Photoconductivity of hydrogenated amorphous silicon

We model the steady-state photoconductivity in hydrogenated amorphous silicon as a function of the energy position and width of the dangling bond states. The model is based on Simmons-Taylor statistics with exponential band tails and trivalent dangling bond states of positive correlation energy; the dangling bond distributions are approximated by peaked double-exponentials. Using the free carrier capture rates of Street, we find three regimes of photoconductivity as a function of generation rate at room temperature. At low generation rate, the recombination traffic is evenly split between electron capture by D^+/0 and D^0/- transitions and subsequent hole capture. At intermediate generation rate, the D^0/- transition accounts for ≈70% of the recombination; however, the energy width of the D^0/- level does not control the photoconductivity according to a simple Rose model. And at high generation rate the recombination through band tail states dominates. The model reveals two complexities: first, the power law exponent γ of photoconductivity can remain constant although the dominant recombination channel changes. Second, the crossover between recombination regimes is a strong function of the dangling bond density in the range 10¹⁵ - 10¹⁶/cm³.