Al-doped Zinc Oxide as a Transparent Conductive Material

Abstract

ZnO powders containing from 1-8 atom% aluminium ions were prepared from aqueous citrate-aminoalcohol-based gels calcined at 500◦C. The powders were characterized using 27Al NMR, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Solid state 27Al NMR clearly distinguished between different Al environments and was effective in determining the relative amounts of incorporation of Al dopant ions into the different Zn lattice sites in the zincite structure. A degree of control over dopant placement is demonstrated by modifying sol precursors and processing parameters, which allowed a syn- thesis protocol to be developed to optimise the doping effectiveness. Relatively minor variations in processing conditions can influence the degree and mode of Al incorporation. Thin films (ca 100 nm) were fabricated by spin-coating the Al-doped Zinc citrate sol-gels onto glass and quartz substrates, followed by a drying step and annealing at 500◦C. The degree of crystal alignment in thin films, deter- mined by X-ray diffraction studies, was found to be influenced by both the dopant level and the heating profile. Resistivity measurements showed films with 0.5-1atom %Al and multiple layers which had been heated at 90◦C before annealing at 500◦C, and subjected to post-annealing treatment under vacuum displayed the lowest resistivities. Further post-annealing treatment at 500◦C under H2/N2 reduced resistivities by an order of magnitude. Transparencies for all films were above 80% in the visible range, and thicker multi-layer films generally demonstrated lower transmittances. Preferential c-axis orientation was observed for all films, but showed higher intensities with increasing film thickness, and for films subjected to directional heating on a hotplate. Increased film thickness gave films with lower resistivities, but also lower transparencies