High field conduction mechanism of the evaporated cadmium arsenide thin films

Abstract

Cadmium arsenide is a II-V semiconductor which exhibits n-type intrinsic conductivity with high mobility. Potential applications include magnetoresistors and both thermal and photodetectors, which require electrical characterisation over a wide range of deposition and measurement conditions. The films were prepared by vacuum evaporation with deposition rates of 0.5 nm s/sup -1/ and substrate temperatures maintained at constant values of 293 K-393 K. Sandwich-type samples were deposited with film thicknesses of 0.1-1.1 /spl mu/m using evaporated electrodes of Ag and occasionally Au or Al. Above a typical electric field F/sub b/ of up to 5/spl times/10/sup 7/ V m/sup -1/, all samples showed instabilities characteristic of dielectric breakdown or electroforming. Below this field, they showed a high-field conduction process with log J/spl prop/V/sup 1/2/, where J is the current density and V the applied voltage. This type of dependence is indicative of carrier excitation over a potential barrier whose effective barrier height has been lowered by the high electric field. The field-lowering coefficient /spl beta/ had a value of (1.2-5.3)/spl times/10/sup -5/ eV m/sup 1/2/ V/sup -1/2/, which is reasonably consistent with the theoretical value of /spl beta//sub PF/=2.19/spl times/10/sup -5/ eV m/sup 1/2/ V/sup -1/2/ expected when the field-lowering occurs at donor-like centres in the semiconductor (Poole-Frenkel effect). For thinner films, Schottky emission was more probable. The effects of the film thickness, electrode materials, deposition rate, and substrate temperature on the conductivity behaviour are discussed.