Epitaxial m-plane (11 ¡Â00) ZnO thin films grown on m-sapphire substrates by atomic layer deposition have been studied. Atomic imaging and electron diffraction conducted in a transmission electron microscope (TEM) and crystallography by X-ray diffractometry all show consistent epitaxial relations with ZnO m-plane // sapphire m-plane, while ZnO [112 ¡Â0] // Al2O3 [0001], and ZnO [0001] // Al2O3 [112 ¡Â0]. The widths (full width at half maximum, or FWHM) of the rocking curves depend on the crystallographic axis of rotation. Dislocations near the interface between the ZnO epi-layers and sapphire substrates can be found from the cross-sectional TEM images when the direction of the incident electron beam, namely, the zone axis, is parallel to ZnO [112 ¡Â0], the a-axis of ZnO. There are stacking faults found in ZnO films away from their interfaces with the substrates. Polarization-dependent photoluminescence by differently polarized incident laser beam have also been investigated. Careful analysis of the spectra via multi-peak fittings revealed optical transitions at 3.32eV for T = 15K, which, however, shifted to 3.28eV at T = 300K. This shift in energy is accounted for by the quadratic temperature dependence of the Fermi level as determined by the positions of the lines of emission corresponding to the band edge transition. The 300K spectrum showed a more distinct peak at 2.48eV when the polarization of the emitted light was along the a-axis of ZnO, as compared to that along the c-axis of ZnO. The origin of this difference remains unaccounted for at the time of writing this thesis. The rest of the peaks have been interpreted in terms of optical transitions involving band gap impurity states, possible exciton formations, and their interactions with phonons.