Micro-photoluminescence studies of shallow boron diffusions below polysilicon passivating contacts

Micro-photoluminescence is utilised in this work to study the shallow boron diffusions that occur underneath p + polysilicon passivating contacts. Regions with high boron concentrations emit a photoluminescence peak at a higher wavelength compared to the substrate crystalline silicon due to band-gap narrowing. We demonstrate the presence of this peak with samples following optimised fabrication procedures based on previous studies for thermally BBr₃ diffused polysilicon passivating contacts. The intensities of the diffusion peaks, when normalised against the substrate crystalline silicon peak, change with the dopant concentration profiles, which in turn vary with the BBr₃ diffusion temperatures as well as the thickness of the interfacial oxide layer. Comparisons are made between the photoluminescence spectra from p + polysilicon passivating contacts and n + polysilicon contacts reported previously [1]. We find that the sub-band-gap photoluminescence peaks arising from the n + polysilicon layers themselves are absent in the p + polysilicon contacts. This simplifies the procedure for photoluminescence spectra measurements in the case of p + polysilicon contacts. Further demonstration of micron-scale spectral photoluminescence mapping with localised p + polysilicon structures demonstrates the feasibility of the method in detecting underlying boron diffused regions with a spatial resolution of below 10 μm. Such micron-scale mapping will be useful for characterising localised polysilicon structures, such as polysilicon under the fingers on the front side of solar cells, or in interdigitated back contact devices.