Spray-Deposited Rare Earth Metal Ion (Sm³⁺, Ce³⁺, Pr³⁺, La³⁺)-Doped CdO Thin Films for Enhanced Formaldehyde Gas Sensing Characteristics

This study shows the electrical conductivity-dependent gas sensing characteristics of spray-deposited rare earth (RE) metal ion (Sm3+, Ce3+, Pr3+, La3+)-doped cadmium oxide (CdO) thin films on soda-lime microscope glass substrates at 300 °C. We examined the deposited films’ structural, surface microstructural, DC electrical, and gas sensing features. The X-ray diffraction study indicates that all samples were polycrystalline, with the favored growth direction shifting from the (111) plane to the (200) plane. The highest root-mean-square values were obtained for the Pr-doped CdO thin film (5.86 nm). The surface microstructure of CdO thin films was significantly influenced by the RE metal ion dopant, with typical grain size values ranging from 64 nm to 134 nm depending on the dopant. The carrier concentration and resistivity of CdO films vary based on the RE metal ions used as dopants. Low resistivity (3.01 × 10–4 Ω.cm) was achieved for the CdO thin film doped with La. High gas sensitivity (71.42%) was achieved for CdO thin films doped with La. The donor dopant regulated the electrical conductivity and gas sensing capabilities of CdO thin films.