Degradation of ZnS:Cu,Au,Al phosphor powder and thin films under prolonged electron bombardment

[摘要] Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by thesame electron beam, were used to monitor changes in surface composition and luminousefficiency during electron bombardment. ZnS:Cu,Al,Au phosphor powders and thin filmswere subjected to prolonged electron beam bombardment of varying beam energies anddifferent electron beam current densities in two different (O2 and CO2) vacuum gasambients. The thin film phosphors were grown on Si (100) substrates by using XeCl(308nm) pulsed laser deposition (PLD) method. X-ray diffraction (XRD) measurementsrevealed that ZnS (100) films were preferentially grown on a Si (100) substrate. The RBSresults show that the growth rate, increased with an increase of the N2 pressure in thedeposition chamber during deposition.Degradation on both the powder and the thin film phosphors was manifested by a nonluminescentZnO layer that formed on the surface of the phosphor according to theelectron stimulated surface chemical reactions (ESSCR) mechanism.Lower current densities lead to a higher surface reaction rate, due to a lower localtemperature beneath the beam, which resulted into a more severe CL degradation. A lowertemperature beneath the electron beam may lead to an increase in the surface reaction ratedue to the longer time spent by the adsorbed molecules on the surface, with a directincrease in the ESSCR probability. Low current densities would also lead to surfacecharging due to a lower electron conductivity of the phosphor resulting in an increase inthe CL degradation rate due to band-bending.In the studies conducted between room temperature and 310 oC, an increase in thetemperature led to a decrease in the surface reaction rate due to a decrease in the meansurface lifetime of the oxygen molecules on the surface, with a direct decrease in theESSCR probability. Without the presence of the electron beam no chemical reactions, upto 310 oC, occurred on the surface. Therefore, local heating due to the electron beamirradiation is not responsible for the chemical reactions on the ZnS phosphor surface. At-125 °C the degradation was controlled by the residual small amount of water vapour inthe system that is frozen at this low temperature. The thermoluminescence (TL) curves ofthe phosphor powder before and after degradation showed the influence of the Osubstitutional atoms that are created during electron bombardment in an O2 ambient. TheO substitutional atoms acted as electron traps.On the electron beam bombardment of thin film phosphors, the degradation was moresevere under O2 ambient compared to the same partial pressure of CO2 during electronbeam bombardment, which is attributed to the free energy of formation of ZnO from ZnSwhen these respective gases are used. The degradation rate also depended on the energyof the electron beam, decreasing with increasing beam energy. This was interpretedaccording to the ionisation energy cross-section profile. The CL brightness increased exponentially with the increasing energy beam as more free carriers that will subsequentlyrecombine yielding CL, are excited at higher beam energies.The thin film phosphor was also subjected to the electron beam bombardment after thephosphor film was coated with a CdO film by using a chemical bath deposition (CBD)method. The surface reactions were electron beam stimulated, resulting in the desorptionof both Cd and S from the surface which happened as soon as the surface adventitious Cwas depleted. Sulphur from the ZnS accumulated on the surface but was soon depleted asvolatile SOx compounds. The CdO was reduced by an electron beam assisted mechanismin the presence of non-reducible ZnO in the CdO-ZnO system as the Zn from theunderlying ZnS layer emerged to the surface. The CL intensity degradation of the coatedfilm showed a dependence on the surface composition. The intensity remained constantuntil the Cd was reduced on the surface before a slight decrease was observed. The effectof the CdO capping layer on the intensity of the phosphor was evident until the CdOeventually disintegrated.

[发布机构] University of the Free State