Effects of annealing temperature on the structural and optical properties of ZnO hexagonal pyramids

ZnO thin films were deposited on quartz substrate at 550 °C by using spray pyrolysis method and subsequently annealed between 600–900 °C with a step of 100 °C. The characterizations of the structural and optical properties of the films have been carried out by means of X-ray diffraction, scanning electron microscopy (SEM) and optical transmittance measurements. XRD ans SEM images indicated that annealing temperature did not play a great role on the microstructure of ZnO films. ZnO thin films are all in hexagonal crystallographic phase and have (0 0 2) preferred orientation, regardless of the annealing temperature. However, SEM studies showed that there exist a high density of micro-rods in the shape of hexagonal pyramid with the width in the order of about 1 μm and height in range of 1–3 μm and the adjacent hexagonal crystals to start fusing with each other along their boundaries at high temperatures. As a result of the optical measurements, it was observed that the films show the low transmittance and optical band gap decreases from 3.15 to 3.10 eV with the increasing of the annealing temperatures up to 800 °C and followed by an increase to 3.20 eV upon further annealing at 900 °C.