Destruction mechanism of color centers and recombination luminescence in NaCl-Cd crystal phosphors

The authors have studied the origin of integral thermoluminescence peaks and the destruction mechanism of color centers during thermal relaxation of X-irradiated NaCl-Cd phosphors. The luminescence spectra have also been studied, and previous similar results are discussed.     

Kinetics of the radical-recombination luminescence of crystal phosphors on excitation by atomic oxygen

The kinetic characteristics of the radical-recombination luminescence of crystal phosphors with a wide forbidden band in atomic oxygen are considered. It is shown that, in order to explain the form of the kinetic curve in the initial stage, simple model representations as to the statistical character of the recombination processes are sufficient. By comparing the theoretical and experimental curves some of the microcharacteristics of recombination between the atoms are determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 135–139, November, 1973.In conclusion, the authors wish to thank D. P. Popov for presenting the experimental data.     

Investigation into the processes of thermal and photothermal destruction of F-color centers in alkali-halide crystal phosphors

Using the concept of thermal and photothermal destruction of color centers in alkali-halide phosphors as an ion mechanism, certain formulas are examined which represent these processes. The phenomenon of thermal and photothermal destruction of F-color centers is investigated experimentally for the case of NaCl-Ag. A comparison of the experimental results obtained with the theory enabled an estimate to be made of a number of kinetic characteristics of the processes in question.The results obtained confirm the usefulness of the idea of ion processes in explaining the phenomena of relaxation in alkali-halide crystal phosphors.     

The interaction of photoexcited color F-centers with microdefects in the crystal lattice of alkali-halide crystal phosphors. II

The interaction of different microdefects with excited and nonexcited F-centers was studied, using the methods of thermal decolorization, thermal luminescence, photothermal decolorization and photothermal luminescence.A comparison of experimental data with the results of theoretical calculations of the corresponding curves enables an estimate to be made of certain kinetic characteristics of the processes in questions.     

Aggregation of impurity centers in ionic crystals

Results of modeling of the aggregation process in complex centers of ionic crystals are presented. Concentration dependences of the number of paired centers are obtained. A nature of the paired luminescence centers is discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 62–65, February, 2007.     

Luminescent impurity doping trends in alternating-current thin-film electroluminescent phosphors

The doping properties of three alternating-current thin-film electroluminescent (ACTFEL) phosphor host/luminescent impurity systems, ZnS:Mn, SrS:Ce, and SrS:Cu, are elucidated, and the ACTFEL device implications of these properties are assessed. Mn is isovalent, Ce is a donor, and Cu is an acceptor. Moreover, Ce is readily ionized in SrS, so that it behaves as a double donor. The distinctly different doping nature of these three luminescent impurities leads to dramatically disparate defect and device physics trends. The donor/acceptor nature of Ce/Cu in SrS results in charge neutrality being achieved in SrS:Ce and SrS:Cu via self-compensation-induced vacancy creation; subsequent defect complexing between oppositely charged luminescent impurities and self-compensation-induced vacancies results in more complex ACTFEL device behaviors such as dynamic space charge, trailing-edge emission, charge collapse, color tuning, and electroluminescence (EL) thermal quenching. In contrast, the isovalent nature of ZnS:Mn leads to more ideal ACTFEL device operation. This suggests that the optimal ACTFEL phosphor luminescent impurity is isovalent.     

Determination of the positions of impurity centers in a dislocation core in a NaCl crystal from magnetoplasticity spectra

The spectra of the mean free paths l(ν) of edge dislocations have been studied in NaCl crystals exposed in the electron paramagnetic resonance scheme to the crossed magnetic fields: the Earth’s field (50 μT) and the pump field (2.5 μT, 5–440 kHz). The spectra have been measured for a series of angles θ = 0°–5° of rotation of the sample around its edge [100] with respect to the Earth’s field. The fine structure of the spectra contains a series of peaks whose resonance frequencies are described by the empirical expression v _i ^± = Asin(θ ± Δθ _i ) ≈ A(θ ± Δθ _i ). The parameters Δθ _i are independent of the angle θ within the experimental errors. Within the model of “frozen” magnetic moments associated with impurity center Ca⁺-Cl⁰, the angles Δθ _i characterize the deviation of the axis of the center from the 〈100〉 direction in the core of a dislocation. These angles can be expressed in terms of the spectra obtained: Δθ _i = (? _i ⁺ ? v _i ^? )/2A. The computer simulation of the edge dislocation core provides the set of the angles Δθ _i close to the measured values. The spin-lattice relaxation time of the center on dislocation has been estimated from the low-frequency edge of the spectrum l(ν) as τ _{s ? l} ～ 10^?4 s.     

Optical spectroscopy of single impurity centers in semiconductors

Using optical spectroscopy with diffraction limited spatial resolution, the possibility of measuring the luminescence from single impurity centers in a semiconductor is demonstrated. Selectively studying individual centers that are formed by two neighboring nitrogen atoms in GaAs makes it possible to unveil their otherwise concealed polarization anisotropy, analyze their selection rules, identify their particular configuration, map their spatial distribution, and demonstrate the presence of a diversity of local environments. Circumventing the limitation imposed by ensemble averaging and the ability to discriminate the individual electronic responses from discrete emitters provides an unprecedented perspective on the nanoscience of impurities.     

Luminescence of AgHal phosphors containing a silver sulfide impurity

In a study of the luminescence spectra of AgHal single crystals containing various levels of a silver sulfide impurity, the mechanism for the luminescence is established. The thermal quenching of the steady-state luminescence and of the light sum of the luminescence of AgHal (Ag₂S) phosphors is due to ionic processes, as in pure silver halides. The activation energy for the thermal quenching of the light sum is found. The ratio of the cross sections for electron recombination with holes localized at the luminescence centers responsible for the various luminescence bands is found in a kinetic study of the decay of the optical emission stimulated in these bands.     

IR-spectroscopy of crystals containing Jahn-Teller impurity centers

Using a novel IR-technique, the low-temperature spectra of CdTe, ZnTe and Cd_0.95Mn_0.05Te crystals, doped by Jahn-Teller Co²⁺ impurity ions, were studied in the spectral range from 1 to 400 μm. The absorption spectrum regions and the energy positions of ZPL, corresponding to the transitions ⁴A₂(⁴F)→⁴T₁(P), ⁴T₁(F), ⁴T₂(F) and ²G were detected. The parameters of the spin-orbit interaction λ, the crystal field parameter D_q, the parameters of electron-electron interaction B and C were obtained. It was found, that Jahn-Teller interaction differently affects the parameter λ for ⁴A₂(F)→⁴T₁(F) and ⁴T₂(F) transitions and, consequently, the energy positions of ZPL. The electron-vibronic spectra were compared with phonon spectra.     

Shallow impurity centers in semiconductor quantum well structures

Interest in the study of the behavior of shallow impurity centers in superlattices and quantum well structures is fairly recent. This paper reviews briefly both the theoretical and experimental work done in this field in the last few years. Several recent calculations of the energy levels of hydrogenic impurity states in quantum well structures, such as Ga_1?xAl_xAsGaAsGa_1?xAl_xAs, are reviewed. The behavior of these levels as a function of the quantum well size is discussed. Recent experimental data concerning the variations of the binding energies of shallow donors and acceptors as a function of the GaAs quantum well size are reviewed. A comparison between these experimental measurements and the results of recent calculations is presented.