Zinc Sulfide Crystals Research Articles

A mechanism of field quenching in zinc sulphide phosphors

A model is proposed to explain the experimental observations on the quenching of alpha particle scintillations in zinc sulphide crystals by the application of an electric field. The existence of two kinds of centre in the forbidden gap is assumed, one associated with the ground state of a luminescence centre and the other with a non-radiative centre. The necessity for such centres, and the time sequence of events in the crystal is considered. Good agreement with experiment is obtained and the height of the ground state of the luminescence centre above the valence band is found to be 0.35 eV. The extention of this model to bombardment conductivity and electroluminescence in these crystals is also considered.

The operation of crystal counters with non-uniform electric field distribution

A simple theoretical analysis of the magnitude of bombardment conductivity pulses produced in a Schottky type exhaustion barrier is carried out. For the energetic particles incident on the crystal normal to the field direction, the variations of pulse size with position of incidence in the barrier and with applied voltage are found. The case of incidence parallel to the field direction is also considered. Comparison with experiment for zinc sulphide crystals shows good agreement with the predicted results, and values of the electron lifetime and donor concentration are obtained.

Determination of Zinc Sulfide and Cadmium Sulfide in Solid Solutions of Small Single Crystals Used for Semiconductors by X-Ray and Chemical Methods

AbstractSingle crystals of cadmium sulfide and zinc sulfide have been grown and studied intensively by the Solid State Physics group at the Aeronautical Research Laboratory. The physical phenomena such as reflection, transmission, ultraviolet-excited emission, and electrical resistivity have been observed and characterized on single crystals of these sulfides. Much interest concerning these phenomena has also been centered on single crystals containing both cadmium sulfide and zinc sulfide.For research purposes, mixed crystals as small as a few tenths of 1 mg or less, to 0.5 g of the mixed sulfides, are being prepared. Special chemical methods are required to determine these constituents in them quantitatively. At times, these chemical methods are not applicable, since these methods invariably consume the sample, and, as a result, other observations on the same crystals cannot be performed.Changes in lattice parameter in single crystals of mixed sulfides as compared to pure zinc sulfide or cadmium sulfide provide excellent means for the determination of the percentage of these sulfides. In the X-ray method, single crystals used for the determination of the lattice parameters remain intact. The equipment adopted, procedure used, and the data obtained are illustrated and discussed.In the present study, crystals of cadmium sulfide (greenockite), alpha-zinc sulfide (wurtzite) and solid solutions of these two sulfides having a hexagonal unit cell were used. Since the lattice parameter a0 is found to follow Vegard's law, single-crystal rotation photographs described in this paper were obtained by rotating crystals around the c axis; the lattice parameter was determined with high precision by scanning along the zero-layer line with a microphotometer.

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)

Growth and Heat Treatment of Zinc Sulfide Single Crystals

Zinc sulfide crystals grow readily from the vapor phase if small traces of certain impurities, such as zinc oxide and copper, are present. The decrease of ambient impurity concentration leads first to very thin, flexible, ribbon‐like crystals which are hexagonal, and finally to nucleation with little growth. The purity of crystals depends not only on the composition of the starting material but also on the purity of the combustion tube employed; contamination during growth leads to an impurity gradient in crystals and to impurity variations among crystals. Crystals usually exhibit disorder of the crystal structure, although pure cubic or hexagonal structure can be achieved by annealing or quenching. Heat treatment also changes the impurity content and attendant properties, such as physical color, luminescence behavior, electrical properties, and disorder of the crystal structure.

Bombardment Conductivity in Synthetic Zinc Sulphide Crystals†

Bombardment conductivity pulses were observed in crystalline zinc sulfide due to irradiation with 5.3 Mev alpha particles. The counting rate and maximum pulse amplitude were examined as a function of the applied voltage. The results are interpreted in terms of an exhaustion barrier at the negative electrode and the properties of this barrier were determined. (auth)

Some Properties of Zinc Sulfide Crystals Grown from the Melt

Hexagonal zinc sulfide crystals were obtained by controlled cooling of melted zinc sulfide. Both pure and activated zinc sulfide powders were used. The density of pure melt grown crystals was found to be higher than that of natural zinc blende crystals or crystals grown by evaporation. The stability of the pure hexagonal crystals towards transformation to the cubic zinc-blende structure in the interval of temperature 700–1150°C was investigated. A full conversion to the cubic phase was never observed. The experimental evidence indicates the transition point to be above 1150°C.

Diffusion and Exchange of Zinc in Crystalline Zinc Sulfide

The diffusion and exchange of zinc vapor with crystals of zinc sulfide has been measured from 925 to 1075°C. The measured exchange rate was observed to be controlled by the diffusion of Zn in ZnS. A critical examination of the log D versus 1/T plot reveals that three distinct diffusion processes are operative in the temperature range studied. At 1 atmos zinc pressure, the diffusion coefficients were calculated as D=3×10−4exp(−35 kcal/RT) cm2 sec−1below 940°C, D=1.5×104exp(−75 kcal/RT) cm2 sec—1 between 940° and 1030°C, and D=1×1016exp(−150 kcal/RT) cm2sec−1above 1030°C. Variation of zinc pressure from 0.25 to 2.0 atmos showed that D varies as PZn1.5. Values of D were observed to increase with increasing penetration of Zn into the ZnS crystals. The kinetic and structural interpretations of these results are discussed.

Optical Properties of Hexagonal ZnS Single Crystals

The optical transmission of hexagonal zinc sulfide crystals has been measured in the spectral range from 0.32 to 15 \ensuremath{\mu}. From the spacing of interference maxima, the dependence of index of refraction on wavelength has been determined. Measurements of the effects of temperature from 20\ifmmode^\circ\else\textdegree\fi{}C to 120\ifmmode^\circ\else\textdegree\fi{}C and of pressure up to 1700 atmos on the width of the forbidden energy band indicate an increase in band gap of 9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ev/atmos with pressure and a decrease of 7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ${\mathrm{e}\mathrm{v}/\mathit{C}}^{0}$ with temperature for both ordinary and extraordinary rays. Approximately one-fifth of the shift with temperature is the result of dilatation of the lattice. The origin of the larger portion of the shift is discussed.

Photovoltages Larger than the Band Gap in Zinc Sulfide Crystals

Photovoltages Larger than the Band Gap in Zinc Sulfide Crystals

Spiral Crystals of Magnesium Sulphate

IN a communication in Nature of March 2, p. 493, there is a photograph of zinc sulphide crystals of spiral shape.

A New Type of Zinc Sulphide Crystals

BY sublimation of zinc sulphide in an atmosphere of hydrogen, I have obtained a new kind of zinc sulphide crystals. Fig. 1 shows two different views of a beautiful sample of these crystals.

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.

Potential Distribution and Localized Photoeffects in Zinc Sulfide Single Crystals

Potential distributions in synthetic zinc sulfide crystals have been measured and correlated with photoconductive regions found by light scanning experiments. Crystals having uniform potential distributions are found to have peak photosensitivity in their middle. An interpretation based on nonuniform light absorption is proposed.

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.

Decoration patterns in natural ZnS crystals

Synopsis Excess zinc or silver has been introduced in natural transparent zinc sulphide crystals, respectively by the Rexer method or by electrophoresis at temperatures between 500°C and 800°C. After such a treatment precipitates of the excess metal are observed in a number of nearly circular zones of variable size. Optical examination proved that they are formed by limited cleavage occuring in the early stages of the heat treatment. They are to be considered as thin, flat, lens shaped cavities with their main dimensions parallel to {110} faces. They are gradually filled in as the heat treatment proceeds, with zinc sulphide and excess metal, different stages of this growth in a same zone could be followed. The specks of excess metal are regularly distributed on lines having definite crystallographic directions, [111], [111], [001] in the central part of the grown in greater cavities. Decoration around their perimeter was similar to that in smaller areas. That dislocation lines have not been decorated is attributed to the absence of charged jogs in this type of crystals.

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.

Spiroidal Crystals of Zinc Sulphide

IN the course of a comprehensive investigation of stacking disorders and polymorphism in zinc sulphide crystals grown by sublimation, we have encountered a new type of crystal form. The term ‘spiroid’ has been used for conveying the topological character of the habit, and to differentiate the phenomenon from the surface texture associated with a spiral dislocation mechanism of growth.

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.