Luminescence properties of Yb³⁺-Tb³⁺ co-doped amorphous silicon oxycarbide thin films

This work analyzes the photoluminescence emission of Yb³⁺ and Tb³⁺ ions in co-doped silicon oxycarbide thin films, their activation by thermal treatment, and reveals their luminescent properties regarding the energy transfer between them. Three samples of silicon oxycarbide were prepared by rf magnetron sputtering from SiC, Yb and Tb targets in an oxygen/argon atmosphere. The first one is the undoped silicon oxycarbide sample, the second one is Tb single doped, and the last one is the Yb-Tb co-doped sample. All three samples are identified as silicon oxycarbides with a low carbon content using energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The latter shows the presence of the vibrational modes of Si–C and Si–O bonds. Subsequent annealing treatments up to temperatures of 750 °C led to the rare earths optical activation in the samples. For each annealing step, we present the photoluminescence spectra using an above-bandgap excitation of 325 nm. The Tb³⁺ and Yb³⁺ -related luminescence lines were identified. The Yb³⁺ ions luminescence for the co-doped sample shows concentration quenching above annealing temperatures of 500 °C whereas the Tb³⁺ ions luminescence intensity remains almost constant. The analysis of the photoluminescence excitation spectra shows the direct excitation of the Tb³⁺ ions in the Tb-doped samples. A high suppression of the direct excitation of the Tb³⁺ ions in the Yb-Tb co-doped samples was observed. The energy transfer from Tb³⁺ to Yb³⁺ ions in co-doped samples is evidenced, first, by the decrease of the Tb³⁺ photoluminescence intensity in the co-doped compared to the Tb doped sample. Second, a change from nearly single exponential to nonexponential decay in the Tb³⁺ photoluminescence decay curves and third, by the reduction of the Tb³⁺ decay time from 1.2 ms in the Tb-doped sample to 0.5 ms in the Yb-Tb co-doped sample.