Optically Allowed Photoluminescence from a Direct-Gap Si-Ge Superstructure on Si0.4Ge0.6

Abstract

Both Si and Ge possess an indirect band gap, which makes them very inefficient light emitters, and band gap engineering has been proposed as a way to overcome this limitation. According to the theoretical work of M. d’Avezac et al. (Phys. Rev. Lett. 108, 027401 (2012)), a SiGe2Si2Ge2SiGen superstructure on Si0.4Ge0.6 should have a direct and dipole-allowed gap of 0.863 eV. Here we report on the growth of such a structure and its optical properties. Two similar samples were prepared by molecular-beam and solid-phase epitaxy. Photoluminescence (PL) spectra were obtained at low temperatures (6-25 K) with excitation at 405 and 458 nm. A strong low-energy PL quadruplet is seen, with peaks near 729, 758, 792 and 823 meV at 6 K, together with a much weaker peak at 871 MeV. The weak peak at 871 meV is assigned to the dipole-allowed direct-gap transition expected in the superstructure.