Nonlinear optical spectroscopy, dynamics and reactivity of buried semiconductor interfaces

We have studied Si and Ge interfaces using Second Harmonic Generation (SHG). In order to study semiconductor interfaces in the region near the gap, an automated, transform limited, picosecond optical parametric amplifier tunable in the 1.1--3.3 mum wavelength range with a 15 cm-1 bandwidth was constructed.; Second Harmonic Generation Rotational Anisotropy (SHG-RA) provides a convenient means to monitor the chemical state of the Si surfaces, and to follow the conversion of a H terminated surface to SiO2 by oxidation as a function of time in ambient. SHG at 800 nm is sensitive to changes in the surface electronic properties induced by oxidation and it probes both the initial stage of oxidation as well as the logarithmic oxide growth. We found that intense ultrashort (4 ps) 800 nm laser pulses accelerate the initial stage of oxidation of Si(111)-H in ambient. The cross-section of the photo-induced process was found to be 1.7 x 10-23 cm2 . A mechanism involving drift of photo-excited electrons toward the surface is proposed to explain the photooxidation.; Time resolved SHG was used to probe carrier relaxation at various Si interfaces and to quantify the electrical passivating nature of the surface. Carriers were created by optical excitation via an interband transition. Surface recombination and Auger recombination are the relaxation processes responsible for the decay of the conduction band electrons. The importance of the surface recombination was highlighted by the dependence of the SHG dynamics on the 'quality' of the H-termination.; The chemical modification of germanium surfaces and their stability was investigated using a number of techniques, including SHG, Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). The XPS spectra revealed rapid reoxidation of nominally H and Cl terminated interfaces against air. We found the alkyl- and S-termination stable against oxidation for >2 month as judged by SHG-RA and XPS. The resonance in the IR-SHG spectrum of the Ge(111)-GeO 2 interface was assigned to the direct interband transition of germanium. The IR-SHG spectrum of S-Ge(111) showed a new resonance, possibly originating from the surface, in addition to the resonance inherent to the bulk Ge.