The effect of oxygen on the properties of transparent conducting Cu-Al-O thin films deposited by rf magnetron sputtering

Cu-Al-O thin films were prepared by rf magnetron sputtering. By adjusting oxygen partial pressure, the effect of oxygen on optical transmission and electrical properties of the films were investigated. The films exhibit good transparency in the visible range. As oxygen partial pressure is above 30%, the transmittance is about 60-70%. The direct and indirect band gaps are about 3.49 and 1.86 eV, respectively. The linear I-V relations exhibit ohmic contacts between Ag and the films, which suggest that the work function of Cu-Al-O thin films is around 4.26 eV. In the temperature range of 250-310 K, the temperature dependence of the conductivity of Cu-Al-O thin films is of semiconducting thermal-activation type. The average conductivity at room temperature successively increases as the oxygen partial pressure augments. The maximum conductivity is 4.6 × 10^-3 S cm^-1 for Cu-Al-O thin film deposited at 40% oxygen partial pressure. When above 40%, the decrease of the conductivity may be due to the inhibition of the carrier transport led by the semiconductor trap states originated from the excess oxygen atoms.