Electron beam-induced degradation of zinc sulfide-based phosphors

The changes in cathodoluminescent (CL) brightness and in the surface chemistry of SiO2-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 SOx or H2S. 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 H2S, 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 SiO2-coated samples after degradation as well as reaction rate data suggest that SiO2 acted as a catalyst for decomposition in hydrogen.