Electron Traps with a Wide Range of Capture Cross Sections in Powdery ZnS-Based Luminophors

Abstract

Using the method of thermally stimulated luminescence, the phenomenon of spatial modulation of the capture cross section (S t ) of electron traps with a level of E c − 0.18 eV in powdery luminophors Zn 0.97 Cd 0.03 S ICu, ClJ with green luminescence and ZnSIAg, ClJ with blue luminescence by a collective electric field of macroinhomogeneity has been investigated. The maximum value of the effect (width of the range of values of S t ) attains five orders of magnitude, with the size of the powder grains being of order 1 µm. A model is suggested, according to which traps with E c − 0.18 eV with an extended range of values of S t represent an isolated anionic vacancy in a region of the collective field of macroscopic inhomogeneity that is the negatively charged surface of the powder grains. The extension of the range of values of S t is the result of modulation of the kinetic parameters of electron traps E c − 0.18 eV depending on the spatial position occupied by the anionic vacancy relative to the surface of the powder grains.Introduction. According to model representations [1-3], slow electron traps with anomalous kinetic properties (almost inexplicable from the viewpoint of both the dynamic mechanism of charge-carrier capture and static charge state of traps) are point defects in the region of large-scale disturbances of crystals with collective electric fields that repel the majority carriers. While the electron capture cross sections (S t ) of fast and slow traps are proper (not disturbed by other defects), the cross sections S t of the traps localized in the region of large-scale disturbances of the crystal acquire an effective character due to being modulated by collective electric fields. This models leads to a number of consequences. In particular, irrespective of the behavior of energy parameters, the section S t of an individual trap can take a range of values [2] as a result of the distribution of the trap atoms over the entire volume of microinhomogeneity, within the limits of which the potential of the collective electric field undergoes a change.In [3], using an electron trap with a level of E c − 0.2 eV in a γ-La 2 S 3 crystal as an example, results of the first experimental investigations are presented for the phenomenon of spatial modulation of the section S t by a collective electric field of microinhomogeneity. The conclusion was drawn that the role of macroinhomogeneities in γ-La 2 S 3 is played by acceptor-type dislocations. The maximum expansion of the range of values of S t attains four orders of magnitude. In this work, we present the results of investigation of thermally stimulated luminescence (TSL) of powdery luminophors Zn 0.97 Cd 0.03 SICu, ClJ with green emission and ZnSIAg, ClJ with blue emission. * * The size of the powder grains is of the order of 1 µm. These results show that the same electron traps with a level of E c − 0.18 eV and a wide range of S t values (five orders of magnitude) are observed in both powders. The role of macroinhomogeneity with which the electron traps E c − 0.1...

Thermally stimulated generation–recombination processes in zinc oxide single crystals and nanocrystals

“…In the latter, they have a discrete cross section S t = 2.1⋅10 ues for the same trapping centers, depending on the modification of the semiconductor, complicates attempts to interpret this phenomenon based on ideas assuming it is connected only with individual features of the corresponding electron center. An electron trapping center with a discrete level E C -0.18 eV and an expanded spectrum of S t values was studied by TSL earlier in [12] in powdered luminophores based on zinc sulfide. According to the proposed model, electron trapping centers with an expanded spectrum of S t values are donor-type centers, localized in the region of the collective field of a macroscopic inhomogeneity: the negatively charged surface of the powder grains.…”

“…However, it considered participation in the thermally stimulated generationrecombination process of only one trapping center with a discrete level and a radiative recombination center (RRC) which does not undergo thermal ionization over the course of the entire process. As shown by experience [2][3][4][5][6][7][8][9][10][11][12][13][14], in real semiconductors and dielectrics, electron and hole trapping centers are realized with a quasidiscrete system of energy levels, and consequently the TSC and TSL spectra ** do not consist of one elementary band but rather a series of…”

“…* Theoretical [6] and experimental [10][11][12][13][14] studies show that in semiconductors and luminophores, fast (ratio of the electron capture rate on trapping centers to their recombination rate R >> 1) and slow (R << 1) electron trapping centers are observed.…”

“…Analysis of expressions (11) and (12) shows that the initial section of the TSC and TSL spectrum can be represented in the form…”

“…As noted above, experimental data accumulated over decades shows that in many semiconductors and dielectrics [2][3][4][5][6][7][8][9][10][11][12][13][14], the TSC and TSL spectra have a complicated structure and consist of a series of overlapping bands. In the general case, when there are j trapping centers in the sample, each of which is characterized by individual parameters E t , S t , and n * , we need to consider a system of j + 1 coupled nonlinear equations of form (8) and (9).…”