Optical properties of sputter-deposited ZnO:Al thin films

Abstract

ZnO:Al coatings were prepared by rf magnetron sputtering of ZnO together with dc magnetron sputtering of Al onto rapidly revolving unheated substrates under weakly oxidizing conditions. Optimized films had ∼1% luminous absorptance, ∼85% thermal infrared reflectance, and ∼5×10−4 Ω cm electrical resistivity at a thickness of ∼0.3 μm. The Al content was ≲2 at. %, as determined by Rutherford backscattering spectrometry. Transmission electron microscopy and electron diffraction showed ∼50-nm average crystallite size and a hexagonal wurtzite structure. Spectrophotometric transmittance and reflectance were recorded in the 0.2–50-μm wavelength interval, and the complex dielectric function was evaluated by computation. The optical data were explained from an effective mass model for n-doped semiconductors. The Al atoms are singly ionized, and the associated electrons occupy the bottom of the conduction band as free-electron gas. The Al ions act as pointlike Coulomb scatterers and are screened by the electrons according to the random phase approximation or an extension thereof. The optical properties of ZnO:Al could be understood by considering the free electrons to be damped primarily by ionized impurity scattering. ZnO:Al films can have high luminous transmittance, high solar ultraviolet absorptance, low thermal infrared emittance, and high electrical conductance; hence, they are of large interest for energy-efficient windows.