Band structure and optical properties of Si-Si1-xGex superlattices

Abstract

We report the band structure and optical properties of Si-Si1-xGex superlattices calculated by k⋅p theory using the envelope-function approximation. In this paper we have demonstrated that direct-band-gap Si-Si1-xGex superlattices can be achieved by a suitable choice of layer thicknesses. We have presented detailed results for Si-Si0.5Ge0.5 superlattices grown on Si0.75Ge0.25 buffer layers with layer thicknesses in the range from 4 to 24 monolayers. Our calculations indicate that the optical absorption strengths can vary by 3–4 orders of magnitude even for layer thickness variations as small as 1–2 monolayers. Thus, it is important to control the layer thicknesses to a monolayer accuracy to obtain the enhanced optical absorption strengths. Although these optical absorption strengths are 3–4 orders of magnitude larger than bulk Si or Ge, they are still 3 orders of magnitude smaller than the absorption strengths due to direct transitions in materials such as GaAs.

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