A comparative study between the simulated and measured cathodoluminescence generated in ZnS phosphor powder

[摘要] In the past few decades cathode ray tubes (CRTs) have dominated the display marketbecause of their excellent image quality, ease and economy of manufacture. Howevertheir bulky packaging and high power consumption make them unsuitable for portableelectronic devices.Field emission displays (FEDs) show the most potential amongst all other types of flatpanel displays (FPDs). These FEDs have several advantages over the FPD market,which is currently dominated by active matrix liquid crystal displays (AMLCDs) andplasma displays (PDPs). FEDs generate their own light by a process referred to ascathodoluminescence (CL) in which phosphor powders inside the screen are excitedin a similar manner to those used in CRTs. However, in contrast to CRTs, theaccelerating voltage of electrons in FEDs is lowered in order to reduce the bulkypackaging and the power consumption. Electrons with the reduced acceleratingvoltage have a shallower penetration depth and therefore the surface condition of thephosphor powder is critical in order to ensure proper functioning of the display.During the prolonged exposure of the phosphors to an electron beam, the phosphorsurface is oxidised to form a non-luminescent layer. This electron stimulated oxideformation is due a chemical reaction between the phosphor and the residual gases inthe sealed vacuum, e.g. oxygen and water vapour. Since the CL is dependent upon theenergy loss of electrons in the phosphors, the CL decreases with the growth of theoxide layer on the phosphor surface. For high acceleration voltages, this oxide layerhas little effect on the brightness of the CL, but as the accelerating voltage decreasesas 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. Inthis study the P22G phosphor powder was bombarded by an electron beam in anoxygen ambient, argon ambient and other mixture of gases. These mixtures consistedof varying concentrations of oxygen, carbon monoxide and argon gas. Auger electron spectroscopy (AES) and cathodoluminescence spectroscopy were used to monitorchanges in surface composition and luminescent properties of the P22G phosphorduring electron bombardment.When the P22G phosphor powder was exposed to an electron beam in water-richoxygen gas, a chemically-limited ZnO layer was formed on the surface. The CLintensity generated from carbon free P22G phosphor decreased linearly with thethickness of the ZnO layer. The experimentally measured thickness of the ZnO layeragrees very well with the calculated value of the theoretical simulation. Thetheoretical simulation of electron trajectories into the ZnO/ZnS powders was based ona Monte Carlo simulation and the CL intensity was quantified from the electronenergy loss profile generated during the simulation. According to the results of thesimulation, the effect of a ZnO layer on the CL is minimised by the use of a highenergy electron beam at a low incident angle.The electron exposure of P22G phosphor powder was also performed in dry oxygengas. A layer of ZnSO4 was formed on the surface after electron exposure. The sulphateformation decayed exponentially with time and it is postulated that this was due to thediffusion of the charge reactants through the sulfate film to reaction interfaces. TheP22G phosphor exposed to the electron beam in argon gas and gas mixtures degradedmore slowly than in oxygen gas. Argon gas and carbon monoxide gas may suppressthe degradation of the P22G phosphor powder.

[发布机构] University of the Free State