Investigation of Gallium-Boron Spin-On Codoping for Poly-Si/SiOx Passivating Contacts

A doping technique for p-type poly-Si/SiOx passivating contacts using a spin-on method for different mixtures of Ga and B glass solutions is presented. Effects of solution mixing ratios on the contact performance (implied open circuit voltage iV_oc, contact resistivity ρ_c) are investigated. For all as-annealed samples at different drive-in temperatures, increasing the percentage of Ga in the solution shows a decrement in iV_oc (from ∼680 to ∼610 mV) and increment in ρ_c (from ∼3 to ∼800 mΩ cm²). After a hydrogenation treatment by depositing a SiN_x/AlO_x stack followed by forming gas annealing, all samples show improved iVoc (∼700 mV with Ga-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 shows negative effects on the surface passivation. Active and total dopant profiles obtained by electrical capacitance voltage and secondary-ion mass spectrometry measurements, respectively, reveal a relatively low percentage of electrically-active Ga and B in the poly-Si and Si layers. These results help understand the different features of the two dopants: a low ρ_c with B, a good passivation with Ga, their degree of activation inside the poly-Si and Si layers, and the annealing effects.