Annealing Effects on Photoconductivity Spectra of CuInSe2 Single Crystals

Photoconductivity spectra of as-grown n-type and p-type CuInSe₂ single crystals and of crystals annealed under maximum and minimum selenium pressure are measured in the photon energy range hv = 0.75 – 3.0 eV and as a function of temperature in the range T = 80 – 320 K. A model with trapping of minority carriers is used to explain the temperature dependence of the photoconductivity. The trap energies present in the samples depend on their preparation conditions.     

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

Photoluminescence spectra of Tl₄GaIn₃S₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λ_exc = 532 nm), and at T = 26 K with intrinsic excitation source (λ_exc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm^–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl₄GaIn₃S₈ crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of source composition on the properties of flash-evaporated thin films in the Cu–In–Se system

Using CuIn₂Se_3.5 as source material epitaxial layers are obtained on (111)A- und (100)-oriented GaAs substrates by flash evaporation in the substrate temperature range T_sub = 720—870 K. The composition of the films is between CuInSe₂ and CuIn₂Se_3.5. The structural properties of the films are similar to those for CuInSe₂ epitaxial layers and refer to a chalcopyrite-like structure. The films are always p-type conducting but different acceptor states are found in dependence on the substrate temperature.     

Influence of substrate orientation on the electrical properties of CuInSe2 epitaxial layers on GaAs substrates

The electrical properties of CuInSe₂ epitaxial layers on (111)A-, (110)-, and (100)-oriented GaAs substrates are investigated. At substrate temperatures T_sub ≧ 820 K the p-type conductivity observed is governed by an acceptor state with an ionization energy of about 110 meV independent of the substrate orientation. At T_sub ≦ 770 K three different acceptor states are found in dependence on the substrate orientation: 390 meV for (111)A-, 110 meV for (110)-, and a very shallow acceptor for (100)-oriented substrates. A possible correlation between these acceptor states and intrinsic defects is proposed.     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.     

Nuclear Spin-Lattice Relaxation in the Linear Chain Metal,NbSe3

Abstract

The nuclear spin-lattice relaxation time T ₁ of ⁹³Nb in NbSe₃ has been observed as a function of frequency v _N in the range 6-40 MHz at 4.2 K and of temperature T in the range 1.3-77 K at 10 MHz. [T ₁ T]^?1 is found to be proportional to 1/√ν corresponding to characteristic divergence of spin correlation function in 1-D system. Below ~10 MHz, a break down of the divergence is observed. The important features of the T dependence observed are a sharp minimum at ~45 K and an asymptotical approach to the metallic relation, T ₁ T= const, with decreasing T down to helium temperature. This [T ₁ T]^?1 minimum is explained by the destruction of the Fermi surface due to the CDW gap formation. The re-increase of [T ₁ T]^?1 below the CDW transition may suggest a partial recovery of the destructed Fermi surface.     

Pyroelectric Properties of Potassium and Rubidium Titanyl—Arsenate Single Crystals in the Temperature Range of 4.2–300 K

The temperature dependences of the pyroelectric coefficients of KTiOAsO₄ and RbTiOAsO₄ single crystals grown by flux crystallization have been investigated in the temperature range of 4.2–300 K. With an increase in temperature, superionic conductivity first arises in KTiOAsO4 (at T > 200 K) and then (at T > 270 K) in RbTiOAsO₄. This conductivity is much higher in the samples polarized at T = 4.2 K. An exponential change in the crystal resistivity along the polar direction is simultaneously observed. The results of measurements in the range of 4.2–200 K indicate larger values of pyroelectric coefficients when compared with potassium and rubidium titanyl-phosphate crystals. A correlation between the pyroelectric coefficients and a change in the lattice constants at isomorphic substitutions of K atoms for Rb and P atoms for As has been revealed within the symmetry approach.     

Low‐temperature elastic and dielectric properties of K3Na(SO4)2 crystals

Elastic and dielectric properties of glaserite K₃Na(SO₄)₂ single crystal were examined using the method of composite oscillators, Brillouin light scattering methods and dielectric spectroscopy. Measurements were performed in the temperature range from 18 K to 300 K. Anomalies in the temperature dependencies of Brillouin shift and dielectric permittivity at about 70 K confirmed the earlier predicted phase transition at 75 ± 25 K. Temperature dependences of the resonance frequency of the vibrating composite oscillator, Brillouin shift measured in the [110] direction, components of dielectric permittivity tensor reveal an anomaly at about 50 K. Moreover, thermal hysteresis of the dielectric permittivity suggested the presence of an incommensurate state between T ₁ = 50 K and T ₂ = 70 K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EPR and pulsed NMR study of (NH4)2ZnBr4:Mn2+

The EPR and pulsed NMR measurements of (NH₄)₂ZnBr₄:Mn²⁺ have been carried out in the temperature range 110–473 K and 83–284 K, respectively. EPR spectra from three different paramagnetic entities have been observed. Intensity changes of certain lines in the EPR spectrum observed at ≈ 430 K indicate the occurrence of a phase transition. Another phase transition at ≈ 235 K is indicated as a change of slope in the T₁ vs 1/T curve.     

Investigation of metastable zone width of ammonium oxalate aqueous solutions

The metastable zone width of pure ammonium oxalate aqueous solutions, as represented by maximum supercooling ΔT_max, is investigated as functions of cooling rate R and saturation temperature T₀ by the polythermal method. The experimental results are discussed by using two recently advanced approaches: (1) self‐consistent Nývlt‐like approach based on a power‐law relationship between nucleation rate J and maximum supersaturation lnS_max, and (2) a novel approach based on the relationship between J and lnS_max described by the classical three‐dimensional nucleation theory. Analysis of the experimental data revealed that both approaches describe the experimental data on metastable zone width by the polythermal method reliably and provide useful information about the physical processes and parameters involved in nucleation kinetics. The values of various physical quantities predicted by both of these approaches are reasonable for a fairly‐soluble compound. A careful examination of the data on ΔT_max as a function of T₀ obtained by polythermal method and from density measurements showed that ΔT_max has a slight tendency to decrease with increasing saturation temperature T₀. The values of lnS_max at saturation temperature 303 K suggest that the metastable zone width of ammonium oxalate aqueous solutions is determined by primary nucleation in the polythermal method and by secondary nucleation during density measurements. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A neutron diffraction study of the phase transition of fully deuterated triglycine sulphate (ND2CD2COOD)3.D2SO4

Using neutron single crystal and powder diffraction, the first thorough investigation of the structure of fully deuterated triglycine sulphate, (ND₂CD₂COOD)₃.D₂SO₄ is presented, including its evolution with T, through its structural phase transition. This includes new precise structural parameters determined at several key temperatures above and below T_C using single crystal diffraction, and for the first time a parametric study has been undertaken over a wide temperature range — from 4 to 500 K in 2 K steps. It was found that fully deuterated TGS shows a structure consistent with hydrogenous TGS and partially deuterated TGS. The evolution of several key hydrogen bond lengths suggests that weakening of the H‐bond network with T is crucial in decoupling the polarising glycine molecules from the other glycines and allowing the long‐range ferroelectric order to break down. A new parameterisation of the phase transition is demonstrated. Contrary to results of physical properties measurements, there is no evidence of a second low temperature phase transition in TGS – no low temperature anomalies were observed in the crystal structure.     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Logarithmic Anomaly in the Debye-Waller Factor and Thermal Expansion Coefficient at Ferroelectric Phase Transition in KDP

Temperature dependence of the thermal expansion coefficient and the Debye-Waller factor along the polar axes in KDP were investigated by means of X-ray diffraction. A logarithmic anomaly was observed in the range from T_c up to 6 K above T_c.     

Invar effect in PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics

High-density lead ferroniobate PbFe_1/2Nb_1/2O₃ (PFN) is prepared by conventional ceramic technology. Its structural properties are studied in a wide temperature range (293 ≤ T ≤ 973 K). The following chain of phase transitions is established in the vicinity of the transition to the polar phase: Rh (rhombohedral phase) (T < 363 K) → Psc (pseudo-cubic phase) (363 < T < 387 K) → C (cubic phase) (T > 387 K). The paraelectric range contains five ranges of constant unit-cell volume (invar effect): I (387 ≤ T ≤ 413 K), II (433 ≤ T ≤ 463 K), III (553 ≤ T ≤ 613 K), IV (743 ≤ T ≤ 773 K), and V (798 ≤ T ≤ 823 K). It is shown that the anomalous behavior of the PFN dielectric characteristics above the Curie temperature, which was revealed previously, is associated with the specific features of its real (defect) structure, which is caused by the crystal-chemical specificity of the main structure-forming agents: α-Fe₂O₃ and α_ht-Nb₂O₅.     

Effect of the magnetic flux trapping upon the temperature dependence of the flow stress change at the superconducting transition

The effect of the superconducting transition on the flow stress in single and polycrystals of Pb-In and Pb-Sn alloys has been studied in the temperature range 1.8 K to T_e. Unlike earlier efforts, these experiments involved investigation of the magnetic flux trapping. Two types of temperature dependences of the magnetic flux trapping, B₀(T), were revealed. In Pb-1.9 at.% In and Pb-6 at.% Sn alloys (subjected to various thermal treatment), B₀ monotonically rises with lowering temperature. In Pb-1.8, 3 at.% Sn single crystals the flux trapping arises at high strains (> 50%) and low temperatures (<2.5 K). In this case B₀ magnitude depends on the strain rate. The difference in the B₀(T) behaviour is associated with dissimilar flux pinnig by pinning centres. The date on B₀(T) obtained give an insight into the two types of the Δσ(T) dependences; taking into account of the flux trapping allowed the Δσ values to be corrected, so that as a result, a single Δσ_SN(T) curve was obtained for all the alloys and structures studied.     

Thermodynamic functions of (InP)x(InAs)1-x from 5 to 300 K

Temperature dependence of the heat capacity of the solid solutions (InP)_x(InAs)_1-x has been investigated over the temperature range from 5 to 300 K. Solid solution samples with x = 1; 0.8; 0.6; 0.4; 0.2; 0.1; 0 produced by means of crystallization from the melt the following annealing and diffusion in the powder at temperature close to solidus have been studied. The absolute values of enthalpy H, entropy S, Gibbs free energy Z as well as their deviations from the additive values have been obtained for the (InP)_x(InAs)_1-x system from the analysis of the thermodynamic properties of the solid solutions of the III–V compounds at low temperatures. The curves for the characteristic temperatures (T) of the solid solutions studied show that the change of the character of the phonon spectrum is negligible, the thermodynamic functions are close to additive, the influence of the enthalpy and entropy of mixing on the deviations from the additive enthalpy and entropy of the alloys is insignificant.     

Irradiation plus annealing-induced 1.20 eV emission band in p-type GaAs

The effect of 2.2 MeV electron irradiation and subsequent annealings on the luminescence of tellurium-compensated p-type GaAs crystals is studied and analyzed. A comparatively strong emission band peaked at hv_m near 1.20 eV (induced by radiative electron recombination in Te_AsV_Ga pairs) appears in irradiated annealed (at T ≧ 250 °C) compensated p-type GaAs. The data obtained testify about the following: a) electron irradiation of GaAs creates stable (at T ≦ 200 °C) defects not only in the arsenic sublattice of GaAs, but in its gallium sublattice too; b) radiation plus annealing (r.p.a.)-induced Te_AsV_Ga pairs are characterized by a rather high (compared to that for grown-in Te_As V_Ga pairs) probability of electron radiative transitions in them. The electrical characterization of the r.p. a.-induced 1.20 eV radiative centres (Te_AsV_Ga pairs) is given.     

Crystal Growth and Elastic Properties of Hopeite,Zn3(PO4) · 4 H2O

Single crystals of orthorhombic Zn₃(PO₄)₃ · 4 H₂O of optical quality with dimensions up to 12 × 10 × 8 mm were grown from aqueous solutions of ZnSO₄ and NaH₂PO₄ by controlled diffusion of NH₃ into the solution. The elastic constants c_ij and the thermoelastic constants T_ij = dc_ij/dT, T temperature, were determined from ultrasonic resonance frequencies of thick plates in the range between 5 and 40 MHz and between 250 and 320 K. In respect to the longitudinal elastic stiffness hopeite behaves quasi isotropically, however, the elastic shear stiffness shows a large anisotropy of about a factor 2.5. Above 260 K the shear stiffness c₆₆ possesses a quite anomalous temperature dependence (T₆₆ > 0). Further, the linear thermal expansion reveals a strong anisotropy (α₁₁ = −3.3, α₂₂ = 3.4, α₃₃ = 33 · 10⁻⁶/K).