Linear optical properties of strained (Si)n/(Ge)nsuperlattices on (001) Si substrates

We report the first full band-structure calculation of the linear optical properties of strained (Si${)}_{\mathit{n}}$/(Ge${)}_{\mathit{n}}$ superlattices on Si(001) substrates for n=2,3,4. We use a semi–ab initio minimal basis orthogonalized linear combination of Gaussian orbitals technique, in conjunction with a linear analytic tetrahedra method, to obtain ε→(ω). We find that the expected anisotropy in these novel materials is large and almost entirely due to the strain in Ge rather than to the lower symmetry of the superlattices compared to the bulk materials; in a simple picture it results from an alignment of more bonds (antibonds) along the superlattice axis than perpendicular to it. The pure superlattice features are only observed near the absorption edges, they differ from superlattice to superlattice, and are the real signature of each superlattice. Most of the features of the response function at frequencies away from the absorption edge are found to be due to bulklike transitions.