STRONG IMPROVEMENT OF DIFFUSION LENGTH BY PHOSPHORUS AND ALUMINUM GETTERING

The minority carrier diffusion length in polycrystalline silicon has been strongly improved by using several gettering processes. These processes include different surface treatments followed by conventional thermal annealing (CTA) performed at temperatures between 800 and 950-degrees-C. The n+p structures with a back lapped surface exhibit a maximum increase of the diffusion length from 35 to 140 mum for 45 min annealing duration at temperatures of 900 and 950-degrees-C. The realization of a back surface field (BSF) on the lapped surface by aluminum deposition followed by a CTA cycle at 950-degrees-C, with slow cooling rate (2-degrees-C/min) induces a strong additional gettering, resulting in the increase of the minority carrier diffusion length up to 350%.