Synthesis and properties of alkaline earth elements (Ca, Sr, and Ba) doped SnO₂ thin films

Transparent conductive oxides (TCOs) are commonly used in many optoelectronic, photovoltaic, and catalytic applications because of their unique optical transparency and electrical conductivity. In this work, we report, for the first time, on the optical and electrical properties of Ca-doped tin thin films, and we shed more light on the structure and properties of Sr- and Ba-doped SnO₂ thin films. A simple and low-cost spray pyrolysis technique (SPT) was used to synthesize SnO₂ thin films doped with alkaline earth elements (Ca, Sr, and Ba) that have ionic radii larger than tin ion radius. The microstructural features of the prepared samples were investigated using a scanning electron microscope, an atomic force microscope, and an x-ray diffractometer. The presence of the dopants and relevant chemical bonds were ascertained through Fourier Transform-Infrared (FT-IR) spectroscopy. The optical and electrical properties were characterized using a spectrophotometer and a four-point probe analyzer, respectively. The synthesized thin films were polycrystalline and had a tetragonal rutile structure with preferred growth orientation along <110> direction. Doping with Ca increased the crystallite size and decreased the dislocation density and micro strain, whereas opposite trend was observed for samples doped with Sr and Ba. The films had smooth, dense, continuous, and homogenous surface. FT-IR analysis revealed the presence of O–Sn–O, Ca–O–Ca, Ba–O and Sr–O bonds. Doping SnO₂ with Ca, Sr and Ba improved its optical transmittance from 67% to around 82%. The band gap of pure SnO₂ (3.57 eV) decreased to 3.08 and 3.05 eV as a result of doping with Ca and Sr, respectively; and increased to 3.60 eV because of doping with Ba. Strontium and calcium increased the electrical conductivity of SnO₂ from 5.79 (Ω cm)⁻¹ to 36.24 and 56.11 (Ω cm)⁻¹, respectively; but barium decreased it to 2.45 (Ω cm)⁻¹. The relatively wide energy gap, high electrical conductivity, and high transmittance of the Ca- and Sr-doped films make them good candidates in diverse functional applications.