Development of phase-pure CuAlO2 thin films grown on c-plane sapphire substrates prepared by RF sputtering

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 °C in air, only the film deposited at 700 °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.

► Annealing Cu–Al–O (Cu:Al=1:1) films on sapphire inevitably yields CuAl2O4 impurities.
► Instead, we prepared Cu–Al–O/Cu2O/sapphire by depositing a thin Cu2O film in between.
► A phase-pure, well-oriented CuAlO2 film was made by annealing the sandwich structure.
► The electric conduction of the CuAlO2 film also improved by two orders of magnitude.