Optical Properties of Bi2Se3‐xAsx Single Crystals

Bi₂Se_3‐xAs_x single crystals with the As content of c_As = 0 to 2.0x10¹⁹ atoms/cm³ prepared from the elements of 5N purity by means of a modified Bridgman method were characterized by measurements of infrared reflectance and transmittance. Values of the plasma resonance frequency omega_p, optical relaxation time tau, and high‐frequency permittivity were determined by fitting the Drude‐Zener formulas to the reflectance spectra. It was found that the substitution of As atoms for Se atoms in the Bi₂Se₃ crystal lattice leads to a decrease in the omega_p values. This effect is accounted for by a model of point defects in the crystal lattice of Bi₂Se_3‐xAs_x. The dependences of the absorption coefficient K on the energy of incident photons were determined from the transmittance spectra. The optical width of the energy gap is found to decrease with increasing As content. The values of the exponent b from the relation of K ∼ lamda^b for the long‐wavelength absorption edge range within the interval 2.0 to 2.3, i.e. the dominant scattering mechanism of free current carriers in Bi₂Se_3‐xAs_x crystals is the scattering by acoustic phonons.     

Transport Coefficients of (Bi1-xSbx)2Se3 Single Crystals

(Bi_1-xSb_x)₂Se₃(x = 0.0 to 0.2) single crystals were prepared using a modified Bridgman method. Measurements of the Hall coefficient R_H, electrical conductivity σ and Seebeck coefficient α showed that incorporation of the antimony atoms into the Bi₂Se₃ crystal lattice results in an increase in the free electron concentration for a low antimony content, whereas the free electron concentration is suppressed in the range of a high antimony content. This effect is explained qualitatively by changes in the concentrations of point defects in the (Bi_1-xSb_x)₂Se₃ crystals. We asume that the substitution of Bi atoms by Sb atoms results in a decrease in the concentration of Se vacancies V and antisite defects Bí_Se. The course of the dependences of In (R_Hσ) vs In T manifests that in the temperature region of 100–400 K transport properties of the studied crystals are characterized by a mixed transport mechanism of free carriers, mainly on acoustical phonons and ionized impurities.     

Thermoelectric properties of Tl‐doped Bi2Se3 single crystals

The single crystals of the ternary system based on Bi_2‐xTl_xSe₃ (nominaly x = 0.0‐0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by the measurement of lattice parameters, electrical conductivity σ_⊥c , Hall coefficient R_H (B∥c), and Seebeck coefficient S (ΔT ⊥c). The measurements indicate that by incorporating Tl in Bi₂Se₃ one lowers the concentration of free electrons and enhances their mobility. This effect is explained in terms of the point defects in the crystal lattice – formation of substitutional defects thallium on the site of bismuth Tl_Bi and the decrease of concentration of selenium vacancies V_Se⁺². We also discuss the temperature dependence of the power factor σS² of the samples. Upon the thallium doping we observe a significant increase of the power factor compare to the parental Bi₂Se₃. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dislocation and microindentation analysis of vapour grown Bi2Te3‐xSex whiskers

The structural defects and microhardness of Bi₂Te_3‐xSe_x whiskers (x = 0, 0.2 and 0.4 at% Se) grown by physical vapour deposition (PVD) method have been investigated. Concentric pairs of dislocation loops were observed on the as‐grown surfaces of short hexagonal prisms. A systematic study of dislocations in these crystals was carried out by chemical etching technique. The effects of Se doping, annealing and quenching on the mechanical properties have also been studied on the prism faces of Bi₂Te_3‐xSe_x whiskers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,structure, and some transport properties of topological insulators based on bismuth chalcogenide single crystals

Single crystals of topological insulators—bismuth chalcogenides Bi₂Te₃, Bi_{2 ? x} Sn _x Te₃, Bi₂Se₃, and Bi_{2 ? x} Cu _x Se₃ with different charge-carrier densities—are grown by the modified Bridgman method. Their composition and structure are investigated and temperature dependences of the electric resistance and magnetic field dependences of the Hall voltage are obtained.     

Nanocrystallites in Bi–As–S system

Bi₂S₃–As₂S₃ composite formation was performed by two methods: by the direct insertion of Bi₂S₃ nanocrystals into a molten As₂S₃ glass which was further solidified and by the crystallization of a rapidly quenched (As₂S₃)_1−x (Bi₂S₃)_x glasses with x = 0.005, 0.01, 0.02 and 0.04 at different conditions. Fine tuning of the annealing of the quenched glass as well as the mixing of nanocrystals in to the molten glass resulted glass-crystalline composites with different amounts and distribution of 20–50 nm large Bi₂S₃ nanocrystals as well as larger, up to few micrometer long, needle-like crystals. Structural and optical investigations support the presence of the Bi₂S₃ crystalline phase in all composites. Optical absorption and the photoconductivity of bulk composite samples follow the structural changes of the structure in the amorphous and amorphous-crystalline phase. In addition, the 290 cm⁻¹ characteristic band in Raman spectra may be used for tracing the formation of the nanocomposites.     

Characterisation of Bi2Se3 Crystals Highly Doped with Pb

Pb-doped Bi₂Se₃ crystals were prepared from starting elements Bi, Se and Pb of 5N purity in the concentration interval c_pb = 0 – 4 × 10²⁵ Pb atoms m⁻³ by a modified Bridgman method. The measured values of the transmittance and reflectance were used to determine the dependence of the absorption coefficient K on the photon energy for crystals with various values of cpb and to prove the shift of the short-wavelength absorption edge with cpb. On the basis of the assumption of the validity of the “single valley” model, which can describe the lowest conductivity band of Bi₂Se₃, and using the values of the freecarrier effective mass in the directions perpendicular and parallel to the trigonal axis c we determined the value of the reduced Fermi energy η 300 K for crystals with various values of cpb. Using the value of η, we calculated the dependence of the Seebeck coefficient on cpb and compared it with the experimentally determined values. The comparison has shown that the increasing content of Pb atoms in the Bi₂Se₃ lattice leads to a suppression of the role of the mechanism of scattering by ionised impurities; at higher concentrations of Pb in the crystal the mechanism of scattering of free carriers by acoustic phonons becomes dominant. Further, the ideas on the nature of the point defects in the Bi₂Se₃(Pb) crystals are presented and the “anomalous” dependence of the free-electron concentration on cpb is qualitatively accounted for.     

Composition Dependency of Lattice Anisotropy of TlBX2‐type Chain Mixed Crystals

Variation of the lattice parameters of TlTl_1‐xIn_xSe₂ chain mixed crystals with composition have been studied by X‐ray diffraction technique. The lattice anisotropy ( c/a ) of the TlBX₂‐type mixed crystals changes linearly with substitution of the atoms located both at the center and at the vertices of the BX4 tetrahedra. A brief survey of the important features of the effect of isomorphic atom substitution on the lattice anisotropy of TlBX₂‐type mixed crystals with chain structure has been presented.     

Effect of temperature and isomorphic atom substitution on optical absorption edge of TlInS2xSe2(1‐x) mixed crystals (0.25 ≤ x ≤ 1)

The optical properties of the TlInS_2xSe_2(1‐x)mixed crystals (0.25 ≤ x ≤ 1) have been investigated through the transmission and reflection measurements in the wavelength range of 400–1100 nm. The optical indirect band gap energies were determined by means of the analysis of the absorption data. It was found that the energy band gaps decrease with the increase of selenium atoms content in the TlInS_2xSe_2(1‐x)mixed crystals. The transmission measurements carried out in the temperature range of 10–300 K revealed that the rates of change of the indirect band gaps with temperature are γ = –9.2×10^–4 eV/K, –6.1×10^–4 eV/K, –4.7×10^–4 eV/K and –5.6×10^–4 eV/K for TlInS₂, TlInS_1.5Se_0.5, TlInSSe and TlInS_0.5Se_1.5 crystals, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transport Coefficient of Gallium-doped Bi2Te3 Single Crystals

Gallium-doped Bi₂Te₃ single crystals were prepared by means of a modified Bridgman method. Temperature dependences of the Hall constant R_H( B ∥ c ), electrical conductivity σ⊥c and Seebeck coefficient S(Δ T ⊥ c) were measured on the samples of these crystals in the temperature interval of 100–400 K. The variations of the investigated transport coefficients with increasing Ga content in the samples showed that Ga atoms in the Bi₂Te₃ crystal lattice behave as donors. This effect is qualitatively explained on the basis of a model of point defects in Bi₂Te₃(Ga) crystals; singly ionised gallium atoms in interstitial sites; Ga_i, are considered to be the most probable defects.     

Crystal growth of the CdGa2(1‐x)Cr2xSe4 compounds by chemical transport method

Single crystals of CdGa_2(1‐x)Cr_2xSe₄ compounds for 0 ≤ x ≤ 1 have been grown by using the chemical vapor transport technique in a closed system. The transporting agent was CdCl₂ in a proportion of 0.75 mg/cc of capsule. The starting material was previously synthetized. The structural characterization on the crystals were done by powder x‐ray diffraction studies. The results show three different phases for various Cr concentration ranges: spinel structure for x ≥ 0.7, rombohedral for 0.6 ≥ x ≥ 0.5 and tetragonal for 0.4 ≥ x ≥ 0. That is, the chromium dilution in the CdCr₂Se₄ compound by Ga atoms produces very significant changes in the structural atomic arrangement. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vapor phase transport and stoichiometry control of cadmium sulfo-selenide

Cadmium sulfo-selenide Cd_1+y(S_xSe_1?x) single crystals were grown by the sublimation method under controlled partial pressures of sulfur and selenium. The partial pressure control was achieved by amounts of sulfur and selenium introduced in a closed growth tube. To control the deviation from stoichiometry y for the specified compositions x, the sum of chalcogen partial pressures Pps(=p_S2+P_Se2) was regulated, holding the partial pressure ratio () constant. The comp osition x of single crystals, grown with appreciable growth rate, was found to be the same one as the source specimen but the electrical conductivity changed extremely from 1 to 10^-12 (ω cm)^-1 with increasing Pps. Hall effect and thermally stimulated current (TSC) were also measured on the high and low conductivity specimens respectively and found to depend upon the controlled Pps. From these experimental results, it was confirmed that the deviation from stoichiometry y was effectively controlled. The transport rate was also precisely measured as a function of controlled Pps.     

Iodine Intercalation of Bi2Sr2Ca(n−1)CunOy(N=2,3) Superconductors by Using Reactants I2 or FeI2

Abstract

The change of superconductivity induced by intercalation is very interesting We have investigated on intercalation into Bi2212 single crystal and Bi2223 polycrystalline superconductors using FeI₂ or I₂ as reactant. The iodine intercalation by using FeI₂ as a reactant into Bi2223 phase is reported for first time as long as we know. In iodine intercalation into Bi2212 by using FeI₂ as reactant, the critical tem T_Conset in out-of-plane measurement has suddenly decreased on X=0.95. Similar changes of T_Conset with higher X have been observed independent of directions and reactants. T_Conset of I_xBi₂Sr₂CaCu₂O_y intercalated by FeI₂ is a little lower than one of I_xBi₂Sr₂CaCu₂O_y intercalated by I₂. This is likely due to the change of hall concentration induced by reduction of host In iodine intercalation into Bi2223, the smaller change of T_Conset with increasing amount of intercalant is likely due to the number of CuO₂ planes between ntercalant layers.     

Raman spectra of GeSbS system glasses in the S-rich region

Raman spectra of the Ge_xSb_yS_z glasses with sulfur concentration ?65 at.% are given and discussed. From the obtained Raman spectra it is concluded that the presence of small concentrations of Ge atoms of more than the (GeS₂)_1?x(Sb₂S₃)_x compositions results in the formation of S₃GeGeS₃ structural units. On the other hand the presence of small concentrations of excess sulfur results in replacing some individual bridging S atoms by ?SS? bridging groups. At higher concentrations of the excess sulfur the S atoms form S₈ ring molecules which are dissolved in the network structure of the glass composed of GeS₄ tetrahedra and SbS₃ pyramids interconnected by one or two sulfur atoms.     

Electrical transport in n-type semiconducting Ge120BixSe70−xTe10 glasses

Resistivity and thermoelectric power were measured as a function of temperature and composition for Ge₂₀Bi_xSe_70?xTe₁₀ glasses (x = 0–11). The results were compared with the case of of Ge₂₀Bi_xSe_80?x glasses to see on the electrical properties the influence of the substitution of Te for a part of Se. The glasses show n-type conduction for x ? 9, which was not affected by the substitution of Te. The resistivity was about three orders of magnitude lower for the glasses with x < 10, and remained almost the same for x ? 10, compared with the glasses not containing Te. From the composition dependence of the calculated concentration of covalent bonds in the glasses, it was proposed that the appearance of n-type conduction was closely related to the formation of a sufficient number of BiSe bonds and the disappearance of the bonds between two chalcogen atoms such as TeSe or SeSe bonds, and that the remarkably low resistivity in the present glasses with x < 10 was likely to be attributed to the formation of TeSe bonds.     

High temperature superconducting thin films on CaGdAlO4 substrates

Single crystals of CaGdAlO₄ that have not been used previously as a substrate material for HTSC films were grown by Czochralski method and applied for the deposition of the Bi₂Sr₂CaCu₂O_8−x and LuBa₂Cu₃O_7−z thin films. The main advantages of the CaGdAlO₄ crystals are sufficiently low dielectric constant, appropriate crystal lattice match with most HTSC, fairly high crystal perfection and reproducibility of the growth process. Bi₂Sr₂CaCu₂O_8−x films were prepared by “off-axis” DC magnetron sputtering and LuBa₂Cu₃O_7−z films were obtained by flash evaporation MOCVD technique on the (0 0 1) oriented CaGdAlO₄ substrates.     

Viscosity of germanium sulfide melts

Viscosities of Ge_xS_1−x melts (0.30⩽x⩽0.44) have been measured by penetration viscometry from 10⁷ to 10¹³ Pa s. The temperature dependence of equilibrium viscosities in this range can be expressed approximately by a simple Arrhenius equation. Both the heat capacity change at the glass transition and the activation energy of viscous flow monotonously increase with germanium content as predicted by the Adam–Gibbs theory. Therefore, the connectivity of the germanium–sulfur network is reduced due to decreasing concentration of sulfur which causes increasing fragility of the undercooled liquid. The glass transition temperature exhibits a maximum near the GeS₂ composition where heteropolar bonds are predominantly formed.     

Contributions to the surface preparation of semiconductor materials of low hardness

Starting from the general demands made upon semiconductor surfaces (measuring samples, substrates) and the critical material properties, the boundary conditions for the surface preparation of substances of low hardness are stated and technological variants for Bi_1−xSb_x, Pb_1−xSn_xTe, and PbS_1−xSe_x are presented. The results show that almost defectfree surfaces of a high quality can be produced in a repeatable way. The depths of significant and detectable damages are determined depending on the technique in question.     

Heterodoped nanoflakes: synthesis and characterization of sulfur‐doped NbSe2 nanoflakes

Doping of NbSe₂ with heteroatoms is an effective way to tailor its properties. Here, solid phase synthesis process has been utilized for growing uniform and excellent crystalline nano‐NbSe₂ by sulfur dopant during the growing process. The sulfur‐doped mechanism of nano‐NbS_2xSe_2(1‐x)(x = 0.03∼0.2) has been investigated: the sulfur atoms not only replace the Se position but also intercalate in layer gap by one‐step reaction. In the structure of NbSe₂, the sulfur atoms replace the positions of Se atoms and then are located in the empty octahedral positions of van der waals gap between the NbSe₂ layers with the increase of sulfur content. The lubrication properties of the as‐prepared NbSe₂ and sulfur‐doped NbSe₂ powders as additives in HVI1500 base oil were discussed. The lubrication properties of base oil was improved by both NbSe₂ and sulfur‐doped NbSe₂. Furthermore, the lubrication properties of sulfur‐doped (0.1at%) NbSe₂ are better than those of pure NbSe₂ in HVI1500 base oil, which could be attributed to their thin laminated structure and the sulfur dopant.     

Effect of the substitution of S for Se on the structure and non-linear optical properties of the glasses in the system Ge0.18Ga0.05Sb0.07S0.70−xSex

Glasses in the system Ge–Ga–Sb–S/Se have been elaborated with different S/Se ratios in order to increase the non-linear optical properties of these glasses. We report results of a systematic study examining the relationship of the physical properties to the structure of the glasses in the system Ge_0.18Ga_0.05Sb_0.07S_0.70−xSe_x with x = 0, 0.02, 0.05, 0.10, 0.20, 0.30 and 0.40 where the replacement of S by Se has been made. The non-linear refractive index has been measured using the Z-scan technique, with picosecond pulses emitted by a 10 Hz Q-switched mode-locked Nd-Yag laser at 1064 nm under conditions suitable to characterize ultrafast non-linearities. The decrease of the glass transition temperature, the increase of the non-linear refractive index and of the density with the progressive replacement of S by Se have been correlated along with the red shift of the absorption band gap, to associated structural reorganization. A corresponding progressive decrease of corner-sharing GeS_4/2 and the formation of mostly two edge-sharing Ge₂S₄S_2/2, S₃Ge–S–GeS₃ as well as mixed GeS_4−xSe_x units have been identified by Raman spectroscopy.