Kinetic Limits to Growth on GaAs by OMCVD
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KINETIC LIMITS TO GROWTH ON GaAs BY OMCVD D. E. ASPNES, R. BHAT, E. COLAS, M. A. KOZA, V. G. KERAMIDAS, and A. A. STUDNA Bellcore, Red Bank, NJ 07701-7040 ABSTRACT Using a real-time surface probe with 0.01 monolayer (ML) sensitivity, we determine the rate-limiting steps for atmospheric-pressure, alternating-layer-epitaxy OMCVD growth on (001) and (110) GaAs with trimethylgallium and arsine sources. The reaction of TMG with AsH 3saturated (001) GaAs is limited by a competition between decomposition (at 39 kcal/mole) and desorption of TMG chemisorbed (at -26 kcal/mole) via an excluded-volume mechanism. The reaction of AsH 3 with TMG-saturated (001) GaAs shows an initial fast transient followed by a slower recovery. On (110) GaAs, TMG reacts essentially instantaneously with an AsH 3-saturated surface while the reaction of AsH 3 with a TMG-saturated surface is relatively slow. In the latter case temperature and pressure dependences indicate a fast AsH3 -surface reaction that is blocked by an adsorbed species that must be desorbed before the AsH 3 -surface reaction can take place. INTRODUCTION Real-time measurements of growth surfaces are needed to obtain the improved understanding and control of growth processes that are necessary to meet material-quality and dimensional-accuracy requirements of evolving semiconductor technologies. However, the high pressure, highly reactive sample environments characteristic of organometallic chemical vapor deposition (OMCVD) preclude the use of conventional surface analysis techniques. Consequently, until very recently our knowledge of OMCVD growth has been indirect.1 16 To circumvent this limitation and to provide a way of comparing OMCVD growth surfaces with the better-known surfaces of molecular beam epitaxy (MBE), we are developing noninvasive, real-time methods of studying such surfaces with 0.01 monola 'er (ML) sensitivity. One of these approaches is reflectance-difference spectroscopy (RDS),'-' a near-normal-incidence visiblenear ultraviolet optical technique that can be used on most growth stations without their modification. RDS takes advantage of symmetry to suppress the normally dominant, but uninteresting, reflectance contribution from the bulk and to enhance the normally weak, but desired, reflectance contribution from the surface. Here, we use RDS to obtain information about the kinetic limits to OMCVD growth on (001) and (110) GaAs with trimethylgallium (TMG, Ga(CH 3 ) 3) and arsine
(AsH3) sources.20-22 EXPERIMENTAL
A schematic diagram of our OMCVD reflectance-difference (RD) spectrometer19 is shown in Fig. 1. The spectrometer is constructed from standard components: a 75 W Xe short-arc lamp, front-surface spherical and plane mirrors, a MgF2 Rochon polarizer, a 50 kHz photoelastic modulator, a quartz Rochon analyzer, a 0.1 m focusing-grating monochromator with 0.5 mm slits, and an extended S-20 photomultiplier detector. The spectral range is 1.5 to 6.0 eV. The monochromator exchanges spectral resolution for compact size, but its resolution is sufficient for typical g
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