p-n Junctions in vacuum deposited polycrystalline silicon thin films

Diffused p-n junctions formed in vacuum deposited polycrystalline silicon films on sapphire substrates were examined as functions of both diffusion and deposition conditions. Thick films (5-33 µm) with large crystallites suitable for solar cell and large area device applications are described. Comparisons are made between amorphous silicon films converted to polycrystalline during diffusion processing and films deposited in polycrystalline form on hot substrates. Standard integrated-circuit boron and phosphorus diffusion techniques, oxidation, and masking were used throughout the study. Sample purity, diffusion profiles, and junction depths were determined by SIMS. The study shows that junction depth can be controlled within polycrystalline silicon films. Samples with different crystallite size (determined by SEM) were formed at different substrate temperatures. Resistivity, current voltage characteristics, and photovoltaic response are discussed in terms of film parameters and device geometry.