Interface resonances in optical second-harmonic generation from oxide-covered Ge(111) and Ge(100)

摘要

Ge surfaces have been investigated with optical second-harmonic generation (SHG) spectroscopy in the range from 1.78 to 3.44 eV. The spectra reveal surface-specific resonances corresponding to the E1 and E1 + Δ_1 bulk transitions. The splitting between the surface E1 and E1 + Δ_1 resonances is found to be larger than the bulk value. It is suggested this is caused by surface-induced band bending through a Rashba effect. By probing metal-oxidesemiconductor structures it is found that contributions from electric-field-induced SHG from the space charge region are negligible for Ge within the probed spectral range. Strong second-harmonic resonances in the 2.6-3.2 eV range are observed and tentatively assigned to Ge-Ge bonds at the interface.