Effect of thermal annealing on the structural, electrical and optical properties of Al-Ni co-doped ZnO thin films prepared using a sol-gel method

Al-Ni co-doped ZnO (NiAl:ZnO) thin films were deposited on glass substrates using a sol-gel method. Based on a previous study, Zn-1 (-) xAlxO (AZO; Al/Zn = 1.5 mol%) thin films optimized with a Ni content of 0.5 mol% were annealed at different temperatures from 450 to 600 degrees C in N-2/H-2 (95/5) forming gas for 1 h. The effects of the annealing temperature on the structural, electrical and optical properties were determined. X-ray diffraction showed that NiAl:ZnO thin film annealed at 500 degrees C exhibited the best crystallization quality. XPS revealed the presence of metallic Ni and Ni2O3 states, as well as Ni and Al atoms were successfully doped in NiAl:ZnO films, which did not result in a change in ZnO crystal structure and orientation. Scanning electron microscopy showed that the films were smooth and compact, and the grain size increased with increasing annealing temperature from similar to 23.8 nm to similar to 34.6 nm. According to the Hall Effect measurements, when the temperature reached 500 degrees C the resistivity of the thin film showed the lowest value of 1.05 x 10(-3) (Omega cm), which is the lowest resistivity reported for NiAl:ZnO films. The UV-Vis transmission spectra showed a high transmittance of more than 80% in the visible light range, and the band gap of the films was increased from 3.30 to 3.55 eV. This study showed that the annealing temperature in the forming gas is a vital factor affecting the quality of thin films. In addition, 500 degrees C was found to be the most appropriate annealing temperature for NiAl:ZnO films. This study provides a simple and efficient method for preparing high quality, high transparency and low resistivity NiAl:ZnO films for optoelectronic applications. (C) 2014 Elsevier B.V. All rights reserved.