Step Dynamics and Morphological Instabilities During Epitaxial Growth

Vicinal surfaces consist of terraces separated by atomic steps. During step-flow epitaxial growth, adsorbed atoms (from the vapor or a beam) diffuse on the terraces until they attach to steps,causing them to advance. This collective lateral migration of steps results in the net growth of the underlying thin crystalline film. For a train of initially equidistant straight steps, two types of morphological instabilities occur: bunching, which leads to regions of high step density separated by wide terraces, and meandering, whereby steps become wavy. Fairly recent experiments indicate that bunching and meandering coexist on some stepped surfaces, in contrast to the stability predictions of the classical Burton-Cabrera-Frank (BCF) model. In this talk, I will review the BCF theory and present a generalization of it, based on nonequilibrium thermodynamics, that resolves this apparent paradox. In particular, I will show that step bunching and meandering can occur simultaneously, provided that the adatom equilibrium coverage is not negligible. I will also compare this thermodynamically consistent theory with various extensions of the BCF framework that attempt to reconcile theory with experiments.