Photoluminescence and cathodoluminescence of spin coated ZnO films with different concentration of Eu³⁺ ions

ZnO thin films doped with Eu³⁺ up to 4 mol% were successfully prepared using a sol-gel spin coating technique. The structure, morphology, chemical analysis and luminescence of the films were studied. X-ray photoelectron spectroscopy (XPS) analysis for the Eu 3d core shells confirmed the presence of divalent (Eu²⁺) and trivalent (Eu³⁺) states. Time of flight secondary ion mass spectroscopy showed that the Eu³⁺ions were homogeneously distributed throughout the thin film with Eu²⁺ mainly only in the surface region of the thin films. The films exhibited ZnO exciton emission around 376 nm, broad defect related emission and the Eu³⁺ characteristic emission simultaneously when excited at 325 nm. For the excitation at 464 nm, the doped samples exhibited only the characteristic emissions of Eu³⁺ which were attributed to the ⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4) transitions, respectively. The film with a 3 mol% of Eu³⁺ has emitted the highest photo/cathodoluminescence (PL/CL) emission. Therefore, it was subjected to electron beam irradiation in vacuum for about 22 h to establish the effect of the electron beam on the film's surface state, chemical and luminescence stability. Minor surface modification during electron beam irradiation was observed. The O 1s XPS peak revealed the creation of new defects associated with oxygen deficiencies due to the irradiation. Surface modification and creation of defects were identified as the major mechanisms for the initial decrease in the CL intensity. This film was very stable during prolonged electron beam irradiation.