Ab initio study of silicon in GW approximation: A direct band gap semiconductor

A theoretical study of physical and electronic properties of silicon has been made by using pseudopotential method within density functional theory (DFT) in generalized gradient approximation (GGA). The effects caused by the expansion of the lattice not studied so far have been investigated. The energy gap for the experimental lattice constant calculated in GW approximation is seen to be in agreement with the experimental value. We observe a crossover from an indirect gap to direct gap at a lattice constant of 5.918 Å corresponding to a lattice expansion of about 8.39%. Silicon is found to be metallic for a lattice constant of 7.61 Å in the tetrahedral structure.