Second-harmonic spectroscopy of interband excitations at the interfaces of strainedSi(100)−Si0.85Ge0.15−SiO2heterostructures

Abstract

We have applied optical second-harmonic generation to the spectroscopy of strained $\mathrm{Si}(100)\ensuremath{-}{\mathrm{Si}}_{0.85}{\mathrm{Ge}}_{0.15}\ensuremath{-}{\mathrm{SiO}}_{2}$ 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 $\mathrm{Si}(100)\ensuremath{-}{\mathrm{SiO}}_{2}$ interfaces. These transitions are assigned to alloy atoms without ${T}_{d}$ symmetry at the boundary between the ${\mathrm{Si}}_{0.85}{\mathrm{Ge}}_{0.15}$ layer and the ${\mathrm{SiO}}_{x}$ transition region.