Effective doping of Al in ZnO films by multi-target reactive sputtering for near-infrared reflection

Thin films of aluminium-doped zinc oxide (ZnO:Al) as heat reflective coatings were prepared by multi-target reactive sputtering using metallic Zn and Al targets. To control the surface oxidation of each target during sputtering, emission intensity of Zn plasma and current-voltage characteristics for the Al target were monitored. This technique allowed to reduce ambient oxygen content to the lower limit of the ZnO:Al formation. The lowering oxygen content in addition to the optimizing Al content achieved the carrier concentration N _e of 11.0×10₂₀ cm^-3 and the Hall mobility μ of 25.0 cm²/Vs. The highest Ne provided the shortest plasma wavelength λp of 1375 nm, which shifted the near-infrared reflectance spectrum closer to the visible region. The high λp reduced the optical absorption and enhanced the reflectance. The local structure and the carrier generation behaviour of the doped Al were evaluated by X-ray absorption fine structure (XAFS) measurements.