Adequate Method for Decoupling Bulk Lifetime and Surface Recombination Velocity in Silicon Wafers

In this paper, we present an appropriate method of decoupling surface and bulk recombination processes in silicon wafers. The study was carried out using the surface passivation of multicrystalline silicon wafers by ethanolic solution of iodine at different molarities varying between 0.01 M and 0.1 M. The effect of the concentration of ethanolic iodine solution on surface passivation effectiveness was investigated by using quasi steady state photoconductance technique. Reproducible experiments have shown that the best passivation is reached for a molarity of around 0.02 M. The carrier lifetime after passivation at 0.02 M has been improved by more than one order of magnitude, compared to that of the same wafer before the passivation. Using an adequate modeling of minority carrier lifetime curves τ (∆n), based on Hornbeck-Haynes model, surface recombination velocity was calculated. The minimum values of surface recombination velocity have been found to be approximately 120 cm/s for 0.02 M. The modeling results indicate that the minority carrier lifetime improvement can be easily correlated with the decrease of the surface recombination velocity for a fixed bulk lifetime τ b = 115 µs

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 n⁺p 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 $(\tau_\text{eff})$ 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 $\tau_\text{eff}$ 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 (5×10¹⁴ cm¯³ to 5×10¹⁵ cm¯³) of Cr from the bulk to the surface gettering layer, as observed in the chromium SIMS profiles. A light degradation of $\tau_\text{eff}$ (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 $\tau_\text{Cr-Impurity}$ lifetime value of about 8.5 μs is extracted, which is the result of interstitial $Cr_{i}$ or $Cr_{i}B_{s}$ pairs, proving their strongest recombination activity in silicon