Abstract
We observe two regimes in the variation of gap size (E04) and density of states (DOS) as a function of H-content (cH) in sputtered a-Si : H. In the first regime, an increase in cH from 5 to 17% is accompanied by an exponential functional decrease of the DOS near midgap, which varies also directly with E04 over a wide range of preparation conditions. In the second regime, additional incorporation of H beyond 17% fails to further decrease the DOS (with cH and E04), which shows a saturation with scatter. This behavior is interpreted in terms of H-induced and residual impurity related defects. Variation of the DOS in the lower half of the gap, responsible for hole trapping, is observed via the hole mobility-lifetime product, which exhibits a dramatic drop with impurity incorporation (P, O) and deteriorates for increasing cH in the second regime.