There has been a great deal of interest in zinc oxide (ZnO) in recent years. Much attention on this semiconductor material is fueled and fanned by its extensive applications in optoelectronic devices owing to its good piezoelectricity, direct wide band gap (~3.3 eV at 300 K), a large exciton binding energy (~60 meV), high carrier mobility, and low-temperature process [1]. Some application aspects of ZnO overlap with GaN, which is another wide gap semiconductor and has been widely used in ultraviolet, blue, green, and white light-emitting devices. Nevertheless, ZnO has some properties superior to GaN, which contains the availability of high-quality ZnO single bulk crystals and thermal energy sufficiently higher than that at room temperature (~27 meV). Moreover, many crystal-growth technologies have been exploited for ZnO bulk crystal and thin film formations, leading to potential low cost for ZnO-based devices.