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 (SLS's). grown on (111) and (110)Si1-xGex substrates. We End that for growth on a Ge(111) substrate the (Si)(2)/(Gr)(m) SLS's, with m = 2 + 4k (k = 0-5), are direct gap materials with transition probabilities one order of magnitude smaller than typical E(0) 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) SLS's with n = 3, 5 are direct gap materials with transition probabilities 2-3 orders of magnitude smaller than a typical E(0) bulk-like transition.