Terbium Doped ITO Thin Films Prepared by an Aqueous Sol-Gel Method

Transparent conducting oxides (TCOs) combine the properties of optical transparency in the visible region of the electromagnetic spectrum and a near-metallic electrical conductivity. This combination make them very applicable in many technological areas. The prime candidate amongst the TCOs is indium oxide doped with tin oxide, usually referred to as indium tin oxide (ITO). The aim of this work was to dope this incredible material with the rare earth element terbium and investigate the effects of the dopant on the properties of the host. If successful, one could possibly enhance the optical properties by introducing luminescence. An environmentally friendly aqueous sol?gel process was used to fabricate nanocrystalline thin films and powders of indium tin oxide doped with terbium. A stable solution was prepared from indium nitrate precursor, tin acetate and terbium nitrate together with acetic acid and ethylene glycol. The ITO:Tb thin films consisted of spherical nanocrystalline particles and were of high homogeneity and phase purity. Grain size and crystallite size of the thin films seemed to be independent on terbium concentration. The powders were also nanocrystalline but showed some impurities at high terbium concentrations. The deposited thin films exhibited very good and reproducible electrical conductivity and optical transparency. The specific resistance of the thin films declined with the terbium concentration, but not to an extent that it was detrimental to the electrical properties. The specific resistance increased from 5.1E-3 for a doping concentration of 0.5 cation% terbium to 5.2E-2 for a doping concentration of 10 cation% terbium. Post-annealing of the prepared thin films in air up to 300C showed a decrease in the conductivity, which became more prominent with the terbium concentration in the thin films. This decrease was attributed to oxidation of terbium and tin and thereby reduction of charge carrier concentration. The decrease in conductivity as a result of the doping by itself seemed to be equally important as the reduction caused by oxidation. The optical transmission seemed independent on terbium concentration and had a maximum of 92 % in the visible spectrum and a band absorption edge around 350 nm. A decrease in the lattice parameter of the thin films was observed for the lowest terbium concentrations followed by an increase with the terbium concentration. The latter effect was attributed to the larger size of Tb3+ compared to In3+. The lattice parameter of the thin films also increased with the number of deposited layers, probably due to strain between the substrate and the thin film.Absorption and emission properties of the materials were not studied, and therefore no conclusions could be made about the luminescent properties of the ITO:Tb materials