Structural Characterization of Thin, Low Temperature Films of GaAs on Si Substrates
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STRUCTURAL CHARACTERIZATION OF THIN, LOW TEMPERATURE FILMS OF GaAs ON Si SUBSTRATES S. J. ROSNER*#, S. M. KOCH#, J. S. HARRIS, Jr.# *Hewlett-Packard, P.O. Box 10350, Palo Alto, CA 94303 and #Stanford University, Stanford, CA 94305
ABSTRACT The technology of heteroepitaxially growing compound semiconductors, particularly GaAs, on Si has attracted an increasing volume of attention in the past 3-5 years. In the past two years there have been few major advances in the growth recipes, which all include a high temperature Si preheat, a two-step growth temperature profile, and the use of misoriented substrates. The need for mechanistic understanding of the effect of these parameters is crucial to advancing the state of the art beyond this current practice. This work focusses on the effect of the misorientation in inducing asymmetry in early stages of the molecular beam epitaxy of GaAs on Si substrates. The strain in the films is found to have greatly different rates of relaxation in the plane of the film when measured in the two orthogonal directions. This asymmetry persists to greater than 30 nm film thickness at 400 0 C. The nucleation morphology was also examined as a function of substrate misorientation. At low substrate tilts, nucleation density was sparse and there was not substantial ordering of the nuclei. As the tilt was increased, a distinct habit emerged where collections of nuclei were quite coherent along the steps for several 100 nm, with dimensions of the order of 10's of nm perpendicular to the steps. The density of steps with nuclei was also substantially less than that predicted by the widely accepted "array of double-height steps" used to explain the curious lack of anti-phase disorder in these films.
INTRODUCTION There is a growing volume of R&D activity focussed on replacing GaAs substrates in current commercial applications with GaAs on Si films; the obvious advantages are the increased strength and thermal conductivity, as well as the decreased mass in space-based applications. In the future, the ability to integrate high-speed logic and III-V based optoelectronics will become increasingly important. The obvious oft-cited difficulties with this technology are substantial: 4% lattice mismatch, the problem of anti-phase disorder, differences in thermal coefficients of expansion, and the possibility of Si out-diffusion. Many of these problems have been solved or at least partially alleviated by clever empirical work on the growth recipe. This has generated the key features of most of the GaAs on Si growth recipes, the most significant of which are the Si cleaning and preheat, the two-step growth process (low temperature buffer layer), and the use of misoriented substrates. This work is intended to provide mechanistic understanding of the effect of these empirically determined recipes by studying the initial stages of epitaxy at low temperatures on substrates with a variety of misorientations. This paper will report on some of the morphological asymmetries that appear in the epitaxial layer as a resul
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