A systematic investigation of the electroluminescent (EL) ZnS thin film devices doped with rare earth centers, SmF3 (red), TbF3 (green), and TmF3 (blue), is made to examine the possibility of multicoloring. Various characteristics, such as brightness, emission efficiency, decay time, EL spectra, and their concentration dependences are measured. The number of photons emitted from these devices is also estimated to discuss quantum efficiencies. For SmF3 and TmF3 activators, it is found that concentration quenching of EL emission takes place through non‐radiative crossrelaxation process. No such quenching is observed for TbF3 activators. In TmF3 activators, most of the radiative energy is found to be emitted in the infrared region. The impact excitation cross‐section for these rare earth compounds is also inferred to be lower compared with that of Mn2+ ions.
A generalized analytical method to determine the density of energy states of electron emission source (EES) is devised by using a thermal excitation and emission model for an exoelectron in the MgO layer and the emission time constants of the exoelectron extracted from experimental stochastic distributions of discharge delay time. When applied to Si-doped MgO, the emission time constant of the exoelectron from the Si EES becomes shorter at high temperature and at short time intervals due to thermal excitation. The density of energy states of the Si EES DSi(E) shows the main peak at 736 meV, a satellite peak at 601 meV, and broad energy structures over the range of 586–896 meV. The effective number of Si EES is 5.5 times larger than that in purified MgO. The excitation energy in a Si-doped MgO cluster with a crystal structure is obtained to be 0.83 eV by using the symmetry-adapted-cluster configuration interaction method and the Si EES contributes to exoelectron emission. The thermal excitation is governed by the transition from the Si–O bound state and the Mg edge state to the antisymmetrical edge states and the extended surface state. The excitation energy in an MgO cluster with a Si-doped atom inside and a nearest oxygen vacancy taking account of structural relaxation is calculated to be 0.75 eV, which shows good agreement with the main peak in DSi(E). The excitation energies of 0.64, 0.73, and 0.78 eV are also obtained in an MgO cluster with a Si-doped atom at the surface and a nearest oxygen vacancy. The first excitation energy corresponds with the satellite peak. The broad energy structures of DSi(E) are caused by the dependence of excitation energy on the position of Si-doped atoms inside and at the surface of the MgO cluster, and on the interatomic distance of Si–O due to structural relaxation. The energy structures can be also attributed to the thermal excitation to the various symmetrical Mg edge states and the surface states. When the number of complex structures of the Si EES with adjacent oxygen vacancies increases, oxygen vacancies are generated from the complex structures and the increase in the electron traps degrades electron emission rate. Therefore, the number of complex structures has an optimum value that leads to the maximum effective number of Si EES.
We report a double insulated CaS:Ce thin-film electroluminescent (EL) device which emits a bright green EL due to Ce 3 + luminescent centers, being characteristic of parity allowed 5d-4f transitions. A brightness level of 500 cd/m2 and emission efficiency of 0.11 Im/W have been obtained under 5-kHz sinusoidal voltage excitation. The CaS:Ce thin film has been fabricated by coevaporation of CaS and sulfur.
The decrease in efficiency of ZnS:Ag,Al phosphor powders under high cathode-ray excitation densities has been investigated in a wide range of Ag concentrations (18–1560 ppm) with the Al/Ag ratio fixed at 1.5. Two distinct regions have been observed in this saturation behavior. For low Ag concentrations up to some 100 ppm (mol) saturation is due to activator ground-state depletion, which manifests itself in a concentration dependence of saturation. For higher doping levels saturation becomes independent on the Ag concentration. Saturation was found to be temperature independent between 83 and 303 K. Diffuse reflectance measurements revealed an onset of optical absorption near 2.8 eV due to Ag incorporation, which continuously increased with Ag concentration giving no evidence for a change in activator incorporation around 100 ppm. Varying the Al/Ag ratio up to 56.2 had no influence on the saturation behavior, while the efficiency at low excitation densities showed a considerable decrease which could not be reconciled with optical-absorption losses, only. The saturation at high Ag concentration is tentatively attributed to a nonradiative Auger process. The probability of different types of Auger processes is discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.