KINETIC ROUGHENING OF THE KOSSEL (100) SURFACE - COMPARISON OF CLASSICAL CRITERIA WITH MONTE-CARLO RESULTS

Kinetic roughening is not a phase transition and, as such, it lacks an exact definition. Many criteria are used to mark the onset of kinetic roughening. Criteria stemming from the classical two-dimensional nucl eation theory are widely used. On the other hand, experimentalists obs erve a transition from flat to rounded crystal facets at certain drivi ng forces. And measuring the growth rate as a function of driving forc e, a change from exponential to linear growth kinetics is frequently f ound. It is assumed that these experimental phenomena coincide with th e onset of kinetic roughening. These experimental criteria, three clas sical criteria for kinetic roughening and statistical mechanical crite ria based on the interface width and the surface roughness, are compar ed with each other by means of Monte Carlo simulations on a Kossel (10 0) SOS model. Surface diffusion is neglected, and only attachment/deta chment kinetics is considered. The change from flat to rounded facets with increasing driving force turns out to be quite gradual. Neverthel ess, this experimental criterion is made explicit by defining a critic al driving force for which the curvature of a facet becomes visible by optical microscopy. The conditions for an experiment to detect kineti c roughening using this criterion are described. The different criteri a for kinetic roughening yield different values for the critical drivi ng force, although most of the criteria studied show a similar, almost linear, dependence of the critical driving force on the nearest neigh bor bond strength. This again indicates that kinetic roughening is dif fuse in nature, and shows that in discussions on kinetic roughening it is imperative to mention the criterion used. Some attention is also p aid to the two-dimensional anisotropy of step motion on a Kossel (100) surface. An anisotropic step velocity is found far below thermal roug hening. The anisotropy is reduced by increasing the driving force. (C) 1998 Elsevier Science B.V. All rights reserved.