Electroluminescent ZnS Research Articles

Electrical properties of amorphous and crystalline phases in electroluminescent ZnS:Mn, Cu, Cl films

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Structural characterization of thin ZnS films by X-ray diffraction

Structural properties of electroluminescent ZnS: Mn thin films grown by atomic layer epitaxy, which emit yellow light with an external efficiency as high as 2%, are investigated. The films grown at 500°C have a strong preferred orientation: one-half of the 00·1 plane normals are aligned within 7° from the normal of the glass substrate. The length of coherently diffracting domains (crystallite size) in the direction of the normal obtained from the Fourier analysis of a line profile ranges from 30 to 160 nm. The average relative strain in the same direction is calculated to be 10 −3. The estimated dislocation density is about 10 10 cm −2.

Some aspects of electromigration in D.C. electroluminescent ZnS devices

The stabilization with time of light emission from electroluminescent (EL) structures still requires investigation. Microscope observations of the EL structure show that the brightness is inhomogeneous and that the light emission is localized on many spots of different sizes. We investigated the distribution of the brightness along the active region of the sample and its dependence on time. The localized emission disappears with increasing time, and the brightness of the sample becomes more homogeneous although its intensity is reduced. This degration of the light emission is tentatively explained in terms of copper ion migration. The migration of copper ions affects the properties of the layer close to the positive electrode which appears bright. Some changes in the electrode surface and the precipitation of substantial amounts of copper were observed.

Optical properties of electroluminescent ZnS doped with Cu+, Mn2+ and Gd3+ ions

ZnS:Gd, ZnS: Cu, Gd and ZnS: Mn, Gd phosphors have been prepared by firing the samples in argon atmosphere. Spectral distributions in these phosphors are discussed with appropriate mechanism. ZnS:Cu, Gd and ZnS:Mn, Gd are found to be examples of multiple band phosphors. Enhancement and quenching of the emission band intensities of all these phosphors have been studied inpel emission. It is observed that Gd3+ ions play an important role in transferring their excitation energy to other centres. The voltage and frequency variation ofel brightness are in agreement with collision excitation mechanism in Schottky barrier at the metal semiconductor interface. Studies in phosphorescence and thermoluminescence of these phosphors have also been carried out. It is observed that trap-depth changes slowly with temperature and dopant concentration. The values of trapping parameters have been evaluated. The irregular variation of the life-time of electrons in the traps. with temperature shows the existence of retrapping in these phosphors.

X-ray diffraction study of thin electroluminescent ZnS films grown by atomic layer epitaxy

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Physical processes in forward-biased ZnS MIS light-emitting diodes with a „thick” interfacial layer

Forward-biased electroluminescent ZnS MIS structures with a „thick” (200 to 3 × 104 A) interfacial layer are investigated. A peculiar feature of the structures studied is that the insertion of the high-resistivity layer between the metal and ZnS results in an increase of the electroluminescence brightness both, at a given voltage bias and at a given forward current. The nature of the effect of the insulator on the diode characteristics as well as the mechanisms of carrier transport through the ”thick” interfacial layer are analysed. [Russian Text Ignored].

Effect of thickness on characteristics of electroluminescent ZnS:Mn films

Effect of thickness on characteristics of electroluminescent ZnS:Mn films

Impact excitation and ionization

Three impact processes relevant to high-field electroluminescence are considered, namely ionization across the band gap, impurity ionization and impurity excitation. The measured impact rate as a function of field is a convolution of the rate as a function of electron energy and of the distribution of electron energies. Experimental data for the three processes are discussed, with particular attention being given to ZnS:Mn and ZnSe:Mn. The data for electroluminescence in bulk crystals with Schottky contacts is presented in terms of quantum efficiency as a function of field. It is found that the rates of both ionization across the gap and excitation of manganese are a function of shallow donor density, and an explanation is advanced in terms of the band structure of ZnS and ZnSe. A maximum in the quantum efficiency of electroluminescence in ZnS:Mn and ZnSe:Mn Schottky diodes is reported, and its implication for device performance is pointed out.

Electrical conduction and degradation mechanisms in powder ZnS:Mn, Cu direct current electroluminescent devices

The electrical properties and degradation mechanisms in direct current electroluminescent (dc EL) ZnS: Mn,Cu powder devices have been investigated. The electrical characteristics of degraded devices were found to be substantially different from freshly prepared devices. The degradation mechanisms were explored by scanning electron microscopy (SEM) and it was found that the layer close to the positive electrode had undergone modifications due to Cu ion migration. The equivalent circuit is also analyzed and the effect of degradation on the values of its parameters is examined. Finally, a method to improve the maintenance characteristics is described.

An investigation of the electrical and optical properties of DC electroluminescent ZnS:Mn, Cu-powder panels

Electrical and optical measurements on ZnS : Mn, Cu-dc electroluminescent (DCEL) powder panels are reported. It is shown that current conduction in these devices depends upon avalanche breakdown in a thin insulating Cu-depleted layer of phosphor of thickness up to approximately 1 µm. However, it is concluded that the overall current-voltage characteristics of a panel are determined by the various components external to this layer. That the current density throughout this layer is highly nonuniform is deduced and the consequences of this result are discussed in terms of the effect on saturation brightness. A new model to explain the details of the observed electrical behavior of DCEL powder panels is introduced and compared with previously proposed models. Finally the various failure modes observed in these devices are identified and their elimination is discussed.

On the Mechanism of the Copper‐induced Hexagonal‐Cubic Transformation of Zinc Sulphide

AbstractThe influence 2H → 3C transition on the lattice constants, electron paramagnetic resonance and electroluminescence of ZnS: Cu, Mn was investigated. The effect of the electron gasionized donors interaction on the lattice constants of crystal is discussed. It is concluded that 2H → 3C transition is the first‐order phase transition.

Bulk and junction effects in d.c. electroluminescent ZnS:Cu,Mn powder panels

Current/voltage, current/brightness and capacitance values have been monitored as d.c. electroluminescent ZnS:Cu,Mn powder panels have been ‘formed’ to a series of voltages. Bulk effects are seen to be at least as important as junction effects to the generation of electroluminescence.

A.C. electroluminescence of ZnS:LnF 3 and ZnS:Mn thin films. Excitation mechanisms and memory effects

Thin film E.L. of ZnS:LnF 3 and ZnS:Mn in MISM and MISIM structures is investigated. It is first demonstrated that E.L. is due to carriers accelerated in barriers uniformly distributed in the ZnS layer. Secondly it is shown that low electric field domains exist where traps can be emptied only under a high voltage. It is finally suggested that these domains could be associated with the observed memory effects.

Emission centres of DC electroluminescence in ZnS:Mn thin films

Electroluminescent thin films of ZnS:Mn with the structure SnO 2-ZnS-Cu 2S-ZnS: Mn-Al 2O 3-Al were examined for their decay of light. The decay of electroluminescence light of the films after electric field switching-off was measured versus the Mn concentration. The present work concerns evidence for the existence of two kinds of emission centres: Mn single ions and Mn pairs.

On the mechanism of the double-comet-shaped electroluminescence in ZnS:Cu crystals

Brightness waves of electroluminescent comets in ZnS:Cu single crystals have been investigated to decide whether the recombination occurs spontaneously or delayed by one half-period. Brightness waves during rising and decay, in addition to modifications of brightness waves due to disturbances of the stationary excitation by voltage pulses, exhibit delayed emission and a reservoir of excited centres exists in the stationary case. These results together with the time dependent blue-to-green ratio of the emission may be explained best on the basis of a bipolar field-emission model as proposed by Fischer.

The memory effect of ZnS : Mn ac thin-film electroluminescence

The electroluminescence of ZnS : Mn thin-film memory devices is observed to occur in two components, one which is spatially homogeneous and one from localized bright regions less than ∼1 μm in diameter. The brightness-voltage hysteresis, or memory effect, is observed to reside exclusively in the luminescence of the localized bright regions in our devices. It is concluded that the memory effect arises from a negative resistance of a filamentary ac conduction mechanism in these regions.

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.

Excitation Mechanism of Electroluminescent ZnS Thin Films Doped with Rare-Earth Ions

Electroluminescence(EL) and photoluminescence(PL) experiments have been carried out on the ZnS: Nd, ZnS: Cu, Cl and ZnS: Cu, Cl, Nd films. The EL emission spectra show two peaks due to Cu-Cl and Nd centers, while the PL emission spectra show only one peak due to Cu-Cl centers. By comparing the efficiencies and the emission spectra of the EL with the PL measurements, it has been clearly shown that the excitation mechanism of the Nd ions in the electroluminescent ZnS: Nd film is via direct impact excitation by hot electrons.

Initial behaviour of the photoelectroluminescent brightness waves

The intensi ty of the photoluminescent (PL) emission in ZnS is known to be controlled by the electric field. I t follows that oscillating fields produce a modulated emission (brightness waves). When this phenomenon takes place in electroluminescent (EL) ZnS, i t is usually referred to as photoelectroluminescence (PEL) (1). Investigations on this effect were recently performed, concerning principally the dependence of the brightness on the oscillating voltage amplitude. Evidence was obtained about the connection between the EL and PEL processes, which turned out to be due to the same lattice defects (2). In the present paper, some experiments on the initial behaviour of the PEL emission are reported. The need of these experiments arises from the fact that investigations on the phenomenon in a t ransient phase are more suited to obtain information on the kinetics than measurements in steady conditions. Condenser-shaped cells about 0.1 mm thick were used, filled with ZnS microcrystals embedded in araldite and doped with about 10 -a Cu and C1 atoms per Zn atom. The cell electrodes were buil t with conducting quartz, in order to allow the UV light to reach the ZnS crystals. The cell was irradiated with parallel light coming from a quartz lens in front of a 250 W d.c. low-pressure Hg lamp. A filter (Jena Glaswerk UG1) and a collimator were used to shield the cell from visible light. Light from the cell, suitably filtered (Jena Glaswerk GG 495) to remove scattered UV light, was detected by a 6810 A RCA phototube which fed directly the a.c. input of the scope (a). While the cell was i r radiated for several minutes with the UV light, a sinusoidal voltage (100 V ...... amplitude, variable frequency) was applied, with an ini t ia l zero phase, to excite the PEL emission. Under these conditions the brightness waves were detected by the scope. In Fig. 1 some typical pictures are shown as recorded on the same t ime scale; a) is the 103 Hz exciting voltage, b) is the customary building-up of the EL emission alone (in this case, of course, UV light was tu rned off and the scope d.c. input was used), c) and d) correspond to the ini t ia l behaviour of the PEL brightness waves. Specifically, while in picture c) during the first half-cycle of the applied voltage the electric field was opposite to the UV flux direction, in picture d) the electric field and the UV flux were, on the contrary, directed in the same way. I t is to be noticed that the

Franz-keldysh effect in emission of electroluminescent ZnS:Cu, Cl films

Franz-keldysh effect in emission of electroluminescent ZnS:Cu, Cl films