Reflection High Energy Electron Diffraction Studies of Diffusion and Cluster Formation During Molecular Beam Epitaxy

Abstract

Understanding the surface processes controlling molecular beam epitaxy (MBE) is crucial to the growth of high quality epitaxial structures. The fundamental mechanisms involved in the growth include adatom and impurity adsorption, surface reconstruction, surface diffusion, cluster formation, step propagation, and evaporation. Reflection high-energy electron diffraction (RHEED) is useful because it probes the surface reconstruction and the step structure during the growth process. In particular, the competing processes of cluster formation and step propagation are followed as a function of growth conditions. The growth of GaAs and Ge by MBE has been investigated in detail with RHEED. Layer-by-layer growth is observed on these surfaces over a range of conditions. In both systems, the growth mode changes from cluster formation at low substrate temperatures to step propagation at high temperatures. The underlying step morphology also affects the growth mode. By introducing an array of steps due to a misorientation from a low-index plane, step propagation is strongly enhanced. The step motion and stability is interpreted within the framework of several growth models. The models give insight into why the MBE layers can be grown so smoothly.