| Changes in the cathodoluminescent (CL) brightness, surface chemistry and morphology of SiO2-coated and uncoated ZnS:Ag,Cl powder, and ZnS:Mn thin films have been investigated using scanning Auger electron spectrometry (AES), optical spectrometry and scanning electron microscopy (SEM). Water partial pressure, surface coatings and temperature were controlled as parameters critical to degradation. In the case of an oxidizing ambient (i.e., high water partial pressure) a non-luminescent oxide layer formed on all materials. In the case of a reducing ambient (i.e., low water and high hydrogen) hydrogen removes S as H2S, leaving elemental Zn which evaporates due to its high vapor pressure.;In the case of SiO2-coated ZnS:Ag,Cl, morphological changes were observed on the surface after extended electron beam exposure. This erosion of ZnS is more rapid at higher power densities. Uncoated ZnS:Ag,Cl phosphors exhibited no change in the surface morphology. The SEM images of SiO-coated samples after degradation and reaction rate data suggest that the SiO 2 particles acted as a catalyst for CL and morphological degradation.;Temperature effects on degradation were isolated by measuring thermal quenching behavior. Upon heating, the CL intensity decreased, the spectral shape changed and shifted to higher wavelengths. This shift was attributed to band gap narrowing and copper contamination. Full recovery of the CL occurred upon cooling. Evaporation of Zn and degradation of ZnS is accelerated by elevated temperatures caused by electron beam heating. CL loss at high temperatures with constant electron beam exposure was less than that at RT for the same coulombic dose. This supports the predictions of the ESSCR model that elevated temperatures causes a decrease in the residence time of physisorbed species, reducing the rate of surface reactions.;After elevated temperature degradation, morphological erosion occurred for uncoated ZnS. At the combined temperature due to electron beam heating and external sample heating, temperatures up to 300°C were calculated. At this temperature, Zn has a high vapor pressure and the removal rate of Zn increases leading to the observed surface erosion. |
