Enhanced band-gap luminescence in strain-symmetrized (Si)m/(Ge)nsuperlattices

We report on band-gap luminescence in strain-symmetrized, (Si${)}_{\mathit{m}}$/(Ge${)}_{\mathit{n}}$ superlattices grown on a step-graded, alloy buffer with a reduced dislocation density, using Sb as a surfactant. The luminescence efficiency for a (Si${)}_9$/(Ge${)}_6$ and (Si${)}_6$/(Ge${)}_4$ superlattice is strongly enhanced compared with a corresponding ${\mathrm{Si}}_{0.6}$ ${\mathrm{Ge}}_{0.4}$ alloy reference sample. The luminescence signals can be attributed to interband transitions of excitons localized at potential fluctuations in the superlattice. The observed systematic shift of the band-gap luminescence to lower energies with increasing period length compares well with results of a simple, effective-mass calculation. An increasing superlattice band gap and a reduction in luminescence intensity is observed if the Si and Ge layers are interdiffused by thermal annealing. The band gap for a (Si${)}_6$/(Ge${)}_4$ superlattice was also measured with absorption spectroscopy. The absorption coefficient, as determined by direct transmission, is in the order of ${10}^3$ ${\mathrm{cm}}^{\mathrm{-}1}$ about 0.1 eV above the band gap.