Mechanisms Affecting Emission in Rare-Earth-Activated Phosphors
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Mechanisms Affecting Emission in Rare-Earth-Activated Phosphors David R. Tallant, Carleton H. Seager and Regina L. Simpson Sandia National Laboratories, Albuquerque, NM, 87185-1411, U.S.A. ABSTRACT The relatively poor efficiency of phosphor materials in cathodoluminescence with low accelerating voltages is a major concern in the design of field emission flat panel displays operated below 5 kV. Our research on rare-earth-activated phosphors indicates that mechanisms involving interactions of excited activators have a significant impact on phosphor efficiency. Persistence measurements in photoluminescence (PL) and cathodoluminescence (CL) show significant deviations from the sequential relaxation model. This model assumes that higher excited manifolds in an activator de-excite primarily by phonon-mediated sequential relaxation to lower energy manifolds in the same activator ion. In addition to sequential relaxation, there appears to be strong coupling between activators, which results in energy transfer interactions. Some of these interactions negatively impact phosphor efficiency by nonradiatively de-exciting activators. Increasing activator concentration enhances these interactions. The net effect is a significant degradation in phosphor efficiency at useful activator concentrations, which is exaggerated when low–energy electron beams are used to excite the emission. INTRODUCTION Phosphors are the light-emitting components used in flat panel displays based on electroluminescent, plasma or field emission excitation technologies. Unlike cathode ray tube (CRT) displays, which use highly energetic electrons and substantial beam currents to excite phosphor emission, design constraints in flat panel displays severely limit the amount of excitation power available to excite phosphors. Consequently, the emission efficiency of phosphor materials becomes an issue of major concern in the design of flat panel displays. Unfortunately, the emission efficiency of rare-earth-activated phosphors appears to be limited by concentration quenching of activators (the dopants which produce characteristic light emission in phosphors) [1] and other effects [2]. The quenching effects are aggravated when low-voltage electrons are used for excitation [3]. It is, therefore, important to understand the mechanisms of quenching in phosphors and so determine whether quenching limitations of phosphor efficiency can be overcome and, if so, how. Our work has identified interactions between activators in excited states which have a major impact on phosphor efficiency. This paper presents evidence for these interactions (primarily) from photoluminescence (PL) experiments but also with data from cathodoluminescence (CL) which show concomitant effects. EXPERIMENTAL DETAILS Powder samples of phosphors from the systems Y2O3:Eu, Y2SiO5:Tb and Zn2SiO4:Mn were prepared by spray pyrolysis (Superior MicroPowders) in dopant concentrations from 0.2 to 10 atomic %. Photoluminescence (PL) emission spectra were obtained using a D2 lamp/monochromator combination for ex
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