Optical absorption of pseudomorphic Si/Ge superlattices

Abstract

The optical properties of pseudomorphic (defect-free) (Si${)}_{\mathit{n}}$/(Ge${)}_{\mathit{n}}$ strained-layer superlattices grown on ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ (001) substrates are studied within the framework of the empirical tight-binding method. Attention is focused on the nature (direct/indirect) of the fundamental energy gap and on the effects of the superlattice periodicity on the optical absorption near the band edge. The band-edge absorption is the main distinguishing feature among different superlattices. It is found that the behavior of the absorption is strongly dependent on light polarization (anisotropy) and substrate lattice constant. The contribution of the lowest interband transitions to absorption spectra is also analyzed. Theoretical results are compared with recent experimental data.