Surface and interface effect in ALD-Grown ZnO : properties and application in rechargeable batteries

Abstract

Owing to its direct and wide band gap (Eg 3.37 eV), large exciton binding energy (60 meV) at room temperature, high thermal and chemical stability, biocompatibility, zinc oxide (ZnO) has gained intensive interest in the research community in the past few years [1-4]. Its distinct properties like wide band gap and large excitonic binding energy (60 meV, which is more than twice exciton binding energy of GaN) [5], ensure efficient excitonic emission in ZnO at room temperature and even at higher temperatures [6, 7], which makes ZnO a promising material for short-wavelength optoelectronic devices, especially for ultraviolet light-emitting diodes and laser diodes [ZnO can crystallize in a variety of crystal structures including wurtzite, rocksalt and zinc blende, as shown in Figure 1.1. But, in general, under ambient conditions, hexagonal wurtzite structure is found to be the most thermodynamically stable form [10, 11]. In wurtzite crystal, the structure is composed of two interconnecting hexagonal close-packed (hcp) sub-lattice in a hexagonal lattice, each of which consisted of one of the Zn2+ and O2- atom along the threefold c-axis involving sp3 covalent bonding where each anion is surrounded by four cations at the corners of a tetrahedron [10]. It belongs to the space group of P63mc which has two lattice parameters; a = 3.25 Å, c = 5.20 Å [11, 12]. The tetrahedral coordination in ZnO results in non-central symmetric structure and consequently induces the characteristic piezoelectricity [12].