Role of hydrogen bonding environment in a-Si:H films for c-Si surface passivation

The search for an ideal surface passivation layer of crystalline silicon (c-Si) to be employed in a silicon heterojunction photovoltaic device has garnered much attention. The leading candidate is a few nanometers of intrinsic amorphous silicon ((i) a-Si:H) film. Reported dependencies of film surface passivation quality on substrate preparation, orientation, and deposition temperature have been extended in this work to include H2 to Si H4 dilution ratio and postdeposition annealing. Simple avoidance of the deposition regimes that lead to epitaxial growth of Si on the c-Si substrate produces decent lifetimes on the order of 500 μs. Subsequent low temperature annealings cause an important restructuring of Si-H bonding at the a-Si:Hc-Si interface increasing the amount of monohydride at the c-Si surface. This restructuring would reduce the c-Si surface defect density and cause an improvement of surface passivation as confirmed by effective lifetime measurements. Final effective carrier lifetimes up to 2550 μs are achieved postannealing. Initial results indicate the improvement depends on surplus Si H2 from the interface region.