Impact of Cd/Sn molar ratio and annealing temperature on the properties of spin coated Cd₂SnO₄ thin films

We have synthesized and characterized spin coated Cd₂SnO₄ (CTO) thin films in the present work by varying the Cd/Sn = 2,3,4, and 5 M ratios, and 450 and 500 ^οC annealing temperatures. The structural studies were done using X-ray diffraction (XRD) and the results show that all the films exhibit a polycrystalline nature with (311) plane preferred orientation corresponding to Cd₂SnO₄ inverse spinel cubic phase. The morphological studies were done using an atomic force microscope (AFM) and field emission scanning electron microscope (FESEM). The thin films obtained with a roughness of 3–40 nm, and uniformly distributed grains without cracks are evident from the micrographs. The elemental analyses were carried out by employing EDAX and XPS techniques. The optical properties such as transmittance, bandgap, and refractive index are obtained using UV–Vis spectroscopy. The films were showing high transmittance (75–93 %) in the visible region. The band gaps of the investigated samples are found to be in the range of 3.4 eV–3.7 eV. The refractive index of CTO thin films in the 350–1000 nm wavelength range is around 1.7 to 1.9. The electrical properties show better results for the films annealed at 500 ^οC. The mobility values obtained are in the range of 0.6–4 cm²/Vs for different Cd/Sn ratio samples. The carrier concentration of CTO thin films is in the range of 4 × 10¹⁸ to 28 × 10¹⁸ cm⁻³. The presence of defects such as oxygen vacancy, Cd/Sn interstitial, and self-doping of Cd and Sn results in increased carrier concentration at higher Cd/Sn ratios. Besides, the wettability analysis was performed by contact angle measurements, which infer a hydrophobic to hydrophilic conversion in terms of the surface roughness of CTO thin films. From optoelectrical results, it is evident that the films exhibited better optical and electrical properties and are suitable for optoelectronic devices.