Luminescence of thin single-crystal rock salt films in strong electric fields

Abstract

The possibility of electrotuminescence in ionic crystals has been considered by several authors, for example, [1, 2] and others. The discovery of the electrical strengthening of solid dielectrics [2-4] provides a basis for the assumption that ionization and excitation and therefore electroluminescence events occur in thin films. Very thin monolayers of rock salt break down in electric fields of approximately 108 V/cm [4], while in the fields with intensities just below the breakdown value (E ~ 107 V/cm) a number of new phenomena can be observed [8], in particular, the luminescence of the salt fi lm [8, 6, 7]. This luminescence can be produced only in monolayers of NaC1 less than 1 .2to 1. g -G thick. The monolayers were prepared in accordance with the method described in [8]. Figure 1 shows a photomicrograph of a luminescing natural rock salt specimen (thickness of the monocrystalline layer 1 g). An electrolyte (saturated solution of NaC1 in butyl alcohol) was used to obtain electric contact with the salt f i lm. In the photomicrograph (Fig. 1) 1 is the luminescing salt layer, and 2 are depressions filled with electrolyte. Figure 2 shows oscillograms of the brightness waves (I) of the luminescence in a specimen to which alternating and pulsed voltage is applied (II). The phase of the brightness waves leads to the conclusion, that the luminescence occurs only at the instant when the layer is subjected to a strong electric field, and is not related to the change in voltage polarity. The pulse width (Fig. 2b) is 47 ~sec. and the amplitude Up = 1.68 kv. It follows from Fig. 2b, that the persistence does not exceed 1 gsec. It is apparent from Fig. 2b that the brightness wave consists of very short, discrete flashes of the order of several microseconds in duraction. The individual flashes can be clearly seen on the oscillograms obtained when the film is excited by a relatively low-amplitude square pulse [7]. This obviously indicates the instability of the processes leading to luminescence. The literature cites instances [9] when the luminescence occurring in the presence of an electric field was caused by ultraviolet irradiation of the specimen due to microdischarges in gaseous inclusions. We were unable to produce luminescence in thin films of NaC1 by exposing them to UV radiation (PRK-4 source with a filter). Butyl aicohoI exposed to UV radiation luminesces very weakly, and the color of the luminescence is violet. It is known that NaC1 crystals with an Ni impurity introduced by electro-thermodiffusion generate bright orange-red luminescence when irradiated with IJV (k =365 my) [10]. We prepared thin singlecrystal fiIms from a natural crystal with Ni impurity introduced using the technique described in [10]. However, the orange-red band characteristic of Ni was not observed when such layers were subjected to an electric field. The use of a photoluminescent electrolyte (saturated NaC1 solution and a small quantity of the dye phodamine 6Y in butyl alcohol) also did not indicate the presence of UV radiation. The luminescence spectrum, measured by means of an SF-4 monochromator did not contain a UV component. In our experiments when the potential was increased, the luminescence usually occurred in one or more regions of the film, but most often in the center (observation was under a microscope through the face of the film (Fig. 1)). However, following a further potential increase of several percent, the luminescent region spread rapidly to include the whole of the dielectric layer, while the intensity of luminescence rapidly increased in response to the increasing potential. Figure 3 shows a plot of the average light flux from the layer in arbitrary units as a function of the field intensity in the specimen. The frequency of the applied voltage was 4 kHz. The character of this Fig. 1 Photomicrograph of thinfi lm NaC1 luminescence.