- Email: [email protected]
Optical reflectivity studies of GaN and AlN chemical beam epitaxy on GaAs(100)
Diamond and Related Materials 8 (1999) 373–376
Optical reflectivity studies of GaN and AlN chemical beam epitaxy on GaAs(100) P.R. Chalker *, T.B. Joyce, T. Farrell Materials Science and Engineering, The University of Liverpool, Liverpool, L69 3BX, UK Received 27 July 1998; accepted 21 October 1998
Abstract The deposition of gallium nitride and aluminium nitride thin films on GaAs(100) substrates by chemical beam epitaxy is reported. In-situ dynamic optical reflectivity has been used to compare growth rates of the nitride layers as a function of substrate temperature with their arsenide analogues. The relative growth efficiency of gallium nitride/gallium arsenide from triethyl gallium was found to be in the range 75–85%. The growth temperature for gallium nitride extends to higher temperatures, compared with gallium arsenide, probably due to lower evaporation rates of Ga bound to the nitride surface. At the same beam equivalent pressure, the growth rate of aluminium nitride from ethyldimethyl aluminium alane is approximately one-third of that for gallium nitride from triethyl gallium. Atomic force microscopy reveals that the gallium nitride surface formed at 500 °C is facetted, whereas an aluminium nitride surface deposited at 400 °C exhibits a rounded columnar type growth habit. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs(100)-c(4×4) As stabilized surface at temperatures as low as 400 °C but without the gross facetting that has been observed at higher temperatures. © 1999 Elsevier Science S.A. All rights reserved. Keywords: nitrides; DOR; RAS; Chemical Beam Epitaxy
1. Introduction Cubic gallium nitride ( b-GaN ) has a number of potentially advantageous properties over the hexagonal material, for example b-GaN exhibits higher carrier mobilities and is more easily cleaved to form laser facets. These advantages have stimulated research into the formation of cubic GaN films via nitridation and deposition on gallium arsenide (GaAs) substrates [1–6 ], but exposure of GaAs to chemically active nitrogen atoms causes surface roughening and a highly facetted interface between the GaN layer and GaAs substrate [7] and the incorporation of more thermodynamically stable hexagonal GaN phases in the growing layer [3]. Ruvimov et al. [8] have recently characterized the microstructure of b-GaN epilayers grown on GaAs(001) grown by molecular beam epitaxy coupled with an r.f. plasma source to generate chemically active nitrogen. Exposure of the surface to an arsenic (As) flux during the growth of the first few monolayers and near-stoichiometric nucleation was found to produce a planar * Tel: +44 151 794 4313; Fax: +44 151 794 4675; e-mail: [email protected]
GaN–GaAs interface in contrast to studies using nitridation. Deviation from near-stoichiometric nucleation conditions caused interface roughening and introduction of the hexagonal phase material within the GaN layer. The nitridation of gallium arsenide surfaces under varying arsenic partial pressures has also been investigated by Jung et al. [9]. At low partial pressures, in-plane reflectance anisotropy spectra indicated that an As-stabilized (2×4)–GaAs(100) surface undergoes nitridation, via a (3×3) reconstruction when exposed to atomic nitrogen due to As desorption, to form a stable GaN layer. Nitridation of the As-stabilized c(4×4)GaAs(100) surface under a high partial pressure of arsenic at 300 °C showed a higher N/Ga ratio, as measured by Auger spectroscopy in comparison with other surface structures. After heating at 500 °C for 5 min, the N/Ga ratio decreased by a third, which was attributed to the loss of nitrogen bound to desorbed arsenic. It was also concluded that high background pressures of As greatly suppressed the nitridation process due to the competitive formation of As–N bonds rather than Ga–N bonds and that this condition is desirable for cubic GaN growth on gallium arsenide. The potential for high-frequency transistors or opto-
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 37 4 - 4
374
P.R. Chalker et al. / Diamond and Related Materials 8 (1999) 373–376
electronic devices with cleaved facets based on a cubic GaN/AlN materials system provides the motivation for characterizing the growth of these materials on a cubic substrate, namely gallium arsenide. This paper reports the deposition of gallium nitride and aluminium nitride on to GaAs(100) substrates by chemical beam epitaxy. The growth rates of the nitride layers have been assessed as a function of substrate temperature by dynamic optical reflectivity, and the surface structure of nitride films deposited at optimum growth temperatures has been investigated by atomic force microscopy. The initial formation of a nitrogen-terminated gallium arsenide surface at low temperatures has been characterized by reflectance anisotropy spectroscopy to investigate disordering of the GaAs(100)–c(4×4) arsenic-terminated surface at low temperatures.
2. Experimental All layers were grown by chemical beam epitaxy (CBE) in a VG V80H system configured for all vapour sources [10]. Triethylgallium (TEGa) and ethyl-dimethylamine-alane ( EDMAA) were used as the group III precursors. Arsine (AsH ) was precracked to give 3 an As flux for wafer cleaning and GaAs buffer 2 layer growth. An Oxford Applied Instruments HD25 13.56-MHz atom source was used to generate the neutral nitrogen atom beam. The source was operated with an applied power of 500 W and was fed with high-purity nitrogen gas, resulting in a deposition chamber pressure between 2 and 4×10−4 mbar during nitridation or growth. The reflectance anisotropy spectroscopy (RAS) monitoring equipment was based on the photo-elastic modulated design reported by Aspnes et al. [11]. Linearly polarized white light was introduced through a strain free window at near normal incidence to the sample. The reflected light entered a photoelastic modulator, a glan analyser and a monochromator and was detected with a silicon photo-diode. The output from the photodiode was fed via a lock-in amplifier tuned to the first harmonic of the photoelastic modulator frequency to a personal computer for real-time acquisition and analysis. In-situ dynamic optical reflectivity (DOR) [12] was used to measure the growth rate. The optical response of a growing thin film is governed by the interference of multiple reflections between the interfaces of layers with different refractive indices. The variation of reflectivity with increasing film thickness is a series of damped oscillations of periodicity l/2n , where l is the wavef length of the probe light and n is the refractive index f of the film. In the current experiments, the 514-nm emission of an argon ion laser was used at normal incidence to the growth surface to dynamically monitor the optical reflectivity.
3. Results and discussion Growth-rate experiments were performed by first depositing a GaAs buffer layer at 550 °C. The substrate was then stabilized at the nitride growth temperature, and the As flux used to stabilize the GaAs surface 2 against arsenic desorption was turned off as the nitrogen atom source was activated. Subsequently, the group III precursor was admitted into the chamber at a beam equivalent pressure of 2×10−5 mbar (TEGa) or 1×10−5 mbar ( EDMAA) for the respective nitride growth-rate measurement. The surface reflectivity varied during GaN growth showing characteristic Fabry–Perottype oscillations under illumination by a coherent source. A typical reflectivity response is shown in the inset of Fig. 1. GaN and AlN growth rates on GaAs(100) wafers were measured as a function of substrate temperature using refractive indices measured by Ambacher et al. [13] to calculate the growth rates from the periodicity of the DOR response. The temperature dependence of the GaN growth rate is compared with that for GaAs grown by CBE in Fig. 1. Both curves show an increase in growth rate as the temperature is raised above 400 °C, with a maximum at around 550 °C. The slow fall in growth rate as the substrate temperature is increased further is due to desorption of diethyl gallium from the surface before it can decompose. GaN does not display the sharp fall in
Fig. 1. GaN and AlN growth rate on GaAs(100) as a function of substrate temperature.
P.R. Chalker et al. / Diamond and Related Materials 8 (1999) 373–376
Fig. 2. (a) AFM image of a GaN film grown at 570 °C; (b) AFM image of an AlN film grown at 520 °C.
growth rate at around 700 °C seen for GaAs; this is attributed to desorption of Ga, which suggests that Ga is more strongly adsorbed at the GaN surface compared with GaAs. At temperatures below 700 °C, the GaN growth rate is lower than that for GaAs at the same TEGa beam equivalent pressure. The relative growth efficiency nitride/arsenide was found to be in the range 75–85%. In a comparable study, Mackenzie et al. [14] have reported a much lower value of 56%, but this may be due to their use of an ECR rather than an r.f. plasma nitrogen source. In the case of AlN, Fig. 1 shows that the growth rate of AlN is lower than would be expected from those of GaN and AlAs. Whereas the AlAs growth rate is approximately half that obtained for GaAs at 550 °C, as would be expected given the lower flux, that for AlN
375
is one-fifth of that of GaN. We have found that the instability of EDMAA can lead to a reduction in AlAs growth rate of the order of 25% due to premature decomposition under certain conditions, but the very much lower AlN growth rate cannot be explained by this alone. There is little variation in growth rate over the temperature range 440–600 °C for either AlN or AlAs, since the EDMAA is entirely decomposed at temperatures below 350 °C, and this suggests that the anomolous AlN growth rate arises from low nitrogen atom incorporation rather than being rate-limited by aluminium adsorption. Fig. 2 shows atomic force microscopy images of GaN and AlN layers deposited at 500 and 400 °C, respectively. The topographic scans were measured using a standard Digital Instruments Nanoscope III with the tip in contact mode. The surface of the GaN layer ( Fig. 2a) shows a very fine overlaid facetted structure with a mean roughness, R , from the image statistics (1×1 mm) of a 7.7 nm with an associated R (RMS) value of 9.9 nm. q By contrast, Fig. 2b shows the AlN layer exhibiting a growth habit of rounded features, which are also suggestive of a columnar structure. For a layer thickness of approximately 1 mm, the measured R is 2.7 nm, with a an R of 3.4 nm from the same scan area. The difference q between the two microstructures reflects the lower surface mobility of the more reactive aluminium adatoms (from fully decomposed EDMAA) compared with that of the gallium surface species. In the latter case, DEGa is known to play an important role, and this is reflected in the temperature dependence of growth rate. The influence of nitrogen on the surface flatness of GaAs has been previously investigated at higher growth temperatures [3]. To investigate the processes occurring at the CBE growth temperatures used in this study, RAS spectra were recorded whilst a GaAs(100) surface was exposed to atomic nitrogen at 400 °C. Fig. 3a shows the characteristic reflection anisotropy spectrum of a GaAs(100)–c(4×4)As stabilized surface at 520 °C. The pronounced dip in the RAS at 2.7 eV is due to As–As dimers, which are known to change their alignment from [110] to [1: 10] between the c(4×4)- and (2×4)like reconstructions. The sample was cooled to 400 °C and the As flux switched off ( Fig. 3b). Within 120 s 2 exposure of this surface to a nitrogen atom flux, the RAS structure dramatically changes ( Fig. 3c) and remains relatively unchanged thereafter. Fig. 3d shows the RAS spectrum of the surface after exposure to N atoms for 660 s. The form of the resulting surface suggests that the N atom treated GaAs surface has an anisotropic cubic character. Gross facetting, which occurs at higher temperatures, would be expected to disrupt the RAS signal, and we conclude that the surface planarity is effectively maintained, at least at these relatively low temperatures. Preliminary RAS measurements of AlN layers deposited on nitrided GaAs sub-
376
P.R. Chalker et al. / Diamond and Related Materials 8 (1999) 373–376
nitride surface deposited at 400 °C has a rounded columnar type growth habit. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs(100)-c(4×4) As stabilized surface at temperatures as low as 400 °C but without the gross facetting that has been observed at higher temperatures.
Acknowledgement The authors would like to thank the Engineering and Physical Sciences Research Council and the University of Liverpool for financial support.
References
Fig. 3. Reflection anisotropy spectra of a GaAs(100) surface exposed to an N atom flux at 400 °C.
strates subsequently annealed at 720 °C show a strong cubic anisotropy, but further surface studies are necessary to confirm these structures.
4. Conclusions The growth rates of gallium nitride and aluminium nitride deposited by chemical beam epitaxy have been compared with their arsenide analogues. The relative growth efficiency R /R was found to be in the GaN GaAs range 75–85%, and the growth temperature for GaN extends to higher temperatures compared with GaAs due to lower evaporation rates of Ga bound to the nitride surface. At the same beam equivalent pressure, the growth rate of aluminium nitride from ethyldimethyl aluminium alane is approximately a third of that for gallium nitride from triethyl gallium. Atomic force microscopy reveals that the gallium nitride surface formed at 500 °C is facetted, whereas an aluminium
[1] S. Strite, J. Ruan, Z. Li, A. Salvador, H. Chen, D.J. Smith, W.J. Choyke, H. Morkoc, J. Vac. Sci. Technol. B 9 (1991) 1924. [2] O. Brandt, H. Yang, B. Jenichen, Y. Suzuki, L. Dweritz, K.H. Ploog, Phys. Rev. B 52 (1992) R2253. [3] A. Kikuchi, H. Hoshi, K. Kishino, Jpn. J. Appl. Phys. 1 (33) (1994) 688. [4] T.S. Cheng, L.C. Jenkins, S.E. Hooper, C.T. Foxon, J.W. Orton, D.E. Lacklison, Appl. Phys. Lett. 66 (1995) 1509. [5] R.J. Hauenstein, D.A. Collins, X.P. Cai, M.L. O’Steen, T.C. McGill, Appl. Phys. Lett. 66 (1995) 2861. [6 ] Y. Yamauchi, K. Uwai, N. Kobayashi, Jpn. J. Appl. Phys. 1 (35) (1996) L80. [7] K. Kuwano, Y. Nagatomo, K. Kobayashi, K. Oki, S. Miyoshi, N. Yagu-chi, K. Onabe, Y. Shiraki, Jpn. J. Appl. Phys. 33 (1994) 18. [8] S. Ruvimov, Z. Liliental-Weber, J. Washburn, T.J. Drummond, M. Hafich, S.R. Lee, Appl. Phys. Lett. 71 (1997) 20. [9] H.D. Jung, N. Kumagai, T. Hanada, Z. Zhu, T. Yao, T. Yasuda, K. Kimura, J. Appl. Phys. 82 (9) (1997) 4684. [10] T.B. Joyce, T.J. Bullough, P. Kightley, Y.R. Xing, P.J. Goodhew, J. Cryst. Growth 120 (1992) 206. [11] D.E. Aspnes, J.P. Harbison, A.A. Studna, L.T. Florez, J. Vac. Sci. Technol. A 6 (1988) 1327. [12] J.V. Armstrong, T. Farrell, T.B. Joyce, P. Kightly, T.J. Bullough, P.J. Goodhew, J. Cryst. Growth 120 (1992) 84. [13] O. Ambacher, M. Arzberger, D. Brunner, H. Angerer, F. Freudenberg, N. Esser, T. Wethkamp, K. Wilmers, W. Richter, M. Stutzmann, MRS Internet J. Nitride Semicond. Res. 2 (1997) 22. [14] J.D. Mackenzie, C.R. Abernathy, J.D. Stewart, G.T. Muhr, J. Cryst. Growth 164 (1996) 143.
Optical reflectivity studies of GaN and AlN chemical beam epitaxy on GaAs(100) P.R. Chalker *, T.B. Joyce, T. Farrell Materials Science and Engineering, The University of Liverpool, Liverpool, L69 3BX, UK Received 27 July 1998; accepted 21 October 1998
Abstract The deposition of gallium nitride and aluminium nitride thin films on GaAs(100) substrates by chemical beam epitaxy is reported. In-situ dynamic optical reflectivity has been used to compare growth rates of the nitride layers as a function of substrate temperature with their arsenide analogues. The relative growth efficiency of gallium nitride/gallium arsenide from triethyl gallium was found to be in the range 75–85%. The growth temperature for gallium nitride extends to higher temperatures, compared with gallium arsenide, probably due to lower evaporation rates of Ga bound to the nitride surface. At the same beam equivalent pressure, the growth rate of aluminium nitride from ethyldimethyl aluminium alane is approximately one-third of that for gallium nitride from triethyl gallium. Atomic force microscopy reveals that the gallium nitride surface formed at 500 °C is facetted, whereas an aluminium nitride surface deposited at 400 °C exhibits a rounded columnar type growth habit. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs(100)-c(4×4) As stabilized surface at temperatures as low as 400 °C but without the gross facetting that has been observed at higher temperatures. © 1999 Elsevier Science S.A. All rights reserved. Keywords: nitrides; DOR; RAS; Chemical Beam Epitaxy
1. Introduction Cubic gallium nitride ( b-GaN ) has a number of potentially advantageous properties over the hexagonal material, for example b-GaN exhibits higher carrier mobilities and is more easily cleaved to form laser facets. These advantages have stimulated research into the formation of cubic GaN films via nitridation and deposition on gallium arsenide (GaAs) substrates [1–6 ], but exposure of GaAs to chemically active nitrogen atoms causes surface roughening and a highly facetted interface between the GaN layer and GaAs substrate [7] and the incorporation of more thermodynamically stable hexagonal GaN phases in the growing layer [3]. Ruvimov et al. [8] have recently characterized the microstructure of b-GaN epilayers grown on GaAs(001) grown by molecular beam epitaxy coupled with an r.f. plasma source to generate chemically active nitrogen. Exposure of the surface to an arsenic (As) flux during the growth of the first few monolayers and near-stoichiometric nucleation was found to produce a planar * Tel: +44 151 794 4313; Fax: +44 151 794 4675; e-mail: [email protected]
GaN–GaAs interface in contrast to studies using nitridation. Deviation from near-stoichiometric nucleation conditions caused interface roughening and introduction of the hexagonal phase material within the GaN layer. The nitridation of gallium arsenide surfaces under varying arsenic partial pressures has also been investigated by Jung et al. [9]. At low partial pressures, in-plane reflectance anisotropy spectra indicated that an As-stabilized (2×4)–GaAs(100) surface undergoes nitridation, via a (3×3) reconstruction when exposed to atomic nitrogen due to As desorption, to form a stable GaN layer. Nitridation of the As-stabilized c(4×4)GaAs(100) surface under a high partial pressure of arsenic at 300 °C showed a higher N/Ga ratio, as measured by Auger spectroscopy in comparison with other surface structures. After heating at 500 °C for 5 min, the N/Ga ratio decreased by a third, which was attributed to the loss of nitrogen bound to desorbed arsenic. It was also concluded that high background pressures of As greatly suppressed the nitridation process due to the competitive formation of As–N bonds rather than Ga–N bonds and that this condition is desirable for cubic GaN growth on gallium arsenide. The potential for high-frequency transistors or opto-
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 37 4 - 4
374
P.R. Chalker et al. / Diamond and Related Materials 8 (1999) 373–376
electronic devices with cleaved facets based on a cubic GaN/AlN materials system provides the motivation for characterizing the growth of these materials on a cubic substrate, namely gallium arsenide. This paper reports the deposition of gallium nitride and aluminium nitride on to GaAs(100) substrates by chemical beam epitaxy. The growth rates of the nitride layers have been assessed as a function of substrate temperature by dynamic optical reflectivity, and the surface structure of nitride films deposited at optimum growth temperatures has been investigated by atomic force microscopy. The initial formation of a nitrogen-terminated gallium arsenide surface at low temperatures has been characterized by reflectance anisotropy spectroscopy to investigate disordering of the GaAs(100)–c(4×4) arsenic-terminated surface at low temperatures.
2. Experimental All layers were grown by chemical beam epitaxy (CBE) in a VG V80H system configured for all vapour sources [10]. Triethylgallium (TEGa) and ethyl-dimethylamine-alane ( EDMAA) were used as the group III precursors. Arsine (AsH ) was precracked to give 3 an As flux for wafer cleaning and GaAs buffer 2 layer growth. An Oxford Applied Instruments HD25 13.56-MHz atom source was used to generate the neutral nitrogen atom beam. The source was operated with an applied power of 500 W and was fed with high-purity nitrogen gas, resulting in a deposition chamber pressure between 2 and 4×10−4 mbar during nitridation or growth. The reflectance anisotropy spectroscopy (RAS) monitoring equipment was based on the photo-elastic modulated design reported by Aspnes et al. [11]. Linearly polarized white light was introduced through a strain free window at near normal incidence to the sample. The reflected light entered a photoelastic modulator, a glan analyser and a monochromator and was detected with a silicon photo-diode. The output from the photodiode was fed via a lock-in amplifier tuned to the first harmonic of the photoelastic modulator frequency to a personal computer for real-time acquisition and analysis. In-situ dynamic optical reflectivity (DOR) [12] was used to measure the growth rate. The optical response of a growing thin film is governed by the interference of multiple reflections between the interfaces of layers with different refractive indices. The variation of reflectivity with increasing film thickness is a series of damped oscillations of periodicity l/2n , where l is the wavef length of the probe light and n is the refractive index f of the film. In the current experiments, the 514-nm emission of an argon ion laser was used at normal incidence to the growth surface to dynamically monitor the optical reflectivity.
3. Results and discussion Growth-rate experiments were performed by first depositing a GaAs buffer layer at 550 °C. The substrate was then stabilized at the nitride growth temperature, and the As flux used to stabilize the GaAs surface 2 against arsenic desorption was turned off as the nitrogen atom source was activated. Subsequently, the group III precursor was admitted into the chamber at a beam equivalent pressure of 2×10−5 mbar (TEGa) or 1×10−5 mbar ( EDMAA) for the respective nitride growth-rate measurement. The surface reflectivity varied during GaN growth showing characteristic Fabry–Perottype oscillations under illumination by a coherent source. A typical reflectivity response is shown in the inset of Fig. 1. GaN and AlN growth rates on GaAs(100) wafers were measured as a function of substrate temperature using refractive indices measured by Ambacher et al. [13] to calculate the growth rates from the periodicity of the DOR response. The temperature dependence of the GaN growth rate is compared with that for GaAs grown by CBE in Fig. 1. Both curves show an increase in growth rate as the temperature is raised above 400 °C, with a maximum at around 550 °C. The slow fall in growth rate as the substrate temperature is increased further is due to desorption of diethyl gallium from the surface before it can decompose. GaN does not display the sharp fall in
Fig. 1. GaN and AlN growth rate on GaAs(100) as a function of substrate temperature.
P.R. Chalker et al. / Diamond and Related Materials 8 (1999) 373–376
Fig. 2. (a) AFM image of a GaN film grown at 570 °C; (b) AFM image of an AlN film grown at 520 °C.
growth rate at around 700 °C seen for GaAs; this is attributed to desorption of Ga, which suggests that Ga is more strongly adsorbed at the GaN surface compared with GaAs. At temperatures below 700 °C, the GaN growth rate is lower than that for GaAs at the same TEGa beam equivalent pressure. The relative growth efficiency nitride/arsenide was found to be in the range 75–85%. In a comparable study, Mackenzie et al. [14] have reported a much lower value of 56%, but this may be due to their use of an ECR rather than an r.f. plasma nitrogen source. In the case of AlN, Fig. 1 shows that the growth rate of AlN is lower than would be expected from those of GaN and AlAs. Whereas the AlAs growth rate is approximately half that obtained for GaAs at 550 °C, as would be expected given the lower flux, that for AlN
375
is one-fifth of that of GaN. We have found that the instability of EDMAA can lead to a reduction in AlAs growth rate of the order of 25% due to premature decomposition under certain conditions, but the very much lower AlN growth rate cannot be explained by this alone. There is little variation in growth rate over the temperature range 440–600 °C for either AlN or AlAs, since the EDMAA is entirely decomposed at temperatures below 350 °C, and this suggests that the anomolous AlN growth rate arises from low nitrogen atom incorporation rather than being rate-limited by aluminium adsorption. Fig. 2 shows atomic force microscopy images of GaN and AlN layers deposited at 500 and 400 °C, respectively. The topographic scans were measured using a standard Digital Instruments Nanoscope III with the tip in contact mode. The surface of the GaN layer ( Fig. 2a) shows a very fine overlaid facetted structure with a mean roughness, R , from the image statistics (1×1 mm) of a 7.7 nm with an associated R (RMS) value of 9.9 nm. q By contrast, Fig. 2b shows the AlN layer exhibiting a growth habit of rounded features, which are also suggestive of a columnar structure. For a layer thickness of approximately 1 mm, the measured R is 2.7 nm, with a an R of 3.4 nm from the same scan area. The difference q between the two microstructures reflects the lower surface mobility of the more reactive aluminium adatoms (from fully decomposed EDMAA) compared with that of the gallium surface species. In the latter case, DEGa is known to play an important role, and this is reflected in the temperature dependence of growth rate. The influence of nitrogen on the surface flatness of GaAs has been previously investigated at higher growth temperatures [3]. To investigate the processes occurring at the CBE growth temperatures used in this study, RAS spectra were recorded whilst a GaAs(100) surface was exposed to atomic nitrogen at 400 °C. Fig. 3a shows the characteristic reflection anisotropy spectrum of a GaAs(100)–c(4×4)As stabilized surface at 520 °C. The pronounced dip in the RAS at 2.7 eV is due to As–As dimers, which are known to change their alignment from [110] to [1: 10] between the c(4×4)- and (2×4)like reconstructions. The sample was cooled to 400 °C and the As flux switched off ( Fig. 3b). Within 120 s 2 exposure of this surface to a nitrogen atom flux, the RAS structure dramatically changes ( Fig. 3c) and remains relatively unchanged thereafter. Fig. 3d shows the RAS spectrum of the surface after exposure to N atoms for 660 s. The form of the resulting surface suggests that the N atom treated GaAs surface has an anisotropic cubic character. Gross facetting, which occurs at higher temperatures, would be expected to disrupt the RAS signal, and we conclude that the surface planarity is effectively maintained, at least at these relatively low temperatures. Preliminary RAS measurements of AlN layers deposited on nitrided GaAs sub-
376
P.R. Chalker et al. / Diamond and Related Materials 8 (1999) 373–376
nitride surface deposited at 400 °C has a rounded columnar type growth habit. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs(100)-c(4×4) As stabilized surface at temperatures as low as 400 °C but without the gross facetting that has been observed at higher temperatures.
Acknowledgement The authors would like to thank the Engineering and Physical Sciences Research Council and the University of Liverpool for financial support.
References
Fig. 3. Reflection anisotropy spectra of a GaAs(100) surface exposed to an N atom flux at 400 °C.
strates subsequently annealed at 720 °C show a strong cubic anisotropy, but further surface studies are necessary to confirm these structures.
4. Conclusions The growth rates of gallium nitride and aluminium nitride deposited by chemical beam epitaxy have been compared with their arsenide analogues. The relative growth efficiency R /R was found to be in the GaN GaAs range 75–85%, and the growth temperature for GaN extends to higher temperatures compared with GaAs due to lower evaporation rates of Ga bound to the nitride surface. At the same beam equivalent pressure, the growth rate of aluminium nitride from ethyldimethyl aluminium alane is approximately a third of that for gallium nitride from triethyl gallium. Atomic force microscopy reveals that the gallium nitride surface formed at 500 °C is facetted, whereas an aluminium
[1] S. Strite, J. Ruan, Z. Li, A. Salvador, H. Chen, D.J. Smith, W.J. Choyke, H. Morkoc, J. Vac. Sci. Technol. B 9 (1991) 1924. [2] O. Brandt, H. Yang, B. Jenichen, Y. Suzuki, L. Dweritz, K.H. Ploog, Phys. Rev. B 52 (1992) R2253. [3] A. Kikuchi, H. Hoshi, K. Kishino, Jpn. J. Appl. Phys. 1 (33) (1994) 688. [4] T.S. Cheng, L.C. Jenkins, S.E. Hooper, C.T. Foxon, J.W. Orton, D.E. Lacklison, Appl. Phys. Lett. 66 (1995) 1509. [5] R.J. Hauenstein, D.A. Collins, X.P. Cai, M.L. O’Steen, T.C. McGill, Appl. Phys. Lett. 66 (1995) 2861. [6 ] Y. Yamauchi, K. Uwai, N. Kobayashi, Jpn. J. Appl. Phys. 1 (35) (1996) L80. [7] K. Kuwano, Y. Nagatomo, K. Kobayashi, K. Oki, S. Miyoshi, N. Yagu-chi, K. Onabe, Y. Shiraki, Jpn. J. Appl. Phys. 33 (1994) 18. [8] S. Ruvimov, Z. Liliental-Weber, J. Washburn, T.J. Drummond, M. Hafich, S.R. Lee, Appl. Phys. Lett. 71 (1997) 20. [9] H.D. Jung, N. Kumagai, T. Hanada, Z. Zhu, T. Yao, T. Yasuda, K. Kimura, J. Appl. Phys. 82 (9) (1997) 4684. [10] T.B. Joyce, T.J. Bullough, P. Kightley, Y.R. Xing, P.J. Goodhew, J. Cryst. Growth 120 (1992) 206. [11] D.E. Aspnes, J.P. Harbison, A.A. Studna, L.T. Florez, J. Vac. Sci. Technol. A 6 (1988) 1327. [12] J.V. Armstrong, T. Farrell, T.B. Joyce, P. Kightly, T.J. Bullough, P.J. Goodhew, J. Cryst. Growth 120 (1992) 84. [13] O. Ambacher, M. Arzberger, D. Brunner, H. Angerer, F. Freudenberg, N. Esser, T. Wethkamp, K. Wilmers, W. Richter, M. Stutzmann, MRS Internet J. Nitride Semicond. Res. 2 (1997) 22. [14] J.D. Mackenzie, C.R. Abernathy, J.D. Stewart, G.T. Muhr, J. Cryst. Growth 164 (1996) 143.