Effect of Sn-Ti co-doping ratio and annealing temperature on the structural, optical and electrical properties of spray-deposited ZnO thin films

The doping of semiconductor materials has proved an effective way to tailor their optical and electronic performance, allowing them to be optimised for use in a given target application. One material in particular that has been of interest is zinc oxide due to its wide direct band gap, high optical transparency in the visible region, good electron mobility, excellent chemical stability, and compatibility with low-cost solution-based deposition techniques. In this work we investigate how the structural, optical, and electrical properties of ZnO thin films are modified when co-doped with Sn and Ti, examining the effect of different dopant ratios and annealing temperatures. The thin films were deposited on a borosilicate glass substrate via spray pyrolysis, with X-ray diffraction (XRD) analysis showing that all the films are polycrystalline with a wurtzite hexagonal structure, and with a crystallite size and surface morphology that varies with the doping ratio and annealing temperature. Atomic force microscopy (AFM) results showed roughness values ranging from 3.96 nm to 4.82 nm for undoped ZnO and 3.69 nm to 6.54 nm for Sn-Ti co-doped ZnO. XPS confirmed the incorporation of Sn–Ti into the ZnO matrix, while SEM was used to examine the surface morphology of the films. The optical characterisation of the films shows a transmittance of greater than 90% in the visible region for undoped ZnO reducing to about 75% for co-doped ZnO, with transmittance improving to between 80% and 90% after annealing. The band gap ranged from 3.19 eV to 3.31 eV. Electrical measurements showed the Ti 2% Sn 1% ZnO 97% composition having the lowest sheet resistance of 66.28 Ω/□, while the calculated Haacke Figure of Merit (FOM) of the Ti 1.5% Sn 0.5% ZnO 98% annealed at 400°C of 1.96 × 10⁻³ Ω⁻¹ indicates its suitability for optoelectronic applications. The results demonstrate that Sn-Ti co-doping and annealing provide an effective route for tailoring ZnO thin films for transparent optoelectronic applications.