Characterization of CuAlO2 Thin Films Prepared on Sapphire Substrates by Reactive Sputtering and Annealing

概要

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As-deposited films were prepared on sapphire substrates at 500–680 °C by alternately sputtering Cu and Al targets in Ar-diluted O2 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 °C had an amorphous structure, while the films as-deposited at 680 °C had CuAl2O4 phase but no CuAlO2 phase. Annealing at 1050 °C in nitrogen flow caused a reduction in the molar fraction of oxygen, i.e., the composition of the annealed films with $\text{[Cu]}/\text{[Al]} \approx 1$ 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 [$\bar{1}100$] of the sapphire substrate. The annealed films had an absorption edge corresponding to the energy gap of CuAlO2 and exhibited p-type conductivity.
2008-01-25