Photoluminescence from Localized Excitons in Si/Ge Superlattices

Short period strained-layer Si/Ge superlattices grown by molecular beam epitaxy have been investigated by photoluminescence spectroscopy. In 16×4, 15×4 and 12×4 superlattices with 80 or 120 periods two new luminescence peaks appear. From their separation in photon energy, the two peaks are assigned to a no-phonon transition and its silicon transverse optical phonon replica. Detailed photoluminescence investigations have confirmed that the new peaks which appear at energies between 0.85 and 1.05 eV give evidence for radiative recombination in the superlattices. From their luminescence intensity as well as lineshape dependence on the excitation power we attribute the new peaks to localized excitons.