Atomic hydrogen induced defects in amorphous silicon : an in situ study of the reaction kinetics

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The motivation of this thesis is to provide quantitative insights into the microscopic interaction mechanisms between atomic hydrogen and hydrogenated amorphous silicon (a-Si:H), particularly the evolution kinetics of the electronic defect states during the interaction. a-Si:H thin films are grown on the total internal reflection surface of a folded miniature optical resonator by thermal decomposition of SiH4 on a hotwire, and are subjected to quantified atomic H fluxes at various substrate temperature. The H-induced defects during the process is monitored in situ by the Evanescent-Wave Cavity Ring-Down Spectroscopy (EW-CRDS), which allows to study the kinetics with a time resolution of 33 ms and sensitivity up to 0.1 ppm in the defect absorption change. The defect evolution process is characterized by a fast increase to a steady state when the H flux is turn on and reversible healing after the H flux is turned off. The effects of the H flux, substrate temperature, film structure and film thickness on the defect evolution process are investigated. The electronic defect state detected by EW-CRDS is recognized as the defect complexes (DC), which are created by insertion of H atoms into strained Si-Si bonds. A comprehensive microscopic kinetic model is proposed to interpret the measured EW-CRDS data, which involves DC creation by H insertion, H-DC recombination, DC self-healing and H diffusion. The kinetic model is studied analytically for the H and DC profiles in the steady state and the DC evolution kinetics in the initial stage. The defect absorption kinetics in the entire time range can be fitted numerically based on the kinetic model, which is consistent with the analytical results. Based on the combined analytical and numerical study, the kinetic parameters of the H/a-Si:H interaction and their temperature dependence can be quantitatively evaluated. The results from the thesis will provide new insights into a number of important H-involved processes in a-Si:H such as the light induced degradation and the H-induced crystallization.