Structural and thermoelectric properties in (Sb<Subscript>1-x</Subscript>Bi<Subscript>x</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> thin films

We have investigated the structural and thermoelectric properties of (Sb_1-xBi_x)₂Te₃ thin films on CdTe(111)B. Analysis of X-ray diffraction patterns (–2 scans and rocking curves) of the films shows that they are of high quality and that they are well aligned with their (00.1) axis normal to the substrates. Measurements of the temperature-dependent thermoelectric power, resistivity, and Hall coefficient of the films were performed with respect to the binary composition, x. For the samples in the range 0.2<x<0.3, the room-temperature thermopower values were in the range 159–184 V/K, the room-temperature carrier concentrations were 3.93–5.13×10¹⁹ cm^-3, and the room-temperature mobilities were 24.6–64.0 cm²V^-1s^-1. PACS 72.20.Pa; 72.80.Jc; 73.6l.Le     

Photoacoustic spectroscopy of thin films of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, As<Subscript>2</Subscript>Se<Subscript>3</Subscript> and GeSe<Subscript>2</Subscript>

Photoacoustic spectroscopy (PAS) is one of the important branches of spectroscopy, which enables one to detect light-induced heat production following the absorption of pulsed radiation by the sample. As₂S₃, As₂Se₃ and GeSe₂ exhibit a wide variety of photo-induced phenomena that enable them to be used as optical imaging or storage medium and various electronic devices, including electro-optic information storage devices and optical mass memories. Therefore, accurate measurement of thermal properties of semiconducting films is necessary to study the memory density. The thermal conductivity of thin films of As₂S₃ (thickness 100 μm and 80 μm), As₂Se₃ (thickness 100 μm and 80 μm) and GeSe₂ (thickness 120 μm and 100 μm) has been measured using PAS technique. Our result shows that the thermal conductivity of thicker films is larger than the thinner films. This can be explained by the thermal resistance effect between the film and the surface of the substrate.      

Fundamental band edge optical absorption in Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

Spectra of optical absorption in Bi_0.5Sb_1.5Te₃ films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi₂Te₃ available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi_0.5Sb_1.5Te₃ and Bi₂Te₃ represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi_2−xSb_xTe₃ (x ≤ 1.5, T = 300 K). For Bi_0.5Sb_1.5Te₃ the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–52, July, 2008.     

Effect of thermal treatment on the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> solid solution thin films

The effect thermal treatment in a vacuum has on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution thin films obtained via ion-beam sputtering in an argon atmosphere is considered. It is established that the specific resistance and thermopower are determined by the type and concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution. The power factor values are found to be comparable to those of nanostructured materials based on (Bi,Sb)₂(Te,Se)₃ solid solutions.     

Magnetotransport in thin epitaxial Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The magnetoconductivity of thin Bi₂Se₃ films covered by a protective Se layer and grown at (111) BaF₂ substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov?de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.     

Effect of heat treatment on the structure and the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> thin films and composites based on them

This work considers the effect of vacuum annealing on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 thin film and Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb_0.9Bi_1.1Te_2.9Se_0.1–C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution, which determine the type of conductivity of Sb_0.9Bi_1.1Te_2.9Se_0.1 granules. The power factors are estimated for films of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution and films of Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)₂(Te,Se)₃ solid solutions.     

Weak antilocalization in thin films of the Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> solid solution

A technology has been developed for the preparation of thin films of the Bi₂Te_2.7Se_0.3 solid solution through the thermal evaporation in a vacuum using the “hot-wall” method. The high quality of the thin films thus prepared has been confirmed by the X-ray diffraction and Raman scattering data. The electron transport has been investigated over wide ranges of temperatures (1.4–300 K) and magnetic fields (up to 8 T). It has been assumed that the observed weak antilocalization is associated with the dominant contribution from the surface states of a topological insulator. The dephasing length has been estimated.     

Selective wet etching of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> phase-change thin films in thermal lithography with tetramethylammonium

In this paper, we study Ge₂Sb₂Te₅ phase-change film as a promising inorganic photoresist using organic alkaline: tetramethylammonium hydroxide (TMAH) solution, instead of inorganic alkali or acid as etchant. The basic etching properties are investigated by prior and posterior annealing Ge₂Sb₂Te₅ films. Selectivity is found to be dependent on concentration of TMAH. There is a good selectivity in the 25% TMAH solution, in which the amorphous state is etched away, whereas the crystalline state remains. The etching rate decreases when the concentration of TMAH is diluted; and an opposite selectivity, compared with 25% TMAH solution, is observed in the 0.125% TMAH solution. Selective etching with laser crystallization in different power levels is also studied, and an excellent wet selectivity in the 25% TMAH solution is obtained. The remaining crystalline lines are observed by atomic force microscopy. The surface roughness after etching is at a good level. The selective wet-etching mechanism is also discussed.     

Impact of Ultrathin Pb Films on the Topological Surface and Quantum-Well States of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulators

The effect of an ultrathin Pb film deposited on the surface of Bi₂Se₃ and Sb₂Te₃ compounds on the electronic state structure of topological insulators is studied experimentally by the angle-resolved photoemission spectroscopy (ARPES) technique. The following features are revealed: formation of two-dimensional quantum-well states in the near-surface region, an increase in the binding energy of the Dirac cone and the core levels, and a simultaneous electronic states intensity redistribution in the system in photoemission spectra. The results obtained show that topological states may coexist at the interface between studied materials and a superconductor, which seems to be promising for application in quantum computers.     

Marangoni effect induced micro/nano-patterning on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> phase change thin film by laser pulse

Thermocapillary and chemicapillary effects are known to coexist in a material molten pool when irradiated by a pulse laser. According to the effects, we fabricate various patterns with different shapes on a Sb₂Te₃ phase change thin film by precisely adjusting the pulse energy. In this process, the laser power is fixed at 5.0 mW, and the pulse width is adjusted from 100 ns to 5 ns. The shape of the patterns gradually changes from a dimple-bowl shape, a dome shape, a “Sombrero” shape to a deep-bowl shape following an increase in the pulse energy, which corresponds to the crystallization-threshold, bump-threshold, rupture-threshold, and ablation-threshold of the material. The different patterns are the results of the competition between the thermocapillary and chemicapillary effects in the molten pool, which determine the nature of the flow and lead to the different patterns in different laser parameters.     

Effect of conditions of preparation on the thermoelectric properties of solid solutions of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

The effect the conditions of preparing thermoelectric solid solutions of Bi_0.5Sb_1.5Te₃ + 0.06 wt % Pb by hardening from the liquid state with subsequent hot pressing have on their thermoelectric properties is studied. It is found that the optimum thermoelectric quality factors are achieved at a 2200–2800 rpm rate of copper disc rotation.     

Stratification of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Single Crystals and Sintering of the Obtained Micro- and Nanoscale Plates

The use of surface active liquids facilitates intense stratification of mechanically strained Bi_0.5Sb_1.5Te₃ crystallites. A Bi_0.5Sb_1.5Te₃ heat element with specified thickness and structure is formed by layer-by-layer deposition of “thermoelectric ink” on its free surface. A heat treatment of the formed thermoelectric element in argon at a temperature of 800 K makes it possible to minimize radically the resistance of the grain boundaries introduced into its bulk.     

Some peculiarities of magnetoresistance and Hall resistance of Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> nanoplates

Antimony telluride (Sb₂Te₃) nanoplates of various thickness were grown by the vapor phase deposition method. The Hall resistance and magnetoresistance of the samples were measured in magnetic fields up to 9 T at temperatures from 2 to 300 K. Temperature dependence of the magnetoresistance and Hall resistance of the nanoplates shows a strong dependence on the thickness of the samples. Relatively thick samples show a nonlinear dependence of the Hall resistance on magnetic field. The measurement data are analyzed within the model of multi-channel transport. The difference in behavior is attributed to the existence of two channels of charge transfer with high and low mobility.     

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped <Emphasis Type="Italic">p</Emphasis>-(Bi<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi_0.5Sb_0.5)₂Te₃ single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.     

Magnetic Resonance Study of the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Doped with Manganese

Crystals of 3D topological insulators, bismuth telluride Bi₂Te₃, doped with manganese were studied using electron spin resonance (ESR) spectroscopy together with the SQUID magnetometry, transport measurements, and X-ray characterization. The obtained ESR data, such as the temperature and the angular dependence of the resonance field, reveal the specific critical behavior and confirm the ferromagnetic ordering of Mn spins even at modest doping. In addition to the studies of the critical behavior of diluted ferromagnet Bi_2?x Mn _x Te_3, we also discuss the effects of the limited solubility of Mn ions giving rise to microscopic inclusions of the spurious magnetic phases which were revealed using ESR technique.     

Optical functions of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions in the range of plasmon excitation and interband transitions

The most narrow sub-Doppler frequency resonances in the linear absorption of monochromatic radiation that propagates in the normal direction through a cell containing a layer of rarefied gas medium with a thickness smaller than or on the order of the wavelength of this radiation are theoretically studied. The calculation is performed using as an example a three-dimensional gas cell shaped like a rectangular parallelepiped. It is shown that the width and amplitude of considered sub-Doppler resonances (in the vicinity of centers of rather weak quantum transitions) significantly depend on the transit relaxation of atomic particles, which is determined by their transit times through the irradiated region of the cell both in longitudinal and in transverse directions. The restrictions of the approximation of the planar one-dimensional cell that was previously used in such calculations are determined. Possible applications of linear absorption resonances in ultrathin (nanometer) gas cells as references for optical frequency standards are discussed.     

Features of the electron density distribution in antimony telluride Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>

Based on the results of electron density functional calculations of the electronic band structure of semiconductors Sb₂Te₃, Ge, Te, and semimetal Sb, the parameters of critical points in the electron density distribution (maxima, minima, and saddle points) in the lattices of the above materials are found. The data obtained are used to analyze the chemical bond nature in Sb₂Te₃.     

Thermal conductivity of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>: Sn and the effect of codoping by Pb and I atoms

The variation of the lattice thermal conductivity of Bi₂Te₃ induced either by alloying it with tin alone or by codoping the lattice with an acceptor or donor impurity was studied. The experimental data obtained at room and liquid nitrogen temperatures argue for the validity of the model of quasi-local impurity states associated with tin atoms.     

NMR study of topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> in a wide temperature range

NMR studies of ¹²⁵Te in the topological insulator bismuth telluride Bi₂Te₃ in a wide temperature range from room temperature to 12.5 K are performed. The pulsed NMR spectrometer Bruker Avance 400 is applied. The NMR spectra are obtained for the powder from Bi2Te3 single crystal and monocrystalline plates with the orientations c || B and c ⊥ B. At room temperature, the spectra consist of two lines related to two nonequivalent positions of tellurium nuclei Te1 and Te2. The parameters of the NMR frequency shift tensor are found from the powder spectrum. The temperature dependences of the spectra for the powder and plates with the orientation c ⊥ B agree with each other. The line shift with decreasing temperature is explained by the reduction of the Knight shift. The thermal activation energy of charge carriers is estimated. The spectra for the plates with the orientation c || B demonstrate peculiar behavior below 91 K. The spin-lattice relaxation time for the powder and monocrystalline plates with both orientations at room temperature is measured.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).