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 (}20, x, }80 mol%) were prepared by a radio-frequency sputtering method in an ArO2 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 × 1016cm−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 × 1020 cm−3 and 12 cm2V−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.
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