Amorphous Silicon Passivation of Surfaces Promoting Epitaxy

AbstractA two-step approach to passivate crystalline silicon (c-Si) with hydrogenated amorphous silicon (a-Si:H) for amor- phous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells is discussed: The combination of low temperature a-Si:H deposition of 7nm (i)a-Si:H and hydrogen plasma post deposition treatments (HPT) is shown to yield charge carrier lifetimes of 8ms on c-Si<100>, i.e. a crystal surface which can be difficult to passivate because it promotes epitaxial growth. It is shown that the passivation improvement upon HPT stems from diffusion of hydrogen atoms to the heterointerface and subsequent dangling bond passivation. Upon HPT, the a-Si:H hydrogen density increases, leading to a valence band offset increase at the heterojunction. However, the film disorder is not increased. Thus, HPTs allow for a-Si:H band gap and a-Si:H/c-Si band offset engineering. Furthermore a HPT enables the application of a-Si:H/c-Si-heterojunction solar cells concepts on surfaces that promote epitaxy, like Si<100> and (nano)structured surfaces