Transformations in the photoluminescent, electrical and structural properties of Tb³⁺ and Eu³⁺ co-doped ZnO films under high-temperature annealing

The effect of thermal annealing on optical, electrical and structural properties of Tb and Eu co-doped ZnO films grown by magnetron sputtering on Si and Al2O3 substrates was investigated by X-ray diffraction, photoluminescence, micro-Raman and IR reflection methods. It is shown that incorporation of rare earth ions in ZnO is accompanied by the formation of intrinsic defects. The as-deposited and annealed at 600 degrees C films demonstrate Tb3+ emission and no Eu3+ one. Higher intensity of Tb3+ photoluminescence in the films on Al2O3 substrate as compared with that on Si is ascribed to higher content of Tb3+ emitting centers. The model of these centers including the substitutional Tb and interstitial oxygen is proposed. In the excitation spectra of Tb3+ emission, no features connected with light absorption in ZnO are observed. An annealing at 900 degrees C is found to result in the formation of crystalline terbium oxide and silicate phases. In the photoluminescence spectra, the decrease of Tb3+ emission and the appearance of two sets of Eu3+ related bands caused by energy transfer from Tb3+ to Eu3+ ions are found. This is ascribed to segregation of rare earth ions in the additional phases and the decrease of the distance between the ions.