Appearance of direct gap in silicon and germanium nanosize slabs

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, (C) 2001 Elsevier Science B.V. All rights reserved.