Understanding the impurity gettering effect of polysilicon/oxide passivating contact structures through experiment and simulation
Polysilicon/oxide (poly-Si/SiOx) passivating contacts are a promising technology for the next-generation of high-efficiency silicon solar cells. The structure can be realised by a range of fabrication techniques, which can induce very different impurity gettering effects during the formation process. Understanding the different gettering effects will enable tailored solutions to optimise the gettering efficiency in device fabrication. This paper demonstrates a method to separately quantify the impact of each component on the overall gettering effect of the poly-Si/SiOx passivating contact structures. These components consist of the heavily doped poly-Si layer, in terms of its gettering strength; the SiOx interlayer, regarding its potential blocking effect for slowing down the diffusion of impurities; and the dopant in-diffused surface regions of the silicon wafer bulk directly below the SiOx interlayer, which may have a small additional gettering effect due to heavy dop ing. Phosphorus in-situ doped poly-Si layers from plasma-enhanced chemical vapour deposition (PECVD), coupled with SiOx interlayers from different growth techniques, were used to demonstrate the method. The experimental and simulation results confirm that the heavily doped poly-Si layer acts as the main gettering sink and the presence of different SiOx interlayers determines the overall gettering rate. For the ultrathin SiOx interlayers studied in this work, which have a similar thickness but different stoichiometry, a standard thermally grown SiOx demonstrates the strongest blocking effect, followed by a chemically grown SiOx from hot nitric acid, and a thermal SiOx of a reduced stoichiometry (grown in a pure nitrogen ambient) demonstrates practically no blocking effect.