Synthesis and characterization of Cr-doped cadmium oxide thin films for NH3 gas-sensing applications

This research focuses on the fabrication of transparent conducting films of cadmium oxide (CdO) and cadmium oxide doped with chromium, utilizing a straightforward chemical spray pyrolysis technique. The investigation delves into the characteristics of these films by employing various methods, including X-ray diffraction, scanning electron microscopy, UV–visible transmittance analysis, Hall measurements and gas-sensing experiments. The study comprehensively examines the structural, morphological, optical, electrical and sensing properties in connection with different levels of chromium doping. The findings reveal that both the pristine and chromium-doped CdO films exhibit a polycrystalline cubic structure, as indicated by the X-ray diffraction analysis. Alterations in the surface microstructure, caused by varying chromium (Cr) doping concentrations, are evident in SEM images, displaying the presence of semi-spherical nanoparticles. The optical transmittance of the films exhibits a non-linear trend, decreasing initially and then increasing as the Cr concentration rises, reaching its peak at a 2.60 eV bandgap for films doped with 7 wt% Cr. Hall measurements confirm that all films exhibit n-type semiconductor behaviour. The CdO film doped with 7 wt% Cr exhibits a minimum resistivity of 0.427 × 10⁻² Ω.cm and a carrier concentration of 2 × 1018 cm⁻³. Gas-sensing experiments conducted with ammonia indicate a maximum response of 26% at an operating temperature of 300°C for the 7 wt% Cr-doped CdO sample, which also displays faster response and recovery times.