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

摘要

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 degrees C to 950 degrees C with time cycle of 90 minutes. Phosphorous profile of n(+)p junction was measured by secondary ion mass spectroscopy (SIMS) from 0.45 mu m to 2.4 mu m. Chromium concentration profile measured on the same samples by SIMS shows a high accumulated concentration of Cr atoms in the gettering layer at 900 degrees C and 950 degrees C, compared to samples obtained at 800 degrees C and 850 degrees C. The effective lifetime (tau(eff)) 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 tau(eff) from 7 mu s before PDG to 26 mu s in the samples after PDG, processed at 900 degrees C. This indicates the extraction of a non-negligible concentration (5 x 10(14) cm(-3) to 5 x 10(15) cm(-3)) of Cr from the bulk to the surface gettering layer, as observed in the chromium SIMS profiles. A light degradation of tau(eff) (18 mu s) is observed in the samples treated at 950 degrees C due probably to a partial dissolution of the metallic precipitates, especially at the grain boundaries and in the dislocations vicinity. The related tau(Cr-Impurity) lifetime value of about 8.5 mu s is extracted, which is the result of interstitial Cri or CriBs pairs, proving their strongest recombination activity in silicon.