Firing Stability of Polysilicon Passivating Contacts: The Role of Hydrogen

Abstract

This work compares the firing response of n-type silicon wafers passivated by n- and p-type polysilicon (polySi)/SiOx contacts, and assesses the changes in their properties using various characterization tools, including Fourier-transform infrared spectroscopy (FTIR), secondary ion mass spectrometry (SIMS) and micro-photoluminescence (μ-PL) spectroscopy. Ptype poly-Si exhibits greater thermal stability than n-type poly-Si, particularly upon firing at 800°C, which results in an increase in the recombination current density parameter J0 from 12 to 15 fA/cm2 for the p-type poly-Si contacts, in comparison to a much larger increase from 8 to 40 fA/cm2 for n-type poly-Si contacts. FTIR measurements indicate that the n-type poly-Si contacts show an increased density of hydrogen bonds after firing, where no changes are observed in p-type poly-Si. SIMS results demonstrate that, for n-type poly-Si, the hydrogen concentration near the interfacial SiOx increases with the firing temperature, whereas the hydrogen profile remains largely unchanged in the p-type poly-Si passivating contacts upon various firing temperatures. It is speculated that the different firing stability of n- and p-type polySi might be due to the different hydrogen concentration injected into the SiOx interlayer upon firing, which could be caused by the difference in hydrogen diffusivity. In addition to hydrogen, other effects, such as the formation of defects, may also contribute to the firing impact, supported by the diminished a-Si:H signal detected by μ-PL in n- and p-type poly-Si contacts.