Ab initiomodels of amorphousSi1−xGex:H

Abstract

We study the structural, dynamical, and electronic properties of amorphous ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}:\mathrm{H}$ alloys using first-principles local basis molecular dynamics techniques. The network topology and defects in the amorphous network have been analyzed. Structural changes, particularly an increase in number of defects and strained bond angles, have been found as the Ge content increases from $x=0.1$ to 0.5. The electronic density of states exhibits a decreasing band gap and additional midgap and band-tail defect states as Ge concentration increases. We report the network structures which are responsible for midgap and band-tail states. The band tails show an exponential (Urbach) behavior. The mobility gap is coarsely estimated as a function of Ge concentration.