Centers In ZnS Research Articles

Effects of lattice distortion on the deep Cu center in ZnS

Self-consistent linear combination of atomic orbitals cluster calculations are performed for the substitutional copper impurity in ZnS, showing the electronic changes induced by the impurity. The new bonding forces lead to a relaxation of the neighboring atoms. Total-energy calculations yield a new energy minimum at an outward breathing distortion of 0.21 \AA{}. Consequently the $e$ and ${t}_{2}$ levels are shifted in the gap, and the crystal-field splitting parameter $10Dq=\ensuremath{\epsilon}({t}_{2})\ensuremath{-}\ensuremath{\epsilon}(e)$ is calculated to be 0.68 eV for the relaxed cluster compared to 0.77 eV from experiment.

Study of nonequivalent radiative Tm 3+ centers in ZnS

There were observed at least three types of nonequivalent centers in quasicubic crystals ZnS:Tm 3+. Fluorescence, polarization, IR and temperature quenching of those centers depend on preparation. The number of nonequivalent Tm 3+ centers in ZnS will be probably more than three as a result of charge compensation (Tm 3+−V Zn complexes create) and due to the different distances of Tm 3+ ion from the stacking faults in the host lattice.

Luminescent properties of ZnS:Ce, Li

Luminescence excitation and emission spectra of ZnS:Ce, Li phosphors have been measured at liquid nitrogen temperature. The samples quenched at 1080°C exhibits a Ce 3+ emission and a strong edge emission band. Their excitation spectra show dips at the exciton absorption energies. Heat-treatment at 400°C changes the shapes of the emission and excitation spectra. The Ce 3+ emission spectra also change according to the wavelength of the exciting light, which indicates multiple structures of the Ce 3+ centers in ZnS.

Triplet to quintet luminescence of Fe 2+ center in ZnS

Two luminescence bands centered at 10 300 cm −1 and 7 000 cm −1 were observed in ZnS:Fe. The higher energy band is interpreted as due to the 3T 1 → 5E transitions within the Fe 2+ ion. Such an interpretation is supported by results of excitation measurements and by the analysis of the zero-phonon structure of this luminescence band.

Photo-EPR and ODMR investigations of radiative processes in ZnS:Cr,Sc

Scandium has been known as coactivator of the luminescence of the ZnS crystals but is role in the photoluminescence and the thermoluminescence processes has not been well understood. The authors have used the EPR of chromium, which can act as both an electron and hole trap, and also ODMR, to investigate these processes. In particular, the measurements of the change of the chromium Cr+ EPR signal intensity in thermal or light-induced processes enable them to determine the energy depths of the important centres in ZnS:Cr,Sc. The ODMR experiments correlated the luminescence transitions with specific defects such as Sc2+ donors and (VZn-Sc) A-centres.

Modeling a.c. thin-film electroluminescent devices

Since the work at Sharp Corporation demonstrated the possibility for long lived thin-film electroluminescent devices, there has been considerable interest in this technology for display applications. As this technology moves out of the research laboratory, there is a need for accurate models that one can use to characterize thin-film EL elements. Such models are necessary for process optimization, for the design of optimum driving circuitry, and also to make projections of expected performance limits in a particular application. In a.c.-coupled thin-film EL devices, the active layer (usually ZnS:Mn) is sandwiched between two insulator layers. This structure is the subject of this paper. The presence of the insulator layer results in certain features that are unique to this particular construction, and we have found certain special techniques for characterizing device operation to be particularly useful. The “ Q-Vfigure,” analogous to the “ H-B” diagram for a magnetic material, provides a graphic representation of device operation that is useful for both qualitative and quantitative descriptions. In this paper, a series of simple-to-complex models for device operation are offered that are useful for analysis in applications. The simpler models, though useful for some purposes, fail to give a complete description. The more complete models require a much more detailed knowledge of the underlying processes and so are more cumbersome to use. They can, however, provide insight into the operation. A detailed exposition of experimental data for typical a.c. thin-film EL devices is given, which includes electrical and optical data along with suggestions of underlying physical mechanisms that may provide a model explaining this data. Special consideration is given to the role of tunneling and space charge in determining the electrical characteristics and to the special features of the Mn ++ center in ZnS. It is suggested that an Auger process involving this center can provide a link between concentration quenching and the occurrence of hysteresis or memory effect. The nature of brightness saturation in ZnS:Mn devices is reviewed. The saturation data strongly suggest an interaction between Mn ++ excited states that provide nonradiative decay channels.

Electro- and photoluminescence properties of Mn 2+ in ZnS and ZnCdS

Some aspects of the nature of the Mn center in ZnS and ZnCdS and of the influence of electric fields on the luminescence mechanism are reviewed. The photoluminescence properties of isolated Mn ions on the four main sites of polymorphic crystals are considered. The influence of more distant stacking faults and the combined action of several stacking faults on the Mn excitation and emission are discussed tentatively. A survey is given on the effects of an increasing amount of Mn: (1) The shift of the zero-phonon lines which is due to inhomogenous line broadening; (2) the appearance of additional zero-phonon lines both in emission and excitation due to Mn-Mn pairs; (3) the observation of additional low-energy emission bands for highly doped ZnS:Mn crystals. The a.c. EL of ZnS:Cu, Mn, X powders where X stands for Fe, Ni, Co is discussed with respect to an energy transfer between deep centers during the low-field period of the EL cycle. It turns out that a strong energy transfer exists between these centers due to thermally controlled hole liberation. The influence of Auger quenching on the Mn emission is considered. It turns out that some effects already reported earlier in a.c. EL can be explained in that way. Finally the field enhancement of the Mn emission (“out-of-phase enhancement”) is considered as due to the control of the Auger quenching by electric a.c. field and/or additional red or infrared irradiation control of the density of electrons in the conduction band, where any reduction of this density involves a reduction of the Auger quenching.

Luminescence of an M center in ZnS?

Selected, differently doped and annealed cubic ZnS monocrystals exhibit a structured luminescence in the spectral region between 815 and 1150 nm. At 4 K more than 70 lines with typical linewidths of only 3–5 cm −1 could be resolved. The influence of temperature, of magnetic fields up to B = 10 T, and of irradiation with additional light were studied, and the excitation spectrum was recorded. The experiments indicate for the luminescence center a three-fold excited state and a singlet ground state with a strong phonon coupling, and allow the determination of the position of the involved electronic levels between valence band and conduction band. The results can be explained satisfactorily under the assumption that the center of luminescence consists of two adjacent F + centers, the so-called M center. The present work marks the first time that any such center has been found in a II–VI compound.

Luminescence of an M center in ZNS?

Luminescence of an M center in ZNS?

Electric and optical properties of the ’’Cu-red’’ center in ZnSe

Emission and capture rates describing the optical and thermal transitions at the ’’Cu-red’’ center in ZnSe have been investigated using a modified photocapacitance technique. By keeping the depletion region of a Schottky diode constant during the measurements of transients highly compensated samples could be used, resulting in larger signals than previously possible. This permitted optical-emission rates for both electrons and holes to be measured with higher accuracy and in a broader temperature region than in previous investigations. The capture cross section of electrons σn was determined in the temperature range 77–200 K. σn increases with decreasing temperature and has a value of 1.4×10−19 cm2 at 77 K.

The excitation mechanism of Mn luminescence centres in ZnS:Mn thin films

physica status solidi (a)Volume 49, Issue 2 p. K201-K203 Short Note The excitation mechanism of Mn luminescence centres in ZnS:Mn thin films E. Walentynowicz, E. Walentynowicz Institute of Physics, Poznañ Technical UniversitySearch for more papers by this authorS. Szuba, S. Szuba Institute of Physics, Poznañ Technical UniversitySearch for more papers by this author E. Walentynowicz, E. Walentynowicz Institute of Physics, Poznañ Technical UniversitySearch for more papers by this authorS. Szuba, S. Szuba Institute of Physics, Poznañ Technical UniversitySearch for more papers by this author First published: 16 October 1978 https://doi.org/10.1002/pssa.2210490271Citations: 7AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume49, Issue216 October 1978Pages K201-K203 RelatedInformation

Electron paramagnetic resonance (EPR) and spin-lattice relaxation of trigonal Co2+ centers in ZnS single crystals

Single crystals with a ZnS structure are of great practical value, in which connection they are studied in detail by different methods including the spectroscopic. In particular, such crystals activated by ion impurities are investigated in detail by the EPR method. However, the spin-lattice relaxation (SLR) of such systems has hardly been studied. Papers studying the SLR of cubic Mn2+ centers [1] and trigonal Nd3+ and Yb3+ [2] centers in ZnS did only appear recently.

Jahn-Teller effect in the fluorescent level ofMn++in ZnSe and ZnS

Uniaxial-stress experiments have been used to analyze the fine-structure lines of the lowest $^{4}T_{1}$ level of ${\mathrm{Mn}}^{++}$ in cubic ZnSe and in cubic ZnS containing stacking faults. We show that the two fine-structure lines due to the cubic centers in ZnSe and ZnS can be interpreted by Ham's model corresponding to a strong Jahn-Teller coupling with $E$ symmetry modes. In both cases, the observed lines correspond to transitions from the $|^{6}A_{1}〉$ fundamental state to the almost degenerate states $|{\ensuremath{\Gamma}}_{7}〉$, $(\frac{3}{\sqrt{10}})|{\ensuremath{\Gamma}}_{8}(\frac{3}{2})〉\ensuremath{-}(\frac{1}{\sqrt{10}})|{\ensuremath{\Gamma}}_{8}(\frac{5}{2})〉$ for the line at lower energy and $|{\ensuremath{\Gamma}}_{6}〉$, $(\frac{1}{\sqrt{10}})|{\ensuremath{\Gamma}}_{8}(\frac{3}{2})〉+(\frac{3}{\sqrt{10}})|{\ensuremath{\Gamma}}_{8}(\frac{5}{2})〉$ for the line at higher energy. The influence of the Jahn-Teller effect on the axial ${\mathrm{Mn}}^{++}$ centers in stacking faults of ZnS is also briefly considered. Finally a comparison is made of the Jahn-Teller effect in the lowest $^{4}T_{1}$ and $^{4}T_{2}$ states of ${\mathrm{Mn}}^{++}$ in ZnS and ZnSe.

Identification of deep centers in ZnSe

Cu-doped, Mn-doped, and nominally undoped ZnSe crystals were prepared by vapor transport or sublimation. The presence of the dopants was established by photoluminescence EPR, atomic absorption, and radiotracer techniques, and their associated energy levels studied by photoluminescence. From measurements of photocurrent and photocapacitance, it was shown that Cu doping introduces an energy level at about 0.68 eV above the valence band. From the good agreement between the optical cross sections obtained from the measurement of photocapacitance and optical quenching of photoluminescence, it was concluded that this center is responsible for the red Cu-emission in photoluminescence. This conclusion was supported by the observation that the absolute concentration of this level agrees with the reduction of the free-carrier concentration caused by Cu doping. Owing, presumably, to residual copper contamination, the 0.68-eV center was sometimes detected in nonintentionally doped and Mn-doped material. Evidence is given that this center is identical to the M center reported previously in Mn-doped ZnSe is identical with the 0.68-eV energy level. Independent of dopant, another level at about 0.58 eV above the valence band was observed.

Further effects of high temperature annealing on the photoluminescence of an ultra high pure ZnS crystal

If an ultra high pure ZnS crystal is annealed at 1200°C and afterwards slowly or rapidly cooled down to room temperature, one obtains reversible and irreversible changes of the photoluminescence in the visible part of the spectrum. Besides two different reversible blue-green conversions, violet and orange fluorescence bands are reversibly created and annihilated. The blue-blue conversion produced by a variation of the cooling rate after annealing at 1000°C is irrecoverably destroyed if the crystal is only once warmed up to 1200°C. High temperature annealing affects luminescence and thermoluminescence in the same direction. Etching the crystals after annealing at 1000°C has no influence on the reversible behaviour of the fluorescence bands. The experimental results indicate that the structure of the deep centers in ZnS has to be discussed from a more uniform aspect.

Evidence for the direct impact excitation of Mn centers in electroluminescent ZnS:Mn films

In order to clarify the excitation mechanism of Mn luminescent centers in ZnS:Mn electroluminescent thin films, time-resolved emission spectra in photoluminescence and electroluminescence were measured. In photoluminescence, both emissions arising from ZnS host and Mn centers were observed. For the emission arising from Mn centers, the time delay showing energy transfer from ZnS host to Mn centers was recognized. In electroluminescence, contrary to this, the emission from the ZnS host was hardly observed, and the emission from Mn centers was observed immediately after excitation. These experimental results strongly suggest that the excitation mechanism of Mn centers in electroluminescence is due to the direct impact excitation of Mn centers by hot electrons accelerated in the high electric field in the Zns host. These results are also supported by efficiency measurements.

The spectroscopy of colour centres in ZnS and CaS

Some data about colour centres of unactivated and activated CaS and ZnS with various impurities are reported. The stimulation (excited absorption) spectrum of ZnS consists at LNT four groups of band: the band at 2.5–3 eV and complex stimulation bands at 1.6 eV, at 1 eV and at 0.5 eV. The structure of stimulation spectra of CaS is analogous: it consists of an intense band at 2–2.5 eV, and bands at 1.6–1.8 eV and 0.8–1.0 eV. At room temperature the low energy band (at 0.5 eV for ZnS and 0.8–1.0 eV for CaS) does not appear. The excited absorption bands appear at energies above 2 eV. The investigation of non-isothermal relaxation of stimulation permitted us to establish the temperature stability of these stimulation bands.

The identity of the EPR A-center and the self-activated luminescence center in ZnS: Cl

Generation and bleaching with polarized light of the EPR signal associated with the zinc vacancy-chlorine donor pair ( A-center) in ZnS : Cl are described. Excitation bands at 3550 and 3300Å and bleaching bands at 9000 and 13,000Å are revealed and their polarization properties determined. The close correlation with the polarized luminescence studies of Koda and Shionoya on the ‘self-activated’ luminescence center confirms that the luminescence arises from the zinc vacancy-chlorine pair.

Irradiation effects in II-VI compounds

Abstract A summary of published results on electron energy threshold measurements in II–VI compounds, and their interpretation in terms of damage on the metal and chalcogenide sublattices, is given. EPR results in irradiated II–VI compounds are also reviewed. These include the F+ center in ZnS; ZnO, and BeO and new results on the zinc vacancy (V) in ZnSe. For the zinc vacancy, optical absorption bands at 4680Å and 8850Å are identified with V− and a band at 5000Å is tentatively assigned to V−. The activation energy for anneal of the vacancy is measured to be 1.26±.06 eV, and this is tentatively identified as the activation energy for vacancy motion. An EPR spectrum produced in a 20.4 °K irradiation is tentatively identified as a zinc vacancy-interstitial pair. Complete annealing occurs for this defect in the range 60–100 °K.

Phosphorus and Arsenic Impurity Centers in ZnSe. II. Optical and Electrical Properties

ZnSe crystals containing phosphorus or arsenic which are grown under conditions enhancing their incorporation as uncompensated acceptors show broad luminescence bands in the red and infrared portions of the spectrum. Excitation bands are found which are resolved from the band-edge absorption. Infrared quenching of photoluminescence and photoconductivity identify the 1.91-eV luminescence of ZnSe: P as originating from the ${\mathrm{P}}_{\mathrm{Se}}$ center identified by EPR. The crystals behave electrically as high-resistivity material showing $p$-type conductivity in the dark at elevated temperatures. Photoconductivity at low temperatures is $n$ type and is sensitized by the ionized ${\mathrm{P}}_{\mathrm{Se}}$ acceptor. Configuration coordinate curves for the ground and excited states of the ${\mathrm{P}}_{\mathrm{Se}}$ center are derived which are consistent with the distortions from ${T}_{d}$ symmetry observed previously by EPR.