Defect-pool model and the hydrogen density of states in hydrogenated amorphous silicon

Abstract

We present a treatment of the defect-pool model, for the calculation of the density of electronic gap states in hydrogenated amorphous silicon, based on the equilibration of elemental chemical reactions involving the separate release and capture of hydrogen. We derive the corresponding hydrogen density of states, describing the distribution of hydrogen binding energies, and show that the two densities of states are completely consistent. Hydrogen can be captured into weak SiSi bonds, which can be occupied by one or two hydrogen atoms. These are the dominant chemical reactions controlling the defect density. The effective hydrogen correlation energy is variable, being negative for most sites but positive where most defects occur. We show that the electronic density of states reproduces the main features of our earlier defect-pool model, with more charged defects than neutral defects for intrinsic amorphous silicon. The electronic density of states and the corresponding hydrogen density of states are consistent with a wide range of experimental results, including hydrogenation-dehydrogenation and hydrogen diffusion. \textcopyright{} 1996 The American Physical Society.