Electron Stimulated Surface Chemical Reaction Research Articles

Low temperature effect on the electron beam induced degradation of ZnS:Cu,Al,Au phosphor powders

ZnS:Cu,Al,Au phosphor powders were bombarded by an electron beam of 2 keV with the current density of 8.7 mA/cm 2 at an oxygen pressure of 2×10 −6 Torr. The studies were carried out at temperatures between −125 and 25 °C. Auger electron spectroscopy (AES) and cathodoluminescence (CL) were used to monitor the changes in the surface chemistry of the phosphor and luminous efficiency during electron bombardment, respectively. Thermoluminescence (TL) glow curves produced by the phosphor warming up from −125 °C were also detected. Degradation was manifested by the non-luminescent ZnO layer that formed on the surface of the phosphor according to the electron stimulated surface chemical reaction (ESSCR) mechanism. A decrease in temperature lead to an increase in the surface stay time of the oxygen molecules on the surface with the increase in the ESSCR probability. The effect of thermal quenching of the initial CL was reduced at lower temperatures. The residual water vapour in the vacuum, however, had a significant effect on the rate of degradation because it was frozen at lower temperatures. The formation of O defects during electron bombardment, which act as electron traps, were confirmed through TL glow curves.

On the degradation of ZnS:Cu,Al,Au phosphor powder: The effects of temperature

The effects of temperature are discussed when the ZnS:Cu,Al,Au (P22G) phosphor powder is bombarded by a 2 keV electron beam with a current density of 88 mA/cm2 at an oxygen pressure of 2 × 10−6 Torr at temperatures between 25 and 300°C. The rate of surface reaction decreases at higher temperatures due to the reduction in the mean stay time of the O2 on the surface which is vital for the reactions according to an electron stimulated surface chemical reaction model. A direct correlation between the temperature and the initial cathodoluminescence (CL) brightness which depicts thermal quenching of the CL was observed.

Degradation effect of a ZnO layer on ZnS: comparison between a Monte Carlo simulation and experimental Auger and CL measurements

AbstractSurface reactions occur on the ZnS:Cu,Al,Au surface during prolonged electron bombardment forming a non‐luminescent ZnO layer with consequent loss in cathodoluminescence (CL) intensity. The layer is formed according to the electron‐stimulated surface chemical reaction (ESSCR) mechanism. There is a direct correlation between the power emitted as luminescence and the energy loss of the electron beam. The electron energy loss within the ZnS was determined by using the Monte Carlo algorithm CASINO, which is a publicly available code. By determining the energy loss, the influence of a non‐luminescent ZnO layer of varying thickness on a ZnS substrate on the CL intensity was simulated. These results were compared also with previous calculations that were based on the results of experiments done on the transmission of electrons through thin films. The energy loss within the ZnS as function of the ZnO thickness was determined at beam voltages of 1, 2 and 5 keV. The results obtained correlate with experimental CL measurements of the phosphor degradation during electron bombardment. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of temperature on the degradation of ZnS FED phosphors

AbstractThe changes in cathodoluminescent brightness and changes in the surface chemistry of ZnS:Cu,Al,Au have been investigated using Auger electron spectroscopy (AES) and a photospectrometer. The ZnS:Cu,Al,Au phosphor powders have been subjected to electron beam bombardment of 2 keV, with an electron beam current density of 88 mA cm−2 at an oxygen pressure of 2 × 10−6 Torr. The studies were carried out between room temperature and 310 °C. Degradation was manifested by a non‐luminescent ZnO layer that formed on the surface of the phosphor according to the electron‐stimulated surface chemical reaction (ESSCR) mechanism. An increase in temperature led to a decrease in the surface reaction rate due to a decrease in the mean surface lifetime of the oxygen molecules on the surface, with a direct decrease in the ESSCR probability. Without the presence of the electron beam, no chemical reactions occurred on the surface up to 310 °C. Local heating due to the electron beam irradiation is not responsible for the chemical reactions on the ZnS phosphor surface. Copyright © 2001 John Wiley & Sons, Ltd.

Degradation of field emission display phosphors

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.

Electron beam-induced degradation of zinc sulfide-based phosphors

The changes in cathodoluminescent (CL) brightness and in the surface chemistry of SiO 2-coated and uncoated ZnS:Ag,Cl powder phosphor have been investigated using a scanning Auger electron spectrometer and an optical spectrometer. The data were collected in a stainless steel UHV chamber with residual gas pressures between 1×10 −8 and 1×10 −6 Torr. The primary electron current density was 272 μA cm −2, while the primary electron beam energy was varied between 2 and 5 keV. In the presence of a 2 keV primary electron beam in 1×10 −6 Torr of water, the amounts of C and S on the surface decreased, that of O increased, and the CL intensity decreased with electron dose. In a vacuum of 1×10 −8 Torr dominated by hydrogen and with a low water content, there was a small increase in the S signal, no rise in the O Auger signal, but the CL intensity still decreased. The model of electron-stimulated surface chemical reactions developed to explain degradation in residual vacuum gases high in water was postulated to apply also to residual vacuum gas dominated by hydrogen. In both cases, the model is based on the postulate that the primary and secondary electrons dissociate physisorbed molecules to form reactive atomic species. These atomic species remove S as volatile SO x or H 2S. In the case of an oxidizing ambient (i.e. high partial pressure of water) a non-luminescent ZnO layer is formed. In the case of a reducing ambient (i.e. low water and high hydrogen partial pressures) hydrogen removes S as H 2S, leaving elemental Zn which evaporates because of a high vapor pressure. Evaporation of Zn and degradation of ZnS is accelerated by elevated temperatures caused by electron beam heating. SEM images of the SiO 2-coated samples after degradation as well as reaction rate data suggest that SiO 2 acted as a catalyst for decomposition in hydrogen.

Degradation of ZnS FED phosphors

The degradation of cathodoluminescent brightness under prolonged electron beam excitation of phosphors has been identified as one of the outstanding critical issues for the flat-panel field emission industries. The ZnS : Cu,Al,Au phosphor powders have been subjected to electron beam bombardment of 2 keV with different electron beam current densities (2.5-88 mA cm -2 ) at an oxygen pressure of 1 × 10 -6 Torr. Auger electron spectroscopy (AES) and cathodoluminescence, both excited by the same electron beam, were used to monitor changes in surface composition and luminous efficiency during electron bombardment. Degradation was manifested by a non-luminescent ZnO layer that formed on the surface of the phosphor according to electron-stimulated surface chemical reaction (ESSCR). Lower current densities lead to a higher surface reaction rate, due to a lower local temperature beneath the beam, which resulted in more severe cathodoluminescence degradation. A lower temperature beneath the electron beam may lead to an increase in the surface reaction rate due to the longer time spent by the adsorbed molecules on the surface, with a direct increase in the ESSCR probability.