Strain effects and optical properties of Si1–xGex/Si superlattices

We demonstrate that quasi direct band gap Si1–xGex/Si superlattices can be obtained by suitable choices of layer thicknesses. We calculate strain dependent conduction-band offsets as functions of the substrate alloy concentration, and of the epilayer alloy concentration. Optical matrix elements are computed for Si0.5Ge0.5/Si superlattices grown on Si0.75Ge0.25 buffer layers with superlattice layer thicknesses of 4 to 24 monolayers. We find that optical absorption and emission strengths can vary by three to four orders of magnitude for layer thickness variations as small as 1–2 monolayers, suggesting that layer thicknesses must be controlled to within one monolayer to obtain enhanced optical properties. Typical optical matrix elements calculated for these Si1–xGex/Si superlattices are three to four orders of magnitude larger than for bulk Si or Ge, but, are still three orders of magnitude smaller than for direct band gap materials such as GaAs