HIGH-FIELD CARRIER TRANSPORT AND KINETICS IN AMORPHOUS GE15TE81SB2S2 THIN-FILMS

Measurements have been carried out on the effect of high electric fiel ds on equilibrium conductivity, steady-state photoconductivity, and ca rrier drift mobility in films of Ge15Te81Sb2S2 over a range of tempera ture from 90 K to 300 K. For each of these properties, the dependence on electric field, E, is of the form exp(-ebE/kT), where the factor b depends on the particular property concerned and on temperature. The c onductivity returns a value for b which decreases with temperature, wh ile the reverse is true for drift mobility. The value obtained for the photoconductivity depends on the prevailing recombination regime. At both high and low temperature, the photoconductivity is linearly depen dent on photon flux, and the field dependence is smallest, while in th e intermediate temperature regime, where square-root (bimolecular) beh aviour prevails, a value of b is found which is just 1/2 that for the dark conductivity. A consistent model for all of this behaviour is des cribed, which involves extended state transport with multiple-trapping in band-tails, negative-U recombination centres appropriate to chalco genides, and an electric field-dependent emission prefactor which is n ecessarily common to all gap-states, i.e., the shallow states controll ing the drift mobility, and also the deeper lying recombination centre s. Such an effect can arise from field-induced delocalisation of state s near the mobility edge. (C) 1998 Elsevier Science B.V. All rights re served.