Optimization of growth parameters for direct band gap SiGe superlattices

Abstract

The criteria affecting the direct/indirect nature of the lowest energy transition across the band gap of a SiGe superlattice are discussed, and a prescription for obtaining a direct gap system is given. A systematic study of the optical properties of Si nGe n superlattices as a function of period and substrate is performed in order to optimize the energy and transition strength. Specific structural parameters are presented for two systems in which the lowest energy transition is direct and of a magnitude large enough to be of practical use. The range of energies for which direct transitions are possible is found to cover an almost continuous spectrum from 50 meV to around 900 meV, the lowest energy transitions being between conduction subbands. Of particular interest to optical fibre applications is the prediction that, with a suitably chosen substrate, the Si 5Ge 5 superlattice has a direct optical band gap corresponding to a wavelength of 1.55 μm.