Bandgap narrowing in CdO doped with europium

Lightly Eu-doped CdO thin films (0.4%, 0.5%, 0.8%, and 1.1%) have been prepared by a vacuum evaporation method on glass and Si wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV-Vis-NIR absorption spectroscopy, and dc-electrical measurements. Experimental data indicate that Eu3+ doping slightly stresses the CdO crystalline structure and changes the optical and electrical properties. The bandgap of CdO was suddenly narrowed by about 20% due to a little (0.4%) doping with Eu3+ ions. Then, as the Eu doping level was increased (for less than 1.1%), the energygap was still narrowing. This variation was explained by the bandgap shrinkage effects. The electrical behaviours of the Eu-doped CdO films show that they are degenerate semiconductors with energygap 1.5-1.8 eV. The 0.8% Eu-doped CdO film shows increase its mobility by about 3.5 times, conductivity by 40 times, and carrier concentration by 11 times, relative to undoped CdO film. From transparent-conducting-oxide point of view, Eu is sufficiently effective for CdO doping. Finally, the absorption in the NIR spectral region was studied in the framework of the classical Drude theory.