Pre-annealing for improved LPCVD deposited boron-doped poly-Si hole-selective contacts

We demonstrate the beneficial effect of a pre-annealing step prior to the boron diffusion on passivation and contact resistivity of industrially LPCVD deposited poly-Si/SiOX hole-selective contacts. We investigate the influence of the pre-annealing temperature on passivation quality, measured as implied open-circuit voltage and recombination current density, and on changes in crystallinity, characterized by Raman spectroscopy. A clear increase in passivation quality is observed on planar and textured surfaces as well as for various poly-Si thicknesses (100-230 nm) and thermal SiOX growth temperatures (600-800 degrees C). On planar surfaces and without the use of atomic hydrogenation, we report an increase in iVOC of around 5 mV with every additional increase of pre-annealing temperature by 50 degrees C (>900 degrees C) leading to an iVOC of 720 mV (J0 = 9.3 fA/cm2). After atomic hydrogenation, the effect of the pre-annealing is less pronounced. Nevertheless a gain in iVOC (reduction in J0) of 5-10 mV (2-5 fA/cm2) is achieved when comparing samples without pre-annealing with samples after a pre-annealing at 1050 degrees C. On textured surfaces on the other hand, this trend is more pronounced after atomic hydrogenation, for which a pre-crystallisation at 1050 degrees C leads to an iVOC (J0) of 705 mV (16.8 fA/cm2), which is a gain (reduction) of 24 mV (21.7 fA/cm2) compared to samples without a pre-annealing step.