Zinc sulfide activated with copper or silver and coactivated with gallium or indium shows two emission bands. The shorter wave-length band does not involve the ground state of the coactivator or donor, whereas the longer wavelength band does. Both bands involve the ground state of the activator or acceptor. The factors contributing to the emission intensities from the various associated donor-acceptor pairs are discussed theoretically. The dependences on temperature and on concentrations of activator and coactivator are in accord with the longer wave-length band involving a highly associated donoracceptor pair. In addition, the energy levels of the donors, as obtained from thermoluminescent data, are correlated on the basis of the model with the differences in the transition energies of the two emission bands of these phosphors.Zinc sulfide phosphors coactivated with Group III elements of the Periodic Table have been reported to have a long wave-length emission band in addition to the short wave-length emission band associated with activation by Group I elements and with self-activation. KrSger and Dikhoff (1), for example, reported long wave-length emission in phosphors coactivated with Sc, Ga, or In. The emission bands peaked at 5250A with Sc, 5700A with Ga, and 6800A with In and were relatively independent of the identity of the activator. They also observed that the substitution of ZnS in part by CdS had less effect on the long wave-length emission spectra than on the short wave-length emission spectra. Klasens (2) attributed these long wave-length bands to the radiative recombination of a free hole with an electron trapped at the coactivator. Froelich (3) reported long wave-length emission for Cu and self-activated ZnS phosphors which were coactivated with quite high concentrations of A1. Emission peaks are at 5880 and 5000A, respectively. Apple (4) has reported that the crystalline structure influences the relative intensities of the two emission bands observed with ZnS: x(Cu, Ag, Au), y(Ga, In) phosphors in which x K y. The long wave-length emission was found to be enhanced in hexagonal ZnS. He has suggested that this effect may be related to a change in association of activator and coactivator.In this investigation a comprehensive study of ZnS:x(Cu or Ag), y(Ga or In) was made. The spectra of the long wave-length emission are found to depend on the identities of both activator and coactivator. The relative intensities of the long and short wave-length emission depend on concentrations of activator and coactivator and on temperature of excitation. These experimental results are explained by means of a model for the long wavelength emission center consisting of a highly associated activator-coactivator center. This center is similar to the associated donor-acceptor system proposed by Prener and Williams (5) to explain the short wave-length emission by radiative transitions from excited states of the donor to the ground state of the acceptor but is different in that the donor and acceptor responsible for the...

No abstract

No abstract

No abstract

No abstract