Limiting factors of phosphorus external gettering efficiency in multicrystalline silicon

Abstract

External gettering by phosphorus diffusion is used to improve the minority carrier diffusion lengths L/sub n/ in multicrystalline silicon solar cell wafers obtained by different growth techniques. Polix, Silso, Semix and Eurosolare materials were characterized. It is shown that gettering efficiency depends strongly on the origin of the material. For Polix and Eurosolare wafers, the optimal diffusion temperature is around 900/spl deg/C; L/sub n/ increases with diffusion time and tends to saturate, after 8 hours, at about 300 to 500 /spl mu/m. However the presence of a too large density of defects, or (and) a high concentration of dissolved oxygen atoms (i.e 10/sup 18/ cm/sup -3/) limits the external gettering efficiency, probably by an internal gettering of impurities. Competition between external and internal gettering seems to occur. >