A comparative study between the simulated and measured cathodoluminescence generated in ZnS phosphor powder
In the past few decades cathode ray tubes (CRTs) have dominated the display market because of their excellent image quality, ease and economy of manufacture. However their bulky packaging and high power consumption make them unsuitable for portable electronic devices. Field emission displays (FEDs) show the most potential amongst all other types of flat panel displays (FPDs). These FEDs have several advantages over the FPD market, which is currently dominated by active matrix liquid crystal displays (AMLCDs) and plasma displays (PDPs). FEDs generate their own light by a process referred to as cathodoluminescence (CL) in which phosphor powders inside the screen are excited in a similar manner to those used in CRTs. However, in contrast to CRTs, the accelerating voltage of electrons in FEDs is lowered in order to reduce the bulky packaging and the power consumption. Electrons with the reduced accelerating voltage have a shallower penetration depth and therefore the surface condition of the phosphor powder is critical in order to ensure proper functioning of the display. During the prolonged exposure of the phosphors to an electron beam, the phosphor surface is oxidised to form a non-luminescent layer. This electron stimulated oxide formation is due a chemical reaction between the phosphor and the residual gases in the sealed vacuum, e.g. oxygen and water vapour. Since the CL is dependent upon the energy loss of electrons in the phosphors, the CL decreases with the growth of the oxide layer on the phosphor surface. For high acceleration voltages, this oxide layer has little effect on the brightness of the CL, but as the accelerating voltage decreases as for FEDs, the layer has a much more profound effect. The ZnS:Cu,Al,Au (P22G) is a standard green phosphor commonly found in CRTs. In this study the P22G phosphor powder was bombarded by an electron beam in an oxygen ambient, argon ambient and other mixture of gases. These mixtures consisted of varying concentrations of oxygen, carbon monoxide and argon gas. Auger electron spectroscopy (AES) and cathodoluminescence spectroscopy were used to monitor changes in surface composition and luminescent properties of the P22G phosphor during electron bombardment. When the P22G phosphor powder was exposed to an electron beam in water-rich oxygen gas, a chemically-limited ZnO layer was formed on the surface. The CL intensity generated from carbon free P22G phosphor decreased linearly with the thickness of the ZnO layer. The experimentally measured thickness of the ZnO layer agrees very well with the calculated value of the theoretical simulation. The theoretical simulation of electron trajectories into the ZnO/ZnS powders was based on a Monte Carlo simulation and the CL intensity was quantified from the electron energy loss profile generated during the simulation. According to the results of the simulation, the effect of a ZnO layer on the CL is minimised by the use of a high energy electron beam at a low incident angle. The electron exposure of P22G phosphor powder was also performed in dry oxygen gas. A layer of ZnSO4 was formed on the surface after electron exposure. The sulphate formation decayed exponentially with time and it is postulated that this was due to the diffusion of the charge reactants through the sulfate film to reaction interfaces. The P22G phosphor exposed to the electron beam in argon gas and gas mixtures degraded more slowly than in oxygen gas. Argon gas and carbon monoxide gas may suppress the degradation of the P22G phosphor powder.