DC reactive magnetron sputtering, annealing, and characterization of CuAlO₂ thin films

CuAlO_x thin films were prepared at three substrate temperatures (T_S =60, 300, and 600 °C) and two oxygen partial pressures (P O₂ =0.5 and 2 mTorr) via dc reactive magnetron sputtering from Cu-Al 50-50 at. % alloy targets and subsequent annealing. As-deposited films with P O₂ =0.5 mTorr were oxygen deficient; although the delafossite structure formed upon annealing, electrical properties were poor. Films deposited with P O₂ =2 mTorr transformed into the delafossite structure and exhibited p -type conductivity after annealing under N₂ at temperatures T_A 750 °C. Conductivity generally increased with increasing T_S and decreasing T_A. A special case of P O₂ =2 mTorr and low T_S (60 °C) resulted in a partially crystalline oxide phase that transformed into the delafossite structure at T_A =700 °C and yielded the highest conductivity of 1.8 S cm-1. In general, a T_A near the phase formation boundary led to an increase in conductivity. Low-temperature hydrothermal annealing was also investigated and shown to produce mixed phase films exhibiting the delafossite structure along with CuO, AlOOH, and Al₂ O₃.