Spectroscopic studies of optical second-harmonic generation from silicon(001) surfaces.

摘要

I present a spectroscopic study of optical second-harmonic generation (SHG) from Si(001) surfaces and interfaces in the vicinity of the direct two-photon E₁ transition using tunable femtosecond lasers. The samples investigated are oxidized Si, hydrogen terminated Si, and Cr-SiO₂-Si structures. I first use a phenomenological theory and susceptibility tensors to predict the symmetry properties of several different SHG contributions and present methods for separating bulk and surface SHG contributions and uniquely determining susceptibility tensor elements. By measuring polarization selected rotational-anisotropy SHG (RA-SHG), I show that both bulk and surface SHG contributions display resonances and that interference between these contributions can shift the apparent resonance energy. The strength of bulk and surface SHG contributions varies with photon energy. Linear optics also plays a role in SHG spectroscopy. For certain photon energies, the peak locations of the RA-SHG signals from oxidized and hydrogen terminated Si(001) surfaces differ. This indicates phase shift between surface SHG fields. For appropriate polarizations, peaks of the RA-SHG signals from oxidized Si surfaces can be turned into valleys by varying the photon energy, and eightfold symmetric RA-SHG signals can be observed at certain photon energies. Comparison of RA-SHG signals from Cr-SiO ₂-Si structures and oxidized Si samples also shows a difference in the peak location at certain photon energies. Further experimental results show that an ultrathin Cr coating film on oxidized Si introduces additional sources of SHG, which modify the spectra and time-dependence of SHG. I also study the effect of thermal oxidation of Si(001) samples on SHG and show that SHG is sensitive to interface width. RA-SHG signals with eightfold symmetry are found for several different polarizations and the corresponding photon energies are sensitive to interface conditions. Thermal oxidation affects the time-dependence and spectroscopy of SHG. These results indicate that SHG spectroscopy is a powerful tool for characterizing Si surfaces or interfaces.