Luminescence of GaN single crystals prepared by heating a Ga melt in Na–N2 atmosphere

Colorless transparent prismatic crystals (0.5‐2.0 mm long) and hopper crystals (1.0‐2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor‐bound exciton (D⁰‐X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full‐width at half‐maximum (FWHM) of (D⁰‐X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D⁰‐A⁰) and its phonon replicas were observed in a lower energy range (2.9‐3.3 eV). The emission peaks of the D⁰‐A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰‐X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal data and some physical properties of Tl2InGaTe4 crystals

The room temperature crystal data, Debye temperature, dark and photoelectrical properties of the Bridgman method grown Tl₂InGaTe₄ crystals are reported for the first time. The X‐ray diffraction technique has revealed that Tl₂InGaTe₄ is a single phase crystal of tetragonal body‐centered structure belonging to the space group. A Debye temperature of 124 K is calculated from the results of the X‐ray data. The current‐voltage measurements have shown the existence of the switching property of the crystals at a critical voltage of 80 V. The dark electrical resistivity and Hall effect measurements indicated the n ‐type conduction with an electrical resistivity, electron density and Hall mobility of 2.49×10³ Ω cm, 4.76×10¹² cm^–3 and 527 cm²V^–1s^–1, respectively. The photosensitivity measurements on the crystal revealed that, the variation of photocurrent with illumination intensity is linear, indicating the domination of monomolecular recombination at room temperature. Moreover, the spectral distribution of the photocurrent allowed the determination of the energy band gap of the crystal studied as 0.88 eV. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SQUID magnetometry,EPR and XPS characterization of Y3Al5O12: Co,Si single crystals

Single crystals of garnet Y₃Al₅O₁₂ (YAG): Co and Co, Si were grown by Czochralski method as materials for optoelectronic applications. Silicon doping is responsible for the change of the Co³⁺ ions into Co²⁺ in order to keep charge neutrality of the material and for a coloration of materials. Magnetization and Electron Paramagnetic Resonance measurements show that the most cobalt ions in the silicon co‐doped crystals were Co²⁺ in octahedral positions. Estimated concentration of the Co²⁺ ions were close to Si⁴⁺ nominal concentration. A little excess of Co²⁺ may be ascribed to defects present in the crystals. For the crystal doped only with Co, the concentration of Co²⁺ was about 13% of the nominal Co amount and might be caused by crystal defects. X‐ray Photoelectron Spectroscopy measurements exhibit aluminium deficiency. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The color of langatate crystals and its relationship with composition and optical properties

Langatate crystals grown by the Czochralski method in 〈0001〉 and 〈01 1〉 directions both without subsequent annealing (from colorless to orange crystals) and with vacuum (colorless and pale‐green crystals) and air (orange crystal) annealing have been investigated by neutron diffraction, X‐ray diffraction and optical spectroscopy. The main types of point defects were compared with absorption bands: oxygen vacancies ‐ V_O^•• (0.35‐0.36 µm), the vacancies in the lanthanum position ‐ V_La‴ (0.29‐0.30 µm); the absorption band at 0.48‐0.49 µm is typical for the colored samples. The relationship between the X color coordinate (red component) and the oxygen contents has been obtained. The green color of the sample annealed in vacuum is due to the presence of Ta⁺³ ions in the La(Ta³⁺,Ga)O₃ phase that has the structural and geometric agreement with langatate structure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compensation of CdTe by Doping With Gallium

Semi‐insulating CdTe single crystals doped with Ga were grown from the vapour phase by the modified Markov technique MMT. The study of the resistivity map in the cross‐sections cut a long the growth direction has been performed. The compensation phenomenon is analysed in the framework of the three levels Fermi‐statistic model. It is shown that a semi‐insulation behaviour throughout the ingot is due to the compensation of shallow impurities by the deep level. From the low ‐temperature photolum inescence spectra it was concluded that shallow donors (Ga_Cd) are partly compensated by (Ga_Cd‐V_Cd)^‐and (Ga_Cd‐Cd_Te) complexes and by residual acceptors (Na_Cd, Cu_Cd) . The microscopic structure of (Ga_Cd‐Cd_Te) complexis proposed based on the value of its local phonon mode and the growth conditions. A native defect like Te_Cd which has a deep level near the middle‐band‐gap is suppose to give a stable compensation and a tolerance for variation in shallow impurity concentrations.     

Physical Properties of Vanadium Oxide Bronzes β-A0.33V2O5 (A = Ca and Sr)

Abstract

The vanadium bronzes.β-A_0.33V₂O₅(A= Ca and Sr) have been synthesized, and their electromagnetic properties investigated. They show phase transitions at 150K and 170K, respectively, accompanying with anomalies in the magnetic susceptibility and resistivity curves. These transitions are of charge order type. Below the transition temperature the magnetic susceptibility showed low dimensional behavior in both compounds, especially spin gap behavior in Sr_0.33V₂O₅. Such behavior is closely related to the crystal structure characteristic of the vanadium β-bronzes and a manner of ordering for magnetic V⁴⁺ ions at the transition.     

The growth of HgI2 crystals

The paper presents a system for αHgI₂ crystal growth by the temperature oscillation method. The system has a capability of crystal growing at an excess I₂ or Hg vapour pressure. Optimum conditions for producing crystals up to 2 cm³ by volume have been established. The crystals grown at an excess I₂ vapour pressure have higher resistivity and higher drift electron and hole mobilities — μ_e = 120 cm² V⁻¹ s⁻¹ and μ_h = 6 cm² V⁻¹ s⁻¹, respectively.     

Growth and characterization of pure and Co2+‐doped Li2B4O7 single crystals

Pure and Co‐doped Li₂B₄O₇ (LBO) single crystals were grown by the Czochralski method. Starting concentrations of Co₂O₃ in the melt were: 0.5, 0.85 and 1 mol% relative to Li₂CO₃. Technological factors affecting the quality of both crystals were discussed. Optical absorption and EPR spectra were analyzed to define the oxidation states and lattice sites of cobalt ions. It was shown that Co²⁺ ions enter LBO crystal at octahedral Li⁺ site positions. Low‐temperature EPR measurements revealed that two types of Co²⁺ complexes can be distinguished in the Li₂B₄O₇:Co crystals. Additional absorption calculated for γ‐irradiated crystals showed V_k type defects suggesting the creation of cation vacancies during growth. The concentration of the defects decreases with an increase of intentional Co concentration. Introduction of cobalt ions to LBO crystal is limited probably by the formation of cobalt ion pairs or by the entrance of cobalt as Co⁺. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence Spectra of GaS0.75Se0.25 Layered Single Crystals

Photoluminescence (PL) spectra of GaS_0.75Se_0.25 layered single crystals have been studied in the wavelength region of 500‐850 nm and in the temperature range of 10‐200 K. Two PL bands centered at 527 ( 2.353 eV, A‐band) and 658 nm (1.884 eV, B‐band) were observed at T = 10 K. Variations of both bands have been studied as a function of excitation laser intensity in the range from 8 × 10^‐3 to 10.7 W cm^‐2. These bands are attributed to recombination of charge carriers through donor‐acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.043 and 0.064 eV below the bottom of conduction band to acceptor levels located 0.088 and 0.536 eV above the top of the valence band are suggested to be responsible for the observed A‐ and B‐bands in the PL spectra, respectively.     

Electrical properties of semi-insulating CdTe0:9Se0:1:Cl crystal and its surface preparation

CdTe_0.9Se_0.1:Cl crystals doped with chlorine at 5×10¹⁷ cm^?3 level were grown by the vertical Bridgman method. The composition of Se throughout the ingot was nearly constant at x=0.110±0.016. The electrical resistivity of CdTeSe:Cl was 4.5×10⁹ Ω cm. Chemical etchants were employed to obtain stoichiometric and flat surfaces for electrode deposition, and the effects of the etchants on CdTeSe surfaces were analyzed by photoluminescence (PL) and AFM with different bromine concentrations and etching times. The mobility-lifetime products of electron and hole in CdTeSe:Cl crystals were of the order of ～10^?2 cm²/V and its values are greater than those for CdTe crystals. The energy resolution of a CdTeSe:Cl detector was tested using a ²⁴¹Am radioactive source.     

Growth of PBI2 single crystals from stoichiometric and Pb excess melts

We have successfully grown high-purity and -quality PbI₂ single crystals by the vertical Bridgman method. The rocking curves of four-crystal X-ray diffraction (XRD) show 120 arcsec in full-width at half-maximum (FWHM). The photoluminescence (PL) spectra at 7.8 K show the resolved intensive exciton emission line and the weak DAP emission band. The deep-level emissions are not observed. The measurement of the electrical and radiographic properties show that Leadiodide (PbI₂) single crystal has a resistivity of 5×10¹⁰ Ω cm and imager lag is 8 s, respectively. In order to improve the controllability of crystal growth, PbI₂ single crystals were also grown from a lead (Pb) excess PbI₂ source. The experimental results show very good reproducibility. In addition, the growth models of crystal are proposed, and the growth mechanism is discussed.     

Photostimulated changes of properties of CdTe films

The effect of illumination during the close‐spaced sublimation (CSS) growth on composition, structural, electrical, optical and photovoltaic properties of CdTe films and CdTe/CdS solar cells were investigated. Data on comparative study by using X‐ray diffraction (XRD), scanning electron microscopy (SEM), absorption spectra and conductivity‐temperature measurements of CdTe films prepared by CSS method in dark (CSSD) and under illumination (CSSI) were presented. It is shown that the growth rate and the grain size of CdTe films grown under illumination is higher (by factor about of 1.5 and 3 respectively) than those for films prepared without illumination. The energy band gap of CdTe films fabricated by both technology, determined from absorption spectra, is same (about of 1.50 eV), however conductivity of the CdTe films produced by CSSI is considerably greater (by factor of 10⁷) than that of films prepared by CSSD. The photovoltaic parameters of pCdTe/nCdS solar cells fabricated by photostimulated CSSI technology (J_sc = 28 mA/cm², V_oc =0.63 V) are considerably larger than those for cells prepared by CSSD method (J_sc = 22 mA/cm², V_oc = 0.52 V). A mechanism of photostimulated changes of properties of CdTe films and improvement of photovoltaic parameters of CdTe/CdS solar cells is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical property analysis of CZT:In single crystals after annealing by a two-step method

Indium-doped Cd_1−xZn_xTe (CZT:In) single crystals were annealed by a two-step method, including a high-temperature step and a low-temperature step in sequence. IR transmittance spectrum, I–V curve and PL spectrum were used to characterize the CZT single crystals. After annealing, the opto-electrical properties of the CZT:In crystals were improved obviously. The average IR transmittance was remarkably increased by about 23%, and the resistivity was enhanced by as high as four orders of magnitude. In the PL spectra, the intensity of the (D⁰, X) peak prominently increased, and the full-width-at-half-maximum was reduced. Meanwhile, the intensity of the DAP peak decreased greatly, and the structure became practically indistinguishable from the background. Moreover, the intensity of the D_complex peak also decreased. The investigation shows that these improvements in the physical properties after annealing are due to variations in the micro-structures. The two-step annealing method can eliminate precipitates/inclusions, remove impurities, compensate Cd vacancies, decrease dislocations and reduce internal stress.     

Micro‐Pulling Down Growth of Co‐doped Lithium Niobate Single Crystal Fibers According Er and Mg Contents and Photolumenescence Properties

The Er³⁺doped Mg:LiNbO₃single crystal fibers employed in our experiment were grown in air by a micro‐pulling down (μ‐PD) method from host materials of a congruent Li/Nb (0.945) ratio which were melt‐doped with a nominal molar concentration of 1, 3, 5% MgO and 0.6% Er₂O₃. The X‐ray diffraction analysis results indicated that the co‐doped crystals main tained the same structural characteristics as the undoped LiNbO₃, however the lattice parameters with Mg differed; c (Å) value decreased, and a (Å) increased than of pure LiNbO₃. The influence of dopants on the photoluminescence (PL) properties of the Er:Mg:LiNbO₃ single crystal fibers excited by laser lines of 514 nm was reported. Also, the PL properties according to temperature and the excitation power of Er:Mg:LiNbO₃ crystal fibers were analyzed.     

Luminescence characteristics of undoped and Eu‐doped GdCa4O(BO3)3 single crystals and nanopowders

Single crystals of GdCa₄O(BO₃)₃ (GdCOB) pure and doped with Eu concentration of 1 and 4 at% were grown by the Czochralski and micropulling‐down methods. The distribution of Eu ions in GdCOB crystals was uniform. The substitutions of Eu³⁺ in Gd, Ca(1) and Ca(2) cation sites and eventually formation Eu²⁺ have been investigated. The spectroscopic properties of crystals are compared with the properties of nanopowders obtained by sol‐gel method. Radioluminescence spectra of undoped GdCOB crystal show the characteristic emission of Gd³⁺ at about 312 nm, whereas this emission dramatically decreases in Eu‐doped crystals upon X‐ray excitation, as well as in Eu‐doped nanopowders excited in vacuum ultraviolet (VUV) region. The VUV excitation in the range 125‐333 nm for Eu‐doped samples leads to strong emission in red coming from the ⁵D₀ multiplet of Eu³⁺, only. In the photoluminescence decay kinetics of 312 nm emissions substantial shortening and departure for single exponential decay in Eu‐doped samples is clearly observed. Higher Eu doping results in further acceleration of the decay. In undoped GdCOB crystal, the lifetime of the Gd^{3+ 6}P_7/2 multiplet is 2.79 ms. The Eu^{3+ 5}D₀ decay kinetics monitored at 613 nm are rather constant. Numerical fitting of fully exponential curves, reveals lifetimes 2.7 ms for nanopowder and 2.5 ms for single crystal. The results suggest that this material may be used as a red phosphor in plasma display panels in nanopowder form because of strong excitation band of Eu³⁺ luminescence in the 160‐200 nm regions. Contrary to nanopowder sample, such an excitation band, attributed to the Gd³⁺–O^2– charge transfer was not observed in crystal obtained by the micropulling‐down method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Controlled synthesis of barium chromate microcrystals

Barium chromate (BaCrO₄) microcrystals with various morphologies such as shuttle‐like, x‐shaped, ellipsoid, leaf‐like crystals were synthesized from aqueous solutions using poly (sodium 4‐styrenesulfonate) (PSS) as template agent at room temperature. The characterization results show that pH values and concentrations of reactants and PSS are important parameters in the morphology and size evolution of BaCrO₄. The possible formation mechanism of BaCrO₄ crystals with different morphologies was proposed. PSS complexes Ba²⁺ first, then affects the nucleation and crystallization process by interacting with the plane face of crystal, which finally results in various morphologies and sizes of crystals. In addition, the possible mechanism of photoluminescence (PL) spectra was also proposed.     

Crystal Data,Photoconductivity and Carrier Scattering Mechanisms in CuIn5S8 Single Crystals

The X‐ray diffraction has revealed that CuIn₅S₈ is a single phase crystal of cubic spinel structure. The value of the unit cell parameter for this crystal is 1.06736 nm. The crystal is assigned to the conventional space group Fd3m. The photocurrent is found to have the characteristic of monomolecular and bimolecular recombination at low and high illumination intensities, respectively. The electrical resistivity and Hall effect of CuIn₅S₈ crystals are measured in the temperature range of 50‐400 K. The crystals are found to be intrinsic and extrinsic above and below 300 K, respectively. An energy band gap of ∼1.35 eV at 0 K, a carrier effective mass of 0.2 m₀ , an acceptor to donor concentration ratio of 0.9, an acoustic phonon deformation potential of 10 eV and a nonpolar optical phonon deformation potential of 15 eV are identified from the resistivity and Hall measurements. The Hall mobility data are analyzed assuming the carrier scattering by polar optical phonons, acoustic combined with nonpolar optical phonons, and ionized impurities.     

Microscopic studies on imperfections in selected YAB single crystals

The as‐grown surface and inner structures of undoped and Nd³⁺‐, Cr³⁺‐, V³⁺‐, Ce³⁺‐, Er³⁺ and Yb³⁺ – and (Er³⁺ + Yb³⁺) – doped yttrium aluminum borate (YAB) single crystals grown from (K₂Mo₃O₁₀ + B₂O₃) flux by spontaneous crystallization or top seeded solution growth (TSSG) technique, were investigated using optical and scanning electron microscopic and analytic chemical methods. Fine and rough growth hillocks of dislocational origin, growth layers, traces of inner planar defects and foreign phase crystalline debris were found and analyzed on the as‐grown faces of crystals. Irregular grains and regular block structures and foreign phase inclusions were observed and studied in the interior of the crystals. The chemical compositions measured by energy dispersive X‐ray spectrometry on perfect and imperfect micro regions are compared with those obtained by flame atomic absorption spectrometry on bulk crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the nature of point defects in in Te-doped gallium antimonide

Precision lattice constant and density measurements held on gallium antimonide single crystals grown from stoichiometric melts showed that doping of such crystals with Te lead to the intensification of Ga-supersaturated solid solution decomposition process with initial Frenkel defect (Ga_i + V_Ga) production. It is supposed that Te_sb — simple donors form complexes with V_Ga which are believed to be acceptors. Doping of GaSb with Te up to the levels above 2 · 10¹⁸ cm⁻³ leads to partial decomposition of GaSb(Te) solid solution supersaturated with Te.     

Growth and magneto-optical properties of Na0.37Tb0.63F2.26 cubic single crystal

Crystals of Na_0.37Tb_0.63F_2.26 solid solution with a fluorite structure have been grown from melt by the Bridgman technique. The measured values of the Verdet constant are V = 0.35 ± 0.02 and 0.105 ± 0.01 arcmin/(Oe cm) for λ = 633 and 1060 nm, respectively. The dispersion of the Verdet constant is studied in the range of 380–1060 nm. The Na_0.37Tb_0.63F_2.26 crystal is somewhat inferior to the previously studied noncubic fluoride crystals containing Tb³⁺ ions yields by V(λ) and is close to KTb₃F₁₀ crystal in totality of properties.