ZnO is a metal oxide semiconductor that continues to attract research interests due to its desirable properties such as its direct wide bandgap, high exciton binding energy, and optical transparency. These properties make ZnO a potential material of choice for optoelectronic applications especially in the blue and UV regions of the electromagnetic spectrum. For many industrial applications of thin-film devices, a cost-effective and high throughput method for depositing good quality metal oxide thin films is crucial. This research work focusses on mist-CVD as a cost-effective method to deposit ZnO thin films at atmospheric conditions. In particular, this work adds to the existing knowledge in mist-CVD growth of ZnO films by utilizing precursors free from volatile and flammable organic solvents in the film deposition process, a recipe that is different from other reports in the literature.

Optical studies by UV-Vis transmission spectroscopy show films to be transparent, with transmission higher than 85% in the visible region and with distinct absorption edge corresponding to the onset of photon absorption by ZnO. Optical bandgap was found to lie between 3.27 - 3.32 eV depending on the thickness of films. Photoluminescence studies show dominant emission peaks at 3.362 eV and 3.321 eV near the band edge associated with donor and acceptor impurities. Crystalline properties of ZnO films deposited on different crystallographic planes of sapphire substrates indicate preferred growth orientation that is consistent with reports on epitaxial relationships between ZnO and various planes of sapphire. For growths on an r-plane sapphire substrate, the dominant diffraction peak shows preferred orientation in the a-plane (112¯0). The dominant diffraction peak for films grown on c-plane sapphire is along the (0002) crystal plane while for growth on an a-plane sapphire substrate, peaks from (0002) and (101¯0) dominate the XRD pattern. Film surface topography is influenced by growth factors including precursor concentration, temperature, and substrate type. Typical surface roughness Rrms is between 2.2 nm - 4.3 nm for ZnO films deposited on an r-plane sapphire substrate at a growth temperature of 500◦C with thicknesses ranging from 20 nm - 260 nm.

To demonstrate applicability of cheaply produced mist-CVD deposited films in electronic devices, ZnO films are employed as active conducting channels in metal-semiconductor field-effect transistor (MESFET) devices. Fabrication techniques involve standard photolithography and lift-off pro- cedures. Performance characteristics of transistor devices are found to be dependent on various factors including channel thickness, physical dimensions, and choice of gate material. MESFETs fabricated with silver oxide and palladium oxide as gate electrodes exhibited excellent transistor characteristics with Schottky barrier height (SBH) approximately 1.18 eV and 0.94 eV respectively. Transistor current modulation ratio Ion/Ioff as high as 107, sub-threshold swing as low as 85 mV/decade and channel mobility >5 cm2/V.s were obtained for fabricated devices.

Under elevated temperature, ZnO-based MESFET maintains the expected field-effect transistor characteristics, though with an increase in leakage current and reduction in Ion/Ioff ratio by about two orders of magnitude for a temperature increase of 25◦C - 130◦C. By subjecting the gate electrode of a MESFET device to positive constant voltage stress (CVS), an increase in off- state current was observed while a CVS of negative polarity has minimal effect on device functioning. Conversely, under illumination with intense UV light, transistor behavior and performance characteristics deteriorate with a break-down of the Schottky barrier, lasting several days before full recovery to the original state.