Growth and Characterization of Undoped ZnO and Ni/Co-, Fe/Co-, and Fe/Ni-Codoped ZnO Thin Films by a Sol-Gel Spin Coating Technique

In this work, undoped ZnO thin films, as well as Ni/Co-, Fe/Co-, and Fe/Ni-codoped ZnO thin films, were deposited onto glass substrates using the spin-coating technique. The influence of co-doping with Ni/Co, Fe/Co, and Fe/Ni on ZnO thin films was investigated using various characterization studies such as X-ray diffraction (XRD), atomic force microscopy (AFM), UV-Vis-NIR spectroscopy, X-ray photonics (XPS), and electrical measurements. XRD analysis confirms that all films are polycrystalline with a hexagonal structure, exhibiting a preferred (110) orientation that shifts to the (211) plane upon Ni/Co, Fe/Co, and Fe/Ni codoping. The average crystallite size is calculated to be about 16–23 nm. AFM analysis indicates that the average surface roughness (Sa) of our samples ranges from 21.40 to 162.30 nm. The average optical transmittance (AT) of the undoped and Fe/Co-codoped ZnO films in the wavelength range of 300–800 nm is approximately 80%, decreasing to about 60% upon Fe/Ni codoping, while the Ni/Co-codoped ZnO film has the best transparency (≈90%). The band gaps (Eg) varied from 3.08 to 3.25 eV. XPS results indicate that transition metal (TM) dopants, such as Ni, Co, and Fe, were incorporated into the ZnO matrix through the photoelectron peaks amplified at the 2p core-level photoelectron peaks (2p1/2 and 2p3/2) of these elements, confirming the successful doping of TM ions into the ZnO lattice. Electrical measurements revealed n-type conductivity across all films, with an observed reduction in sheet resistance in the codoped ZnO films, which enhances free carrier concentration and improves charge transport through the film. These findings provide valuable insights into the role of transition metal co-doping in the design of zinc oxide thin films for potential applications in optoelectronics and transparent electrodes.