Abstract
We have measured reflected optical second harmonic (SH) intensity from the Ge-oxide/Ge(111) interface as a function of the incident photon energy ranging from \hbarω=1.10 to 1.65 eV. The oxide layers were formed either by oxidizing a Ge(111) substrate in pure oxygen gas at 823 K or by oxidizing a Ge(111) substrate in dry air at room temperature. We observed that SH spectra from the thermally oxidized film have resonant peaks around \hbarω=1.16 eV and 1.48 eV. We suggest that the peaks at \hbarω=1.16 eV and 1.48 eV originate from the one-photon resonance of the electronic level of the GeO2/Ge(111) interface and the two-photon resonance of the electronic level of bulk germanium, respectively.