Electric field induced second harmonic (EFISH) measurements of highly boron doped p-type Si/SiO2
Thesis (PhD (Physics))--Stellenbosch University, 2008.
The advent of high intensity short pulse lasers has opened the door to investigating buried solid-solid interfaces through the technique of optical second harmonic generation (SHG). This has led to extensive study of technologically important systems such as the Si/SiO2 interface. In this study, SHG is employed to study the interface between highly boron doped p+-type Si and its native oxide layer (SiO2). Previous studies from this laboratory have extensively investigated the photo-induced charge transfer process across the Si/SiO2 interface in the case of undoped natively oxidized Si by means of SHG, with initial SHG measurements being performed on boron doped p+-type Si. The natively oxidized p+-type Si/SiO2 sample was placed on a computer controlled positioning system which allowed for translation of the sample and rotation around the azimuth. The laser system employed was characterized in terms of spectral composition, pulse duration, pulse repetition rate, spatial pro le and pulse energy in order to ensure quantitative measurements. The SHG signal generated from the sample interface was recorded in re ection. Under the applied irradiation conditions, defects are created at the interface by the near infra red (NIR) femtosecond radiation from the laser. These defects are then populated via multi-photon processes by electrons and to a lesser extent holes. The charge transfer across the interface induces an interfacial electric eld. This photo-induced electric eld is in addition to the built-in interfacial electric eld caused by positive ionization of naturally occurring interfacial defects due to the strong doping of the bulk Si. It is this interfacial electric eld, consisting of the built-in doping induced eld and the photo-induced electron and hole elds, that is probed by SHG. The SHG signal is strongly dependent on the magnitude of this interfacial electric eld as the electric eld induced second harmonic (EFISH) signal dominates all other contributions to the observed SHG signal in the case of the Si/SiO2 system. The temporal evolution of the SHG signal is recorded for di erent intensities from virgin as well as the pre-irradiated samples. This yields information about the time scales on which the charge separation occurs as well as the in- uence of existing photo-induced trap sites on the charge separation process, since the strength of the SHG signal is an indirect measure of the interfacial electric eld strength. The angular dependence of the SHG signal (SH rotational anisotropy measurements) for both the initial signal (when the doping induced electric eld dominates) and the saturated signal (when the electron induced electric eld dominates) is measured. Both these measurements show a four fold symmetry but with a relative 45 phase shift between them. This iii is taken as con rmation of the reversal of the interfacial electric eld direction. The initial SHG signal as a function of intensity is also recorded for di erent incident wavelengths. The variation in the non-quadratic dependence of the initial SHG signal on the incident intensity is attributed to a resonant enhancement of two-photon absorption and subsequent screening of the interfacial electric eld by charge carriers. The measurement performed and the results obtained contribute to the understanding of the photo-induced charge separation process across buried solid-solid interfaces, speci cally as it applies to the important Si/SiO2 interface.