(Invited) Microscopic Study of Dopant Distribution in Phosphors: Impact on Thermal Quenching and Phosphor Performance

Inorganic luminescent materials or phosphors are currently applied on a large scale in white light-emitting diodes (LEDs) for lighting and display applications. These technologies rely on high-performance phosphors and require subtle fine-tuning of the material composition and the device manufacturing to achieve white LED devices with the desired characteristics1. Synthesis conditions strongly affect phosphor properties, often in an unpredictable and hard-to-control way. This can be attributed to an incomplete understanding of the effects of local variations in composition, morphology and emission properties on the overall performance of the phosphor. Since luminescent characterization methods are generally performed on a macroscopic scale, local variations are averaged out, and the detailed information on the interplay between structure and luminescence is lost. To gain more insight in how performance-determining mechanisms are influenced by synthesis conditions and sample composition, we combine cathodoluminescence (CL) spectroscopy in a scanning electron microscope (SEM) with an energy-dispersive X-ray (EDX) analysis. In SEM-CL-EDX, simultaneous identification of the morphology, the chemical composition and the spectral distribution of a phosphor with a spatial resolution down to 1 µm or less is achieved. For this work a heating stage was added to the setup, allowing to determine thermal quenching profiles on sub-micrometer sized areas. Temperature-dependent SEM-CL-EDX mappings were performed on SrGa2S4:Eu2+, a material which is known as a saturated green phosphor, especially suitable for display applications2 . This case study revealed that samples with ill-distributed dopant ions show a broad range of local quenching temperatures, as can be seen in Fig. 1. For the associated activation energy an upper limit of 0.61 eV was identified, corresponding to the intrinsic thermal quenching of isolated europium ions. Furthermore, the results confirm a previously suggested thermal quenching model which involves the presence of both isolated and clustered dopant ions3. References Smet, P.F.; Parmentier, A.B.; Poelman, D. J. Electrochem. Soc., 2011. 158(6): p. R37-R54. Joos, J.J.; Meert, K.W.; Parmentier, A.B.; Poelman, D.; Smet, P.F. Opt. Mater., 2012. 34(11): p. 1902-1907. Martin, L.I.D.J.; Poelman, D.; Smet, P.F.; Joos, J.J. ECS J. Solid State Sci. Technol., 2018. 7 (1) p. R3052-R3056. Figure 1: Distribution of local Eu concentration related to the distribution of local T 0.5 within a single phosphor grain for Sr1-x Ga2S4:Eu x with x = 0.01, 0.03 and 0.07. Figure 1