Temperature-Induced Stoichiometric Changes in Thermally Grown Interfacial Oxide in Tunnel-Oxide Passivating Contacts
A vital part of poly-Si passivating contacts is the ultrathin interfacial oxide layer sandwiched between heavily-doped poly-Si and c-Si absorber that will herein be investigated in detail. The stoichiometry of differently prepared interfacial oxides in the as-grown state was investigated by XPS. The findings are correlated with the thermal stability of the oxide layers integrated in tunnel-oxide passivating contacts (TOPCon) which experience further structural modification during the high-temperature annealing step. More specifically, it was observed that a more stoichiometric interfacial oxide is a more effective diffusion barrier for dopants and, thereby enhanced the thermal stability of TOPCon structures with respect to the passivation quality. Furthermore, stoichiometric changes in the oxide layer upon subsequent contact formation (thermal annealing) were analyzed by XPS after selective etch back of the doped poly-Si layer. It is found that the interfacial oxide becomes more stoichiometric during annealing. In addition, a slight increase in oxide thickness was observed which tentatively could be explained by a large amount of unbound interstitial oxygen in the interface region.