Silicon interband transitions observed at ${\mathrm{Si}\left(100\right)-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ interfaces

We report on a type of Si interband transitions at ${\mathrm{Si}\left(100\right)-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ interfaces that has no equivalent in the bulk of crystalline silicon. These transitions, leading to strong resonances in optical second-harmonic-generation spectra with energies of 3.6–3.8 eV, are energetically located between the $E_1$ and $E_2$ critical points of bulk Si. We assign these transitions to Si atoms without $T_d$ lattice symmetry at the boundary between crystalline Si and the ${\mathrm{SiO}}_x$ transition region. We also report on a strong blueshift of the $E_2$ transitions at the interface.