Band gap of strain-symmetrized, short-period Si/Ge superlattices

We report an identification and determination of the band-gap energies in a series of strain-symmetrized ${\mathrm{Si}}_{\mathit{n}}$/${\mathrm{Ge}}_{\mathit{n}}$ superlattices. Absorption onsets are observed that shift toward higher energies with decreasing period length in superlattices with identical Si/Ge ratio. Band-gap energies of 0.67, 0.76, and 0.88 eV for ${\mathrm{Si}}_6$/${\mathrm{Ge}}_6$, ${\mathrm{Si}}_5$/${\mathrm{Ge}}_5$, and ${\mathrm{Si}}_4$/${\mathrm{Ge}}_4$ superlattices, respectively, are determined by a fitting procedure. Strong photoluminescence and electroluminescence are observed for the ${\mathrm{Si}}_5$/${\mathrm{Ge}}_5$ superlattices. The energetic position indicates that the luminescence is related to interband transitions.