Photoluminescence investigation of short period silicon-germanium heterostructures grown using molecular beam epitaxy.

High quality (Si6Ge4)p heterostructures, strained on Si(100) substrates, were investigated by photoluminescence (PL) measurements. The films were grown at 400°C by molecular beam epitaxy, using Sb as a surfactant to avoid interdiffusion and clustering effects at the Ge/Si interface as evidenced by X-ray photoelectron diffraction polar patterns. The optical spectra have been obtained as a function of the progressive increasing repetition number (p). The PL results of the whole set of samples show similar spectra, for both the single Ge quantum well (p = 1) and the thicker heterostructure (p = 30). The phonon assisted transverse optical line is measured at ca 40 meV far from the no-phonon (NP) one, and this corresponds to the Ge-Ge vibration. This result reveals that excitonic recombination occurs in the Ge layers and it is indirect in nature, whatever the repetition number (p) is. Our data show a high localization of the PL process excluding any superperiodicity effect.