Upconversion luminescence from magnetron-sputtered Er3+-doped TiO2 films: Influence of deposition- and annealing temperatures and correlation to decay times

Abstract

The optical properties of radio-frequency magnetron sputtered TiO2 thin films doped with Er3+ are strongly influenced by the deposition and post-annealing temperatures. This has an impact on the applications of the material for upconversion, i.e., the merging of two low-energy photons to one photon of higher energy. Maximum upconversion luminescence (UCL) yield is obtained using a deposition temperature of 350 °C without post-annealing. Motivated by the possibilities that become available by sequential depositions (several layers), the effect of post-annealing is systematically investigated. In general, post-annealing treatments reduce the UCL; however, for the lowest deposition temperatures, post-annealing has a positive impact on the UCL provided that the samples are not exposed to ambient air prior to the annealing step. These observations are further analyzed using time-resolved photoluminescence spectroscopy for determining the characteristic decay times of the Er3+ energy levels in the different samples. It is found that the UCL yield scales to a good approximation linearly with the product of the decay times of the two lowest-lying Er3+ excited energy levels (4I11/2 and 4I13/2). The combined data provide strong evidence that the reduction in UCL is due to the opening of non-radiative decay channels from the Er3+ excited levels. Structural measurements show no change of the amorphous samples upon annealing, so these decay channels are most likely related to energy transfer between Er3+ and defect states in the TiO2 bandgap. The non-radiative decay could possibly be related to the loss of hydrogen termination of dangling bonds or related to the oxygen vacancies in TiO2.