Influence of annealing temperature on the Optical and structural proper-ties of Zinc Oxide (ZnO) thin film as a transparent conductive oxide electrode

Abstract

This paper presents the modification of ZnO thin film structural and optical properties based on optimized deposition spin-speed and post-deposition heat treatment. ZnO is a semiconductor material in its natural state that has strong n-type conductivity with related flaws such as native defects, interstitial hydrogen and high resistivity. These inherent challenges must be surmounted for pure ZnO to be used as transparent conducting oxide (TCO). ZnO thin films were deposited onto a glass substrate by spin coating technique at 3000 rpm for 30 seconds. The deposited thin films were heated at 100 oC to evaporate unwanted impurities. The films were then annealed at temperature of 300 oC to 600 oC at an interval of 25 oC for 2 h under ambient temperature. The thin films were characterized and the resistance measurement was performed using four-point probe equipment after annealing. Optical, morphology, X-ray diffraction, and electrical properties at ambient temperature conditions were reported. Optical properties using UV-Vis spectrophotometer show that the transmittance of ZnO thin film is >75 and 95% in the visible region of the electromagnetic wavelength. The optical band gap energy ranged from 3.25 eV to 3.6 eV. Scanning electron microscopy (SEM) images reveal the formation of nearly spherical ZnO with slight agglomeration. An appreciable change in its size as the annealing temperature increases is seen affirming the larger size of ZnO at higher annealing temperatures. An increase in ZnO grain size with higher annealing temperatures suggests that the crystalline quality of the ZnO improves with the increase in annealing temperature. X-ray diffraction shows that ZnO thin films are polycrystalline and have a hexagonal (wurtzite) structure. The crystallites are oriented with (100) and (101) planes parallel to the substrate surface and current-voltage (I-V) measurement was taken with the use of a four-point probe with a Kiethley measuring source unit to determine the resistivity and conductivity of ZnO thin film.