Electroluminescence of Alkali-Halide Crystals at Temperatures Exceeding Room Temperature

Abstract

To study the influence of elevated temperatures on the intensity of electroluminescence (EL) of alkali-halide crystals (AHC), we measured EL of nonactivated AHC and of AHC activated with europium from room temperature (20°C) to temperatures of the order of 100°C. The maximum temperature of our measurements was determined by the properties of the electrolytic electrodes (in particular, by their boiling temperature) used in the experiments. The electric field was applied to a thin dielectric layer (with a thickness of a few micrometers) placed inside a massive sample [1], by means of an electrolyte on the basis of butanol [2]. The solid-state electrodes (including metal ones) do not allow stable EL of AHC to be produced due to the electric field inhomogeneities arising on the sample surface that are cased by microtips presented on the electrodes. The electrolytic electrodes have high resistivity permitting field amplification to be decreased near microtips and the increase of local currents to be limited [3]. Among the disadvantages of the electrolytic electrodes are a sharp increase of their resistivity with the decreasing temperature and fast evaporation from the pits in the sample at high temperatures. Luminescence was excited with a 100-μs high-voltage pulse. The current pulse ran through the sample and then was fed to one of the channels of a storage oscillograph. Observations of luminescence and current running through the sample allowed us to control these parameters and to terminate measurements for samples in which electric breakdown occurred. The sample temperature was changed as follows. In the conductor, the AHC sample was connected to a copper rod used as a heat conductor with an adjustable source of heating. The period of leveling temperatures between the heat conductor and the sample layer adjacent to it lasted several minutes. The error in measuring the sample temperature in the given setup did not exceed one degree. Figure 1 shows the dependences of the emission intensity on the conduction current at the indicated temperatures. The AHC EL quantum yield can be judged from the slope of the curve to the abscissa [3]. As can be seen from the figure, the quantum yield of nonactivated EL decreases as the temperature increases. Generally, the decrease of the EL quantum yield can be caused by temperature quenching of emission centers, that is, by the increased probability of nonradiative transitions of the centers from the exited to the ground state, with changes under conditions of color center excitation, or with the decreased concentration of the emission centers at temperatures exceeding a certain critical level. The last mechanism is confirmed by the data on the increased probability of creation of dynamic bivacancy formations with the increasing temperature presented in [4]. Our experiments were conducted both for direct (heating) and inverse (cooling) modes and yielded dependences that coincided within 10%. For activated crystals we obtained that at temperatures up to the temperature of electrolyte boiling (118°C), the EL quantum yield was independent of the temperature. The results obtained confirm once again the principal difference between the structure of the native and impurity emission centers. In addition, as demonstrated experimentally, the degradation processes in the AHC layer are accelerated at temperatures exceeding room temperature. While at low temperatures (below –20°C) EL of the AHC layer could last several hours [5], at room temperature the samples withstand ~100 pulses before the breakdown, and at higher temperatures