Process enhanced polaron conductivity of infrared transparent nickel-cobalt oxide

The effects of gas composition during combinatorial sputtering of p-type polaron conducting films and the effects of total chamber gas pressure and different target-to-substrate distance show correlated variations in electrical and optical properties. Optimum conductivity from combinatorially deposited films containing equal parts of nickel and cobalt is achieved when sputtered in a gas mixture of 50% argon and 50% oxygen from oxide targets, with the best film conductivity of 375 S cm−1 achieved using growth conditions that promote complete oxidation. The gas pressure study shows that deposition at relatively low chamber pressures followed by subsequent quenching of heated samples in air leads to enhanced conductivity. Conductivity is enhanced even further at lower chamber pressure deposition due to a higher growth rate and increased adatom mobility during growth due to the larger molecular mean free path. Increased distance of target-to-substrate decreased the film density, increased film porosity and decreased conductivity, but increased optical transparency between 2 and 10 μm. Post deposition heat treatment improves film conductivity when followed by rapid quenching, but degrades film conductivity when followed by slow cooling in air. Optical transparency degrades as electrical conductivity improves and vice versa. The temperature dependence of conductivity showed that the activation energy of electrical conductivity is lower for films rapidly quenched than for films that are slowly cooled following heat treatment. Rapid cooling of films from 375 °C to room temperature is postulated to increase the concentration of defects or induce a disordered charge distribution state that increases the concentration of polarons and lowers the activation energy for electrical conductivity. Slow cooling allows defects to anneal out, resulting in improved transparency across the infrared region but a lower electrical conductivity.