New process for synthesis of ZnO thin films: Microstructural, optical and electrical characterization

Abstract

Nanocrystalline ZnO thin films were prepared on glass substrates by using spin coating technique. The effect of annealing temperature (400–700 °C) on structural, compositional, microstructural, morphological, electrical and optical properties of ZnO thin films were studied by X-ray diffraction (XRD), Energy dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM), High Resolution Transmission Microscopy (HRTEM), Scanning Electron Microscopy (SEM), Electrical conductivity and UV–visible Spectroscopy (UV–vis). XRD measurements show that all the films are nanocrystallized in the hexagonal wurtzite structure and present a random orientation. The crystallite size increases with increasing annealing temperature. These modifications influence the optical properties. The AFM analysis revealed that the surface morphology is smooth. The HRTEM analysis of ZnO thin film annealed at 700 °C confirms nanocrystalline nature of film. The SEM results shows that a uniform surface morphology and the nanoparticles are fine with an average grain size of about 40–60 nm. The dc room temperature electrical conductivity of ZnO thin films were increased from 10 −6 to 10 −5 (Ω cm) −1 with increase in annealing temperature. The electron carrier concentration ( n) and mobility ( μ) of ZnO films annealed at 400–700 °C were estimated to be of the order of 4.75–7.10 × 10 19 cm −3 and 2.98–5.20 × 10 −5 cm 2 V −1 S −1. The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 3.32 eV to 3.18 eV with increasing annealing temperature between 400 and 700 °C. This means that the optical quality of ZnO films is improved by annealing. It is observed that the ZnO thin film annealing at 700 °C has a smooth and flat texture suited for different optoelectronic applications.