The mechanism of dc-electric-field-induced second-harmonic generation ͑EISH͒ was studied at the buried Si͑111͒-SiO 2 interface in transmission through a planar Si-SiO 2 -Cr MOS structure. The second-harmonic contribution of the field-induced quadratic polarization generated in the space-charge region is determined. The role of the spatial distribution of the dc electric field inside the silicon space-charge region is demonstrated, as well as the influence of the oxide thickness. We have developed a phenomenological model of the EISH taking into account the interference between field-dependent and field-independent contributions to the nonlinear polarization ͑nonlinear interference͒ as well as the retardation of the EISH wave. We show that, due to these interference effects, the minima of the EISH curves do not coincide with the flatband voltage.

Spectroscopic electric-field-induced second harmonic generation on a Si(111)-SiO2-C r metal-oxide-silicon structure shows a bias-independent " interface" resonance at 3.25 eV and a ''bulk'' resonance at 3.43 eV which is strongly bias dependent. The symmetry forbidden bulk dipole contribution becomes observable, and even dominating, due to the bias-induced band-bending that breaks the bulk inversion symmetry. The origin of these resonances is discussed, as well as the prospects for using second harmonic generation as a probe of metal-oxide-silicon characteristics.

By modifying a Si(lll) surface through the introduction of a variable number of steps, the anisotropic surface and bulk contributions to the optical second-harmonic generation could be determined separately and were found to be of the same order of magnitude for a Si-Si02 interface. This approach is based on the lowering of the surface symmetry by the steps and by the apparent enhancement of the nonlinear susceptibility at the step edges.

Recently a number of second-harmonic-generation (SHG) experiments on (thick) oxide films on Si were performed as studies of the possible presence of strain, crystalline Si0 2 , a static electric field, and roughness at the S i-S i0 2 interface. Large enhancements of the SHG anisotropy have been observed for thick oxide films. We show here that the SHG for thick thermal oxide films on S i ( ll l) as a function of oxide thickness and angle of incidence is dominated by linear optics, owing to multiple reflections in the oxide film.

W e show here that the oxide-thickness dependence o f the s-poîarized SHG from S i ( l l l ) covered with a thick thermal oxide is completely described by multiple reflections in the oxide film. For the p-polarized response, a strong enhancement with thickness is observed, which cannot be explained in this way. These measurements show that one should be cautious in analyzing the SHG from a buried interface, and carefully take into account the linear optics involved.