Relation between crystal structure and electroluminescence in ZnS-Mn crystals

The electroluminescence of ZnS-Mn crystals annealed in zinc melt is investigated. When an external field of 10³-10⁴ V/cm is applied, continuous radiation is emitted over the whole volume of the spcimen. The volt-brightness characteristics are approximated by various functions and depend on the conditions of excitation. The results obtained are explained by the concentration of the electric field at barrier formed by partial dislocations at the place where they cut the hexagonal layers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 33–37, September. 1985.     

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.     

Variation of electrical conductance of ZnS single crystals under the influence of an alternating excitation potential

The variation of electrical conductance was investigated in electroluminescent ZnS-Cu and ZnS-Mn single crystals by measurements in a weak constant field under the influence of an alternating excitation potential. It was found that the conductivity variation for these single crystals differed substantially, and it was concluded that ZnS-Cu and ZnS-Mn have different electroluminescence mechanisms.     

The electric field gradient at iodine impurities in zinc and cadmium

The electric quadrupole interaction at iodine implanted in zinc and cadmium single crystals is investigated, using the - perturbed angular correlation technique. The results show that the implanted¹²⁹Te and¹³²Te isotopes occupy two well-defined sites, identified by the electric field gradient parameters. One fraction of the impurities is substitutionally implanted, witheq _zz=–1.49(9)10¹⁸ and –1.58(15)10¹⁸V/cm², in Zn and Cd respectively. The other fraction is identified as an impurity-divacancy configuration with |eq _zz |=3.81(8)10¹⁸V/cm²; =0.72(4) for Zn and |eq _zz |=3.55(11)10¹⁸V/cm²; =0.53(13) for Cd. The electronic enhancement in the electric fieldgradient at the substitutional site equalsK=–0.66 and –0.56 respectively and follows the empirical trend ofK versus the impurity valence for the s-p elements in Zinc.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

On the dependence of the coercive field of BaTiO3 single crystals on their thickness

The paper gives the results of measuring the coercive field of single-domain single crystals of BaTiO₃, the thickness of which was lowered by successive etching. The initial decrease in the thicknessd is accompanied by a sudden increase in the coercive fieldE _c (E _c /d –1·5 × × 10⁵ V/cm²); after etching off layers larger than 10^–3 ÷ 2×10^–3cm the coercive field grows much more slowly (E _c /d –7×10³ V/cm²). The high initial growth ofE _c is interpreted by means of Schottky exhaustion layers with non-zero gradient of the electric potential.In conclusion, the authors would like to thank H. Arend, P. Coufová and J. Jarý for providing high-quality single crystals and for much valuable advice during the work and V. Dvoák and K. Pátek for remarks on this paper.     

Precision PAC measurements in Er2O3 and Ho2O3 single crystals and structure refinement

The structure of C-form Ho₂O₃ and Er₂O₃ single crystals and powder samples was investigated by the electric quadrupole hyperfine interaction of¹¹¹In(EC)¹¹¹Cd probe ions using the perturbed - angular correlation method (PAC). The resulting set of refined atomic coordinates is compared to X-ray data and used to calculate the orientations of the electric field gradients (EFG) which are reproduced by the PAC measurements in single crystals. The temperature dependence of the coordinates was measured for both substances.     

Cooperative emission of photons by weakly coupled chromium ions in Al2O3

Under strongcw-excitation ruby crystals exhibit complex anti-Stokes fluorescence spectra in the near ultraviolet. Based on ring dye laser excitation spectra, fluorescence decay and concentration dependence, we show that these spectra can be decomposed into single ion emissions and a cooperative emission at the double energy of the² E⁴ A ₂ transition. The unusual vibronic sideband of the latter suggests a general mechanism for the coupling of distant ions in polar insulators involving the electric field of longitudinal optical phonons.Work supported by Sonderforschungsbereich 65 Frankfurt/Darmstadt     

Quadrupole interaction studies of Hg in Zn and lattice location of Hg and Au in Zn

¹⁹⁷Hg,¹⁹⁹Tl and¹⁹⁹Au ions were implanted into Zn single crystals. From time differential perturbed e^–- and - angular correlation experiments with the 5^–/2 states in¹⁹⁷Hg and¹⁹⁹Hg, respectively, the quadrupole interaction for Hg in Zn is reinvestigated. Only the data for the¹⁹⁹Au sample exhibit a single unique quadrupole precession pattern in agreement with the result of the lattice location studies by use of the channeling technique, which show Hg to occupy random lattice sites in Zn while Au is substitutional.From the measured interaction frequency, sign and magnitude of the effective electric field gradient for Hg on the substitutional site in Zn is determined asV _zz ^eff = +15.2(1.8)×10¹⁷V·cm^–2. By comparison of this value to the corresponding lattice field gradient contribution an electronic enhancement factor ofK(HgZn)=2.6 is deduced in disagreement with a previous derivation of this quantity, but which supports the proposal that the electronic field gradient depends linearly upon the valence of the impurity probe atom.     

Electroluminescence of alkali halide crystals activated with europium ions

The prebreakdown electroluminescence of NaCl-Eu, KCl-Eu, and KBr-Eu crystals has been studied. The activator concentration in the crystals was on the order of 10¹⁸ ions per cubic centimeter. The emission spectra, the brightness wave during pulsed excitation, and the concentration dependence of the quantum yield were measured. The experimental results support the suggestion of an impact mechanism for the excitation of the electroluminescence of alkali halide crystals. Recombination and exciton mechanisms for energy migration play an insignificant role. There is no significant contribution from impurity-vacancy dipoles, which act as emission centers, in electric fields up to 10⁶ V/cm.Tomsk Academy of Control and Electronic Systems. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 105–109, February, 1994.     

Influence of the defect state of samples on the luminescence spectra of CdS single crystals irradiated by electrons

The photoluminescence spectra of CdS single crystals irradiated by electrons (E = 1.2 MeV, Φ = 2×10¹⁷ cm^?2) are investigated in the visible and near-infrared regions of electromagnetic radiation. Some samples of the CdS single crystals are preliminarily irradiated by neutrons (E = 2 MeV, Φ = 2 × 10¹⁸ cm^?2) with the aim of increasing the concentration of initial structural defects. From analyzing the peak intensities of photoluminescence in the irradiated single crystals at the wavelengths λ_m = 0.720, 1.030, and 0.605 μm, it is concluded that the CdS samples with a low concentration of structural defects in the initial state possess the highest resistance to electron radiation. It is assumed that the observed transformation of the photoluminescence spectra of the imperfect CdS single crystals subjected to electron irradiation is determined by either the mechanisms of subthreshold defect formation or the transformation of the defect complexes in elastic and electric fields near the large structural damages of the crystal lattice.     

Luminescence of ZnO Having a Superstoichiometric Content of Oxygen

Using a unique method of radical-beam heteroepitaxy (RBHE) based on annealing of the crystals of the II–VI (ZnSe) compounds in a flow of radicals of a metalloid component — oxygen O — ZnO layers with a superstoichiometric content of oxygen were obtained. The conductivity of the layers = 10² ·cm, the mobility of the holes = 23 cm²/V·sec, and the concentration N _A = 10¹⁵ cm^–3. The luminescence spectra of pure ZnO single crystals and those doped with Li and Na, both treated by the RBHE method, are studied at helium temperatures. The luminescence centers associated with the intrinsic defects (V _Zn) are identified.     

New determination of quadrupole coupling constants of Li isotopes in single crystals LiIO3 andLiNbO3

Quadrupole effects in NMR spectra of⁸Li(I =2⁺;T _1/2=0.84 s) in LiIO₃ andLiNbO₃ single crystals have been detected by use of a modified -NMR. The coupling constants for both crystals are in agreement with known ones. Field gradients in the crystals were measured by detecting pulsed-Fourier transformed NMR of⁷Li. The quadrupole moments deduced from both samples agree, and|Q(⁸Li; 2⁺)|=32.7±0.6 mb has been determined.     

Anisotropy of the nonlinear optical susceptibility χ(3) in polydiacetylene single crystals

The anisotropy of ⁽³⁾ in Poly-bis(p-Toluene Sulfonate) of 2.4-hexadiyne-1.6-diol (PTS) PolyDiAcetylene (PDA) single crystals is investigated by Degenerate Four-Wave Mixing (DFWM) at a wavelength of 720 nm. The symmetries of ⁽³⁾(–;, –,) identify the non-vanishing and independent components of the tensor. For the calculation of the ⁽³⁾ elements from the measured values, the anisotropy of both the linear optical index of refraction and the absorption coefficient is included in the coupled wave approach. A magnitude of 10^–10 esu is measured for the ⁽³⁾ component if all beam polarizations are parallel to the polymer chains. The least upper bound for all other measurable components is estimated to be 10^–12 esu. The results for the intensity dependence of the DFWM signal are explained in terms of a thermooptic effect. Measurements on the orientational dependence of the DFWM signal in the samples are carried out, and the influence of the anisotropy of the linear optical parameters on the measured curves is discussed. A method for the preparation of thin layered polymer single crystals of poly-bis (4-ButoxyCarbonyle-Methylene-Urethane) of 4.6-decadiyne-1.1-diol (4-BCMU) and the first DFWM measurements on these samples are presented.     

Search for giant Franz-Keldysh-like effects in GaSe and other layered semiconductors

Recently, an anomalously large redshift of the absorption edge with electric field was claimed for -GaSe_1–x S _x layered crystals. We have studied Bridgman grown -GaSe crystals as well as vapor transport grown -GaS, and 2H-WSe₂. While we have observed a shift in the absorption edge for -GaSe similar to that reported in previous work, our results demonstrate that the redshift arises from Joule heating, and is thus temperature induced, rather than intrinsic. For -GaS, much larger resistivities virtually eliminate Joule heating, and our measurements of the electric field induced absorption edge shift yield an estimated upper limit of approximately 0.04 meV · cm/kV for a field of 2.4×10³ kV/cm, in good agreement with the theoretical value expected for the Franz-Keldysh effect.Also at University of Konstanz     

Charge transfer over localized states in a TlS single crystal

It is revealed that TlS single crystals exhibit a variable range hopping conduction along a normal to their natural layers at temperatures T ≤ 230 K in a dc electric field and a nonactivated hopping conduction at low temperatures in strong electric fields. Estimates are made for the density of states near the Fermi level (N _F = 2.8 × 10²⁰ eV^?1 cm^?3 and their energy spread (ΔW = 0.02 eV), the localization radius (a = 33 Å), the average jump distance in the region of activated (R _av(T) = 40 Å) and nonactivated (R _av(F) = 78 Å) hopping conduction, and also the drop in the charge carrier potential energy along the jump distance in an electric field F: eFR = 0.006 and 0.009 eV at F = 7.50 × 10³ and 1.25 × 10⁴ V/cm, respectively.     

Spin dynamics in RENi5 ferromagnets byμSR measurements

We report a zero fieldSR study of single crystals of theRENi ₅ ferromagnets where RE=Gd, Er, Dy or Tm. Our work points out the strong influence of the crystal electric field on the rare earth total momentum dynamics: whereas the spin-lattice relaxation process belowT _c is controlled by a two magnon mechanism for a weakly anisotropic magnet (GdNi ₅), the phonons drive the relaxation for a strongly anisotropic magnet such asErNi ₅. ForTmNi ₅ a comparison is made between the fluctuation time measured bySR and¹⁶⁹ Tm Mössbauer spectroscopy.     

Drift mobility and diffusion with an electric field of Ba2+ ions in NaCl crystals

Using a carrier-free tracer¹³³Ba the temperature dependences of the diffusion coefficientD and mobility of Ba²⁺ ions in an electric field have been measured on NaCl crystals in the temperature range from 500 to 750 °C. It is shown that the obtained values ofD are not affected by an electric field (30 V/cm) and that the temperature dependence of the mobility of Ba²⁺ ions is given by(T)=(12·3±1·9) exp [-(1·51±0·09) eV/kT]. Vacancy jump frequencies near a Ba²⁺ ion and the Gibbs free energy of association of this ion with a vacancy have been calculated in terms of a simple 5-frequency model for impurity diffusion by a vacancy mechanism.     

Optical and Photoelectric Inhomogeneity of CdTe:V Crystals

The optical and photoelectric properties of CdTe:V crystals with the doping impurity concentration N _V = 5·10¹⁸–5·10¹⁹ cm^–3 are investigated and the possibility of their use as a photorefractive material is considered. As is seen from the spectra of optical transmission, the crystals of both types possess high transparency (50–65%), which for CdTe:V specimens with N _V = 5·10¹⁹ cm^–3 decreases sharply and in the range 12–14 m does not exceed 5%, whereas for CdTe:V crystals with vanadium concentration of 5·10¹⁸ cm^–3 such a value of transmission remains unchanged up to 25 m, implying a good optical quality of the latter crystals and their possible application in the spectral range 1.06–1.25 m in modern fiber-optic communication lines.     

Optical and Paramagnetic Properties of Synthetic Diamond Single Crystals with a Low Content of Nitrogen

Optical and paramagnetic properties of synthetic diamond single crystals grown by the temperature gradient method in the Fe-Co-C and Fe-Co-Al-C systems using split-sphere high-pressure apparatus have been investigated. For crystals with a high (>5·10¹⁷ cm^–3) nitrogen concentration a good agreement of the concentrations of single substitution nitrogen atoms obtained from EPR, IR, and UV measurements was observed. For crystals with a low ((0.5–5.0)·10¹⁷ cm^–3) nitrogen concentration, there was no correlation between the results of substituting nitrogen concentrations obtained by the methods mentioned. It is shown that this behavior is attributable to the transition of the substituting nitrogen atoms from the paramagnetic neutral to nonparamagnetic positive charge state due to compensation by the boron impurity.