P-type poly-Si/SiOx contact by aluminium-induced crystallization of amorphous silicon

Abstract

In this work, we explored aluminium-induced crystallization (AIC) of amorphous Si (a-Si) as a low-temperature alternative to make p-type poly-Si/SiOx contacts for Si solar cells. In this approach, a stack of Al and a-Si deposited on a SiOx-passivated Si wafer surface is annealed at temperatures below 577 degrees C (the eutectic temperature of Al-Si mixture), which induces the exchange of Al and Si layer positions in the stack and the simultaneous crystallization of a-Si to Al-doped p-type poly-Si, potentially leading to the formation of an Al/polySi/SiOx passivating contact. The Al-doped AIC poly-Si has low carrier concentration, and thus to compensate for this, we used highly B-doped a-Si for AIC and could achieve a tenfold increase in the carrier concentration. We found that the typical thickness of 1.5 nm of the passivating SiOx layer is too low for this AIC-based approach, as Al reduces SiOx during the process and degrades its passivation quality. By starting with a thicker SiOx layer, we could demonstrate that AIC poly-Si/SiOx contacts with low contact recombination is achievable. However, the contact characteristic remains non-ohmic. This can be improved by extended annealing after AIC, during which Al diffuses into the Si substrate. The Al diffusion, however, damages the SiOx layer and in turn severely increases contact recombination. We conclude that with AIC poly-Si/SiOx contacts the process window to achieve low contact resistivity and low contact recombination seems to be very narrow, if existing at all.