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 H-2 to SiH4 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 mu s. Subsequent low temperature annealings cause an important restructuring of Si-H bonding at the a-Si:H/c-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 mu s are achieved postannealing. Initial results indicate the improvement depends on surplus SiH2 from the interface region. (C) 2008 American Vacuum Society.