Al-doping effect on the photoluminescence behavior of MTiO₃:Pr^(3+)[M=Ca, Sr, Ba] and thin film phosphors

Abstract

This work reports the correlation between the crystallization and luminescent properties on Al-doped MTiO₃:Pr^(3+)[M = Ca, Sr, Ba] powder and thin film phosphors. Ti-substitution in host lattice by Al-doping improves the crystallization and enhances the luminescent properties. From the result of PL, CL spectra, and microstructure characteristics, we conclude that the Al-doping improves not only the crystallinity and grain size, but also the efficiency of the host-to-activator energy transfer. The intra-4f emissions from the excited state 3P0 and 1D2 to the ground state 3H4 of Pr3+ are mainly observed around from 613 to 617 nm, and the MTiO₃:Pr^(3+) powders sintered at 1200 oC show the better PL and CL properties that those of powders sintered at 1300 oC. Al-doped CaTiO₃:Pr^(3+) sintered at 1200 oC powder among the powders have made the best red luminescence properties. After the mixture of Al(OH)3, thedissolution of sharp edges of particle solids will make the particle surfaces smoother, followed by the formation of larger grains with a rounded shape. The optimization of chemical composition, particle size distribution, and morphology is required for the more efficient luminescence of phosphor materials. Among them, the spherical morphology is an important factor for lower light scattering at the surfaces as well as higher packing densities. According to that observed at SEM image and PL properties of our powders, too. The decay time at the initial stage of luminescence is decreased by Al-doping. The PL and CL intensity of the red emission peak for 1D2 → 3H4 transition, was increased by Al-doping, respectively. Low temperature PL spectra shows that the weak emission peak at 468, 484, and 499 nm are assigned to 4f-4f transitionsfrom the 3P2, 3P1, and 3P0 states to ground state, respectively. The red emission peaks at 610, 612, 614, 626 and 637 nm are assigned to 4f-5d transition from the 1D2 states to ground state. The PL intensity measured at 12 K was increased more than 3 times by Al-doping. For CL spectra, Al-doped CaTiO₃:Pr^(3+) stable and no change of crystal field under the low acceleration voltage. In thin film phosphors, high quality CaTiO₃:Pr^(3+) and Al-doped CaTiO₃:Pr^(3+) film phosphors have been deposited on Al2O3(0001) substrate using a pulsed laser deposition technique. The results presented in this letter suggest that Al2O3 (0001) is one of the most promised substrates for the fabrication of high quality CaTiO₃:Pr^(3+) phosphor thin films. The brightness of Al-doped CaTiO₃:Pr^(3+) films was increased by a factor of 14.3 in comparison with that of CaTiO₃:Pr^(3+) films. The remarkable improvement in PL performance with the Al-doping may result from not only improved crystallinity leading to higher oscillating strengths for the optical transitions, but also reduced internal reflections caused by rougher surfaces. For the enhanced PL intensity, it can be suggested that the incorporation of Al3+ ions creates the oxygen vacancies, which might act as a sensitizer for the effective energy transfer due to the strong mixing of charge transfer states. As the oxygen pressure decreases the crystallinity of the films improved. The ratio of peak values, I(121)/I(042), which was one of the most important factors to determine the PL brightness of Al-doped CaTiO₃:Pr^(3+) films, was the function of the substrate temperature and oxygen pressure. The PL intensity and the ratio of peak values, I(121)/I(042) behave similarly as a function of oxygen pressure. An increase in oxygen pressure from 150 to 300 mtorr resulted in an increase in both the ratio of peak values, I(121)/I(042) and PL intensity of the films . The PL intensity and rms roughness have similar behavior as a function of oxygen pressure. The increase in oxygen pressure from 150 to 300 mtorr resulted in an increase in surface roughness which in turn increased the PL intensity. Growth of as-deposited Al-doped CaTiO₃:Pr^(3+) films with such a high brightness are very encouraging for the application of thin film phosphors in display technologies.