Intra-grain defects - Limiting factor for low-temperature polycrystalline silicon films?

We investigate the electronic properties of silicon films, deposited by ion-assisted deposition at low deposition temperatures T-dep < 650 degreesC. Silicon films deposited on monocrystalline silicon substrates with different crystallographic orientations serve to evaluate the perspectives of polycrystalline thin film epitaxy. We find large differences in the electronic properties of the films depending on the substrate orientation. Diffusion length is up to an order of magnitude lower in (1 1 1)-oriented Si films as compared to (100)-oriented Si films, resulting in inhomogeneous current collection in light beam induced current measurements of polycrystalline Si solar cells. Photoluminescence and deep level transient spectroscopy reveal broad defect distributions for low deposition temperatures T-dep less than or equal to 550 degreesC. Point defect densities are up to four orders of magnitude lower in (100)-oriented Si epitaxial layers than in (1 1 1)-oriented Si films.