A study was made of the possibility of gettering transition metal impurities from solar-grade crystalline Si. Porous Si layers were formed, by stain-etching, on both sides of the Si wafer. The diffusion of Al was performed through the porous Si layer in an infra-red furnace; under a (N2/O2) controlled atmosphere. This permitted the gettering of any metal impurities towards the porous Si layer. The gettering effect was evaluated by measuring the majority carrier density and mobility and the minority carrier diffusion length (Ld) of the Si substrate. For this purpose, Wander Pauw and Hall Effect measurements and the light beam induced current technique were used. It was noticed that the best gettering corresponded to heat treatment at 850C for 0.5h. In that case, an evident decrease in the majority carrier density and an enhancement of the mobility were observed. After gettering, an apparent improvement in the minority carrier diffusion length was found. These results provided evidence for the effectiveness of external gettering treatments.

Gettering Impurities from Crystalline Silicon by Aluminum Diffusion Using a Porous Silicon Layer. N.Khedher, M.Hajji, B.Bessaïs, H.Ezzaouia, A.Selmi, R.Bennaceur: Physica Status Solidi C, 2005, 2[9], 3486-90