Single Crystals Of Zinc Sulphide Research Articles

Excitation spectra and structure of luminescence centers of manganese ions in single crystals of zinc sulfide

We investigate the photoluminescence excitation spectra in ZnS:Mn single crystals at room temperature and at the temperature of liquid nitrogen with a different concentration of Mn2+ ions. The strongest bands peaking at 557, 578, 600, and 637 nm are associated with a different position of the Mn ion in the lattice of the crystals under investigation. The difference obtained in the excitation spectra can be explained by the resonance transfer of energy between the Mn ions.

Conductivity switching in single crystals of zinc sulphide - I

A study of switching effects in vapour-grown zinc sulphide platelets (1-100 μm thickness) contacted with silver or indium is described. Depending on temperature, both `monostable' and `bistable' switching was observed. In the `bistable' region crystals exhibited a variety of metastable conduction states; these are discussed in terms of filamentary behaviour and a `variable donor density' model in the accompanying paper (II). This paper attempts to develop an overall model of the switching by concentrating on transient behaviour. Some attempt is made to describe the type of centre necessary to explain the observed properties.

Sphalerite-wurtzite equilibria and stoichiometry

Abstract Evidence from natural occurrences and from published syntheses indicates that the sphalerite-wurtzite inversion is not an invariant reaction at 1020°C, 1 atm but is a univariant function of fs2 and temperature. Single crystals of zinc sulfide, grown in aqueous NaOH solutions which were selected to control fs2, have shown that a univariant boundary exists between sphalerite and wurtzite near 500 atm from 465 ± 4°C to 517 ± 2°C over a corresponding calculated fs2 range of 10−9.5 to 10−8.7 atm. Direct determination of fs2 for coexisting sphalerite and wurtzite, made by passing H2 + H2S mixtures over ZnS powder, gave fs2 of 10−5 atm at 890°C, between 10−5.5 and 10−6.4 atm at 800°C, and between 10−6.5 and 10−8.5 atm at 700°C. The fs2 -dependence of this phase change demonstrates that wurtzite is sulfur-deficient relative to sphalerite. Nonstoichiometry in zinc sulfide is also indicated by its color and by published luminescence studies, electrical measurements, and chemical analyses. Electrical measurements show the defects to be zinc vacancies in sphalerite and sulfur vacancies in wurtzite. The combined range of nonstoichiometry in ‘ZnS’ is on the order of 0.9 at per cent. The hypothesis that wurtzite might be formed metastably at low temperatures due to oxygen substitution for sulfur is untenable. No oxygen was detected by cell-edge measurements of sphalerite or wurtzite from the hydrothermal experiments. In the gas-mixing experiments in which wurtzite was produced from sphalerite below 900°C, oxygen was not present. Wurtzite is thermodynamically stable at lower fs2 than sphalerite. Above about 250°C, this stability field lies well outside of the normal sulfidation state encountered in ore-forming environments; however, at lower temperatures wurtzite may be deposited in highly reducing environments. Because it is stable throughout the geologically most important pH range within 2 or 3 units of neutrality, highly acid solutions are not necessary for its precipitation as advocated by Allen et al. (1914).

Electron-Induced Traps in Zinc Sulfide Single Crystals

The thermoluminescence characteristic of single crystals of zinc sulfide was studied before and after room-temperature irradiation with monoenergetic electrons. A new trapping state with a threshold energy at 175-195 keV was observed and, in the same specimens, the production of a trapping state with a threshold energy of 235-245 keV was confirmed.

The temperature dependence of infra-red enhanced electroluminescence in zinc sulphide crystals

Infra-red enhancement of electroluminescence in single crystals of zinc sulphide, previously studied only at room temperature, is investigated over the temperature range 97-383°K. At low temperatures normal electroluminescence is strongly limited by space charge due to trapped carriers in the bulk crystal but the latter can be removed by infra-red irradiation which then enhances electroluminescence giving similar characteristics to those found at room temperature. Above room temperature thermal generation of mobile positive holes adds to the enhancement process produced by infra-red liberation of holes. The Schottky barrier model, previously developed to explain the room-temperature effects, is successfully applied to the behaviour at other temperatures.

Growth of ZnS single crystals from the melt at 1850 °C under argon pressure of 50 atm

Single crystals of zinc sulphide 17 mm in diameter and 30–40 mm in length have been grown from the melt at 1850 °C. The process was carried out in the atmosphere of argon under a pressure of 50 atm. The crystals were grown by the Bridgman technique, that is by slowly lowering the crucible inside the graphite heater, and using monocrystalline seed. On cleavage planes of the grown crystals, (112̄0), as well as on etched (0001) planes, only single dislocations were observed.

The enhancement of visible and infra-red electroluminescence in zinc sulphide crystals by infra-red irradiation

When single crystals of zinc sulphide are made with only a few parts per million of copper and chlorine impurities and provided with blocking-type electrodes, they show visible electroluminescence in d.c. fields. This emission can be considerably enhanced by simultaneous application of near infra-red radiation. A quantiative study is made of the enhancement effect and its dependence on field and infra-red intensity. A quantitative model is established in which it is assumed that positive holes released by the infra-red irradiation migrate to the cathode contact and there modulate the blocking potential barrier to give enhanced electron flow into the crystal with increased occurrence of impact excitation of the crystal. Further experiments in a.c. fields at 77°K show that combined electric field and infra-red irradiation produce infra-red emission in the known bands between 14 and 2 μm. A qualitative explanation is provided in this case for the dependence on applied field and infra-red irradiation intensities.

The effect of radiation damage at room temperature on the thermoluminescence characteristic of single crystals of zinc sulphide

A study of the thermoluminescence curves of single crystals of zinc sulphide before and after bombardment with monoenergetic electrons in the range 100–400 keV reveals a threshold energy at 240–250 keV above which a new trapping state is formed.

Disordering of Defects in Single Crystals of Zinc Sulfide

Stacking faults are observed in zinc sulfide (wurtzite) single crystals on basal planes (0001) and on prismatic planes (112̄0) and (12̄10) to the growth surface. The degradation of these stacking faults in different environments is described. The loop contrast observed to form at stacking faults on prismatic planes when exposed to high electron beam intensities in the electron microscope is explained by a thermally activated mass-transport mechanism. Migration of vacancies or atoms occurs depending on whether the stacking fault is extrinsic or intrinsic, respectively. Subjecting crystals to high pressures (about 60 kbar) results in an increase in the number of basal plane stacking faults. This is equivalent to a gradual phase change occurring over several atomic planes. Thermal etch figures observed by transmission electron microscopy in crystals annealed at 700° and 900°C are identified as a crystallographic polarity effect.

The influence of pressure on the luminescence of zinc sulphide single crystals

Experiments were made which show that luminescence originated under the application of pressure on the single crystals of zinc sulphide. The calculated formula gives the same dependence of luminescence on the change of pressure dP/dt as in [1], [2].

Electroluminescence of Zinc Sulfide Single Crystals

In a series of experiments designed to distinguish between ionization and recombination processes during electroluminescence, rectangular voltage pulses were applied to single crystals of zinc sulfide. During the period, the emission gradually increased to a stationary level. Removal of the voltage resulted in a burst of emission, which decayed slowly. This behavior is explained by the assumption of three field-dependent processes, namely: (a) ionization of centers in barrier regions; (b) sweeping out of the liberated electrons from the vicinity of the centers; and (c) tunnelling of electrons into the barrier regions. It was found that for a considerable voltage range the product of the light-sum (measured during the off period) by the current was proportional to the stationary emission level during the period. This proportionality strongly supports the model suggested. In many cases the current was found to be more strongly dependent on the voltage than the light sum. This result favors an ionization mechanism which is independent of the current. The model suggests furthermore an alternative explanation for the Neumark-effect, i.e., the enhancement of the thermal glow by electric fields. The explanation does not involve an impact-ionization mechanism. (auth)

XXIX. Luminescence in ZnS : Cu, CI Single Crystals

The impurity responsible for the green and blue bands in ‘ spectrographically pure’ single crystals of zinc sulphide is proved to be copper, in conjunction with chlorine. The crystal growth process tends to exclude chlorine so that introduction of copper results in luminescence typical of a low chlorine content, i.e. there is no green-2 band and the electroluminescence is blue at room temperature. The diffusion rate of chlorine is found to be much less than that of copper so that attempts to introduce both copper and chlorine into the crystals were unsuccessful. The provision of copper during crystal growth permits the retention of a greater chlorine content. This gives both the green-2 band and green electroluminescence.

Electroluminescence in Disordered Zinc Sulphide

IN recent times a detailed study has been made of the structure of zinc sulphide crystals1, and of the solid-state properties, particularly electroluminescence2, of this compound. of the many factors which may influence the electrical and optical properties of zinc sulphide, little attention has so far been paid to the effect of the modifications which can occur in the crystal structure. In addition to the well-known cubic (sphalerite) and hexagonal (wurtzite) forms, single crystals of zinc sulphide can also be obtained with a one-dimensional stacking disorder of the hexagonal (0001) planes3. This disorder may exhibit a periodic arrangement resulting in the formation of polytypes1. Although no experimental work seems to have been published linking electroluminescence with disorder, Schneer4 has, from theoretical considerations, suggested that there may be some connexion; in addition, the electroluminescent properties of some zinc sulphide phosphors have been tentatively ascribed to a partial transformation from hexagonal to cubic5.

Electroluminescence of Zinc Sulfide Single Crystals

Quantitative measurements of the electroluminescent emission from synthetic single crystals of zinc sulfide with silver paint contacts under 60-cycle half-wave voltage excitation have been carried out at several temperatures. Other workers have shown that the emission intensity has two major peaks per cycle, the first coincident with the voltage maximum and the second as the voltage is falling to zero. These peaks have the same color, and both appear to originate throughout the crystals, but they have different voltage dependencies. The second peak is the larger at low-voltage amplitudes, but the first peak grows the more rapidly with increasing voltage amplitude. Emission intensities of the order of ${10}^{12}$ photons per second, with energy efficiency of the order of ${10}^{\ensuremath{-}5}$, were obtained. The total current consists of a displacement current which is linear with voltage and a conduction current which is roughly exponential with voltage. No evidence of space-charge polarization was detected. It is tentatively suggested that excitation occurs by impact of conduction electrons accelerated through internal barriers, that the two emission peaks result, respectively, from immediate recombination and from recombination delayed by trapping of the excited electron, and that the electric field tends to hold the electron in the trap state.

Electroluminescence in Single Crystals of Zinc Sulphide

An apparatus for the measurement of electroluminescence in single crystals of zinc sulphide under a range of conditions is described. The results of the measurements are compared with existing theories, which are shown to be unsatisfactory. Modified forms of these theories are developed which are in much better agreement with experiment.

Some Electrical and Optical Properties of Synthetic Single Crystals of Zinc Sulfide

The fundamental optical absorption edge and the temperature dependence of electrical conductivity of synthetic single crystals of hexagonal zinc sulfide have been measured, as well as the photoconductivity near the absorption edge. The fundamental absorption coefficient is 1 ${\mathrm{mm}}^{\ensuremath{-}1}$ at a photon energy of (3.58\ifmmode\pm\else\textpm\fi{}0.02) ev and increases exponentially by a factor of ten every 0.07 ev up to ${10}^{3}$ ${\mathrm{mm}}^{\ensuremath{-}1}$. The logarithm of the electric resistivity varies linearly as a function of the inverse absolute temperature with a slope of (3.77\ifmmode\pm\else\textpm\fi{}0.1) ev/2k. A peak in the spectral variation of photoconductivity has been observed at the optical absorption edge.

Conductivity Pulses Induced in Single Crystals of Zinc Sulfide by Alpha-Particle Bombardment

Conductivity Pulses Induced in Single Crystals of Zinc Sulfide by Alpha-Particle Bombardment

Oxidation of Single Crystals of Zinc Sulphide studied by Electron Diffraction

CRYSTALS of sphalerite were heated in air until covered by a thin film showing interference colours. These crystals were photographed in an electron diffraction camera, which will be described elsewhere. The angle between the incident beam and the crystal face was very small. The wave-length was 0.053 A. (calibrated from a powder photograph of MgO and a transmission photograph of MoS2). The distance between the crystal edge and the photographic plate was 303.5 mm. Figs. 1, 2 and 3 represent the diffraction patterns in three different cases. In Figs. 1 and 2 the oxidised face was (111), and the incident beam was perpendicular (Fig. 1) and parallel (Fig. 2) to the octahedron edge. By photographing along two different octahedron edges the same pattern was obtained. In Fig. 3 the oxidised face was (110), and the incident beam was perpendicular to the cleavage edge [001]. This direction is in fact parallel to an octahedron edge. The diffraction patterns are all of the transmission type, on which is superposed a faint powder photograph. They form a series of photographs such as would be expected if we had photographed a single crystal structure in different directions, the crystal having for each direction of the incident beam the form and dimensions necessary for obtaining a point pattern.