaAs surfaces, indicating that the adsorbed species on (0 0 0 1) GaN have a stronger bond than P on GaP and As on GaAs. The measured activation energies are 1.88 eV (in N ) and 0.91 eV (in H ); these energies 2 2 are much smaller than that reported for the evaporation of Ga (2.8 eV) [9]. In addition, the vapor pressure of N is much higher than that of Ga, and N atoms desorb more easily than Ga atoms. Therefore, we conclude that the reflectivity drop observed corresponds to the N atom desorption from the GaN surface when the NH supply is stopped. 3 When NH was supplied to the Ga-rich surface, the 3 reflectivity recovered in the N-rich surface because of the decomposition of NH . In the N carrier gas, 3 2 a reflectivity change resulting from the N desorp-
303
Fig. 2. Typical reflectivity change when NH supply is turned 3 on and off at 1000°C in N . 2
Fig. 3. Arrhenius plots of desorption rate constants from GaN in N and H , GaP, and GaAs. 2 2
tion could be observed at temperatures up to 1030°C with good reproducibility, indicating that in this gas the N-rich surface (region A) was stable at up to 1030°C under the NH flow. In the H carrier 3 2 gas, even when the NH supply to the GaN surface 3 was stopped, the same reflectivity change could be observed in region A, indicating that in this region the GaN surface was N-rich during the NH sup3 ply. In contrast, the amplitude of the reflectivity change in the transition region decreased as the substrate temperature increased, and in region B no reflectivity change was observed. These results indicate that the GaN surfaces in H carrier gas 2 changed from N-rich to Ga-rich surfaces in the transition region and a stable Ga-rich surface was
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formed in region B. In this region the residence lifetime of adsorbed N atoms on the Ga-rich surfaces is too small to form N-rich surface and therefore no reflectivity change was observed. Thus, in H carrier gas, the surface below 720°C is N-rich 2 and the surface above 850°C is Ga-rich. To summarize these results, at the growth temperature used for GaN MOVPE, the surface is N-rich in N carrier gas and is Ga-rich in H . These results 2 2 also suggest that N desorption from the GaN surface in N carrier gas is remarkably suppressed 2 compared with that in H (the lifetime of an 2 N atom on the GaN surface is 4 s at 1000°C). The activation energy (1.88 eV) of N desorption in N was larger than that (0.98 eV) in H . One pos2 2 sible explanation for this difference is that H car2 rier gas may react with N atoms on the surface and enhance the N desorption by etching from the GaN surface, whereas the reaction may not occur in N carrier gas. These results also support the use of 2 N carrier gas in InGaN MOVPE because N 2 2 carrier gas should suppress the N desorption from the surface, and, as a result, suppress In droplet formation on the surface or In desorption from the surface.
4. Conclusions We monitored the N desorption process on a GaN MOVPE surface in N carrier gas in situ 2 using SPA and compared it with that in H . Under 2 a NH flow, a Ga-rich surface was formed at tem3
peratures above 850°C in H carrier gas, whereas 2 a stable N-rich surface is formed at temperatures up to 1030°C in N carrier gas. The N desorption 2 measurement indicates that the N carrier gas sup2 presses N desorption compared with H carrier 2 gas.
Acknowledgements We would like to thank Naosi Uesugi and Tatsuo Izawa for their encouragement throughout this study.
References [1] S. Nakamura, T. Mukai, Jpn. J. Appl. Phys. 31 (1992) L1457. [2] A. Koukitu, N. Takahashi, T. Taki, H. Seki, J. Crystal Growth 170 (1997) 306. [3] F. Scholz, V. Harle, F. Steuber, H. Bolay, A. Dornen, B. Kaufmann, V. Syganow, A. Hangleiter, J. Crystal Growth 170 (1997) 321. [4] B. Beaumont, M. Vaille, T. Boufaden, B. el Jani, P. Gibart, J. Crystal Growth 170 (1997) 316. [5] D.E. Aspnes, J.P. Harbison, A.A. Studna, L.T. Florez, Phys. Rev. Lett. 59 (1987) 1687. [6] N. Kobayshi, Y. Horikoshi, Jpn. J. Appl. Phys. 28 (1989) L1880. [7] N. Kobayashi, Y. Kobayashi, K. Uwai, J. Crystal Growth 170 (1997) 225. [8] N. Kobayashi, Y. Kobayashi, Jpn. J. Appl. Phys. 30 (1991) L1699. [9] Determined from R.E. Honig, D.A. Kramer, RCA Rev. 30 (1969) 285.