High field conduction mechanism of the evaporated cadmium arsenide thin films

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^-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 μm using evaporated electrodes of Ag and occasionally Au or Al. Above a typical electric field F_b of up to 5×10⁷ V m^-1 , all samples showed instabilities characteristic of dielectric breakdown or electroforming. Below this field, they showed a high-field conduction process with log J∝V^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 β had a value of (1.2-5.3)×10 ^-5 eV m^1/2 V^-1/2, which is reasonably consistent with the theoretical value of β_PF=2.19×10^-5 eV m^1/2 V^-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