NONLINEAR-OPTICAL SPECTROSCOPY AT SILICON INTERFACES

Interface properties of oxidized, clean and hydrogen-terminated silicon surfaces have been studied by optical second-harmonic generation (SHG) and sum-frequency generation (SFG) using tunable laser pulses. A strong resonance band at 3.3 eV photon energy is observed in SHG and SFG spectra of clean and various oxidized Si(100) and Si(111) surfaces. In the case of oxidized surfaces, the appearance of the band strongly depends on the treatment of the sample, but not on oxide thickness. Spectra of hydrogen-terminated surfaces prepared by wet chemistry do not reveal appreciable resonant intensity around 3.3eV thus proving the distinct interface character of the band. Chemisorption experiments performed on clean silicon surfaces rule out transitions between surface states as the reason for the observed band. It is concluded that the resonance is caused by direct band gap transitions, derived from the E(1) transitions at 3.4eV in bulk silicon, that take place in a thin layer of silicon atoms with strained bonds underneath the surface atoms of clean Si surfaces and at the Si-SiO2 interface. The commonly observed U-shaped distribution of interface defect states in Si-SiO2 structures is discussed in the light of our results.