The migration of H in solid-state crystallized and low-pressure chemical-vapor deposited polycrystalline material was investigated by performing D diffusion experiments (table 43). The concentration profiles, of D which was introduced into the samples from a remote D plasma or from a deuterated amorphous Si layer, were measured as a function of time and temperature. At high D concentrations, the diffusion was dispersive and depended upon the exposure time. The dispersion was consistent with multiple trapping within a distribution of hopping barriers. The data could be explained in terms of a 2-level model that was used to explain diffusion in hydrogenated amorphous Si. The energy difference between the transport level, and the D chemical potential, was found to be some 1.2 to 1.3eV. The shallow levels for H trapping were about 0.5eV below the transport level, while the deep levels were about 1.5 to 1.7eV below. The H chemical potential decreased as the temperature increased. At lower concentrations, the H chemical potential depended markedly upon the method which was used to prepare the material. This was suggested to be partly due to the dependence of the crystallite size upon the deposition process. Clear evidence for D deep traps was found only in solid-state crystallized material. The low-pressure chemical-vapor deposited material, with columnar grains that extended through the film thickness, displayed little evidence of deep trapping and exhibited enhanced D diffusion. Many of the concentration profiles in columnar chemical-vapor deposited material reflected a complex diffusion behavior. The latter was attributed to spatial variations in trap density, complex formation and/or multiple transport paths. Many aspects of the present diffusion behavior were consistent with diffusivity data for amorphous Si.

N.H.Nickel, W.B.Jackson, J.Walker: Physical Review B, 1996, 53[12], 7750-61

Table 43

Diffusivity of H in Polycrystalline Si