Metal oxide thin films for optoelectronic applications

This thesis details the use of aerosol assisted chemical vapour deposition to deposit transparent conducting oxide thin films. Transparent conducting oxides are a special class of materials that exhibit high optical transparency as well as good electrical conductivity, two properties usually in contradiction with each other. The combination of these properties in one material has established an essential role for transparent conducting oxides in a range of applications such as flat screen displays, photovoltaic cells, gas sensors, low-emissive coatings and light emitting diodes. Aerosol assisted chemical vapour deposition is increasingly becoming recognised as a simple, low-cost and reliable technique for depositing thin films. It involves generating an aerosol mist from a solution containing the precursors that is transported with the aid of an inert or reactive carrier gas into the reaction chamber where deposition takes place on a heated substrate. Two of the attractive features of this method are its versatility in allowing the use of precursors that are not suitable for conventional chemical vapour deposition methods as the method depends on solubility rather than volatility and the facility to use multiple precursors simultaneously within a single vessel. The focus of this work is on doping and co-doping of metal oxide thin films, namely ZnO and SnO2, to enhance their optoelectronic properties. The ZnO films were doped with group III elements aluminium or gallium, and the SnO2 films were doped with multivalent elements antimony or tungsten. All four systems were co-doped by introducing fluorine to replace the oxygen ion in the lattice. Fluorine was used as the co-dopant because of its established use in fluorine doped tin(IV) oxide transparent conducting oxides, a commercially available product. Co-doping has received less attention compared with single cation doping largely because of the limitations of other deposition methods. The rationale for co-doping is that it would allow greater tuning of the optoelectronic properties of the transparent conducting oxides to suit specific applications. All films synthesised in this investigation were characterised using a wide range of techniques including X-ray diffraction, energy and/or wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-visible-near infrared spectroscopy and Hall effect measurements