EPR studies on defects in sol–gel derived alumina films

Abstract

We have studied the defect structures and mechanism for the luminescence of non-doped, Tb 3+-doped and Tb 3+–Zn 2+ codoped alumina films treated at various temperatures from 300 to 800 °C. The electron paramagnetic resonance (EPR) spectra observed before UV-irradiation are attributable to the non-bridging oxygen radicals; Al–O–O at treatment temperatures of 300–500 °C and Al–O at 800 °C. The UV-light irradiation of the non-doped alumina films gives the EPR spectra with the 27Al 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 Tb 3+ ion were observed in the Tb 3+-doped and Tb 3+–Zn 2+ codoped alumina films. The Tb 3+ concentration dependence of the EPR signal intensity due to the non-bridging oxygens suggests that Tb 3+ 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 Tb 3+–Zn 2+ 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.