HYDROGEN MIGRATION IN POLYCRYSTALLINE SILICON

Hydrogen migration in solid-state crystallized and low-pressure chemic al-vapor-deposited (LPCVD) polycrystalline silicon (poly-Si) was inves tigated by deuterium diffusion experiments. The concentration profiles of deuterium, introduced into the poly-Si samples either from a remot e D plasma or from a deuterated amorphous-silicon layer, were measured as a function of time and temperature. At high deuterium concentratio ns the diffusion was dispersive depending on exposure time. The disper sion is consistent with multiple trapping within a distribution of hop ping barriers. The data can be explained by a two-level model used to explain diffusion in hydrogenated amorphous silicon. The energy differ ence between the transport level and the deuterium chemical potential was found to be about 1.2-1.3 eV. The shallow levels for hydrogen trap ping are about 0.5 eV below the transport level, while the deep levels are about 1.5-1.7 eV below. The hydrogen chemical potential mu(H) dec reases as the temperature increases. At lower concentrations, mu(H) wa s found to depend markedly on the method used to prepare the poly-Si, a result doe in part to the dependence of crystallite size on the depo sition process. Clear evidence for deuterium deep traps was found only in the solid-state crystallized material. The LPCVD-grown poly-Si, wi th columnar grains extending through the film thickness, displayed lit tle evidence of deep trapping, and exhibited enhanced D diffusion. Man y concentration profiles in the columnar LPCVD material indicated comp lex diffusion behavior, perhaps reflecting spatial variations of trap densities, complex formation, and/or multiple transport paths. Many as pects of the diffusion in poly-Si are consistent with diffusion data o btained in amorphous silicon.