Near-infrared reflection from periodically aluminium-doped zinc oxide thin films

Multilayers of aluminium-doped zinc oxide (Al:ZnO) as a heat-reflection material were prepared by a reactive sputtering method with intermittent Al doping or Al content modulation. A drop in the refractive indices n around the plasma wavelength λp of 1456 nm for the optimally-doped Al:ZnO layers formed the periodic distribution of n in the multilayers. The periodic n provided selective reflection of approximately 60% under λp and shielded near-infrared solar radiation containing high energy, which was impossible to be reflected from the Al:ZnO monolayer. The selective reflection was accompanied by infrared reflection above λp, low emissivity and no subsequent visible reflection, which allowed the multilayers to achieve compatibility between the solar heat gain coefficient of 0.6 and the visible transmittance of nearly 80%.