Optical, electrical and structural properties of sol gel ZnO:Al coatings

Single and multilayer transparent conducting aluminium doped zinc oxide films have been prepared on DESAG AF45 substrates by the sol gel dip coating process. Zinc acetate solutions of 0.1-0.5 M in isopropanol stabilised by diethanolamine and doped with a concentrated solution of aluminium nitrate in ethanol were used. Each layer was fired at 600°C in a conventional furnace for 15 min. The final coatings were then tempered under a flux of forming gas (N2/H2) at 400°C for 2 h. The coatings have been characterised by surface stylus profiling, optical spectroscopy (UV-NIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and combined Hall and van der Pauw techniques. Single layers with thickness, d, ranging from 25 to 186 nm are polycrystalline with the zincite structure and exhibit a small preferential orientation with the (002) direction perpendicular to the surface (texture coefficient, TC, ranging from 1.8 to 2.8). They consist of almost spherical particles with size ranging from 15 to 25 nm (thin sample) to 40 nm (thick sample) with crystallite sizes ranging from 23 to 40 nm (002), respectively. The film resistivity decreases from 5 x 10-1 Ω cm (d=25 nm) to 4 x 10-2 Ω cm (d=186 nm). Multilayer coatings built with about 20 nm thick layers are also slightly textured along the (002) direction (TC ranging from 1.9 to 2.8). A structural evolution in morphology from spherical to columnar growth was observed. The crystallite size calculated from the (002) reflex increases with the number of layers from 23 nm for a single layer to over 100 nm for ten layers. The resistivity decreases from 5 x 10-1 Ω cm for a single layer to 5 x 10-3 Ω cm for ten layers (d=174 nm). A model for the growth of the crystallites in sol gel multilayer coatings is presented