Transparent, conducting, amorphous oxides: Effect of chemical composition on electrical and optical properties of cadmium germanates

Amorphous cadmium germanate thin films with the chemical composition xCdO-(100 - x)GeO2 (similar to 20 < x < similar to 80 mol%) were prepared by a radio-frequency sputtering method in an Ar-O-2 atmosphere, The electrical and optical properties of the as-sputtered films and films after proton (H+) implantation were measured in the temperature range 300-6 K, The optical band gaps of the as-sputtered firms were >3.1 eV for all specimens; therefore, they were transparent to visible radiation, Direct-current conductivities of all the films prepared were <10(-9) S . cm(-1) at room temperature, Upon proton implantation to a fluence of 2 x 10(16) cm(-2), the conductivity of the film with x approximate to 78 at room temperature was enhanced to 210 S . cm(-1), 11 orders of magnitude greater than the conductivity of the as-sputtered film, and the conductivities remained almost constant to similar to 6 K, whereas those of films with x < similar to 55 remained at similar to 10(-9) S . cm(-1). The carrier electron density and the mobility of carriers in the former films, both of which mere evaluated from the Hall measurements, were similar to 1 x 10(20) cm(-3) and 12 cm(2) . V-1. s(-1), respectively. Effects of the chemical composition on persistent photocurrents also mere examined. Significant photocurrents mere observed in the films with x > similar to 50, and they were greatly enhanced in the films with x > similar to 75, This result indicated that the bottom part of the conduction band, which was composed mainly of 5s orbitals of cadmium cations, was extended for the films with x > similar to 75.