Atomistic analysis of the surface reactivity, roughness, and crystallinity of plasma-deposited amorphous silicon thin films.

摘要

Hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) thin films grown by plasma-enhanced chemical vapor deposition from silane-containing discharges are widely used in electronic devices such as solar cells and flat-panel displays. The reactivities and mobilities of reactive species from the plasma, which impinge on the growth surface during film deposition or on the surface of the deposited film during its post-deposition treatment, determine the film properties such as crystallinity and surface roughness. These film properties, in turn, are crucial in determining the performance of the resulting optoelectronic or photovoltaic devices. Specifically, smooth, device-quality a-Si:H films are deposited under conditions where the dominant deposition precursor is the SiH3 radical. Furthermore, the exposure of a-Si:H films to H atoms from an H2 plasma leads to the deposition of nc-Si:H thin films.; In this dissertation, an integrated atomic-scale computational analysis of the fundamental interactions of the SiH3 radical and H atoms with a-Si:H deposition surfaces is presented. The analysis is based on a synergistic approach that combines molecular-dynamics simulations of radical precursor impingement on a-Si:H growth surfaces and H atom impingement on surfaces of smooth a-Si:H films with targeted, accurate first-principles density functional theory calculations of the corresponding diffusion and reaction pathways. Following this approach, we have addressed a number of outstanding problems in the plasma deposition of a-Si:H thin films and provided a comprehensive interpretation to a large body of measurements from silicon thin-film growth and characterization experiments. Specifically, three major unresolved issues in a-Si:H literature are addressed: analysis of the reactions of the SiH 3 radical on a-Si:H growth surfaces to explain the temperature independent surface reactivity and growth rate of a-Si:H films, elucidation of the atomic-scale mechanisms responsible for the surface smoothness of a-Si:H films, and, finally, comprehensive analysis of the key reaction and transport processes of atomic H, which mediate the amorphous-to-nanocrystalline transition during the post-deposition exposure of a-Si:H films to H atoms.