Steric interaction model of roughening and vacancy reorganization on halogen-terminated Si(100)-2x1 surfaces

We present a steric interaction (SI) model that describes roughening and reorganization on halogen-terminated Si(100)-2x1 surfaces. This model is based on parameters derived from density-functional theory total-energy calculations. Using these parameters together with the requirements for rebonding and atom conservation, it is possible to explain the types of reorganization events that can occur and the different steady-state morphologies observed in the presence of different halogen terminations. Using scanning-tunneling microscopy we show that dimer vacancy (DV) string elongation can occur by a process called primary roughening, i.e., the removal of pairs of dimers from the terrace and the formation/elongation of terrace islands. We show that experimentally and within the confines of the SI model end-on coalescence of DV strings is unfavorable but side-on coalescence can result in the formation of wider vacancy pits. In this manner the SI model explains why elongated vacancy pits are formed in the case of Br termination, whereas short, wider structures are observed with Cl. The model is also used to explain the nucleation of terrace islands and the attachment of dimers at islands and surface steps. The model predicts larger numbers of islands and enhanced roughness at S-B-type steps on the Br surface, in excellent agreement with experiment.