Atomic structure of surfactant monolayers and its role in epitaxial growth

The energetically preferred structures of group V adsorbate monolayers on Si and SiGe substrates were studied using first-principles total energy calculations. By comparing structural features of the optimal adsorbate geometries we infer that structures involving self-bonded adsorbate units, such as dimers, trimers or chains, are most likely to segregate easily during growth. Among the group V elements considered, Sb forms structures involving self-bonded adsorbate units on the substrates considered, making it the most promising candidate for a surfactant. This prediction is consistent with recent experiments on Si(111) homoepitaxy. We also consider a simple solid-on-solid model that captures the essential features of surfactant behavior on Si substrates. The model involves a single variable εa, the activation energy for exchange between a newly deposited atom and a surfactant unit on a terrace. We find that depending on the temperature, the model leads to smooth or rough overlayers. The transition takes place at kBT ∼ 0.1 εa. The qualitative difference in growth mode is demonstrated by examining the topological features of films grown under different conditions.