Firing stability of phosphorus-doped polysilicon passivating contacts: Factors affecting the degradation behavior

We investigate the impact of firing treatments on n-type silicon samples passivated by ex-situ phosphorus-doped polysilicon (poly-Si)/SiO_x structures, and identify factors affecting the firing response. Our samples show stable surface passivation upon firing at temperatures from 600 °C to 750 °C but exhibit a substantial increase in the recombination current density parameter J₀ when the peak firing temperature reaches 800 °C. The extent of degradation is found to also be affected by various processing parameters, such as the means of oxide growth, the poly-Si deposition conditions, and the subsequent phosphorus diffusion. Particularly, the degradation extent appears to increase with poly-Si deposition temperature, possibly associated with changes in the crystal structure. Moreover, phosphorus diffusions performed at a lower temperature leads to stronger firing impact, which could be attributed to the lighter doping concentration in the poly-Si film. In addition, dielectric coatings show the most obvious influence on the firing behavior. Samples fired without the presence of dielectric capping layers suffered the most pronounced degradations in J₀, whereas samples coated with SiN_x/AlO_x stacks or SiN_x single layer with high refractive index above 2 exhibit minimum firing impact. It is speculated that hydrogen diffusion is responsible for the changes in surface passivation quality of the poly-Si/SiO_x passivating contacts. The hypothesis explains the stronger firing impact on samples with lighter doping and lower crystallinity, which determines the diffusion of hydrogen upon firing and hence the amount of hydrogen present in the poly-Si/SiO_x structure, and especially at the oxide interface.