Comparison of firing stability between p- and n-type polysilicon passivating contacts

Abstract

This work compares the firing response of ex-situ doped p- and n-type polysilicon (poly-Si) passivating contacts and identifies possible mechanisms underlying their distinct firing behavior. The p-type poly-Si shows greater firing stability than n-type poly-Si, particularly at a higher firing temperature, which results in a substantial increase in the recombination current density parameter J0 from 9 to 96 fA/cm2 upon firing at 900°C for n-type poly-Si, in comparison to an increase from 11 to 30 fA/cm2 for p-type poly-Si. It is observed that p-type poly-Si contacts only suffer a slight degradation or even exhibit a small improvement in J0 after firing at 800°C, depending on the boron diffusion temperature. Secondary ion mass spectrometry (SIMS) results demonstrate that the hydrogen concentration near the interfacial SiOx increases with the peak firing temperature in n-type poly-Si, whereas the hydrogen profile remains unchanged for p-type poly-Si upon firing at various temperatures. Moreover, we observe that injecting additional hydrogen into the poly-Si/SiOx stacks fired with SiNx coating layers further degrades n-type poly-Si, but recovers the J0 of p-type poly-Si to the value before firing. In contrast, removing hydrogen from the fired poly-Si/SiOx stacks leads to an initial recovery and then a second degradation of J0 in n-type poly-Si, but no substantial impact on p-type poly-Si. It is hypothesized that the distinct difference in the firing impact on p- and n-type poly-Si is related to the different effective hydrogen diffusivity, which determines the hydrogen content surrounding the SiOx layer and hence the passivation quality after firing.