Structural models of a-Si:H with a low defect concentration: a first-principles molecular dynamics study

We present a theoretical study of hydrogenated amorphous silicon (a-Si:H) with a device quality hydrogen concentration of 11 at%. We used a first principle, parameters-free method. The interaction between the atoms was treated quantum mechanically within the density functional theory approximation. Amorphous structures were prepared by cooling from the liquid phase. When using a cooling rate of 0.02¿K/fs defect-free structures were prepared. All silicon atoms were fourfold coordinated and there were no defect states in the band gap. The calculated short range order showed a good agreement with available neutron scattering measurements. We further calculated the formation energy of dangling bonds (DBs; threefold coordinated Si atom) in all three charge states (negative, neutral, and positive) as a function of the Fermi energy. Interestingly, the DB correlation energies can have both positive and negative values.