Si1−xGexSi(001) layers under external uniaxial stress: photoluminescence studies

MBE grown pseudomorphic Si1−xGexSi(001) quantum wells (QWʹs) with germanium contents from 10% to 36% and various thicknesses were studied by photoluminescence (PL) under external stress applied parallel or perpendicular to the growth direction. Stress perpendicular to the growth direction reduces the symmetry and leads to additional splittings in both barrier and layer material. Bending the whole wafer, we create compressive or tensile in-plane strains in the sample. In both cases we could not observe the splittings in the SiGe layers, but the SiGe PL shifts like the SiGe band edge, as expected from deformation potential theory. Parallel stress conserves the tetragonal symmetry and reduces the strain in the SiGe-layer, and at the same time leads to a splitting of valence and conduction band states in the Si barrier. For stress values up to 600 MPa we find an upshift of the SiGe PL, whereas the lowest branch of the bound exciton emission of the Si barriers shifts down. The observed shift rates depend on both Ge concentration and layer thickness. For thick layers, the shift rates are as expected from deformation potential theory for the SiGe band edge. For thin layers significant deviations occur. To interpret these deviations, calculations have been carried out which take into account quantization energies in the QWʹs, stress dependent exciton binding energies, band bending effects, and stress dependence of band offsets.