Passivation of n⁺-Type Si Surfaces by Low Temperature Processed SiO₂/Al₂O₃ Stacks

The surface passivation of SiO2/Al2O3stacks prepared at low process temperatures was investigated on phosphorous diffused n+-type Si surfaces with a broad range of sheet resistances. Two kinds of SiO2films were prepared, the first with plasma-enhanced chemical vapor deposition (PECVD) and the second in a wet chemical process. After atomic layer deposition of the Al2O3capping layer, the resulting SiO2/Al2O3stacks differ in the polarity of their fixed charge density, i.e., the PECVD SiO2stacks had a positive and the wet chemically grown SiO2stacks a negative fixed charge density. The PECVD SiO2/Al2O3stacks resulted in a high surface passivation over a broad range of sheet resistances whereas the wet chemically grown SiO2stacks were only feasible for diffused surfaces with low sheet resistances (<; 100 Ω/□). By corona charging experiments, it was established that the field effect based on a negative fixed charge density was the reason for the loss in surface passivation in the specific range of diffused surfaces.