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)/SiOx structures, and identify factors affecting the firing response. Our samples show stable surface passivation upon firing at temperatures from 600 degrees C to 750 degrees C but exhibit a substantial increase in the recombination current density parameter J(0) when the peak firing temperature reaches 800 degrees 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 struc-ture. 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 SiNx/AlOx stacks or SiNx 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/SiOx passivating contacts. The hy-pothesis explains the stronger firing impact on samples with lighter doping and lower crystallinity, which de-termines the diffusion of hydrogen upon firing and hence the amount of hydrogen present in the poly-Si/SiOx structure, and especially at the oxide interface.

Automated summary

The study found that firing treatments have a stable effect on the surface passivation of phosphorus-doped silicon samples, but can lead to a significant increase in recombination current density at 800 degrees Celsius. Factors affecting the degradation extent include the means of oxide growth, poly-Si deposition conditions, and phosphorus diffusion, particularly the deposition temperature of poly-Si.