Thermoelectric properties of Bi2Se3/Bi2Te3/Bi2Se3 and Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems

In this work, we develop a theory of thermoelectric transport properties in two-dimensional semiconducting quantum well structures. Calculations are performed for n-type 0.1 wt.% CuBr-doped Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ and p-type 3 wt.% Te-doped Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ quantum well systems in the temperature range 50–600 K. It is found that reducing the well thickness has a pronounced effect on enhancing the thermoelectric figure of merit (ZT). For the n-type Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ with 7 nm well width, the maximum value of ZT is estimated to be 0.97 at 350 K and for the p-type Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ with well width 10 nm the highest value of the ZT is found to be 1.945 at 440 K. An explanation is provided for the resulting higher ZT value of the p-type system compared to the n-type system.     

Infrared spectroscopy of Bi2Te2Se

The infrared reflection and transmission spectra of Bi₂Te₂Se single crystals grown by the modified Bridgeman method have been studied in the spectral range of 30–10000 cm^?1 at temperatures of 5–300 K. The bandgap and its temperature dependence, optical function spectra, and concentration of free charge carriers in Bi₂Te₂Se have been determined.     

Photoelectron spectroscopy of Se2 and Te2

The He(I) photoelectron spectra of Se₂ and Te₂ are presented. In both these molecules, ground-state Ω splitting, described by Hund's case (c)     

Scanning tunneling microscope light emission spectra of polycrystalline Ge2Sb2Te5 and Sb2Te3

We have observed scanning tunneling microscope light emission (STM-LE) spectra of Ge₂Sb₂Te₅ and Sb₂Te₃. Although these chalcogenide alloys exhibit band gaps less than 0.5 eV, the STM-LE was observed with a narrow spectral width at a photon energy of 1.5 eV for both materials. By analyzing its bias voltage, polarity, and temperature dependencies combined with recently reported theoretical electronic structures, we concluded that the STM-LE is excited by electronic transitions taking place in the local electronic structure having a direct gap-like shape with a band gap of 1.5 eV, commonly found in the electronic structures of both materials.     

The electrical properties of Cu2Te

The conductivity, Hall effect, and thermo-emf are reported for thin films and polycrystalline bulk specimens of Cu₂Te, which is found to be an impurity semiconductor whose forbidden band has a width of 0.9–1.0 eV. The compound dissociates on vacuum evaporation, so it is very difficult to produce thin films.I am indebted to Professor D. N. Naskedov for his interest and for valuable comments.     

Density functional study of Bi2Te3 and Bi4Te6 molecules

Results of a density functional study for the molecules Bi₂Te₃ and Bi₄Te₆ are reported here. For Bi₂Te₃, calculations yield eight stable conformations. For Bi₄Te₆, eight stable isomers are identified. Equilibrium geometries, adiabatic ionisation potentials, atomisation energies, and vibrational bands are estimated. The lowest energy conformations in both cases are clusters of C_s symmetry with all Te atoms two-fold coordinated and all Bi atoms three-fold coordinated. The predicted low energy conformation for Bi₄Te₆ has alternating rows of Bi and Te atoms. This molecule seems a reasonable precursor to solid bismuth telluride, which has alternating Bi and Te layers.     

High pressure transport characteristics of Bi2Te3, Sb2Te3, and BiSbTe3

This paper presents ambient and high pressure measurements of transport properties of the Bi₂Te₃–Sb₂Te₃ series of materials. The electrical resistivity, thermal conductivity, and Seebeck coefficient have been measured on both end compounds and the direct solid solution of the two at pressure up to 10 GPa. An additional discussion involving the high pressure structure will be presented. From this, it was determined that these materials undergo at least two structural phase transitions between 0 and 20 GPa and a discussion is presented regarding this and the changes in the transport properties.     

Plasma screening of the fundamental absorption edge in crystals of Bi2Te3-Sb2Te3 solid solutions with more than 80 mol % Sb2Te3

The regularities of changes in the optical properties of crystals of Bi₂Te₃-Sb₂Te₃ solid solutions in the range of the effects caused by free-carrier plasma oscillations at a variation in the ratio of Bi₂Te₃ and Sb₂Te₃ components are investigated. It is established that, when the Sb₂Te₃ content in a solid solution exceeds 80 mol %, the fundamental absorption edge undergoes plasma screening. Doping a Sb₂Te₃ crystal with tin increases the free-carrier concentration and plasma frequencies, thus enhancing the screening effect, whereas introduction of 0.1 mol % selenium reduced the plasma frequency and thus removes the fundamental-absorption-edge screening.     

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge₂Sb₂Te₅ (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In₂Te₃ crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \) . Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.     

Electronic properties of glassy As2Te3

Bulk samples of glassy As₂Te₃ and its alloys with Ge, Si or Tl were prepared by quenching of the melt and their electrical, photoelectrical, optical and thermodynamical properties were studied. The addition of Ge, Si or Tl produces three different types of glass from the thermodynamical point of view; the physical properties studied differ only quantitatively.Shortened version of this work was presented at the 4th International Conference on Amorphous and Liquid Semiconductors, Ann Arbor USA, August 1971.Cukrovarnická 10, Praha 6, Czechoslovakia.The authors wish to express their thanks to Mrs. J. Beránková for preparation of the As₂Te₃ glasses, to Mrs. M. ilhavá for her assistance with photoconductivity measurements and to Mr. S. Hotmar for his help with the conductivity measurements.     

Heterodiffusion of selenium in Bi2Te3

We present results for the anisotropic diffusion coefficient of selenium, isoelectronic impurity, in Bi₂Te₃ along the solidus. The experimental conditions permitted us to obtain a definite stoichiometric deviation, and diffusion profiles were obtained by SIMS.     

相变存储材料Ge1Sb2Te4和Ge2Sb2Te5薄膜的结构和电学特性研究

采用射频磁控溅射方法制备了两种用于相变存储器的Ge₁Sb₂Te₄和Ge₂Sb₂Te₅相变薄膜材料,对其结构、电学输运性质和恒温下电阻随时间的变化关系进行了比较和分析.X射线衍射(XRD)和原子力显微镜(AFM)的结果表明:随着退火温度的升高,Ge₁Sb₂Te₄薄膜逐步晶化,由非晶态转变为多晶态,表面出现均匀的、 关键词： 硫系相变材料 1Sb₂Te₄')" href="#">Ge₁Sb₂Te₄ 2Sb₂Te₅')" href="#">Ge₂Sb₂Te₅     

Symmetry of lattice vibrations of hexagonal Cu2Te

The vibrational representation is expanded in terms of irreducible representations at the principal symmetry points of the Brillouin zone. Selection rules are found for single-phonon IR absorption, Raman scattering, and Mandel'shtam-Brillouin scattering of light. In IR absorption, a single frequency is active for light polarized parallel to the sixth order axis and a single frequency for perpendicular polarization. Five frequencies are active in Raman scattering of light. Expansions of the direct product of irreducible representations (selection rules) are found.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–48, November, 1981.     

Collective polynuclear superradiance rather thanstimulated emission of Mössbauer radiation from125m2Te and 123m2Te

Acts attributed to single-particle stimulated emission, ^125m2Te(γ,2γ)^125m1Te, had been observed previously in a Be^125m2Te sample as a variation of the line intensity at Eγ=109.27 keV and by the appearance of a line of double energy at E_2γ=218.54 keV. This effect occurred when the sample temperature, T was lowered from 300 to 4 K. In the present work, the first observation is reported of stimulated emission events in the reaction ^123m2Te(γ,2γ)^123m1Te which were registered from a Mg₃ ^123m2TeO⁶ sample when cooled in a similar fashion. It is shown here that the mechanism of these experimentally observed events is polynuclear superradiance rather than a single-particle stimulated emission of Mössbauer radiation as earlier suggested.     

Dipole excitations in 122Te, 126Te and 130Te

Excited states of the nuclei ¹²²Te, ¹²⁶Te and ¹³⁰Te were populated via the (γ, γ') reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL-CLUSTER detector and a single detector. In all nuclei several dipole transitions were identified at energies around 3 MeV. The lowest corresponding J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict one 1^? state arising from the coupling of the first quadrupole and the first octupole phonon and one 1⁺ state arising from the coupling of the first and the isovector second quadrupole phonon at about 3 MeV. The calculated transition strengths are compatible with experimental ones.