Zinc oxide is a II-VI semiconductor that has commanded significant research interest for a number of years and continues to do so due to its wide band gap of 3.4 eV and high exciton binding energy (60 meV), that make it a strong candidate for novel applications in opto-electronics. These novel applications rely on the development and accurate characterization of high quality thin film materials.

In this thesis epitaxial layers of ZnO were grown on c-plane (0001) sapphire substrates by plasma assisted molecular beam epitaxy. The oxygen:zinc flux ratio was found to be crucial in obtaining a film with a smooth surface and good crystallinity. Increasing film thickness resulted in an increase in the streakiness of RHEED images, and a reduction in crystal strain and increase in crystal alignment. The films were all found to be orientated, with the growth direction in the c-axis (000¯1) of the ZnO crystal structure. Best growth conditions were achieved by first depositing a 2-3 nm low temperature ZnO buffer layer at 450⁰C and then depositing the ZnO film at a substrate temperature of 720⁰C with a Zn flux of 1.9 × 1014 atoms/cm².s and an oxygen partial pressure of 7 × 10−⁷ Torr generated by a 200 W oxygen plasma. Films grown with these parameters were found to be highly crystalline with XRD rocking curve FWHMs as low as 0.07◦. Hexagonal pits and atomically flat triangular island like structures appeared on the surfaces of some of the films. The triangular structures have previously only been reported on the zinc polar face and are thought to form as a stabilisation mechanism on the polar faces of ZnO.

The films were found to be highly transparent in the visible region with over 90% transmission above 400 nm. A sharp absorption edge was visible at around 380 nm. Using the Tauc method the band gaps of the films were found to range from 3.29-3.295 eV. The splitting between the A and B excitons in the films was measured between 7 and 8 meV from both PL and optical absorption measurements, which is slightly wider than the generally accepted value of 5.5-6.5 meV for bulk ZnO. In addition optical absorption from the C exciton was visible in the films at low temperatures.

The electrical resistivity of the ZnO films varied significantly after exposure to UV light. This change in resistivity was found to be largely driven by a change in mobility rather than carrier concentration. When left in a dark environment for a week Hall effect measurements showed typical mobilities for our films below 10 cm²/V.s. We found that by illuminating the samples with UV light we were able to increase the mobility in the films up to 140 cm²/V.s. After removing the illumination source and storing the samples in the dark the mobility required almost a week to return to its original value. The effect is believed to be due to oxygen absorbing to the surface of the films, passivating the surface and lowering the mobility. Upon UV illumination the oxygen is desorbed and the films mobility is increased.

ZnO films were doped with varying concentrations of either magnesium or antimony using in situ doping and ion implantation methods. Magnesium was found to increase the optical gap of the films while antimony was found to decrease the optical gap. While Mg was found to have little affect on the electrical properties of the films, the carrier concentration of the films doped with antimony were able to be altered over three orders of magnitude from -10¹⁶ to -10¹⁹ cm⁻³.