Properties of ZnO thin films grown at room temperature by using ionized cluster beam deposition

Thin ZnO films are of importance due to their unique optical and physical properties. They have a wide variety of applications, such as surface acoustic wave (SAW) bandpass filters, optical waveguides, and laser deflectors using piezoelectric or piezooptic properties [1,2]. Also, they are used as transparent conducting oxide coatings, gas sensors [3], and varistors. The band gap of ZnO is known to be 3.2 – 3.3 eV, thus, it has received a great attention as a luminescence material for display panels [4]. The main property of ZnO for these applications is due to the polar nature of the crystalline structure of ZnO thin films. Many efforts have been made to grow high-quality c-axis-oriented ZnO films. Since ZnO has a high melting temperature and relatively low heat of formation among oxide materials, the magnetron sputtering technique has been preferred to deposit ZnO [5]. Pulsed laser deposition [6], spray pyrolysis [7], and chemical vapor deposition [8] have also been used to obtain ZnO films, which have shown crystallinity only above a substrate temperature of 200 ◦C. Recently, the ionized cluster beam deposition (ICBD) technique has received a great interest because of it′s advantages for film growth, such as easy control for the preferred crystalline orientation, epitaxial growth at relatively low temperature, good interfacial adhesion to the substrate, and the high packing density of the grown film [9]. These properties are mainly caused by the kinetic energy of the clusters given by the electric field. This