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dc.contributor.advisorSwart, H. C.
dc.contributor.authorGreeff, Abraham Petrus
dc.date.accessioned2018-09-04T07:12:10Z
dc.date.available2018-09-04T07:12:10Z
dc.date.issued2001
dc.identifier.urihttp://hdl.handle.net/11660/9208
dc.description.abstractEnglish: 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.en_ZA
dc.description.abstractAfrikaans: Katodestraalbuise (Cathode ray tubes, CRTs) is tans 'n gewilde keuse wat vertooneenhede betref omrede die goeie beeldkwaliteit, asook die maklike en ekonomiese vervaardigingsproses. Ongelukkig is hierdie vertooneenhede groot en swaar en het dit 'n hoë energieverbruik wat dit ongeskik maak vir draagbare of handhoubare elektroniese toestelle. Indien die huidige uitbouing van die mark vir hierdie tipe toestelle in ag geneem word met die beloftes van toekomstige groei, sal 'n dun en ligte vertooneenheid 'n baie gesogte kommoditeit wees in die vertooneen heid mark. Daar is verskillende tipes van hierdie plat vertooneenhede tans beskikbaar, met die Aktiewe matriks vloeikristal (Active matrix liquid crystal display, AMLCD) eenhede die gewildste keuse vir draagbare of handhoubare toestelle. Een moontlike alternatief vir vloeikristal -vertooneenhede is die Veldemmissievertooneenheid (Field emission display, FED). Dit werk op dieselfde basiese beginsels as CRTs, maar in plaas van drie elektrongewere, het die FED 'n matriks van klein metaalpunte wat optree as elektronbronne. Hierdie matriks is geleë digby die agterkant van die fosforskerm. Hierdie uiters kompakte opstelling wek lig op deur 'n proses van katodeluminessensie (Cathodoluminescence, CL).Om die energieverbruik van FEDs te verminder, kan die versnelspanning van elektrone tussen die elektronbronne en die fosforskerm verminder word. Die laer versnelspanning skep egter probleme wat beeldkwaliteit en die leeftyd van die skerm betref. Huidiglik word konvensionele ZnS-tipe fosforpoeiers, dieselfde wat gebruik word in CRTs, gebruik om lig in FEDs op te wek. Tydens lang blootstelling aan elektronbombardering oksideer die fosforpoeier tot 'n nielumineserende ZnO lagie waar die oppervlak blootgestel word aan die elektronbundel. Die vorming van hierdie oksiedlagie is die gevolg van oppervlak chemiese reaksies tussen die ZnS fosforpoeier en waterdamp wat teenwoordig is in die ultra hoë vakuum omgewing. Die reaksie self word gestimuleer deur die elektronbundel. Die lae-energie elektrone in FEDs het 'n vlakker indringingsdiepte as dié wat gebruik word in CRTs. Aangesien die CL afhanklik is van die energieverlies van die elektrone in die fosforpoeier, neem die CL intensiteit af as gevolg van die groei van die ZnO lagie en die energieverlies binne-in die lagie. Dit lei dus tot' n afname in beeldkwaliteit en die leeftyd van die fosforskerm. In hierdie studie is die invloed van die ZnO lagie op die CL intensiteit bestudeer deur gebruik te maak van Monte Carlo simulasiemetodes. Die CL intensiteit kan gekwantifiseer word deur die proses van ligopwekking in drie afsonderlike stappe te beskou: die indringing van elektrone in die fosforpoeier, die energieverlies van die elektrone en dan die opwekking en absorpsie van fotone deur die fosformateriaal. Die fosforpoeier bestaan uit 'n distribusie van sferiese en plat korrels. As gevolg van die vorm van die sferiese korrels, asook die willekeurige oriëntasie van die plat korrels, varieër die dikte van die gevormde ZnO lagie met die invalshoek van die elektrone. In die eerste stap is 'n Monte Carlo metode gebruik om 'n invalshoekdistribusie te simuleer om sodoende die struktuur van die fosforpoeier in ag te neem. Die invalshoeke is gesimuleer deur die elektronpaaie te versprei oor 'n oppervlak wat gemodelleer is volgens die fosforpoeier se struktuur. Tweedens is die bane van die lae energie elektrone gesimuleer soos die elektrone die ZnO lagie indring en deurbeweeg na die ZnS fosformateriaal. Die simulasie is uitgevoer deur 'n gewone enkelverstrooiings Monte Carlo metode te gebruik, maar is verbeter deur 'n diffusie-intervlak te gebruik om die elektrone se energieverlies in die gebied tussen die ZnO en ZnS akkuraat te simuleer. Vanuit hierdie simulasies is energieverliesprofiele verkry vir spesifieke ZnO diktes, elektronbundelenergieë en diffusie-intervlakdiktes. Derdens is die elektronenergieverlies in ZnS bereken deur die energieverliesprofiele te gebruik en te aanvaar dat die diffusie-intervlak nie-Iumineserend is nie. Die energieverlies in ZnS lei tot die vorming van elektron-holte pare wat weer kombineer en moontlik kan lei tot fotonopwekking. 'n Uitdrukking is afgelei om die CL intensiteit te kwantifiseer. Die uitdrukking kompenseer vir die absorpsie van fotone deur die fosformateriaal en elimineer kwantummeganiese en optiese aspekte soos totale interne weerkaatsing deur middel van normalisering. Deur hierdie uitdrukking toe te pas op die elektronenergieverlies in ZnS kan 'n kromme verkry word wat die CL intensiteit as funksie van ZnO dikte weergee vir 'n spesifieke bundelenergie. In hierdie studie is die kwantifiseringsuitdrukking toegepas op die eksperimentele resultate van twee tipes fosforpoeiers. Die ZnS:Cu,AI,Au poeier word gebruik om groen lig op te wek, terwyl die ZnS:Ag,CI poeier gebruik word om blou lig op te wek. Vir ZnS:Cu,AI,Au vergelyk die gesimuleerde ZnO dikte baie goed met eksperimentele gemete waardes vir die oksieddikte. lndien dieselfde simulasieparameters gebruik word vir ZnS:Ag,CI is die eksperimentele gemete oksieddikte baie dunner as die voorspelde waarde. Hierdie verskil kan toegeskryf word aan die versameling van lading oor die gebied van die primêre elektrone tydens elektronbombardering. Dit verlaag die tempo van oksiedgroei asook die waarskynlikheid van elektron-holte pare herkom binasie.en_ZA
dc.language.isoenen_ZA
dc.publisherUniversity of the Free Stateen_ZA
dc.subjectZnS phosphor powderen_ZA
dc.subjectCathodoluminescence degradationen_ZA
dc.subjectMonte Carlo simulationen_ZA
dc.subjectCL quantificationen_ZA
dc.subjectField emission displaysen_ZA
dc.subjectElectron beam irradiationen_ZA
dc.subjectMonte Carlo Methoden_ZA
dc.subjectCathodoluminescenceen_ZA
dc.subjectPhosphorsen_ZA
dc.subjectThesis (Ph.D. (Physics))--University of the Free State, 2001en_ZA
dc.titleA Monte Carlo simulation of the effect of a ZnO layer on the cathodoluminescence generated in a ZnS phosphor powderen_ZA
dc.typeThesisen_ZA
dc.rights.holderUniversity of the Free Stateen_ZA


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