Optimized Temperature in Phosphorous Diffusion Gettering Setup of Chromium Transition Metal in Solar Grade Multicrystalline p-Type Silicon Wafer

We have investigated in this work the effect of the temperature profile during homogeneous phosphorous diffusion gettering (PDG) on multicrystalline (mc-Si) silicon p-type wafers destined for photovoltaic solar cells. Temperatures were varied from 800 ◦C to 950 ◦C with time cycle of 90 minutes. Phosphorous profile of np junction was measured by secondary ion mass spectroscopy (SIMS) from 0.45 μm to 2.4 μm. Chromium concentration profile measured on the same samples by SIMS shows a high accumulated concentration of Cr atoms in the gettering layer at 900 ◦C and 950 ◦C, compared to samples obtained at 800 ◦C and 850 ◦C. The effective lifetime (τeff) of minority charge carriers characterized by quasi-steady state photoconductance (QSSPC) is in correlation with these results. From the QSSPC measurements we have observed an amelioration of τeff from 7 μs before PDG to 26 μs in the samples after PDG, processed at 900 ◦C. This indicates the extraction of a non-negligible concentration (5× 10 cm−3 to 5× 10 cm−3) of Cr from the bulk to the surface gettering layer, as observed in the chromium SIMS profiles. A light degradation of τeff (18 μs) is observed in the samples treated at 950 ◦C due probably to a partial dissolution of the metallic precipitates, especially at the grain boundaries and in the dislocations vicinity. The related τCr-Impurity lifetime value of about 8.5 μs is extracted, which is the result of interstitial Cri or CriBs pairs, proving their strongest recombination activity in silicon.