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

Electrical and optical properties of CuAlO2, 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 similar to 1.8 and similar to 3.5 eV, respectively. The conductivity at 300 K was similar to 3 x 10(-1) S cm(-1) and its temperature dependence is of the thermal-activation type (activation energy approximate to 0.2 eV) at temperatures > 220 K but is of the variable-range hopping type (log sigma proportional to T-1/4) at < 220 K. It was inferred that an admixed state of Cu 3d and O 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. (C) 2000 American Institute of Physics. [S0021-8979(00)04720-4].