Drift mobility of amorphous semiconductors measured by the traveling-wave technique

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Abstract
The traveling-wave technique for measuring drift mobility is examined within the context of practical application to thin films of low-mobility amorphous semiconductors. The basic theory that relates the drift mobility to the direct current produced by the nonlinear interaction of carriers with a traveling electric field is reviewed. Extending the theory to include carrier diffusion is shown to produce only small corrections even if the diffusion length exceeds the sample’s thickness. The effect of an inhomogeneous conductivity on the direct current is computed and is shown to be small for an exponential conductivity profile, as would occur in an illuminated semiconductor. A general prescription is given for interpreting the traveling-wave drift-mobility measurements with an arbitrary model of charge transport. Two specific models are solved—multiple trapping and low-temperature hopping. Experimental techniques are then discussed in relation to sources of error. Examples are given of drift-mobility data for n-type a-Si:H from 200 to 450 K in the as-deposited state, after quenching, and after annealing at high temperatures. Finally, the data obtained from illuminated a-Si:H at temperatures down to 1.6 K are discussed.