DC-electric-field-induced and low-frequency electromodulation second-harmonic generation spectroscopy of Si(001)-SiO$_2$ interfaces

摘要

The mechanism of DC-Electric-Field-Induced Second-Harmonic (EFISH) generationat weakly nonlinear buried Si(001)-SiO$_2$ interfaces is studied experimentallyin planar Si(001)-SiO$_2$-Cr MOS structures by optical second-harmonicgeneration (SHG) spectroscopy with a tunable Ti:sapphire femtosecond laser. Thespectral dependence of the EFISH contribution near the direct two-photon $E_1$transition of silicon is extracted. A systematic phenomenological model of theEFISH phenomenon, including a detailed description of the space charge region(SCR) at the semiconductor-dielectric interface in accumulation, depletion, andinversion regimes, has been developed. The influence of surface quantizationeffects, interface states, charge traps in the oxide layer, dopingconcentration and oxide thickness on nonlocal screening of the DC-electricfield and on breaking of inversion symmetry in the SCR is considered. The modeldescribes EFISH generation in the SCR using a Green function formalism whichtakes into account all retardation and absorption effects of the fundamentaland second harmonic (SH) waves, optical interference between field-dependentand field-independent contributions to the SH field and multiple reflectioninterference in the SiO$_2$ layer. Good agreement between the phenomenologicalmodel and our recent and new EFISH spectroscopic results is demonstrated.Finally, low-frequency electromodulated EFISH is demonstrated as a usefuldifferential spectroscopic technique for studies of the Si-SiO$_2$ interface insilicon-based MOS structures.