Influence of surface morphology on the Si(100) and (111) ionization potentials

Using an ab initio pseudopotential approach, we have investigated the dependence of the Si ionization potential on surface atomic relaxation, reconstruction, and crystallographic orientation. Surface reconstruction and relaxation tend to decrease the Si (100) and (111) ionization potentials. The Si(100) ionization potential, however, is systematically larger than the Si(lll) potential for all surface geometries examined, i.e., bulk-truncated, relaxed, and (2 x 1) reconstructed configurations. Such a trend is at variance with the behavior of most metals, whose ionization potential increases with increasing surface atomic packing (Smoluchowski's rule). Based on an atomic-scale analysis of the surface electronic charge, we explain this reversed trend, and relate it to an anomaly previously observed in the Al ionization potential and attributed to the open p-shell nature of this element.