Degradation of ZnS:Cu,Au,Al phosphor powder and thin films under prolonged electron bombardment
Hillie, Kenneth Thembela
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Auger electron spectroscopy (AES) and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in surface composition and luminous efficiency during electron bombardment. ZnS:Cu,Al,Au phosphor powders and thin films were subjected to prolonged electron beam bombardment of varying beam energies and different electron beam current densities in two different (O2 and CO2) vacuum gas ambients. The thin film phosphors were grown on Si (100) substrates by using XeCl (308nm) pulsed laser deposition (PLD) method. X-ray diffraction (XRD) measurements revealed that ZnS (100) films were preferentially grown on a Si (100) substrate. The RBS results show that the growth rate, increased with an increase of the N2 pressure in the deposition chamber during deposition. Degradation on both the powder and the thin film phosphors was manifested by a nonluminescent ZnO layer that formed on the surface of the phosphor according to the electron stimulated surface chemical reactions (ESSCR) mechanism. Lower current densities lead to a higher surface reaction rate, due to a lower local temperature beneath the beam, which resulted into a more severe CL degradation. A lower temperature beneath the electron beam may lead to an increase in the surface reaction rate due to the longer time spent by the adsorbed molecules on the surface, with a direct increase in the ESSCR probability. Low current densities would also lead to surface charging due to a lower electron conductivity of the phosphor resulting in an increase in the CL degradation rate due to band-bending. In the studies conducted between room temperature and 310 oC, an increase in the temperature led to a decrease in the surface reaction rate due to a decrease in the mean surface lifetime of the oxygen molecules on the surface, with a direct decrease in the ESSCR probability. Without the presence of the electron beam no chemical reactions, up to 310 oC, occurred on the surface. Therefore, local heating due to the electron beam irradiation 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 in the system that is frozen at this low temperature. The thermoluminescence (TL) curves of the phosphor powder before and after degradation showed the influence of the O substitutional atoms that are created during electron bombardment in an O2 ambient. The O substitutional atoms acted as electron traps. On the electron beam bombardment of thin film phosphors, the degradation was more severe under O2 ambient compared to the same partial pressure of CO2 during electron beam bombardment, which is attributed to the free energy of formation of ZnO from ZnS when these respective gases are used. The degradation rate also depended on the energy of the electron beam, decreasing with increasing beam energy. This was interpreted according to the ionisation energy cross-section profile. The CL brightness increased exponentially with the increasing energy beam as more free carriers that will subsequently recombine yielding CL, are excited at higher beam energies. The thin film phosphor was also subjected to the electron beam bombardment after the phosphor 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 desorption of both Cd and S from the surface which happened as soon as the surface adventitious C was depleted. Sulphur from the ZnS accumulated on the surface but was soon depleted as volatile SOx compounds. The CdO was reduced by an electron beam assisted mechanism in the presence of non-reducible ZnO in the CdO-ZnO system as the Zn from the underlying ZnS layer emerged to the surface. The CL intensity degradation of the coated film showed a dependence on the surface composition. The intensity remained constant until the Cd was reduced on the surface before a slight decrease was observed. The effect of the CdO capping layer on the intensity of the phosphor was evident until the CdO eventually disintegrated.