Frequency dependence of electroluminescence processes in ZnS crystals

The frequency dependence of the integral electroluminescence brightness and the electrical conductivity of ZnS-Cu, Cl and ZnS-Mn single crystals was examined. It was established that the correlation between the integral electroluminescence brightness and the electrical conductivity in ZuS-Cu, Cl crystals improves slightly at high frequencies of the exciting field, whereas no such correlation exists for the frequency range 30 Hz-18 Hz in ZnS-Mn crystals. Conclusions regarding the various excitation mechanisms of the electroluminescence in ZnS-Cu, Cl and ZnS-Mn crystals are drawn.In conclusion, the authors express their sincere gratitude to P. E. Ramazanov for his valuable advice.     

Size effect in the magnetic field dependence of nonequilibrium electron tunneling

The electron bottleneck in normal metal-insulator-metal tunnel junctions due to nonequilibrium effects has been investigated in the presence of a magnetic field. The observed results are due to a new geometrical size effect and are explained in an extension of our nonequilibrium theory.     

Pulsed laser induced absorption (one–four) dependence of electron mobility in CdI2 and ZnS crystals

The photoconductivity and electron mobility of CdI₂ and ZnS crystals have been studied using N₂-laser, fundamental and frequency doubled Nd:YAG laser. Low values of the electron mobilities obtained in the present case have been attributed to laser-induced-absorption. It is low in one photon excitation and increases with the order of absorption.     

Temperature dependence of photoluminescence and electroluminescence of ZnS:Mn crystals

By means of comprehensive analysis of the temperature dependences of the photoluminescence for ZnS:Mn crystals, we have observed persistent changes in the intensity, shape, and position of the maximum in the emission spectrum associated with changes in the immediate environment of the manganese luminescence centers. We have also observed inflection points on the voltage vs. brightness characteristics of the samples, the position of which depends on the frequency of the exciting voltage and the temperature. The observed dependences are explained using concepts describing the mechanisms of pre-breakdown luminescence. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 631–636, September–October, 2006.     

Excitation and temperature dependence of the blue luminescence of ZnS crystals

The blue luminescence of pure and Al-, Au-, Ag-, Cu-, and Mn-doped ZnS crystals has been studied in the temperature range between LNT and RT and for different wavelengths of the exciting UV-light. All investigated crystals emit two blue bands. The energies of the maxima of these emission bands are independent of the temperature. The temperature dependence of the intensity ratio of the two blue luminescence bands can simulate a temperature shift of the blue emission. The experimental results support the assumption of an essentially uniform structure of the luminescence complex responsible for the visible luminescence of pure and doped ZnS phosphors.     

Temperature dependence of unthermalized nonequilibrium charge carrier mobility in cubic ZnS far from a phase transition

It is shown that if we assume the energy of nonthermalized nonequilibrium charge carriers is independent of temperature, the character of the relationship between temperature and mobility varies from nonlinear to linear and at a certain ratio between scattering mechanisms it is temperature independent.     

Anomalous temperature dependence of the near-infrared absorption of ZnS:Cr crystals

We report an anomalous temperature dependence of the broad-band ⁵T₂→⁵E transition at 5700 cm^-1 in the Cr impurity in ZnS. The band half-width and peak position increase linearly with temperature. These dependences are consistent with a large difference in the frequency of the Jahn-Teller modes between the excited and ground states.     

Melting of polydisperse colloidal crystals in nonequilibrium

The influence of a time-dependent oscillatory external field on the melting transition of a polydisperse colloidal crystal is examined by theory and computer simulation. In a monodisperse crystal the field just induces an overall dynamical mode which does not affect the melting line. For a polydisperse sample, on the other hand, the field shifts the melting line towards smaller temperatures. Combining a solid cell approach and a Lindemann criterion in nonequilibrium, a simple theory is presented showing that the temperature shift scales with the square of the relative polydispersity. The theory is in reasonable agreement with nonequilibrium Brownian dynamics computer simulations.     

The elastic behaviour of tetrahedral materials with vacant sites

As part of a study of defect tetrahedral structure compounds, the elastic constants of single crystal specimens of Hg₃Ga₂□Te₆ and HgIn₂□Te₄ have been measured between 77 K and room temperature using the pulse superposition technique. These compounds are included in the series HgTe, Hg₅In₂□Te₈, Hg₃In₂□Te₆ and HgIn₂□Te₄ and HgTe, Hg₅Ga₂□Te₈, Hg₃Ga₂□Te₆ where there is a progressive increase in the concentration of vacant sites. While the other compounds studied are cubic, HgIn₂□Te₄ has a tetragonal structure with a $\text{c}\text{a}$ ratio of 2.0. The components of the elastic stiffness tensor of this material at 77 K are (in units of 10¹¹ dyne cm^?2) C₁₁ = 4.31 C₁₂ = 2.54 C₄₄ = 2.14 C₃₃ = 4.47 C₁₃ = 2.18 C₆₆ = 2.41. In a cubic material C₁₁ = C₃₃, C₄₄ = C₆₆ and C₁₂ = C₁₃: the elastic behaviour of this tetragonal compound closely resembles that of a cubic material, as might be anticipated from its structure. This similarity is further illustrated by reference to the symmetry of phase velocity and Young's modulus surfaces. Examination of the elastic constants and reduced elastic constants of these compounds shows a regular trend, the elastic stiffness decreases as the number of vacant sites increases. There is an approximately linear relationship between the reduced bulk modulus and the number of sited vacancies.     

Anomalous electric-field dependence of the temperature-independent electron mobilities in organic molecular crystals

The model proposed by the present author (this journal $\text{28}$, 309 (1978)), which explained successfully the temperature (T)-independent mobilities above about the Debye temperature θ, predicts that the drift velocity becomes saturated in the high-electric-field region when T?θ, and that it makes a peak in the low-field one with an intermediate minimum when $\text{T « θ}$. The boundary between the two regions is on the order of 10⁵ V/cm.     

The Urbach rule in ZnS crystals

The slope parameters of the exponential absorption edges of 3C cubic, 4H polytypic and 2H hexagonal ZnS crystals were measured as a function of temperature. Their behavior show that different phonons interact with the electron transitions in different temperature ranges, an LA piezoelectric phonon below approximately 100°K and an LO phonon above this temperature up to 300°K. In cubic ZnS the slope parameter was measured up to 750°K and the same LO phonon was found to be involved at these temperatures.     

Polarized emission of doped ZnS crystals

Polarization of light emitted in various spectral ranges was studied in two single crystals ZnS:Cu, Cl and ZnS:Ag, Cu, Al. The G-Cu, B-Cu and a small amount of S-A centers have been identified in the ZnS:Cu, Cl crystal by the spectral and polarization methods. The B-Ag band was found in the spectrum of the ZnS:Ag, Cu, Al crystal and its polarization properties investigated. This emission appears to be always polarized perpendicular to the [111]_c axis of the stacking faults independently of the polarization of the exciting light. The symmetry of the B-Ag center is not lower than that of the host lattice. Analogy with G-Cu centers suggests a model for the B-Ag center in which the polarization comes from the symmetry properties of the Ag²⁺ orbitals in the trigonal field of stacking faults.     

Low-voltage cathodoluminescence of ZnS single crystals

Bright blue and green cathodoluminescence from low resistivity ZnS crystals has been observed under the excitation of low-energy electron beams of several tens of volts; i.e., 40 fL at 50 V. Properties of the surface of the crystals are studied by the dependence of current and brightness on applied voltage and by the spectra of cathodoluminescence and photoluminescence.     

Flowing crystals: nonequilibrium structure of foam

Bubbles pushed through a quasi-two-dimensional channel self-organize into a variety of periodic lattices. The structures of these lattices correspond to local minima of the interfacial energy. The "flowing crystals" are long-lived metastable states, a small subset of possible local minima of confined quasi-two-dimensional foams [P. Garstecki and G. M. Whitesides, Phys. Rev. E 73, 031603 (2006)10.1103/PhysRevE.73.031603]. Experimental results suggest that the choice of the structures that we observe is dictated by the dynamic stability of the cyclic processes of their formation. Thus, the dynamic system that we report provides a unique example of nonequilibrium self-organization that results in structures that correspond to local minima of the relevant energy functional.     

Orientation of luminescence centers in self-activated ZnS crystals

Results concerning polarization of the self-activated (SA) luminescence in ZnS crystals are presented. It is assumed that the orientations of absorption centers — circular oscillators (rotators) — and emission centers — rotators and dipoles — are along the crystal 3-fold symmetry axes. The calculated polarization diagrams fit well to experimental results.     

Temperature dependence of high field transport in ZnS

High field transport process in ZnS in the temperature range of (10–500) K was simulated by help of Monte Carlo method. The band structure of ZnS is described by analytical fitting of real band structure. Phonon scattering, spatial charge scattering, and impact ionization process are included in the simulation. The phonon scattering rates at different temperatures are calculated and compared. The transient acceleration time of electrons in ZnS is found to be temperature-independent. We attribute this result to the compensation of two opposite factors in ZnS. Average energy of electrons decreases with temperature.     

Red luminescence of ZnS: Fe crystals

In order to elucidate the nature of the red emission band at 660 nm in ZnS: Fe crystals, the equilibrium luminescence, the thermoluminescence and the photoinduced EPR signals were measured, using UV, or the simultaneous UV + IR excitations. In addition, the polarization properties and the light-pressure shift of the red luminescence were studied. On the basis of the obtained results and of the data reported in the literature and concerning the red and IR emissions in ZnS: Fe a donor-acceptor model is proposed, in which the transition Cl^2?→Fe³⁺ would give rise to the 660 nm band.