Recent advances in the phosphors used for field emission displays (FEDs) are discussed. After reviewing the range of voltages and phosphors being used in first generation devices, the improved properties of future generation phosphors are reviewed. Specifically, next generation displays will require better low voltage efficiencies, chromaticity, saturation behavior, and maintenance. Possible routes to achieve these improvements are discussed. The improved understanding of the role of charging and surface recombination effects on cathodoluminescent intensity and efficiency is reviewed. An improved understanding of electron beam-stimulated surface chemical reaction effects on the degradation of phosphor is presented. It is concluded that recent research efforts have created a new level of understanding of FED phosphors, and this should lead to the necessary improvements in properties.
Degradation of ZnS and Y2O2S cathodoluminescent (CL) phosphors has been studied at 1–4 keV using Auger electron spectroscopy simultaneous with CL. The data are consistent with an electron stimulated surface chemical reaction (ESSCR) which led to destruction of ZnS and formation of a surface nonluminescent ZnO layer as well as injection of oxygen point defects into the near-surface region. In the case of Y2O2S:Eu, the electron beam stimulated removal of S and formation of Y2O3:Eu in the presence of 1×10−6 Torr of oxygen. A model is presented which predicts that degradation by the ESSCR should increase with pressure in the vacuum, depend exponentially on electron dose, increase as the primary beam energy was reduced below 4 keV, and depend upon the type of gas present in the vacuum. These trends were demonstrated from the experimental data.
Degradation of phosphors in field emission displays (FED's) is described and related to electron beam stimulated surface reactions between ZnS and residual gas in the vacuum system. The requirements for producing and maintaining vacuums in FED packages is reviewed and limitations associated with the size of the FED's are discussed. It is concluded that vacuum production and maintenance is critical to the performance of FED's, and this is not a simple task.
Phosphor powder of ZnS:Cu,Al,Au has been subjected to electron bombardment (2 keV, 2 mA/cm2) in residual gas pressures ranging from 0.6×10−8 to 7.0×10−8 Torr. Auger electron spectroscopy and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in the surface chemistry and cathodoluminescent brightness versus vacuum conditions during electron bombardment. A direct correlation between the surface reactions and the degradation of CL brightness was observed. The formation of a nonluminescent ZnO layer on the surface of the phosphor was largely responsible for the degradation of the ZnS. The aging of the phosphor was not only a function of the charge per unit area (Coulomb dose) bombarding the surface, but also a function of residual gas pressure and composition. In particular, H2O had the greatest effect on the rate of degradation. The results are interpreted in terms of an electron-beam stimulated surface chemical reaction.
ZnS phosphor powders have been subjected to electron bombardment (2 keV, 2 mA/cm2) at a residual gas pressure of 1.2×10−8 Torr and oxygen pressures of 10−6 Torr. Auger electron spectroscopy and cathodoluminescence (CL), both excited by the same electron beam were used to monitor changes in surface state and luminous efficiency during electron bombardment. A direct correlation between the surface reactions and the degradation of CL brightness was observed. Both C and S were depleted from the surface during electron bombardment. The postulated mechanism for the electron stimulated reactions on the phosphor surface is electron beam dissociation of molecular species to atomic species which subsequently react with C to form volatile compounds (COx, CH4, etc.) and with ZnS to form a nonluminescence layer of either ZnO or ZnSO4. The growth in thickness of the nonluminescence surface layer is directly responsible for the degradation in CL brightness.
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