Quantifying the cathodoluminescence generated in ZnS-based phosphor powders

Abstract

Field emission displays (FEDs) are a possible alternative to other display technologies. They work similar to cathode ray tubes, but instead of an electron gun they have an array of tiny metallic tips acting as electron emitters. This array is situated in close proximity to the phosphor screen and produces light by a process of cathodoluminescence (CL). During prolonged electron beam irradiation a non-luminescent ZnO layer forms on the irradiated surface of the ZnS phosphor powder. This is due to electron-beam-stimulated surface chemical reactions between the ZnS phosphor and water vapour present in the ultrahigh vacuum environment. The low-energy electrons used in FEDs have a shallow penetration depth and, because the CL is dependent upon the energy loss in the ZnS bulk itself, growth of the ZnO layer significantly degrades the CL intensity. Energy loss profiles in the ZnS for different ZnO thicknesses were determined using Monte-Carlo electron trajectory simulations. These profiles, along with photon absorption profiles, were used to determine a curve relating the normalized CL intensity to the ZnO thickness. It was compared with experimentally measured values obtained during CL degradation experiments on standard ZnS : Cu,Al,Au phosphor powders and ZnO thickness measurements with sputter depth profiling. Assuming the presence of a 20 nm thick diffusion interface between the ZnO layer and the ZnS bulk, the theoretical and experimental results agree well. Initially the interface represents the initial stages of oxidation, but after the oxide layer starts to grow as a uniform film it represents a physical interface formed due to diffusion processes. Copyright © 2002 John Wiley & Sons, Ltd.