Near-infrared reflective characteristics of aluminum-doped ZnO thin films fabricated by off-axis sputtering

抄録

Aluminum-doped ZnO (AZO) thin films was deposited by sputtering on glass substrates. Normally, the radio frequency method is applied for sputtering of insulators. However, in this study, the AZO target was made conductive, and then, the thin AZO films were deposited on it by direct current sputtering. We found that there is a significant problem involving the creation of oxygen anions at the target surface, which are then driven into the AZO films, thus resulting in a loss of conductivity due to the formation of oxygen vacancies. At first, the AZO films were deposited on silicon wafers to search for the off-axis positions. After deposition, we observed a white circle on the wafers. We investigated the distribution of surface sheet resistances on the wafer, and found that these were high within the circle (diameter: 87 mm), whereas outside of it, the sheet resistances were quite low and the incidence of oxygen anions was negligible. A small aluminum fitting was made to hold the substrates in the off-axis position, and although the sputtering was carried out at room temperature, crystalline AZO films were obtained. Sputtering at 400 W for 5 min resulted in the best reflectance, with the AZO film reflected in the near-infrared region above 1250 nm. The maximum specular reflectance was 60% at 2500 nm and the film thickness was about 300 nm (although it was somewhat inhomogeneous, probably because of differences in the sputter-particle density). Hall measurements were performed to determine the quality of the resulting AZO films. The carrier concentration was excellent, whereas the resistivity and Hall mobility were average.