Gallium-Boron Spin-on co-Doping for Polycrystalline Silicon Passivating Contacts

Herein, we present an alternative doping technique for p-type poly-Si/SiOx passivating contacts using a spin-on method for different mixtures of Ga and B glass solutions. Effects of solution mixing ratios on the contact performance, represented by an implied open circuit voltage iVoc and a contact resistivity ϱC, are investigated. For all as-annealed samples at different drive-in temperatures, increasing the percentage of Ga in the solution shows a decrement in iVoc (from ~680 to ~610 m V) and increment in ϱC (from ~3 to ~800 mΩ.cm2). After a hydrogenation treatment by depositing a SiNx/A10x stack followed by forming gas annealing, all samples show improved iVoc (up to ~700 mV with Ga and B co-doped, and ~720 mV with all Ga). Interestingly, when co-doping Ga with B, even a small amount of B in the mixing solution is likely to have negative effects on the surface passivation. Free carrier density and total dopant density profiles obtained by electrical capacitance voltage and secondary ion mass spectrometry measurements, respectively, reveal a relatively low percentage of electrically active Ga and B after their diffusion into the poly-Si and the Si substrate. These results help to understand the different features of the two dopants: a low contact resistivity with B, a good passivation with Ga, their degree of activation inside the poly-Si and c-Si layers, and the effects of the annealing temperature.