Raman scattering and photoluminescence characterization of Ge/Si strained-layer superlattices grown by phase-locked epitaxy

Abstract

Raman and photoluminescence spectra of Ge_nSi_m (m=4n, 3n) strained-layer superlattices with periods of several tens of ångströms grown on Si (001) substrates by phase-locked epitaxy method were measured. The superlattice periodicity was observed as zone-folded acoustic phonon peaks for the samples with n. It was found that the structures of the zone-folded acoustic phonons are described by an elastic model considering the lattice strain in the Ge layers. The phonon structures above 200 cm^-1 for the samples with n ≧ 4 are described by a linear-chain model and the concept of superlattice zone-folding. However, those for the samples with n≦ 2 are close to those of GeSi alloys and cannot be explained as superlattices. Ge_nSi_m superlattices showed emission in the spectral region from 0.7 to 0.9 eV at 4.2 K. Quite strong emission was observed for a Ge₄Si₁₂ superlattice.