Effect of Post-annealing Temperature on Microstructural, Morphological and Optical Properties of Sol–gel Spin Coated Cu Doped SnO₂ Thin Films

Copper (Cu) doped SnO₂ thin films were prepared on glass substrates using sol–gel spin coating by varying post-annealing temperature from 150 to 350 °C. The influence of post-annealing temperature on microstructural, surface morphological, and optical properties of Cu-doped SnO₂ thin films have been investigated by X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, and Ultraviolet–Visible diffuse reflectance spectroscopy. X-ray diffraction results confirm that Cu-doped SnO₂ thin films possess a tetragonal rutile structure. The variation observed in lattice constant values of post-annealed Cu-doped SnO₂ thin films is due to the successful substitution of Sn⁴⁺ ions by Cu²⁺ ions in SnO₂ crystal lattice, which exhibit a lattice expansion by causing the reduction of dislocation density and micro-strain. The surface morphology of the films shows that grain size increased when the annealing temperature increased. The optical band gap of Cu-doped SnO₂ thin films decreased from 3.50 to 3.28 eV with the increase in post-annealing temperature. The spectroscopic ellipsometer is employed to study the extinction coefficient and refractive index of the Cu-doped SnO₂ thin films.