Near-infrared reflection from Al-doped ZnO films prepared by multi-target reactive sputtering

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. An optimization of Al content and a reduction in oxygen partial pressure were crucial in increasing the carrier concentration N-e and the Hall mobility mu. The ZnO:Al film with the highest N-e achieved the shortest plasma wavelength lambda(p) of 1375 nm, which shifted the near-infrared reflectance spectrum closer to the visible region. The high mu reduced the optical absorption and enhanced the reflectance. Moreover, the multi-target system enabled intermittent doping of Al, which was applied to stack multilayers consisting of non-doped and Al-doped ZnO layers. A drop in the refractive indices n above lambda(p) for the ZnO:Al layers formed the periodic distribution of n in the thickness direction, which provided a high reflectance zone from 1000 to 1400 nm in wavelength.