Silicon interband transitions observed at Si(100)-SiO2 interfaces

We report on a type of Si interband transitions at Si(100)-SiO2 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 SiOx transition region. We also report on a strong blueshift of the E-2 transitions at the interface.