Direct gap Si/Ge superlattices strained along the [110] and [111] directions

We present calculations for the energy band structure of Si and Ge coherently grown on (110) and (111) Si1−xGex alloy substrates. We find that strained Si and Ge remain indirect gap semiconductors for all substrates. In addition, we calculate the electronic and optical properties of (Si)n/(Ge)m strained layer superlattices (SLSʹs), grown on (111) and (110) Si1−xGex substrates. We find that for growth on a Ge(111) substrate the (Si)2(Ge)m SLSʹs, with m=2+4k (k=0−5), are direct gap materials with transition probabilities one order of magnitude smaller than typical E0 bulk-like transition. We also find that for growth along the [110] direction and on a substrate rich in Ge, the (Si)n(Ge)n SLSʹs with n = 3,5 are direct gap materials with transition probabilities 2–3 orders of magnitude smaller than a typical E0 bulk-like transition.