Diffusion of Impurities in Polycrystalline Silicon

The diffusion of electrically active impurities into polycrystalline silicon deposited by the thermal decomposition of silane has been studied. Both boron and phosphorus have been found to diffuse more rapidly into polycrystalline silicon than into single-crystal silicon. The diffusivity is greatest in films deposited at about 1000°C although this maximum is a function of the deposition rate, film thickness, and substrate material as well as the deposition temperature. The maximum diffusivity is associated with a structure which has the maximum {110} texture. Although most of the films have been deposited onto amorphous SiO2, a nucleating layer of polycrystalline silicon has been found to influence the diffusion of impurities into a thick layer of polycrystalline silicon deposited over the nucleating layer. An optimum structure is obtained with a nucleating layer containing maximum {110} texture. The enhanced diffusion is tentatively attributed to diffusion down defects in the polycrystalline silicon. The activation energy for diffusion of impurities into the polycrystalline silicon is less than that for diffusion into single-crystal silicon.