KINETIC INSTABILITY OF SEMICONDUCTOR ALLOY GROWTH

A kinetic theory of the instability of homogeneous alloy growth with r espect to fluctuations of alloy composition is developed. The growth t h mechanism studied is the step-flow growth of an alloy from the vapor on a surface vicinal to the (001) surface of a cubic substrate. The e pitaxial growth implies that the adsorbed atoms migrate on the surface during the growth of each monolayer, and that their motion is ''froze n'' after the completion of the monolayer. ''Frozen'' fluctuations of alloy composition in all completed monolayers create, via a compositio n-dependent lattice parameter, an elastic strain that influences the m igration of adatoms of the growing monolayer. The migration consists o f diffusion-and strain-induced drift in an effective potential. For te mperatures lower than a certain critical temperature T-c, strain-induc ed drift dominates diffusion and results in the kinetic instability of the homogeneous alloy growth. In an approximation linear in the fluct uation amplitude, the instability means the exponential increase of th e fluctuation amplitude with the thickness of the epitaxial film. It i s shown that the critical temperature of the kinetic instability T, in creases with the increase of elastic effects. The wave vector k(c) of the most unstable mode of composition fluctuations is determined by th e interplay of the anisotropic elastic interaction and the anisotropic diffusion of the adatoms on a stepped vicinal surface. The direction of the wave vector k, differs from the lowest-stiffness direction of t he crystal. Regions in k space of both stable and unstable modes are f ound by model calculations.