PREPARATION AND CHARACTERIZATION OF ZnO/MgO NANOCOMPOSITES

Abstract

The theoretical and experimental studies on the nanoparticles indicate that the optical response in such materials depends on their sizes, especially in the nanometre region where quantum confinement (QC) effect is predominant [4]. Yet as the sizes of the particles decrease, the higher surface to volume ratio can incorporate significant surface related defects which may in turn decrease the excitonic emission efficiency. These surface 1 defects can be minimized by capping or embedding the materials in an appropriate matrix [2]. Dielectric host materials with higher optical band gap may be used to confine the growth and to tailor the properties of the guest 'materials so as to _suit the desired technological applications [5, 6]. Many researchers have attempted to passivate the surface defects of ZnO in nanoforms by embedding it in suitable dielectric materials like A1203, Si02 and MgO by atomic layer deposition, sol-gel method and pulse laser deposition, respectively [2]. ZnO is a very promising material for semiconductor device applications. It has a direct and wide band gap in the near-UV spectral region, and a large free-exciton binding energy so that excitonic emission processes can persist at or even above room temperature [1, 4, 5]. ZnO crystallizes in the wurtzite structure (Fig. 1), and it is available as large bulk single crystals [7, 8]. Its properties have been studied since the early days of semiconductor electronics, but the use of ZnO as a semiconductor in electronic devices has been hindered by the lack of control over its electrical conductivity. ZnO crystals are almost always n-type, the cause of which has been a matter of extensive debate and research. With the recent success of nitrides in optoelectronics, ZnO has been considered as a substrate to GaN, to which it provides a close match [5]. Over the past decade, we have witnessed a significant improvement in the quality of ZnO single-crystal substrates and epitaxial films. The prospect of using ZnO as a complement or alternative to GaN in optoelectronics has driven many research groups worldwide to focus on its semiconductor properties, trying to control the unintentional n-type conductivity and to achieve p-type conductivity [9]. Theoretical studies, in particular, first-principles calculations based on density functional theory (DFT), have also contributed to a deeper understanding of the role of native point defects and impurities on the unintentional n-type conductivity in ZnO [11]. Acceptor doping has remained challenging, however, and the key factors that would lead to reproducible and stable p-type doping have not yet been identified.