Semiconductor Bi2Te3 Research Articles

Stacking fault-induced strengthening mechanism in thermoelectric semiconductor Bi2Te3

Stacking fault-induced strengthening mechanism in thermoelectric semiconductor Bi2Te3

125Te NMR study of the bulk of topological insulators Bi2Te3 and Sb2Te3

AbstractThe narrow band gap semiconductors Bi2Te3 and Sb2Te3 are well known for their room temperature thermoelectric performance. Recently, they were shown to serve as model systems of three‐dimensional topological insulators with a bulk band gap and very robust, spin‐resolved surface states due to a special band inversion in their periodic bulk. Evidently, it is of interest to investigate the special surface states with a local probe like nuclear magnetic resonance (NMR). However, especially the bulk NMR of these materials shows peculiar and rather unexpected phenomena, e. g., a magnetic field induced deformation of the local charge symmetry and excessive line broadening due to a special internuclear coupling. Here we report a comprehensive account of the room temperature NMR properties of the spin‐1/2 nucleus 125Te of single crystalline Bi2Te3 and Sb2Te3. This includes a very unusual spin‐lattice relaxation that seems not to be reflected in the NMR shift, as well as an exchange enhanced NMR line broadening.

Study of vibrational properties of Bi2 − xMnxTe3 nanocrystals in host glass: Effect of xMn‐concentration

AbstractThe Raman and infrared (IR) active lattice vibrations of xMn‐concentration Bi2 − xMnxTe3 nanocrystals (NCs) as a dopant in semiconductor Bi2Te3 nanocrystals grown in a host glass matrix, whose symmetries correspond to the space group, are investigated by Raman scattering. Transmission electron microscopy images confirm the formation of Bi2 − xMnxTe3 nanocrystals with an average size of around 5.4 nm. The observation of the six hyperfine structure lines in electron paramagnetic resonance spectra confirms the incorporation of Mn2+ ions in Bi2Te3 nanocrystals. The vibrational modes of quintuple layer (QL) of the diluted magnetic semiconductor Bi2 − xMnxTe3 nanocrystals are modified in relation to pure Bi2Te3 nanocrystals. The appearance of the (95.5 cm−1), (114 cm−1), and (103 cm−1) IR‐active modes is mainly due to quantum size effects. Indeed, the redshifts of the (95.5 cm−1), (103 cm−1), (110 cm−1), (114 cm−1), and (141 cm−1) vibrational modes give strong indications of the substitutional and interstitial incorporation of Mn2+ in the Bi2Te3 crystalline structure. In addition, the Raman spectra show the formation of Bi2OTe2 semiconductor due to the appearance of the (123 cm−1) mode. The investigation of the Bi2 − xMnxTe3 nanocrystal has promising potential to design new devices with magnetic and optical properties adjusted according to the xMn‐concentration.

Non-universal scaling of thermoelectric efficiency in 3D and 2D thermoelectric semiconductors

We performed the first-principles calculation on common thermoelectric semiconductors Bi2Te3, Bi2Se3, SiGe, and PbTe in bulk three-dimension (3D) and two-dimension (2D). We found that miniaturisation of materials does not generally increase the thermoelectric figure of merit (ZT) according to the Hicks and Dresselhaus (HD) theory. For example, ZT values of 2D PbTe (0.32) and 2D SiGe (0.04) are smaller than their 3D counterparts (0.49 and 0.09, respectively). Meanwhile, the ZT values of 2D Bi2Te3 (0.57) and 2D Bi2Se3 (0.43) are larger than the bulks (0.54 and 0.18, respectively), which agrees with HD theory. The HD theory breakdown occurs because the band gap and band flatness of the materials change upon dimensional reduction. We found that flat bands give a larger electrical conductivity (σ) and electronic thermal conductivity (κ el) in 3D materials, and smaller values in 2D materials. In all cases, maximum ZT values increase proportionally with the band gap and saturate for the band gap above 10 k B T. The 2D Bi2Te3 and Bi2Se3 obtain a higher ZT due to the flat corrugated bands and narrow peaks in their DOS. Meanwhile, the 2D PbTe violates HD theory due to the flatter bands it exhibits, while 2D SiGe possesses a small gap Dirac-cone band.

Photon treatment effect on the surface and figure of merit of thermoelectric material Bi2Te3\u2212xSex

The phase composition, structure, morphology, and thermal conductivity of Bi2Te3−xSex-based semiconductor solid solution after photon treatment (PT) were studied by X-ray diffractometry, SEM, TEM, and the laser flash techniques. It was revealed that PT leads to recrystallization of the subsurface layers of the solid solution with the formation of a heterogeneous nanocrystalline structure. The thermoelectric figure of merit of the semiconductor Bi2Te3−xSex (n-type) solid solution increases after PT due to modification of the surface layers. This is due to the decrease of the thermal conductivity of the studied material after PT.

Fabrication of Bi2Te3 films on the surface of silica optical fibres by MOCVD

Crystalline 2D bismuth and antimony chalcogenides, as well as graphene, can successively operate as saturable absorbers in various schemes of passively mode-locked fibre lasers. An attractive benefit of these saturable absorbers is their power to effectively operate at different wavelengths in the wideband from 1 to 2 microns. However, the problem is how to provide a reliable interaction between light in a fibre and the absorber. The main disadvantage of a standard solution to the problem essentially is to use exfoliated flakes dissolved in a coupling agent, which deposits additional heat that negatively affects the absorber. In the present research, thin crystalline films of a narrow-band semiconductor Bi2Te3 were deposited directly on silica optical fibres by means of metal-organic chemical vapour deposition (MOCVD). A ZnTe buffer layer was deposited first in a single growth run, providing the necessary condition for better adhesion of bismuth telluride film to the silica surface. Two types of fibre coatings were prepared and tested: the coating upon the cleaved end of the fibre and the one upon the lateral surface of the light-guiding fibre core. In the latter case, prior to deposition a fibre section of about 1 cm in length was uniformly thinned via chemical etching to a diameter of about 20 microns. Reflection and transmission of the growing films were monitored in situ during the deposition.

Separation of Contributions from the Ion Core and Free Charge Carriers to the Magnetic Susceptibility of an Anisotropic Semiconductor Bi2Te3–Sb2Te3 Crystal

A technique is presented, by which the magnetic susceptibility χ | of the ion core of an anisotropic semiconductor Bi2Te3–Sb2Te3 crystal is determined from experimental data on the magnetic susceptibility χ ∥ and χ ⊥ obtained with allowance for the orientation of the magnetic field vector H with respect to the trigonal C3 axis of the crystal in accordance with the expression χ ∥/χ ⊥ = (χ ∥ + χ G )/(χ ⊥ + χ G ).In this expression, the value of the magnetic susceptibility of free charge carriers χ ∥ and χ ⊥ depending on their effective masses m ∥ * and m ⊥ * known from the experiment is calculated within the framework of the Pauli and Landau- Peierls approaches. The found value of χ | for Bi2Te3–Sb2Te3 crystals is in good agreement with experimental data, as well as with the estimates obtained in the framework of the Larmor approach explaining, in particular, a linear dependence of the molar magnetic susceptibility on the number of electrons in the molecule observed for a large number of compounds. The proposed technique can be extended to other anisotropic semiconductors.

Quantum Hall effect in the bulk semiconductors bismuth and antimony tellurides: Proof of the existence of a current-carrier reservoir

The quantization of the Hall resistivity ρxy in the form of plateaus in the dependence of ρxy on the magnetic field B is observed in the semiconductors Bi2Te3 and Sb2Te3; the minima of the transverse magnetoresistivity ρxx correspond to the start of the plateaus. The quantization of ρxy is due to the presence of a current-carrier reservoir. An impurity band with a high density of states or a different band with a much higher current-carrier effective mass serves as the reservoir.

Improvement of thermal sensors based on Bi2Te3, Sb2Te3 and Bi0.1Sb1.9Te3

Thermoelectric properties of materials are essentially based on the thermoelectrical figure of merit Z = S2σ/K, where S is the thermoelectrical power, σ the electrical conductivity and K the thermal conductivity. Narrow-bandgap semiconductors Bi2Te3(n), Sb2Te3(p) layered onto amorphous substrates using the molecular beam technique are investigated. These materials present the highest values of Z. Following the previous work on these materials, the variations in mobility μ and carrier density n as a function of film thickness are performed. It is found that μ and n tend to constant values when the film thickness is higher than 6000 Å. We also undertook the study of the thermoelectric properties of BixSb2-xTe3 alloys as a function of the bismuth concentration x. The optimal value of S was obtained at x=0.1. The improvement of S in comparison with that of Sb2Te3 results in the reduction in antisite defects or the carrier compensation. Thermopile and pressure sensors based on [Bi0.1Sb1.9Te3(p)Bi2Te3(n)] are constructed. These [p-n] active junctions constitute the sensitive elements of both the thermopiles and the vacuum gauges. In comparison with the [BiSb] couple, an improvement in the sensitivity by a factor 2.8±0.2 was achieved. Thermal simulation based on COSMOS/M software confirmed the experimental findings.

Crystal growth of Bi2Te3on single-crystal substrate Sb2Te3by molecular beam epitaxy†

Abstract We have grown stoichiometrically thin films of the narrow bandgap semiconductor Bi2Te3 by molecular beam epitaxy in an ultra-high vacuum on single-crystal substrate Sb2Te3 in order to optimize the growth conditions for this technique. High resolution X-ray diffraction, scanning electron microscope observations and energy dispersive X-ray spectroscopy analysis are used to characterize the substrate and the deposited layers. In addition, secondary ion mass spectroscopy depth profiling is used to study the diffusion and the concentration profiles of layer elements. We have also studied the effect of heat treatment on some of the electrical properties of single-crystal substrate as a function of temperature in order to analyse its semiconducting behaviour at low temperature. The sticking coefficients Ks(Te) and Ks(Bi) of the elements Te and Bi, respectively, are determined as a function of substrate temperature Ts and flux ratio FR = [F(Te)/F(Bi)]. It is found that Ks(Te) is very sensitive to both Ts...

Thermal Parameters and Probability Density Functions of the Narrow Gap Semiconductor Bi2Te3 under Hydrostatic Pressure

AbstractThe thermal parameters and the probability density functions of the atoms in the narrow gap semiconductor Bi2Te3 are investigated by neutron diffraction experiments under hydrostatic pressure. The results show rather complex intrasandwich bonding properties of the atoms in this sandwich structure. Especially Bi shows relatively large vibration amplitudes parallel to the trigonal axis in contradiction to former X‐ray studies. The experiments show evidence for healing effects of crystal distortions under hydrostatic pressure.

ELLIPSOMETRIC STUDY OF NUCLEATION AND GROWTH IN THE ANODIC FILM FORMATION ON Bi2 Te3

The present paper shows from the ellipsometric investigation that the anodic film formation on layered semiconductor Bi2Te3 proceeds through the two-dimensional nucleation and growth. The present study indicates that the refractive index, the monolayer thickness and the formation voltage of the anodic film are 2,10 ± 0,05, 15 ± 1 A and 0,23 ± 0,02 v/monolayer, respectively.

The Optical Properties of Bismuth Telluride

The shape of the absorption edge in the semiconductor Bi2Te3 has been determined from transmission measurements on cleavage sections. The edge is of the form expected for indirect transitions, but an interpretation in terms of phonons characterized by a single energy is not applicable. A brief study of anisotropy effects is included.

Heat Conduction in Bismuth Telluride

The Lorenz number of the semiconductor Bi2Te3 has been evaluated, for the range of partial degeneracy, from data provided by the measurements of electrical conductivity and thermoelectric power. The calculated electronic component of the thermal conductivity, for p-type material, has been found to agree with the experimental results, assuming the lattice thermal conductivity to be independent of the electrical conductivity. Agreement has also been obtained for non-halogen-doped n-type Bi2Te3, but not for halogen-doped material. It has been shown that the reason for this is the high effective scattering cross section, for phonons, of the halogen atoms. The theory of the transport of ionization energy has been found to agree very closely with experiment.

The Electrical Conductivity and Thermoelectric Power of Bismuth Telluride

The electrical conductivity and thermoelectric power of the semiconductor Bi2Te3 have been measured between 150°K and 300°K. n-type and p-type samples with a wide range of carrier concentration have been included. Most samples have shown extrinsic conduction and have been partially degenerate over at least part of the temperature range, so it has been necessary to use Fermi-Dirac statistics in interpreting the results. A few samples have exhibited mixed and intrinsic conduction at the higher temperatures. It has been possible to determine the variation of carrier mobility with temperature and to estimate the energy dependence of the relaxation time, as well as a number of the semiconductor parameters.