| Over the past decades, the Si/SiO2 interface has been the subject of extensive studies because of its importance to device technology. Recent advances in non-linear ultra-fast laser optics studies have shown optical second-harmonic generation to be a sensitive probe of semiconductor-to-insulator interfaces. Significant progress has been made in understanding the sensitivity of second-harmonic generation to interface defects, step, strain, roughness, and chemical modification.; As a part of these important advances, we have developed a contactless optical technique that allows us to monitor carrier transport by multiphoton internal-photoemission induced second-harmonic generation. We have applied this technique to studies of semiconductor-insulator heterostructures, such as Silicon-on-insulator (SOI) and Si/SiO2/MgO.; We have applied laser-induced electric fields simultaneously with external bias to monitor charge carrier dynamics in the SOI system. The large penetration depth of optical irradiation allowed us to monitor charge re-distribution caused by external electric field at both Si-island/buried-oxide and Si-substrate/buried-oxide interfaces. We demonstrated that it is possible to distinguish between laser-induced electric fields arising from each interface. Our optical technique allowed us to obtain SHG vs. bias characteristics similar to those obtained by conventional C-V and I-V methods, but in a contactless, non-invasive fashion.; In Si/SiO2 systems, electrons injected into the oxide from silicon, are trapped at the Si/SiO2 interface or on the oxide surface via ambient oxygen. Deposition of an MgO layer on SiO2 creates a potential-well structure arising from the band alignment of the two oxides: the conduction band of MgO is at a lower level than the SiO2 conduction band. New electronic states become available in the potential-well. The energy levels of these states are higher than the electron traps on the surface of SiO2, thus quantum-mechanical tunneling through SiO2 is facilitated and becomes observable when the injection process is interrupted. We observed remarkably different behavior of time-dependent SHG signals at high and low laser intensities. These measurements have provided deep insights into the fundamentals of charge carrier dynamics in semiconductor/insulator systems. |
