STEP MOTIONS ON HIGH-TEMPERATURE VICINAL SURFACES

The fluctuations of a train of steps on a vicinal surface at equilibri um and during evaporation are studied in the framework of Burton, Cabr era and Frank's theory. Step fluctuations are treated as perturbations of the straight step's shape and within a linear analysis the step mo rphological stability is investigated in a general way. Previous resul ts of Bales and Zangwill and of Uwaha and Saito are obtained as specia l cases when asymmetry between kinetic coefficients for adatom attachm ent at step edges (the 'Schwoebel effect') is included. A strong Schwo ebel effect is known to lead to step bunching and to smoothing of an i solated 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 neglig ible on vicinal surfaces with widely spaced steps at evaporation tempe ratures. For both reasons, we conclude, in contrast with Uwaha and Sai to, that the observed phenomenon of kinetic step smoothing during sili con evaporation cannot be justified by the presence of a strong Schwoe bel effect. The experimental situation is discussed and a scenario is proposed that does not invoke a strong Schwoebel effect.