Noor, N; (2014) The Chemical Vapour Deposition of Transparent Conducting Oxides: Exploring Routes Toward Improved Functionality. Doctoral thesis , UCL (University College London).

Abstract

Transparent conducting oxide (TCO) thin films were synthesised by different chemical vapour deposition (CVD) techniques, including aerosol-assisted CVD (AACVD) and atmospheric-pressure CVD (APCVD). Such methods were used to control deposition of primarily SnO2-based TCO films, as well as TiO2, allowing for in-depth analysis of properties for improved control and functionality. Investigations were carried out using a variety of techniques including x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), ultraviolet (UV) Transmittance-Reflectance, Raman spectroscopy and Hall Effect probe measurements. Variation of AACVD deposition parameters in an attempt to improve overall SnO2-TCO properties illustrated how a metathesis reaction (a halide exchange at the Sn-centre) in the precursor is an important step for the formation of high-performing films. Such deposited films exhibit values comparable to the best-performing commercial materials (sheet resistances as low as 3 ohm/square for a 600 nm thick film and figures-of-merit often greater than 2) depending on the defect site identity of the extrinsic dopant. Exploration of various dopant effects on the SnO2 system were also carried out. Fluorine, commonly regarded as the best halide dopant, was confirmed as such (with optical transmittances of 81 % and sheet resistances of 7 ohm/square for a 698 nm thick film), as compared to the other halides. Furthermore, a non-competitive, double-substitutional doping of the SnO2 system, concurrently utilising fluorine and various metal dopants, yields novel methods for control of TCO film morphology (and the attendant optical haze characteristics) and improved electrical properties (resistivities of the order of 10-4 ohm.cm and carrier concentrations of 1020 cm-3 or higher). Such control is highly useful in tailoring TCO functionality to its use, so maximising effectiveness and output for commercial applications. The final chapter explores a novel TCO system, TiO2, and its potential as a TCO material, coupled to its more established materials (e.g.; photocatalytic and wetting) properties. Experimental APCVD values of systems doped with dopants such as uorine, niobium, nitrogen and tungsten (with resistivities ranging 10-1 - 102 Ω.cm and optical transmittances of approximately 70 %, extending into long-wavelength regions) show it is an interesting material worthy of study but that further improvements are clearly required to make it a commercially viable TCO system. The improved mechanistic understanding and synthetic control afforded by the work presented in this thesis should help prepare thin film crystalline TCO systems to meet demands for the next-generation of functional materials applications.

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