Characterization of CuAlO2 thin films prepared on sapphire substrates by reactive sputtering and annealing

As-deposited films were prepared on sapphire substrates at 500-680 degrees C by alternately sputtering Cu and Al targets in Ar-diluted 02 gas atmosphere. The composition of the as-deposited films corresponded to that of the slightly oxygen-rich region of the CuO-CuAl2O4-Al2O3 system. The films as-deposited at 500 degrees C had an amorphous structure, while the films as-deposited at 680 degrees C had CuAl2O4 phase but no CuAlO2 phase. Annealing at 1050 degrees C in nitrogen flow caused a reduction in the molar fraction of oxygen, i.e., the composition of the annealed films with [Cu]/[Al] approximate to 1 corresponded to CuAlO2. The annealed films were predominated by the CuAlO2 Phase. The preferential orientation of the films toward the c-axis normal to the substrate surface is due to the small lattice mismatch between the rhombohedral [010] of delafossite-type CuAlO2 and the hexagonal [(1) over bar 100] of the sapphire substrate. The annealed films had an absorption edge corresponding to the energy gap of CuAlO2 and exhibited p-type conductivity.