dc-electric-field-induced and low-frequency electromodulation second-harmonic generation spectroscopy of Si(001)-SiO2 interfaces

The mechanism of dc-electric-field-induced second-harmonic (EFISH) generati on at weakly nonlinear buried Si(001)-SiO2 interfaces is studied experiment ally in planar Si(001)-SiO2-Cr MOS structures by optical second-harmonic ge neration spectroscopy with a tunable Ti:sapphire femtosecond laser. The spe ctral dependence of the EFISH contribution near the direct two-photon El tr ansition of silicon is extracted. A systematic phenomenological model of th e EFISH phenomenon, including a detailed description of the space-charge re gion (SCR) at the semiconductor-dielectric interface in accumulation, deple tion, and inversion regimes, has been developed. The influence of surface q uantization effects, interface states, charge traps in the oxide layer, dop ing concentration, and oxide thickness on nonlocal screening of the de-elec tric field and on breaking of inversion symmetry in the SCR is considered. The model describes EFISH generation in the SCR using a Green's-function fo rmalism which takes into account all retardation and absorption effects of the fundamental and second-harmonic (SH) waves, and multiple reflection int erference in the SiO2 layer. The optical interference between field-depende nt and -independent contributions to the SH field is considered as an inter nal homodyne amplifier of the EFISH effects. Good agreement between the phe nomenological model and our EFISH spectroscopic results is demonstrated. Fi nally, low-frequency electromodulated EFISH is demonstrated as a useful dif ferential spectroscopic technique for studies of the Si-SiO2 interface in s ilicon-based metaloxide-semiconductor structures.