Study of degradation in bulk lifetime of n-type silicon wafer due to oxidation of boron-rich layer

Various boron (B) diffusion techniques are being investigated to fabricate n-type Si solar cells. Thermal oxidation is often used to remove boron-rich layer (BRL) formed as a byproduct of B diffusion because BRL interferes with surface passivation of boron emitter. However, oxidizing the BRL can cause significant degradation in bulk lifetime. In this paper, high resolution electron microscopy (HREM) was performed to detect the presence of BRL after B diffusion and its removal after subsequent oxidation. In addition, bulk lifetime of n-type Si with BRL was measured after various oxidation conditions to systematically investigate the mechanism of oxidation-induced lifetime degradation in n-type Si. Detailed analysis of the oxidized samples revealed that iron (Fe) is primary metal impurity responsible for the bulk lifetime degradation after oxidation. This happens because Fe is gettered in BRL after B diffusion and during the oxidation, when the BRL is consumed, Fe is released into the bulk to degrade lifetime.