Appearance of direct gap in silicon and germanium nanosize slabs

Abstract Ab initio calculations have been performed to obtain electronic structure of hydrogen terminated silicon and germanium slabs of 0.5–2.0 nm in thickness. Comparative results for all low-index orientations are presented. Both silicon and germanium (1 0 0) slabs are found to possess a direct band gap, though the indirect transition is only slightly higher than the direct one. In (1 1 0) oriented silicon slabs there is a direct band gap, whereas germanium slabs are actually indirect band gap semiconductors. Quite opposite situation is observed for (1 1 1) orientation where electronic properties of silicon slabs are very similar to those of the bulk, whereas band structure of germanium slabs has a well-resolved direct band gap.