ON THE ELECTROLUMINESCENCE OF KJ–Cu CRYSTALS

R. V. GUSAROVA, V. A. SOKOLOV

From the literature there is as yet only one known case in which electroluminescence of alkali-halide compounds has been observed: this was done in the work of A. N. Georgobiani and N. P. Golubeva, who obtained luminescence in an electric field from a thin sublimated film of cesium iodide activated with thallium [1].

The difficulty of exciting electroluminescence in alkali-halide crystals, which possess a considerable fraction of ionic bonding, is due to the following reasons: 1) the large width of the forbidden band; 2) the short mean free path of electrons and holes before scattering by thermal vibrations of the lattice; 3) the comparatively small number of free current carriers, owing to which, in ionic crystals, as compared with crystals possessing covalent bonding, local regions of electric-field concentration are formed with greater difficulty ($p$–$n$ junctions, depletion barriers, etc.).

The difficulties listed above may be circumvented by exciting alkali-halide compounds with an electric field in thin layers of the crystal. In this case there occurs the effect of electrical strengthening predicted by A. F. Ioffe [2], which makes it possible to obtain considerable field strengths in a thin layer; this field can cause direct ionization of luminescence centers, i.e., lead to the Zener mechanism of electroluminescence excitation [3].

Apparently, it was precisely this effect that occurred in the work of A. N. Georgobiani and N. P. Golubeva: they succeeded in obtaining a sublimated layer of cesium iodide of micron thickness, in which the development of avalanche breakdown was impeded, as a result of which the film gave electroluminescence without breakdown in a field of $\sim 2 \cdot 10^{6}$ V/cm.

We attempted to obtain electroluminescence of alkali-halide compounds not in a sublimated layer, but in a thin single-crystal layer; for this purpose we used the method developed by V. A. Kostrygin for obtaining a thin layer by counter-drilling a crystal under a microscope [4]. Several different alkali-halide compounds were tested, as a result of which it was possible to detect

traces of luminescence under the action of a pulsed electric field in a thin layer of potassium iodide activated with copper. However, the very small area of the thin luminous layer obtained in these experiments (the cross section of the drill hole was less than \(0.3\ \mathrm{mm}^2\)) did not permit an experimental study of the character of the luminescence that arose.

Fig. 1.

But it turned out that potassium iodide crystals that give luminescence under these conditions also glow in powder form in an ordinary electroluminescent cell used for observing the luminescence of powdered zinc-sulfide phosphors—a capacitor, one of whose plates is conducting glass.

Fig. 2.

A finely dispersed KJ—Cu powder was placed by us in such a capacitor, suspended in castor oil. In addition, a thin dielectric spacer of mica or cellophane was introduced into the capacitor. When this alkali-halide compound was excited by an alternating field, we recorded the brightness wave (Fig. 1), and also investigated the dependence of the luminescence intensity of the powder on the frequency and field strength of the exciting field (the voltage was supplied through a step-up transformer from a ZG-10 audio generator). It was established that the luminescence intensity of the powder depends only weakly on the frequency (Fig. 2), but depends strongly on the strength of the applied field (Fig. 3), especially in the case of a dielectric spacer made of cellophane, when a sharp increase in brightness occurs with increasing field strength*). An increase in brightness also occurs when the cell is heated to \(50\)—\(60^\circ\mathrm{C}\).

With regard to the nature of the observed luminescence of KJ—Cu in an alternating field, the following assumption may be made. It is possible that in small grains (\(d \approx 3\)—\(5\ \mu\)) of the ground crystal, located in the thin capacitor, the same effects of electrical hardening arise as in thin layers, as a result of which conditions are created for ionization of luminescence centers by the field through the Zener mechanism, which is enhanced when high-field regions are localized at microdefects of the crystallites. At the same time, intensive development of prebreakdown processes of impact ionization is also possible.

*) In Figs. 2 and 3, curves 1 are given for the case of a cellophane spacer; curves 2, for mica.

The luminescence of KJ—Cu powder in a constant field is observed only when a spacer made of a “poor” dielectric (cellophane) is used; this is apparently associated

Fig. 3.

therefore with the presence of a through current and evidently has the same nature as the electroluminescence in a constant field of zinc-sulfide luminophores activated with manganese [5, 6].

LITERATURE

1. A. N. Georgobiani and N. P. Golubeva. Opt. i spektr., 12, 802, 1962.
2. A. F. Ioffe. Physics of Crystals, GIZ, 1929.
3. A. N. Georgobiani. Proceedings of the Physics Institute of the Academy of Sciences of the USSR, 23, 4, 1963.
4. V. A. Kostrygin. FTT, 2, 1841, 1960.
5. V. N. Favorin, G. S. Kozina, L. K. Tikhonova. Opt. i spektr., 7, 703, 1959.
6. V. N. Favorin, L. P. Poskacheva. Opt. i spektr., 7, 706, 1959.