Step motions on high-temperature vicinal surfaces

The fluctuations of a train of steps on a vicinal surface at equilibrium and during evaporation are studied in the framework of Burton, Cabrera and Frank's theory. Step fluctuations are treated as perturbations of the straight step's shape and within a linear analysis the step morphological stability is investigated in a general way. Previous results of Bales and Zangwill and of Uwaha and Saito are obtained as special cases when asymmetry between kinetic coefficients for adatom attachment at step edges (the 'Schwoebel effect') is included. A strong Schwoebel effect is known to lead to step bunching and to smoothing of an isolated step during evaporation. Here we show that a strong Schwoebel effect leads instead to roughening of steps in a train at evaporation temperatures. Furthermore, we show that the Schwoebel effect is negligible on vicinal surfaces with widely spaced steps at evaporation temperatures. For both reasons, we conclude, in contrast with Uwaha and Saito, that the observed phenomenon of kinetic step smoothing during silicon evaporation cannot be justified by the presence of a strong Schwoebel effect. The experimental situation is discussed and a scenario is proposed that does not invoke a strong Schwoebel effect.