CuAlO2 films were deposited on c-plane sapphire by radio-frequency (RF) reactive sputtering using a CuAlO2 ceramic target at different deposition temperatures. The as-deposited films were found to be amorphous regardless of the deposition temperatures. After annealing at 1050 degrees C in air, only the film deposited at 700 degrees C showed the crystalline CuAlO2 structure but with a small amount of CuAl2O4 impurity. During post-annealing, Al in the sapphire substrate reacted with the as-deposited Cu-Al-O film so the whole film contained excess of Al, leading to the formation of CuAl2O4. To suppress CuAl2O4. a layer of 100 nm Cu2O film was deposited between the Cu-Al-O film and sapphire. In the annealing process, Cu2O reacted with sapphire to form CuAlO2 so the ratio of Cu to Al of the whole film was maintained, and a highly preferred c-oriented CuAlO2 film without impurity was fabricated. The room temperature electric conductivity of the phase-pure CuAlO2 thin film is 1.01 S cm(-1), which is two orders of magnitude better than that of the film with CuAl2O4 impurity. The CuAlO2 film exhibited both p-type conductivity and transparency in the visible region. (C) 2011 Elsevier B.V. All rights reserved.