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.     

Optical Properties of Pb-Doped Bi2Te3 Single Crystals

The reflectance and the transmittance spectra in the IR region were measured on Pb-doped Bi₂Te₃ single crystal samples grown by a modified Bridgman technique. The plasma resonance frequency, the optical relaxation time, and the high-frequency permittivity were determined by fitting the Drude-Zener formulas to the reflectance spectra. It was found that Pb impurities in Bi₂Te₃ behave as acceptors. A part of incorporated Pb atoms behaves as electrically inactive. This effect is explained as due to the fact that electrically active Pb-atoms form substitutional defects, Pb′_Bi, and the others form inactive two-dimensional defects — seven-layer lamellae Te—Bi—Te—Pb—Te—Bi—Te. The transmittance spectra were used for the determination of the dependence of the absorption coefficient K on the energy of incident photons. The optical width of the energy gap increases with increasing Pb content. The values of the exponent α from the relation of K ∼ λ^α for the long-wavelength absorption edge range within the interval of 1.6 to 2.8, i.e. the dominant scattering mechanism of free current carriers in Pb-doped Bi₂Te₃ crystals is the scattering by acoustic phonons. By comparison of the effect of doping atoms of the IV.B group of the periodic table on the concentration of holes in Bi₂Te₃ crystal lattice was concluded that the tendency to form substitutional defects Me′_Bi (Me = Ge, Sn, Pb) in these crystals increases from Ge to Pb, whereas the tendency to the formation of seven-layer lamellae Te—Bi—Te—Me—Te—Bi—Te decreases.     

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)     

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.     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electric Characterization of Indium Sesquiselenide Single Crystals

Conductivity, Hall-effect measurements were performed on δ-phase In₂Se₃ single crystals, grown by the Bridgman method over the temperature range 150–428 K, in the directions perpendicular and parallel to the c-axis. The anisotropy of the electrical conductivity and of the Hall coefficient of n-type In₂Se₃ had been investigated. The values of the Hall coefficient and electrical conductivity at room temperature spreads from an order of R_H11 = 1.36 × 10⁴ cm³/coul, σ₁₁ = 4.138 × 10⁻³ Ω⁻¹ cm⁻¹ and R_H = 66.55 × 10⁴ cm³/coul, σ = 0.799 × 10⁻³ Ω⁻¹ cm⁻¹ for parallel and perpendicular to c-axis, respectively. The temperature dependence of Hall mobility and carrier concentration are also studied.     

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.     

Lithium Intercalation into the Tetradymite-type Selenides Sulfides Bi2Se3-xSx

The reaction rate of Li intercalation into Bi₂Se_3−xS_x single crystals (x = 0, 0.025 and 0.05) increases with an increasing sulfur content in the crystals. This effect has been explained qualitatively by a change in the bond polarity in the Bi₂Se₃ due to the substitution of selenium atoms by sulfur atoms. This substitution results in a change in the electron density distribution and thus also in the width of the van der Waals gap between the structural layers, responsible for the course of intercalation. This explanation is supported by a quantum-chemical calculation of the charges on the atoms of the Bi₂Se_3−xS_x lattice manifesting and increase in the negative charge on Se atoms with an increasing value of x.     

Growth and characterization of photorefractive Bi12TiO20 single crystals

Photorefractive Bi₁₂TiO₂₀ single crystals of high optical quality were grown in a resistive heating furnace from high temperature nonstoichiometric (10:1) solutions of Bi₂O₃ and TiO₂ at pulling rates 0.3–0.8 mm/h at rotation 20–30 rpm along the <001> and <011> axis. Powder X-ray analysis, Laue method, and electron-probe microanalysis were used for characterization. BTO crystals have the bcc structure of sillenite type with a₀ = 10.178(8) Å. The chemical composition of the crystals can be written down as Bi_{12.1 ± 0.2}Tio_{0.96 ± 0.09}O_20.1. Natural optical activity ρ of BTO crystals is 6.3 ± 0.2 deg/mm at λ = 0.633 μm and 11.9 ± 0.2 deg/mm at λ = 0.5145 μm, optical absorption coefficient α = 0.42 ± 0.04 cm⁻¹ at λ = 0.633 μm and linear electro-optic coefficient r₄₁ = r₅₂ = r₆₃ = 5.3 pm/V. Fanning effect in the “fiber-like” BTO sample was studied and double phase conjugation with conversion efficiency up to 8% was observed in a wide range of incidence angles of the pump at λ = 0.633 μm for 2 × 3 mW input light power.     

On the Reasons Leading to an Appearance of Correlated and Anticorrelated Dependences of the Minority Carrier Lifetime on the Dislocation Density in Undoped Semi-Insulating “Stoichiometric” GaAs Crystals

It is shown that in undoped semi-insulating GaAs crystals grown under the stoichiometric conditions both correlated and anticorrelated dependences of the minority carrier lifetime τ on the dislocation density N_d could be observed. The above-pointed effect is connected with a slight excess of Ga atoms (then the correlated dependence τ vs Nd appears) or of As atoms (then the anticorrelated dependence τ vs N_d appears) which inevitably exists even in “stoichiometric” GaAs crystals (i.e. in GaAs crystals of “stoichiometric” composition).     

Characterization of Ag-Doped Bi1.5Sb0.5Te3 Single Crystals

Single crystals of Bi_1.5Sb_0.5Te₃ doped with Ag atoms (c_Ag = 0 - 4.34x10¹⁹ atoms/cm³) were characterized by measurement of Hall coefficient, electrical conductivity, Seebeck coefficient and the figure of merit. It was found that Ag atoms in the crystal structure of Bi_1.5Sb_0.5Te₃ behave like donors. It is necessary to add more than one Ag atom to the Bi_1.5Sb_0.5Te₃ compound in order to suppress the hole concentration by one hole. Such a behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant interstitial defects of Ag* silver atoms form also positively charged substitutional defects Ag″, Ag′, and a four-layer-lamellae of the type [Ag_0.5Sb_0.5]-Te-[Ag_0.5Sb_0.5]-Te. The incorporation of Ag atoms into the Bi_1.5Sb_0.5Te₃ lattice, besides the suppression of the hole concentration, leads to a decrease in the figure of merit Z.     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

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)     

Growth and optical characterization of cerium and lead‐doped Bi12TiO20 sillenite single crystals

Bi₁₂TiO₂₀ (BTO) single crystals doped with PbO and CeO₂ were grown by the Top Seeded Solution Growth (TSSG) technique from the liquid phase with nominal compositions of 10Bi₂O₃ : (1–x)TiO₂ : x PbO and 10Bi₂O₃ : (1–x)TiO₂ : xCeO₂ with x = 0.25 and 0.10. No growth‐related difficulties were encountered other than those typical of sillenite crystals. Samples with good optical quality were obtained and were characterized by optical absorption, dark current, spectral photocurrent dependence, optical activity and electro‐optic coefficient measurements. A comparison is made of the results of the optical measurements of doped and undoped BTO crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical and photoconductive properties of GaS0.75Se0.25 mixed crystals

The conductivity, mobility, photoconductivity and photo response measurements in GaS_0.75Se_0.25 mixed crystals were carried out in the temperature range of 150‐450 K. The room temperature conductivity, mobility and electron concentration values were 10^‐9 (Ω‐cm)^‐1, 48 cm²V^‐1s^‐1 and ∼10⁹ cm^‐3, respectively. Two donor levels were obtained from temperature‐dependent conductivity and carrier concentration, located at energies of about 755 and 465 meV below the conduction band. Single donor‐single acceptor analysis yields the same donor level at 465 meV with donor and acceptor concentrations of 8.7 × 10¹⁴ and 5.3 × 10¹³ cm^‐3, respectively. The mobility‐temperature dependence shows that ionized impurity scattering dominates the conduction up to the temperature 310 K with different temperature exponent, while above this critical temperature; the phonon scattering is dominant conduction mechanism. From the photo‐response spectra, the maximum photocurrent was observed for all the samples at 2.42 eV, and varied slightly with temperature. Moreover, the photocurrent‐light intensity dependence in these crystals obeys the power law, I_ph ∝ ϕ^γ with γ between 1.7 and 2.0 for various applied fields and temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three-dimensional Bi2Se3 nanopattern films self-assembled with ultrathin nanosheets on the surface of Se nanotubes

Regular three-dimensional (3D) rose-like Bi₂Se₃ nanopattern film is fabricated through a simple chemical route. This nanopattern film is self-assembled with ultrathin Bi₂Se₃ nanosheets having thicknesses of less than 8 nm. The Bi₂Se₃ nanosheets were formed on the surface of Se nanotubes, and the Se nanotubes were used as both the Se resource and the substrate to support the growth of the Bi₂Se₃ nanopattern film. Since several length scales are involved in this confined 3D structure, which are developed at different time, during the formation of the micro-structure, a careful control of these length scales is expected to provide new opportunities for engineering the boundary scattering of phonons of different wavelengths and developing new thermoelectric materials with novel properties.     

Optical absorption in vitreous GeSb2Se4

Optical absorption of vitreous GeSb₂Se₄ was studied in spectral region 0.7–25 μm. At low absorption levels near the edge the absorption coefficient K depends exponentially on energy. At high absorption levels the quadratical energy dependence of K is observed. In the present work we determined the optical energy gap E^opt_g = 1.29 eV and discussed the temperature dependence of the absorption coefficient. Measured reflectivity curves were used to estimate the value of the refractive index of GeSb₂Se₄ ($\text{n = 3.13–3.56}\text{at}\text{h}\text{?}\text{ω = 1.00–1.70}\text{eV}$). Vitreous GeSb₂Se₄ is also transparent in the spectral interval 2–15.7 μm.     

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.     

Structure of phases of the sillenite family in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript> system

X-ray diffraction studies of sillenite Bi₂₄V₂O₄₀ single crystals grown by the hydrothermal method are performed for a separate crystal and powdered crystals. It is found that the composition of the two specimens is described by the (Bi_{24 − x} ▭ _x )[Bi _y ³⁺V_1−y ⁵⁺]₂ O₄₀ general formula with completely populated oxygen sites but differs in the content of vacancies at the bismuth site (this was established for the first time) and the Bi: V ratio at the tetrahedral site. The structural models of all the vanadium-containing sillenites reported in the literature are considered, and the possibility that Bi atoms are located at the centers of BiO₄ tetrahedra is established.     

Effective electro‐optic coefficient of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals

Refractive indices and effective electro‐optic coefficient γ_c of (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γ_c = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γ_c of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γ_c of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO₃ (γ_c = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.