Optical second harmonic generation studies of charge transfer and trapping and chemical control of these phenomena at semiconductor interfaces

Charge transfer at semiconductor interfaces is of fundamental and technological interest. Second harmonic generation (SHG) is a versatile, contactless, in-situ probe of buried interfaces. The unifying theme of our investigations is the SHG sensitivity to charge trapping at the semiconductor interface and how this can be controlled chemically. The sensitivity of SHG to charges at semiconductor interfaces occurs through electric-field induced second harmonic generation, EFISH. Charging of the semiconductor interface can lead to changes in SHG rotational anisotropy (SHG-RA) and time dependent SHG (TD-SHG).;Charge trapping at semiconductor interfaces leads to anomalous SHG-photomodulation of SHG-RA and non-quadratic second-harmonic generation. These phenomena are strongly sensitive to the chemical state of the interface and the wavelength of the excitation light. An estimation of the fixed oxide charge at the interface is derived from the SHG response.;Self-assembled monolayers (SAMs) were investigated as a means to control interface charging and shown to attenuate ambient-assisted charge trapping at the Si-SiO2 interface. TD SHG is a sensitive probe not only of the amount but also of the sign of the charge trapped at Si-SiO2 interface. Electrons and holes photoinjected in SiO2 lead to distinctively different time-dependent SHG behavior that allowed us to separately probe the relaxation dynamics of electrons and holes photoinjected in SiO2 . The relaxation of holes was found to be several orders of magnitude slower than that of the electrons.;SHG-RA and TD-SHG were used to determine the band bending, dopant density and type at Si interfaces. The sensitivity of SHG-RA to the chemical state of the interface was shown in experiments with chemically-modified Ge and was used to evaluate the relative contribution of the surface- and bulk-originated terms to the SHG signal.