A Sims Study of Electroluminescent Phosphors: SrS:Cu
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Lily H.Zhang, Larry Wang, Wusheng Tong*, YongBao Xin*, Charles Evans & Associates, 301 Chesapeake Dr., Redwood City, CA 94063, [email protected] *Phosphor Technology Center of Excellence, Manufacturing Research Center Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332 ABSTRACT This study has used secondary ion mass spectrometry (SIMS) as a technique for thin film EL material characterization. It has shown that the Cu dopant concentration in the SrS films directly correlates with the luminescent brightness of the EL devices. A series of SrS:Cu,Y were grown using MBE to study the Y co-doping effects. It has been found that Y peak concentration and areal density in the SrS increased as the Y evaporation cell temperature was increased. The maximum PL intensity was found in the sample grown in the middle of the Y cell temperature range used. The Y co-doping has shown to reduce the thermal quenching effects in SrS EL devices. Therefore, in this series of samples, a good correlation has been found between Y and Cu concentration and the EL device performance characteristics. INTRODUCTION Strontium sulfide (SrS) has recently emerged as an important phosphor host material in thin film electroluminescent displays (TFELD) [1]. The Cu and Mn are doped in SrS as color activators for blue light emission [2], [3]. The Y is added as a codopant for charge compensation in SrS [4]. There has been significant development recently in the growth and processing of SrS as EL phosphor [1]. However, in order for these materials to compete with the liquid crystal displays, the brightness has to be
increased about 1.5x for blue color, 3x for green color. Reliable and precise characterization techniques are needed to correlate film growth with device performance to better understand the basic mechanisms and processing techniques relating to EL device performance. Because of its high sensitivity and depth profiling and imaging capabilities, SIMS was used in this study to determine the depth profile and lateral distribution of Cu, Mn and Y in MBE grown SrS films. Part of the SIMS results for Mn measurements was reported in ref [3]. Results for SrS:Cu,Y are presented here. A series of SrS:Cu, Y were grown using different conditions. SIMS results have shown a correlation of Cu concentration with luminescent brightness. The Y has been observed to reduce thermal quenching effects in SrS. It is shown that Y peak concentration and areal densities as well as the Cu concentration are all correlating well with the device performance characteristics. EXPERIMENTS SrS thin films were grown at Georgia Tech Research Institute using molecular beam epitaxy (MBE). The substrate was ATO/ITO/glass. Elemental Sr and Cu sources
41 Mat. Res. Soc. Symp. Proc. Vol. 558 © 2000 Materials Research Society
were thermally evaporated from PBN crucibles. The t-BuSH or H2 S was used as sulfur precursors. YCI3 and Y were used for co-doping studies. A nude ion gauge which can be rotated to the substrate position was used to measure the growth flu
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