PASSIVATION OF GRAIN-BOUNDARIES IN POLYCRYSTALLINE SILICON

Preferential diffusion of various gases down the grain boundaries in polycrystalline silicon is shown to promote significant changes in the density of defect states in these regions. A plasma of monatomic hydrogen provides a significant reduction in both the state density and the accompanying grain-boundary potential barrier while plasmas of oxygen, nitrogen, and sulfur hexafluoride are shown to increase this density of states. Boundaries passivated with hydrogen have as much as a factor of 1000 larger transconductance after treatment. Hydrogenated barriers are stable over long periods at 375°C and essentially indefinitely at 23°C. The results have important implications for the development of low-cost thin-film silicon photovoltaic devices.