Thermal ionization of an electron trap in Zn0.97Cd0.03S〈Cu,Cl〉 luminophore facilitated by electromodulation of the effective capture cross section

Abstract

0.97 Cd 0.03 S〈Cu,Cl〉 powder luminophore with an average grain size of 1 μm belongs to the class of semiconductors contain ing electron traps with original kinetic properties [1][2][3][4]. These electron traps are characterized by a monoenergetic level and extended range of the elec tron capture cross section (S t ). The range of S t distri bution, defined as the ratio of its upper and lower boundaries varies, depending of the luminophore type and trap nature, within one to five orders of magni tude.In order to gain new knowledge about electron traps with extended spectrum of the electron capture cross section (S t ), we have studied the effect of electric field on the luminescence (the Gudden-Pohl effect [5]) of metal-dielectric-luminophore-metal (MDLM) structures with Zn 0.97 Cd 0.03 S〈Cu,Cl〉 pow der luminophore. The MDLM structure was pre pared by pressing a 0.3 mm thick layer of the lumi nophore between glass palates, followed by the chemical deposition of transparent indium oxide electrodes. A 0.03 mm thick dielectric (polyethyl ene) layer was situated between the lower electrode and the luminophore powder. Both excitation of lumi nophore and detection of luminescence were per formed through the upper glass plate.In addition to electroluminescence (EL), we have measured the characteristics of thermostimulated luminescence (TSL). The results of these measure ments allowed us to establish the following factors that influence the process of capturing carriers on traps in the luminophore layer occurring in an external electric field.(i) The behavior of MDLM structures in a constant electric field depends on the state of the electron sub system of Zn 0.97 Cd 0.03 S〈Cu,Cl〉 luminophore. In the equilibrium state, the MDLM structure is insensitive with respect to an external electric field. In contrast, the exposure of this structure to an electric field with a strength of ε > 10 4 V/m after a preliminary low tempera ture (T < 200 K) photoexcitation (capable of driving Zn 0.97 Cd 0.03 S〈Cu,Cl〉 luminophore to a nonequilibrium state) is accompanied by a flash of EL. Figure 1 shows the curves of EL relaxation measured for opposite polari ties of the applied field (inset) and the plots of EL intensity versus electric field strength in semilogarith mic coordinates [, ε], which are linear and inde pendent of the field polarity (curves 1 and 2).(ii) The integral TSL spectrum of the MDLM structure (Fig. 2) measured in the absence of an exter nal electric field displays a broad emission band. An analysis of this spectrum by the method of "thermal" cleaning [6] reveals a set of three elementary lines (curves 1-3). The ionization energy (E t ) can be esti mated from the slope of straight lines [7] (1) in the initial low temperature region of TSL buildup. The electron capture cross sections can be evaluated using the formula [8,9] (2) where β is the rate of sample heating during the mea surement of TSL spectrum, v is the thermal velocity of electrons, N c is the effective density of states in the conduction band, ΔT is the TSL b...