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-xAl_xO (AZO; Al/Zn=1.5mol%) thin films optimized with a Ni content of 0.5mol% were annealed at different temperatures from 450 to 600°C in N₂/H₂ (95/5) forming gas for 1h. 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°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 ~23.8nm to ~34.6nm. According to the Hall Effect measurements, when the temperature reached 500°C, the resistivity of the thin film showed the lowest value of 1.05×10^-3 (Ωcm), 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 3.55eV. This study showed that the annealing temperature in the forming gas is a vital factor affecting the quality of thin films. In addition, 500°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.