Hole trapping, light soaking, and secondary photocurrent transients in amorphous silicon

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Abstract
We measured and analyzed the long-time photocurrent decays in films of undoped amorphous silicon with coplanar Ohmic contacts. We interpret these as a secondary-electron current which flows as long as holes are safely trapped in states 0.3 to 0.5 eV above the valence-band edge. A new analysis of these transients gives the distribution of the trapped holes and implicates thermally assisted tunneling to dangling bonds as the rate-limiting step in hole recombination. Photoconductive gain is lost and the decay time shortened after small amounts of light soaking. The energetically deeper hole traps with the longer residence times are lost first and in the same number as expected for the increase in dangling bonds with a similar light soaking; this result supports a model which has hole trapping in valence-band-tail states as a precursor to light-induced dangling-bond formation. Two possible origins of these traps are discussed.