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 synthesized by sol–gel method.
• NiAl:ZnO film annealed at 500 °C showed the lowest resistivity value of 1.05 × 10− 3 Ω cm.
• All NiAl:ZnO films showed a high transmittance of more than 80%.
• The optical band gap was increased from 3.30 to 3.55 eV with increasing annealing temperature from 450 to 600 °C.

Al–Ni co-doped ZnO (NiAl:ZnO) thin films were deposited on glass substrates using a sol–gel method. Based on a previous study, Zn1 − 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 °C in N2/H2 (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 °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 ~ 23.8 nm to ~ 34.6 nm. 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 (Ω 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 °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.