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Boron (B) doping is an important process to improve the electrical conductivity of p-type Czochralski silicon. The high concentration of B atoms causes long-range hole redistribution during the thermal B activation anneal. The aim of the present work is to study and model this complex phenomenon by analyzing the instantaneous kinetics of B transfer, trapping, clustering and segregation in strongly doped bi-layers using Secondary Ion Mass Spectroscopy and Hall effect measurements.
A heterojunction boron-doped solar cell was developed on the as-cut boron doped silicon wafer and its performance was evaluated by photoluminescence, open circuit voltage, and short circuit current density. The results showed that the boron doping induced an increase in the carrier density, leading to higher open circuit voltage and lower contact resistance than the un-doped device. It also led to a decrease in the quasi-fermi level of holes, which resulted in a reduction of the short circuit current density.
After ion implantation, the p-type silicon was doped with boron by diffusion in an aqueous TMAH solution. The boron concentration was optimized to be above 4 x 1019 cm-3 in the heavily doped layer, which showed excellent etch-stop selectivity. This is due to the strong decomposition of weakly bound hydrogen atoms, which can be explained by an advanced diffusion model.
After the boron diffusion, the p-type silicon was textured by EBL and anisotropic ICP etching. The textured wafers show a reduced surface reflectance. The characterization of the boron doped surfaces with different HF cleaning solutions indicates that the boron doped layers are less susceptible to the formation of Si-O oxide than the un-doped ones.