DEFECTS IN A-SI AND A-SI-H - A NUMERICAL STUDY

We present a numerical study of the electronic properties of various s tructural models of amorphous silicon and hydrogenated amorphous silic on. Starting from an ideal random network, dangling bonds, floating bo nds, double bonds, microvoids, hydrogenated dangling bonds, and hydrog enated floating bonds are introduced. The concentrations of these defe cts can be varied independently, the amount of disorder introduced to the system is therefore strictly controllable, Two continuous random n etworks, the vacancy model of Duffy, Boudreaux, and polk and the bond switching model of Wooten, Winer, and Weaire (WWW model) are investiga ted. For the relaxation of the structures the potentials of Keating an d of Stillinger and Weber are employed. The electronic structure is de scribed by a tight-binding Hamiltonian; the localized or extended char acter of the eigenstates is investigated via a scaling approach. The v acancy model shows a band gap for small defect concentrations but this fills up with increasing disorder. Similar behavior is found for the case of the other models. Zn general defects introduce states into the gap region of a-Si, where the dangling bonds lead to the largest dens ity of states in the gap region for a given defect concentration. This model turns out to be unique. For small system sizes an impurity band results that dramatically changes its character for large systems abo ve 4000 atoms to a nearly uniform density of states as observed experi mentally. In a-Si:H the dangling and floating bonds are removed and a mobility gap results with a width in good agreement with experiment. T he experimentally observed tailing of the band into the gap region (fi rst linear, then exponential) is well described only for the a-Si:H mo del derived from the vacancy model and for very large system sizes abo ve 4000 atoms. The WWW model does not lead to this tail behavior. Loca lized states are found at all band edges but states at the bottom of t he conduction band are more strongly localized than those at the top o f the valence band.