A Monte Carlo simulation of the effect of a ZnO layer on the cathodoluminescence generated in a ZnS phosphor powder

English: Today Cathode ray tubes (CRTs) are the standard in display technology due to their good image quality, ease of manufacturing and economy. Unfortunately, these displays are bulky and have a high power consumption making it unsuitable for portable or hand held electronic devices. With the current market expansion of these devices and the prospects claimed by future projections, a thin lightweight display with low power consumption and excellent image quality will be a very sought after commodity in the display market. There are various types of flat panel displays on ofFer, with the Active matrix liquid crystal display (AM LCD) the most popular choice for portable or hand held devices. One possible alternative to liquid crystal displays are Field emission displays (FEDs). It works on a similar principle as an ordinary CRT, but instead of three electrons guns it has an array of tiny metallic tips acting as electron emitters. They are situated in close proximity at the back of the phosphor screen. This extremely compact setup produces light by a process of cathodoluminescence (CL). To lower the power consumption of FEDs, the accelerating voltage of electrons between the emitters and phosphor screen can be reduced. The lower acceleration voltage results in some difFiculties concerning image quality and the lifetime of the phosphor screen. Currently conventional ZnS-based phosphor powders, the same used in CRTs, are used to generate light in FEDs. During prolonged exposure to the electron beam the phosphor powder oxidizes to a non-luminescent ZnO layer where the surface is irradiated by the electron beam. The formation of this oxide layer is due to surface chemical reactions between the ZnS phosphor and water vapor which is present in the ultra high vacuum environment. The reaction itself is stimulated by the electron beam. The low energy electrons in FEDs have a shallower penetration depth than those used in CRTs. Since the CL is dependent upon the energy loss in the phosphor powder, the CL decreases due to the growth of the ZnO layer and the energy loss inside the layer. This leads to a decrease in the image quality and lifetime of the screen. In this study the influence of the ZnO layer on the CL intensity was investigated using Monte Carlo simulation methods. The CL intensity can be quantified by separating the light generation process into three steps: the penetration of the electrons into the powder, the energy loss of the electrons and the generation and absorption of photons by the phosphor material. The phosphor powder consists of a distribution of spherical and flat grains. Due to the shape of the spherical grains as well as the random orientation of the flat grains, the thickness ofthe oxide layer varies with the incident angle of the electron beam. In the first step a Monte Carlo method was used to simulate a distribution for the incident angles to take into account the structure of the phosphor powder. The incident angles were simulated by spreading the electron paths over a surface modeled according to the structure of the phosphor powder. Secondly, the trajectories of the low energy electrons were simulated as it penetrated the ZnO layer and moved into the ZnS phosphor material. The simulation was performed using an ordinary single scattering Monte Carlo method, but was improved by using a diffusion interface to accurately simulate the energy loss of electron in the interface region between ZnO and ZnS. From these simulations energy loss profiles were obtained for specific ZnO thicknesses, electron beam energies and diffusion interface thicknesses. Thirdly, the electron energy loss in the ZnS was calculated by using the energy loss profiles and assuming that the diffusion interface was non-luminescent. The energy loss in ZnS leads to creation of electronhole pairs that may recombine radiatively and generate photons. An expression was derived to quantify the generated CL. The expression compensates for the absorption of photons by the phosphor material and eliminates quantum mechanical and other optical aspects like total internal reflection by normalization. Applying the quantification expression to the electron energy loss in ZnS a curve relating the CL intensity to the ZnO thickness for a specific beam energy was determined. VI In this study the quantification expression was applied to the experimental results of two types of phosphor powders. The ZnS:Cu,AI,Au powder is used to generate green light, while the ZnS:Ag,CI powder is used for blue light. For ZnS:Cu,AI,Au the predicted ZnO thickness compare extremely well with experimental measurements. However, using the same simulation parameters, the experimentally measured oxide thickness on ZnS:Ag,CI is much thinner than the predicted value. This difference can be attributed to the trapping of charge over the range of the primary electrons during electron irradiation. This lowers the rate of oxide formation as well as the probability of electron-hole pair recombination.