In situ ellipsometric study of As capping and low temperature molecular-beam epitaxy GaAs growth and implications for the low temperature critical thickness

Abstract

Low temperature GaAs (LT–GaAs) has been demonstrated to be a useful high resistivity buffer layer for subsequent growth of a variety of epitaxial device layers. It has proven difficult to reproducibly grow these films due to the low (∼250 °C) substrate temperatures involved and the need for tight III/V flux control. In this work in situ ellipsometry was used to reproducibly set the substrate temperature through a study of the As capping process on a GaAs surface. A hysteresis of ∼200 °C occurred in the temperature at which As was deposited and subsequently removed. Subtle changes occur in the ellipsometric response in this temperature window which suggest 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 followed by ellipsometry. A uniform growth spiral was initially obtained indicating the formation of a homogeneous LT–GaAs layer. The ellipsometric 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 reported [Z. Lilental-Weber, W. Swider, K. M. Yu, J. Kortright, F. W. Smith, and A. R. Calawa, Appl. Phys. Lett. 58, 2153 (1991)], [D. J. Eagelsham, L. N. Pfeiffer, K. W. West, and D. R. Dykaar, Appl. Phys. Lett. 58, 65 (1991)]. Subsequent low angle thin film x-ray diffractometry showed that the formation of polycrystalline LT–GaAs at submicron thicknesses was commensurate with the above signature. Finally variations in the index for LT–GaAs were determined from the growth spirals.