Instabilities in MBE growth.

Abstract

The morphology of GaAs (001) surfaces grown using molecular beam epitaxy (MBE) was examined using scanning tunneling microscopy and force microscopy. Surprisingly, these surfaces show mound formation under certain growth conditions. In particular, when the growth conditions favor the nucleation of islands, regular mounds form. On the other hand, when the growth proceeds via step-flow these mounds are not seen. These multilayered features can be explained by the presence of a barrier at step-edges which hinders the diffusion of adatoms between layers. We show, using a Monte-Carlo simulation, that the addition of this step-edge barrier is sufficient to produce the mounding. We also develop a continuum model to study the large-scale, long-time limit. MBE grown films have traditionally been viewed as self-affine surfaces which conform to the scaling hypothesis. The observation of a characteristic scale (the mound size) rather than many length scales, requires that this view be changed. Our work has played a central role in creating a new paradigm for film growth. We combined experimental observation with modeling and put these together with a physical picture of the dynamical process. This work has far reaching effects because many materials are expected to have step-edge barriers and thus the mounding instability.