Abstract

Zinc oxide (ZnO), a II-VI compound semiconductor, has been in the focus of current research due to its size- and morphology-dependent electrical, optical, and chemical properties at the nanoscale. The thermodynamically stable phase of ZnO is the wurtzite hexagonal one with the lattice parameters a = 0.3249 and c = 0.5207 nm. This structure belongs to the point group 6mm and space group P6₃mc. Besides wurtzite ZnO, it can also exist in other crystalline phases, such as zinc blende (ZB) and rock salt. A few experimental and theoretical studies have addressed the growth and fundamental properties of metastable ZB ZnO [1]. However, ZnO is most stable in the wurtzite structure rather than the ZB form under ambient conditions due to its ionicity, which lies at the border line between those of covalent and ionic materials. Some important physical properties of the wurtzite and ZB ZnO structures are summarized in Table 2.1. Due to the noncentrosymmetric structure in the wurtzite form and large electromechanical coupling of ZnO, it also exhibits excellent piezoelectric and pyroelectric properties. The high breakdown strength and high saturation velocity of ZnO are attractive for electronic applications. With a wide direct bandgap of 3.34 eV at room temperature, ZnO is useful for high-temperature applications, transparent electrodes, and light emitting devices. It also has a large exciton binding energy of 60 meV, which is higher than the thermal energy at room temperature (26 meV). Therefore, it is expected to have an excitonic gain at room temperature for ZnO-based blue/UV light emitting diodes.