EPR studies on defects in sol-gel derived alumina films

We have studied the defect structures and mechanism for the luminescence of non-doped, Tb3+ -doped and Tb3+- Zn2+ codoped alumina films treated at various temperatures from 300 to 800 degreesC. The electron paramagnetic resonance (EPR) spectra observed before UV-irradiation are attributable to the non-bridging oxygen radicals; AI-O-O-. at treatment temperatures of 300-500degreesC and AI-O-. at 800degreesC. The UV-light irradiation of the non-doped alumina films gives the EPR spectra with the Al-27 hyperfine splitting. There exists a good correlation between the treatment temperature dependence on the light-induced EPR signal intensity of the trapped electron and the NMR peak intensity due to the five-coordinated aluminum ion. We concluded that the electron is captured at the oxygen vacancy connected with the five-coordinated aluminum. The broad EPR spectra due to the ground state of the Tb3+ ion were observed in the Tb3+-doped and Tb3+ - Zn2+ codoped alumina films. The Tb3+ concentration dependence of the EPR signal intensity due to the non-bridging oxygens suggests that Tb3+ prefers to form Al-O-Tb bonds and prevent to form a cluster of rare earth ions itself. The light-induced EPR signal of the electron trapped at oxygen vacancy next the five-coordinated aluminum was also observed for the Tb3+ - Zn2+ codoped alumina film, which produced a long-lasting luminescence. The photogenerated trapped electron remained for several minutes under the room temperature. These experimental facts indicate that the five-coordinated aluminum ions act as the trapping sites to produce the long-lasting luminescence. (C) 2002 Published by Elsevier Science Ltd.