IN-SITU ELLIPSOMETRIC STUDY OF AS CAPPING AND LOW-TEMPERATURE MOLECULAR-BEAM EPITAXY GAAS GROWTH AND IMPLICATIONS FOR THE LOW-TEMPERATURE CRITICAL THICKNESS

Low temperature GaAs (LT-GaAs) has been demonstrated to be a useful hi gh resistivity buffer layer for subsequent growth of a variety of epit axial device layers. It has proven difficult to reproducibly grow thes e films due to the low (approximately 250-degrees-C) substrate tempera tures involved and the need for tight III/V flux control. In this work in situ ellipsometry was used to reproducibly set the substrate tempe rature through a study of the As capping process on a GaAs surface. A hysteresis of approximately 200-degrees-C occurred in the temperature at which As was deposited and subsequently removed. Subtle changes occ ur in the ellipsometric response in this temperature window which sugg est minimal variation in the adsorbed As behavior on the GaAs surface. A point occurs in the ellipsometric response which characterizes that temperature at which As deposition begins for a given As overpressure . Using this signature, reproducible LT-GaAs films were grown and foll owed by ellipsometry. A uniform growth spiral was initially obtained i ndicating the formation of a homogeneous LT-GaAs layer. The ellipsomet ric response deviates substantially from the uniform growth spiral as the film thickness exceeds a critical value and may correspond to the formation of an amorphous or polycrystalline structure previously repo rted [Z. Lilental-Weber, W. Swider, K. M. Yu, J. Kortright, F. W. Smit h, and A. R. Calawa, Appl. Phys. Lett. 58, 2153 (1991)], [D. J. Eagels ham, L. N. Pfeiffer, K. W. West, and D. R. Dykaar, Appl. Phys. Lett. 5 8, 65 (1991)]. Subsequent low angle thin film x-ray diffractometry sho wed that the formation of polycrystalline LT-GaAs at submicron thickne sses was commensurate with the above signature. Finally variations in the index for LT-GaAs were determined from the growth spirals.