ADATOM DIFFUSION AND DISORDERING AT THE GE(111)-C(2X8)-(1X1) SURFACE TRANSITION

We present an ab initio study of the diffusion of Ge adatoms and the disordering of Ge(111)-c(2 x 8) at approximately 300-degrees-C. In the absence of adatom vacancies, we find that the most likely diffusion mechanism is an anisotropic correlated process in which several adatoms of a [110BAR] row hop concertedly along the same row. When surface vacancies are present, adatoms can instead hop one by one along [110BAR] in an uncorrelated way. Anisotropy arises from the special bonding topology of the c(2 x 8) structure. Calculated scanning tunneling microscopy images of the diffusion process show that adatom jumps are accompanied by an important electron backflow.