Advacancy-induced step bunching on vicinal surfaces

Step bunching at the surface of a sublimating crystal Joule-heated by a dc electric current is investigated in the presence of both adatoms and surface vacancies (advacancies). We show that the inclusion of advacancies in the step flow model of Burton, Cabrera, and Frank is crucial in order to reproduce the high-temperature behavior of real Si(111) vicinal surfaces. This provides a complete qualitative picture of the morphologies reported by Latyshev et al. [Surf. Sci. 213, 157 (1989)], and strongly supports the hypothesis of electromigration. Agreement with experiments is obtained only assuming that the force exerted on adatoms and advacancies by the electric current is opposite to the latter.