Effect of Annealing on ZnO Thin Films Deposited by Thermal Evaporation on Si (100) Substrates

Abstract

Zinc oxide (ZnO) semiconductor materials which have wide band gap of 3.37 eV and high exciton binding energy of 60meV are very important for optoelectronic device applications. Due to these superior optical/electrical properties, many applications such as LEDs, laser diodes and photodetectors are available for ZnO. Several techniques were used to produce ZnO thin films, namely, sputtering, spray pyrolysis, sol–gel and evaporation. In this study, ZnO thin films have been prepared by thermal evaporation method. In this technique has many andvantage to others in terms of low cost, simple method and also parameters are easily controllable. ZnO thin films of the same thickness were prepared by evaporation of ZnO pellets. These samples were then annealed at different temperatures to obtain a ZnO film. In this study, ZnO formation induced by thermal annealing process at 200-400 °C in air for two hours in ZnO/ Si(100) thin film systems are investigated as a function of the temperature, initial ZnO film thickness of 300 nm, using structural and electrical characterization. The effect of thickness on the growth of ZnO films on Si was systematically characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), AFM (atomic force microscopy) and classical van der Pauw resistivity measurement. Based on the XRD data, as deposited films contain only Zn and Si phases. By increasing the temperature, it was found that XRD peaks related to Zn decrease and at 200°C, ZnO peaks starts to appear because of the film oxidation. After 300°C, polycrystalline ZnO phases with higher intensity are formed. SEM and AFM analyses reveals that ZnO nanorods are formed on the surface of all ZnO/Si(100) films. It has been observed that these nanorods on the surface have a random distribution and after annealing from 200C to 400C they grow in a columnar way and completely cover the surface. Electrical sheet resistance and resistivity measurements show the transition from conductive Zn thin films to lower conductive ZnO films on Si. At 300 degree, electrical resistivity shows a dramatic increase due to the transformation from Zn rich phases to ZnO phases.

Keywords: ZnO thin film, Thermal evaporation, Annealing