Structural and Optical Characteristics of Thin GaAs Films. II

Abstract

Free carrier absorption measurements have been carried out on GaAs films which exhibit an absorption edge shift to lower energies; acceptor concentrations of >1019/cm3 are associated with this shift and causally related to it according to a model proposed by Stern. Absorption measurements on films deposited on a number of substrates and at different temperatures eliminate strain as a major cause in the shift of the edge for polycrystalline films. Reflectivity measurements show a marked dependence on both substrate temperature and thickness. The fall off in reflectivity values for very thick films is associated with the growth of microcrystals in the surface of the films. Examination of these surfaces by electron microscope shows extensive lamellar growth for deposition at temperatures above 350°C; this lamellar growth is explained on the basis of a twin-plane re-entrant growth mechanism. Examination of the films by electron and x-ray diffraction has shown the existence of extra reflections which can be indexed as due to a hcp phase; the existence of these reflections is associated with the lamellar growth in the films.