Abstract
Luminescent properties of commercial ZnS(Cu) electroluminescent powder are investigated under different excitation conditions: photo-excitation with 254mμ and 365mμ U. V. light and electrical excitation with sine wave voltage. The differences in luminescent properties, especially the difference in intensity ratio of blue to green bands and in the temperature dependence of the ratio, caused by these different excitation conditions are explained by the excitation mechanism in each case. The spectral distribution by 254mμ light excitation is typical of host excitation: photons are absorbed in the host lattice and free electron-hole pairs are produced. On the other hand, 365mμ light excites directly only the blue centers. Temperature dependence of the spectral distribution in the latter case shows the existence of some thermal process in which the energy is transferred from excited blue center to green center. However, this process does not seem to be the simple hole migration process.
Under A. C. voltage excitation, electroluminescence spectrum is characterized by the frequency and decay time of the emission. Decay time of blue center (about 0.42 msec.) estimeted from frequency dependence of the emission agrees fairly well with that measured by photo-excitation.
Although this effect may explain the electroluminescence spectrum at low temperatures, interaction between the centers mentioned above for 365mμ excitation must be taken into consideration for the electroluminescence at room temperature or thereabout. Concerning this mechanism, details are not yet known.
