Photoelectric properties of ordered-vacancy Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals

We have studied the photoconductivity spectrum, thermally stimulated current, current-light characteristics, and temperature-dependent photocurrent in Bridgman-grown ordered-vacancy Ga₂Se₃ crystals. The observed temperature quenching of photoconductivity and two regions of its thermal activation in Ga₂Se₃ crystals are interpreted in terms of a multicenter recombination model which incorporates an s-channel of active recombination, r-centers of photosensitivity, and traps for nonequilibrium majority carriers.     

New forms of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> and Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> prepared by reacting InAs and GaAs substrates with selenium

Ga₂Se₃ and In₂Se₃ prepared through heterovalent substitution during thermal annealing of single-crystal gallium arsenide and indium arsenide substrates in selenium vapor in a quasi-closed system have been characterized by electron diffraction, scanning electron microscopy, and X-ray microanalysis. Cubic phases of In₂Se₃ (a ₀ = 1.1243 nm and a ₀ = 1.6890 nm) and Ga₂Se₃ (a ₀ = 1.0893 nm and a ₀ = 1.6293 nm) have been identified for the first time.     

Crystal structure of Gd<Subscript>14</Subscript>Cu<Subscript>48</Subscript>Ga<Subscript>3</Subscript> and Tb<Subscript>14</Subscript>Cu<Subscript>48</Subscript>Ga<Subscript>3</Subscript>

The compounds Gd₁₄Cu₄₈Ga₃ and Tb₁₄Cu₄₈Ga₃ have been synthesized, and their structures have been determined by powder x-ray diffraction (Gd₁₄Ag₅₁ type).     

Cathodo- and photoluminescence of Cu<Subscript>3</Subscript>Ga<Subscript>5</Subscript>Se<Subscript>9</Subscript> single crystals

The cathodo- and photoluminescence spectra of Cu₃Ga₅Se₉ single crystals have been measured at different excitation intensities and temperatures. The results indicate that the radiative recombination of nonequilibrium charge carriers occurs primarily through impurity levels due to anion and cation vacancies. The anion and cation vacancy concentrations can be controlled by thermal and laser anneals.     

X-ray diffraction study of the Cu<Subscript>2</Subscript>Se-In<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>Se<Subscript>3</Subscript> system near CuInCr<Subscript>2</Subscript>Se<Subscript>5</Subscript>

The structure of three compounds in the Cu₂Se-In₂Se₃-Cr₂Se₃ system near CuInCr₂Se₅ is determined by single-crystal x-ray diffraction: CuInCr₄Se₈ (I), Cu₂In₂Se₄ (II), and Cu_0.5In_0.5Se (III). I has a cubic (spinel type) structure: a = 10.606(4) Å, Z = 4, sp. gr. F43m. II has a pseudotetragonal (sphalerite type) structure: a = 5.774(2) Å, c = 11.617(6) Å. The structure of II was solved in a reduced unit cell with a = 5.774(2) Å, b = 5.774(2) Å, c = 7.095(6) Å, = 113.95(5)°, = 113.95(5)°, = 90.00(4)°, Z = 1, sp. gr. P1. III has a triclinic cell (disordered structure of II): a = 4.088(1) Å, b = 4.091(2) Å, c = 4.101(1) Å, = 60.05(1)°, = 60.08(1)°, = 89.98(4)°, Z = 1, sp. gr. P1. The Cu and In atoms in I sit in inequivalent tetrahedral sites, and the Cr atoms reside in octahedral interstices of the close packing of Se atoms. The bond lengths are In–Se = 2.538(6), Cr(1)–Se(1) = 2.514(7), Cr(1)–Se(2) = 2.576(8), and Cu–Se = 2.437(5) Å. In II, all of the atoms sit in tetrahedral sites; the mean bond lengths are In–Se = 2.578(6) and Cu–Se = 2.44(1) Å. In III, the Cu and In atoms are fully disordered in the same tetrahedral site; the mean Cu(In)–Se bond length is 2.508(6) Å.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1435–1439.Original Russian Text Copyright © 2004 by Antsyshkina, Sadikov, Koneshova, Sergienko.     

Phase equilibria in the Ag<Subscript>2</Subscript>Se-As<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

We have studied phase equilibria in the pseudoternary system Ag₂Se-As₂Se₃-Bi₂Se₃ and constructed the 300-, 600-, and 800-K isothermal sections, a number of partial phase diagrams, and the liquidus projection of this system. The AgAsSe₂-AgBiSe₂ and As₂Se₃-AgBiSe₂ joins are shown to be pseudobinary, and the Ag₃AsSe₃-AgBiSe₂ and AgAs₃Se₅-AgAsSe₂ joins are pseudobinary below the liquidus. Several in- and univariant peritectic, eutectic, and eutectoid equilibria and a broad region of AgBiSe₂-based solid solutions are identified. The homogeneity region of the AgBiSe₂-based phase has the largest extent along the AgAsSe₂-AgBiSe₂ join: 40 mol % (650 K) for the high-temperature form of AgBiSe₂ and 20 mol % (300 K) for its low-temperature form.     

Combustion synthesis of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

Nanophase of Ga₂O₃ has potentially important applications in photocatalysis. We report the synthesis of nanophase of the metastable γ- and stable β-Ga₂O₃ and demonstrate that it is possible to prepare a continuously varying mixture starting from the pure metastable γ- to the pure β-phase. This is achieved by employing a facile and reliable combustion route, using urea as a fuel. Typical grain sizes, as estimated from XRD studies, are about 3 nm. Given the importance of surface chemistry for potential applications, thermogravimetric coupled with mass spectrometry is used in conjunction with FTIR to elucidate the chemistry of the adsorbed surface layer. Studies on the γ-Ga₂O₃ phase indicate the occurrence of weight loss of 8.1% in multiple steps. Evolved gas analysis and FTIR studies show presence of physisorbed H₂O molecules and chemisorbed –(OH) ions bonded to active surface states and accounts predominantly for the observed weight loss.     

Production technology and optical absorption characteristics of CuIn<Subscript>0.95</Subscript>Ga<Subscript>0.05</Subscript>Se<Subscript>2</Subscript> semiconductor solid solution films

A production technology of thin CuIn_0.95Ga_0.05Se₂ films has been developed based on the method of two-stage selenization of CuIn_0.95Ga_0.05 precursor by a reactive component (selenium) in a carrier gas (nitrogen) flow. The morphology and structure of obtained films were studied by electron microscopy and X-ray diffraction techniques. The spectral dependence of the optical absorption coefficient was measured.     

High-Temperature Heat Capacity and Thermodynamic Properties of HoBiGeO<Subscript>5</Subscript> and ErBiGeO<Subscript>5</Subscript>

Polycrystalline HoBiGeO₅ and ErBiGeO₅ samples have been prepared by solid-state reactions, by firing stoichiometric mixtures of Ho₂O₃ (Er₂O₃), Bi₂O₃, and GeO₂. The effect of temperature on the heat capacity of the synthesized compounds has been investigated by differential scanning calorimetry in the range 350–1000 K. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of bismuth holmium and bismuth erbium germanates: enthalpy increment, entropy change, and reduced Gibbs energy.     

Physicochemical properties of La<Subscript>3</Subscript>Ga<Subscript>5.5</Subscript>Ta<Subscript>0.5</Subscript>O<Subscript>14</Subscript>

We report the optical and dielectric properties and microhardness of La₃Ga_5.5Ta_0.5O₁₄ lanthanum gallium tantalate (langatate) crystals. Analysis of the optical transmission spectra of the crystals in relation to their refined compositions indicates that the bands at 34000–35000 and 27000–28000 cm⁻¹ are due to lanthanum and oxygen vacancies, respectively, and that the band at 20000–21000 cm⁻¹ is responsible for the yellow (orange) coloration of the crystals. Their resistivity and microhardness decrease with increasing oxygen vacancy concentration.     

Photoluminescence of Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>:Eu nanocrystals

The photoluminescence (PL) of Ga₂S₃-5 mol % Eu₂O₃ nanocrystals prepared by mechanical comminution of the initial compound has been studied in the temperature range from 77 to 300 K. It is established that the PL spectrum of nanocrystals, in comparison to that of a massive sample, extends over a broader wavelength interval (430–620 nm) and has two maxima (at 507 and 556 nm) instead of one. The intensity of emission from nanocrystals is significantly higher than that from the massive crystal. The halfwidth in both cases varies with the temperature in proportion to T ^1/2. The intensity of emission at 556 nm for nanocrystals depends on the temperature as lgI ～ 1/T, this dependence having three linear portions corresponding to an activation energy of 0.04, 0.16, and 0.43 eV. The PL bands with maxima at 507 and 556 nm are assigned to the intracenter 4f ⁶5d4f ⁷ transitions in Eu²⁺ ions.     

Annealing behavior of TiO<Subscript>2</Subscript>-sheathed Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

TiO₂-sheathed Ga₂O₃ one-dimensional (1D) nanostructures were synthesized by thermal evaporation of GaN powders and then sputter-deposition of TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis results indicate that the Ga₂O₃ cores are of a single crystal nature with a monoclinic structure while the TiO₂ shells are amorphous. Photoluminescence (PL) emission is slightly decreased in intensity by TiO₂ coating, but it is significantly increased by thermal annealing in an oxygen atmosphere. The emission peak is also shifted from ~500 to ~550 nm by oxygen annealing. The increase in the green emission is due to the increase in the concentration of the Ga vacancies in the cores by the inflow of oxygen during oxygen annealing. On the other hand, annealing in a nitrogen atmosphere leads to a red shift of the emission to ~700 nm originating from nitrogen doping.     

Crystal growth and elastic properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript>

Perfect α-In₂Se₃ single crystals have been grown, and ultrasound velocities, v _i (i = 1–7), have been measured in single-crystal α-In₂Se₃ in various directions for different polarizations. We have determined the components of its elastic tensor (C _ij ) and calculated its elastic characteristics: elastic compliance, Young’s modulus, shear modulus, linear and volume compressibilities, bulk modulus, and Poisson’s ratio.     

High-Field Magneto-Conductivity Analysis of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystal

We report the high-field (up to 14 Tesla) magneto-conductivity analysis of Bi₂Se₃ topological insulator grown via the self-flux method. The detailed experimental investigations including crystal growth as well as the electrical, thermal, and spectroscopic characterizations of the resultant Bi₂Se₃ single crystal are already reported by some of us. The current letter deals with high-field magneto-conductivity analysis in terms of Hikami Larkin Nagaoka (HLN) model, which revealed that the electronic conduction is dominated by both surface state-driven weak anti-localization (WAL), as well as the bulk weak localization (WL) states. Further, by applying the HLN equation, we have extracted the fitting parameters, i.e., phase coherence length (l_φ) and the pre-factor (α). Here, the magneto-conductivity data is fitted up to ± 5 Tesla, but in order to extract reliable fitting parameters, the same is fitted at much lower magnetic fields, i.e., up to ± 1 Tesla. The value of the HLN coefficient (α), extracted from the HLN equation exhibited values close to ? 1.0, indicating both WAL and WL contributions. On the other hand, the extracted \(l_{\varphi }\) is seen to decrease from 11.125 to 5.576 nm as the temperature is increased from 5 to 200 K, respectively. Summarily, the short letter discusses primarily the temperature-dependent magneto-conductivity analysis of pristine Bi₂Se₃ single crystal by the HLN model.     

Extruded thermoelectric materials based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> solid solutions

Extruded n-type materials based on Bi₂Te₃-Bi₂Se₃ alloys containing 6 to 40 mol % Bi₂Se₃ have been investigated using microstructural analysis and thermoelectric measurements at room temperature and in the range 100–400 K. Their electrical properties have been compared to those of single-crystal analogs. Compositions have been found at which the extruded materials offer the highest thermoelectric performance in different temperature ranges.     

Solid solutions in the Ag<Subscript>2</Subscript>Se-PbSe-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

The phase equilibria in the pseudoternary system Ag₂Se-PbSe-Bi₂Se₃ have been studied using differential thermal analysis, x-ray diffraction, and microhardness measurements. The results have been used to construct the T-x phase diagram along the AgBiSe₂-PbSe join and the 900-K section of the ternary phase diagram. The AgBiSe₂-PbSe system contains a continuous series of cubic solid solutions with the NaCl structure. Their lattice parameter is an almost linear function of composition (a = 5.822–6.125 Å). The formation of the solid solutions stabilizes the high-temperature phase of AgBiSe₂: PbSe dissolution in this compound markedly reduces its polymorphic transformation temperature (590 K), down to room temperature at ? 10 mol % PbSe. In the Ag₂Se-PbSe-Bi₂Se₃ system, the γ-phase exists in a broad region around the AgBiSe₂-PbSe pseudobinary join.     

Thermodynamic properties and homogeneity regions of Tl<Subscript>6</Subscript>SCl<Subscript>4</Subscript> and Tl<Subscript>5</Subscript>Se<Subscript>2</Subscript>Cl

In this activity system Tl-Tl₂X-X (X = S, Se)are studied using emf measurements of concentration chains relative thallic electrode. The solid phase diagrams of these systems are clarified, homogeneity areas of the compounds Tl₆SCl₄ and Tl₅Se₂Cl are determined. On the basis of emf measurement results, relative partial molar functions of thallium in alloys and standard integral thermodynamic functions (ΔG ⁰(298 K), ΔH ⁰ (298 K), ΔS ⁰ (298 K)) of the ternary compounds Tl₆SCl₄ and Tl₅Se₂ Cl and phases of variable composition based on the latter are calculated.     

Bridgman growth and defects of Nd<Superscript>3+</Superscript> : Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> laser crystals

Nd³⁺ : Sr₃Ga₂Ge₄O₁₄ crystals have been grown by the modified Bridgman method. The growth defects, such as striations, scattering particles and dislocations were investigated. Some featherlike striations were observed in as-grown crystals. EPMA analysis suggested that these inclusions were caused by the segregation of Nd₂O₃ from the melt. Chemical etching results showed that the dislocation density was in the range of 10³ ∼ 10⁵/cm².     

Crystal Growth and Magneto-transport of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals

In this letter, we report on the growth and characterization of bulk Bi ₂Se ₃ single crystals. The studied Bi ₂Se ₃ crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 °C) melt of constituent elements and slow cooling (2 ℃/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) Å. Temperature versus resistivity (ρ?T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ _{300 K}) of around 0.53 m Ω-cm and residual resistivity (ρ _{0 K}) of 0.12 m Ω-cm. Resistivity under magnetic field [ ρ(T)H] measurements exhibited large + ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [ ρH] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Δρ/ ρ _oversus B/ ρ, were seen consolidating on one plot. Interestingly, the studied Bi ₂Se ₃ single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T.