Recombination at dangling bonds and steady-state photoconductivity in a-Si:H

A simple model of recombination at dangling bonds in a-Si:H is proposed to explain the steady-state photoconductivity and γ-exponent variations with the equilibrium Fermi-level position. The appropriate statistics for correlated defects and the Shockley-Read formalism are used to obtain a parametrical representation of photoconductivity versus optical generation rate. Oscillations of γ between 0.5 and 1 when $E_F$ is shifted in the central region of the gap depend mainly on the density of dangling bonds and the energy positions of the singly ($T_3^0$) and doubly ($T_3^{\mathrm{-}}$) occupied levels. Experimental results on lightly-boron-doped glow-discharge a-Si:H are in agreement with the model and give a location of the $T_3^0$ level at 0.95 eV from $E_c$, an effective correlation energy of 0.4 eV, and a ratio of charge-to-neutral-state capture cross sections of 50. Finally, the dangling-bond-state occupation probabilities are shown to be weakly modified by illumination even at high photon fluxes. Consequences for the interpretation of ESR experiments are also discussed.