Second-harmonic spectroscopy of interband excitations at the interfaces of strained Si(100)-Si0.85Ge0.15-SiO2 heterostructures

We have applied optical second-harmonic generation to the spectroscopy of strained Si(100)-Si0.85Ge0.15-SiO2 heterostructures, covering the full range of the fundamental critical-point interband transitions. By analyzing spectra from samples with different thicknesses of the alloy layer ranging from 4 to 28 nm, we were able to resolve different interband excitations localized at the two buried interfaces of this system. The contribution of the substrate-alloy interface to the spectra consists of bulklike, E-1- and E-2-type critical-point transitions, consistent with a pseudomorphic structure of this interface. The dominating excitations at the alloy-oxide interface, comprised of a broad resonance centered around 3.6 eV, have no equivalent in the bulk, and were also observed at Si(100)-SiO2 interfaces. These transitions are assigned to alloy atoms without T-d symmetry at the boundary between the Si0.85Ge0.15 layer and the SiOx transition region. [S0163-1829(99)13903-1].