Control of boron diffusion from APCVD BSG layers by interface oxidation

We investigate the impact of oxygen concentration during boron diffusion at peak temperatures varying from 875°C to 950°C using borosilicate glass layers (BSG) deposited by atmospheric pressure chemical vapor deposition (APCVD) as doping source. Therefore, we vary the boron concentration in the BSG layer and the oxygen concentration in the gaseous ambient of the high temperature step. We characterize the process combinations with respect to their resulting sheet resistance and charge carrier concentration profile. In addition, we perform quasi steady-state photoconductance measurements to investigate the influence of the process parameters on the recombination properties at the boron-diffused and passivated surface. Low saturation current densities J0 = 16 fA/cm2 (planar surface, Al2O3/SiNx passivation) are observed at a sheet resistance of Rsh = 121 O/sq at peak temperature Tpeak = 875°C. Finally, we show that the oxygen concentration is a crucial parameter to prevent the formation of a boron rich layer at the silicon surface. This parameter needs to be selected carefully, as too high oxygen concentrations lead to complete decoupling of the doping source, depending on boron concentration and process temperature.