Coulomb effect on doping in amorphous semiconductors

Traditionally, the low doping efficiency in $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ has been explained by the argument that dopant atoms are incorporated into under- or over-coordinated sites and, therefore, inert in such configurations. However, recent molecular dynamic simulations proved that this view is not generally correct. In the present paper we suggest a purely electronic analytic model explaining the low doping efficiency in amorphous semiconductors. The model shows that, in a random network of localized states, the Coulomb interaction between ionized dopant atoms and the resulting localized charge carriers leads to changes in the electronic density-of-states (DOS) distribution which counter the intended shift of the Fermi-level position.