Electron traps and dielectric changes in phosphorescent solids

Abstract

Some luminescent solids, such as zinc sulphide with a specific impurity, exhibit considerable increases in their dielectric constant and loss when excited in the presence of a small electric field by ultra-violet light or other suitable radiation. The effect is found to be confined to those phosphors which show photoconductivity associated with the luminescence process. The total change in dielectric constant of such phosphors may increase by as much as 75 % of the normal value (about 8) for quite moderate intensities of excitation. The loss increase, expressed as the change in equivalent a.c. conductance, may change from less than 10 -8 to 10 -5 ohm -1 during excitation. Experimental investigations and theoretical considerations show that the dielectric changes occur when electron traps are filled by electrons. The relatively low binding energy of the trapped electron to its trap (from 0.1 to 0.7 eV in most zinc sulphide phosphors) permits large displacements of the electron from its mean position when an external field is applied. This gives rise to the increase in dielectric constant. Increase of dielectric loss of the phosphor is associated with the existence of a finite relaxation time of the filled electron traps which causes phase differences between the variation of the applied alternating field and the movement of the trapped electrons. This gives rise to energy absorption from the applied field by the filled traps. It is found from experiment that electrons moving in the conduction energy levels of the phosphor crystal make a negligible contribution to the dielectric effects when compared with the contribution from trapped electrons. The dielectric changes are found to increase rapidly with temperature following an exponential variation. They are also dependent on the applied field frequency over the frequency range comparable in order to the reciprocal of the relaxation time of filled electron traps which is found to be of the order of 10 7 sec. -1 . The relaxation time does not vary rapidly with the temperature. The dielectric changes show small variation with the wave-length of the exciting light between 0.35 and 0.41μ for most zinc sulphide phosphors, but decrease rapidly as the wave-length decreases to 0.25μ, due to a rapid rise in the absorption coefficient of the phosphors. The long wave-length limit for production of the dielectric changes corresponds to the wave-length limit for photoconductivity and for the filling of electron traps and is dependent upon the constitution and structure of the particular specimen. The electron-trap theory of phosphorescence established by previous workers enables many of the dielectric effects to be correlated with the luminescence processes in phosphors. The studies confirm the results of previous workers and enable their observations on the dielectric changes to be given a theoretical interpretation.