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 x CdO·(100 — x)GeO 2 (}20, x , }80 mol%) were prepared by a radio‐frequency sputtering method in an ArO 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 films 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 × 10 16 cm −2 , the conductivity of the film with x 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 }6 K, whereas those of films with x , }55 remained at 10 −9 S·cm −1 . The carrier electron density and the mobility of carriers in the former films, both of which were evaluated from the Hall measurements, were }1 × 10 20 cm −3 and 12 cm 2 V −1 ·s −1 , respectively. Effects of the chemical composition on persistent photocurrents also were examined. Significant photocurrents were observed in the films with x · }50, and they were greatly enhanced in the films with x · }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 · }75.