Aqueous sol-gel processing of transparent conducting rare earth doped indium tin oxide

Abstract

Transparent conducting oxides (TCOs) demonstrate optical transparency in the visible region of the electromagnetic spectrum combined with near-metallic electrical conductivity. Owing to this unique combination of properties, TCOs have found numerous technological applications. Among the TCOs, indium tin oxide (ITO) is widely recognised as having the best combination of transparency and conductivity. The aim of this work was to develop an aqueous sol-gel process to prepare ITO and also doped ITO materials. The motivation was to make a simple, inexpensive and environmentally friendly process, while retaining the ability to prepare excellent materials. Wet chemical deposition techniques have many advantages compared to commercial physical deposition techniques, like sputtering, such as cost, simplicity and readily control of homogeneity and composition. Moreover, by using water instead of organic solvents, the process has the potential to be environmentally friendly and less expensive and thereby more relevant for up-scaling to industrial levels. The challenges related to the frequently used In- or Sn-chloride precursors in soft chemistry synthesis were circumvented by using indium nitrate and tin acetate as precursors.

The aqueous sol-gel process developed in this work was demonstrated to give phase-pure In2O3 and ITO thin films as well as nano-crystalline powders. A gel was formed after evaporation of the solvent, and the amorphous nature of the gel demonstrated homogeneous cation distribution. Calcination of the gel caused decomposition and crystallisation to the desired oxide material. The chemistry of the sol-gel process was investigated by varying the initial cation concentration and the organic complexing agents. These parameters were demonstrated to influence on the decomposition/crystallisation of the gel during thermal treatment and the phase purity of the final oxide materials. The presence of hydroxyl groups appeared to be important regarding complexing and immobilisation of the cations, and the possible formation of the metastable rhombohedral polymorph of In2O3 could be controlled by the choice of the organic additives.

ITO thin films were successfully deposited on substrates by spin coating using the aqueous sol-gel route. The ITO thin films were demonstrated to have excellent optical properties, such as a high transmittance in the visible region and band gap similar to reported values. The electrical properties of the as-deposited films were also quite promising. Particularly after heat treatment at high temperatures and annealing in reducing atmospheres, the specific resistance was excellent compared to ITO thin films prepared by other sol-gel methods and comparable to the best reported values for ITO. In situ conductivity measurements confirmed the effect of the annealing atmosphere on the conductivity of the films.

It has been known for decades that it is difficult to fabricate polycrystalline In2O3 and ITO with high density. Nevertheless, the sintering of ITO is industrially important due to an industrial demand for dense ITO-targets used in sputtering. A comprehensive sintering study of the nano-crystalline, phase-pure powders of In2O3 and ITO was performed. Particularly the phase purity of the powder was important with respect to previous similar sintering studies. The mechanisms governing the sintering, with particular focus on the mass transport mechanisms, the effect of the tin doping and the sintering atmosphere were investigated. Mass transport below 1200 °C was given particular consideration since this temperature region has received very little attention in the literature. Hence, the present findings are also relevant for mass transport during heat treatment of ITO thin films, which is performed at significantly lower temperatures than the sintering of sputtering targets.

One of the main motivations for working with ITO was the possibility to dope ITO with rare earth elements (REEs), thereby enabling the combination of the excellent properties of the ITO host with the characteristic luminescence of the REEs. A thorough investigation of the equilibrium phase composition and solid solubility of neodymium, europium and terbium in In2O3 and ITO at 1400 °C was performed. It was confirmed that the cubic In2O3 crystal structure is a promising host for REEs, as expected from the crystal structure of the pure rare earth oxides. The solubility was shown to decrease with increasing size mismatch between the ionic size of the dopant and the host. Phase-pure materials of In2O3 and ITO doped with REEs were successfully prepared in form of nano-crystalline powders and thin films. In this form the solubility limit for the REEs could be circumvented by synthesis of metastable materials. The effect of the REE-doping on the optical and electrical properties of the two host materials was investigated by various spectroscopic techniques and electrical conductivity and thermopower measurements. Neither the conductivity nor the transparency of ITO thin films was significantly deteriorated by the REE-doping. Finally, strong emissions at around 611 nm were observed for Eu-doped In2O3, demonstrating the possibility of obtaining photoluminescence in a TCO host material.