Electronic structure and optoelectronic properties of transparent p-type conducting CuAlO2

Abstract

Electrical and optical properties of ${\mathrm{CuAlO}}_{\mathrm{2}},$ a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique. The indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively. The conductivity at 300 K was ${\text{∼3×10}}^{\text{−1}} {\mathrm{S cm}}^{\mathrm{-1}}$ and its temperature dependence is of the thermal-activation type (activation energy ≈0.2 eV) at temperatures >220 K but is of the variable-range hopping type $(\log {\mathrm{ \sigma \propto T}}^{\text{−1/4}})$ at <220 K. It was inferred that an admixed state of $\mathrm{Cu}\text{\hspace{0.05em}3d}$ and $\mathrm{O}\text{\hspace{0.05em}2p}$ primarily constitutes the upper valence band, which controls transport of positive holes, from a combined information on ultraviolet photoemission spectrum with x-ray photoemission spectrum. An energy band calculation by full-potential linearized augmented plane wave method substantiated the experimental findings. The present results gave a solid basis for our working hypothesis [Nature (London) 389, 939 (1997)] for chemical design of p-type conducting transparent oxides.