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P -type Bi2 Se3 for topological insulator and low-temperature thermoelectric applications
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The growth and elementary properties of p-type Bi2Se3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi2Se3, the p-type behavior has been induced through low-level substitutions (1% or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle-resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi1.98Ca0.02Se3 relative to the n-type material. p-type single crystals with ab-plane Seebeck coefficients of +180 μV/K at room temperature are reported. These crystals show an anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 μV K−1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi2Se3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.
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... A charge density wave system in polycrystalline Bi 2 Te 3 supports high electron-phonon coupling, reducing the impact of Peierls distortion on the lattice thermal conductivity mechanism. The lower lattice thermal conductivity seen in single crystals of Bi 2 Te 3 [52] is caused by the quantum confinement effect and the charge density wave caused by Periels distortion. ...
The physical parameters of solid-state produced tin and tellurium co-doped bismuth selenide polycrystalline crystals were described. Powder X-ray diffraction revealed the hexagonal structure in the samples’ phase domination. A field emission scanning electron microscope was used to analyze the surface microstructure. Thermoelectric properties such as Seebeck coefficient, electrical resistivity, and thermal conductivity were analyzed in the temperature range 10–350 K. The electrical resistivity of (Bi0.96Sn0.04)2Se2.7Te0.3 was found to be four times lower than that of pure Bi2Se3. Due to donor-like effects and antisite defects, the Seebeck coefficient demonstrates a p- to n-type semiconducting transition. When compared to pure Bi2Se3, power factor and thermoelectric figure of merit of (Bi0.96Sn0.04)2Se2.7Te0.3 is found to increase by 15 and 9 times respectively. Tellurium excess boosts tin vacancies, promoting the p to n-type transition in (Bi0.96Sn0.04)2Se2.7Te0.3, making it a good option for low temperature thermoelectric and sensor applications.
... Temperature-dependent Seebeck coefficient S(T) of pristine Bi 2 Se 3 and (Bi 1−x Sb x ) 2 Se 2.7 Te 0.3 samples in the temperature range 10-350 K is presented in Fig. 9. The negative value of S implies that charged Se vacancies operate as electron donors, causing n-type conduction [31]. It is noted that the absolute value of S increases with increasing temperature, presumably attributed to the diffusive contribution of the Seebeck coefficient. ...
As tellurides and selenides of Bismuth are extensively used thermoelectric materials in the low-temperature range with refrigeration application, A concerted effort is put forth to boost the ZT by co-doping the pristine Bi2Se3 with antimony (Sb) on cation end and tellurium (Te) on anion side of the compound. The double sintered solid-state reaction method is employed in the preparation of Bi2Se3 and (Bi1−xSbx)2Se2.7Te0.3 polycrystalline pellets, with x = 0.02, 0.04, and 0.06. The sample exhibits a rhombohedral structure with the space group of R 3¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{3 }$$\end{document} m. FESEM and EDAX analysis are used to confirm surface morphology and elemental composition of the produced samples. The thermoelectric measurements for the pristine and co-doped samples were conducted by the physical property measurement system as well as a considerable increase in the Seebeck coefficient of − 115 µV/K is observed for 0.02 doping of the Sb which is 4.2 times greater than as for pristine (S = − 28 µV/K). Higher concentration doping (x = 0.04, 0.06) has not reflected any Seebeck coefficient advancement. In contrast, the lowest total thermal conductivity at a low-temperature regime (< 50 K) is observed for x = 0.06 doping concentration which at near room temperature increases beyond the pristine thermal conductivity; whereas, x = 0.02, 0.04 doping concentration shows a decrement in total thermal conductivity at low, at room temperature region the values are almost equivalent to that of pristine. The co-doped samples’ electrical resistivity is substantially less than pristine sample. The highest power factor of 3 × 10–4 µW/mK² is obtained for the x = 0.02 sample. The highest ZT value for the x = 0.02 sample is almost 28 times more than a pristine sample. We conducted a study where we combined experimental and DFT methods to investigate thermoelectric properties incorporated into pristine and doped with antimony and tellurium on Bi2Se3. We have employed this combination to investigate the partial density of states, Seebeck coefficient, power factor using the Boltzmann transport equation. Theoretical thermoelectric data were derived and compared to experimental observations. Hence, using combined experimental and theoretical investigations helps to predict with higher accuracy of thermoelectric properties of semiconducting materials.
... Covalent bonding within each quintuple layer is significantly stronger than weak Van der Waal forces that bind neighboring layers together. The presence of selenium vacancies in the material makes it easy ndoped across a broad range of carrier concentrations [16,17], although recent efforts have also enabled p-doping [18]. Further research has delved into the specifics of interband transitions and phonons. ...
In this paper, Bi 2 Zn x Se 3-x with compositions x = 0 and x = 0.25 were synthesized using the melt technique at 850 C. The crystal structures and surface morphologies of both the samples were investigated with X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM), respectively. XRD spectra of both the bulk alloys of Bi 2 Zn x Se 3-x (x = 0 and x = 0.25) revealed a polycrystalline nature with a hexagonal structure. UV-Vis absorbance spectra have shown that Zn doping increases the optical band gap while decreasing the absorbance edge. The activation energy (E a) as calculated using Arrhenius plot revealed that Zn doping in place of Se results in a decrease in E a. The variation in resistance with temperature highlights the prevalence of phonon-phonon and electron-phonon scattering as the dominant mechanisms. Topological in-sulators, due to their ability to suppress intrinsic electron backscattering, are viable recommendations for use as counter electrode materials in photovoltaic cells. This approach is poised to create a novel pathway for the future commercialization of flexible photovoltaic applications.
... Thermoelectric effects convert temperature gradients to useful electric power and are broadly classified into two categories: first, the Seebeck effect, where a voltage is induced parallel to the heat gradient, and second, the Nernst effect, where the voltage appears in a direction perpendicular to the heat flow. While both the Seebeck and Nernst effects of narrow-gap semiconductors have been intensively studied in the context of energy-saving technology for more than two decades [1][2][3], the further technological potential of Nernst-type phenomena was pointed out recently [4]. The two most well-known contributions to this thermoelectric response are the normal Nernst effect from the orbital motion of electrons and the anomalous Nernst effect that appears in the ground state of magnetic materials, which is typically proportional to the magnetization [5][6][7][8]. ...
The thermoelectric Nernst effect of solids converts heat flow to beneficial electronic voltages. Here, using a correlated topological semimetal with high carrier mobility μ in the presence of magnetic fluctuations, we demonstrate an enhancement of the Nernst effect close to a magnetic phase transition. A magnetic instability in NdAlSi modifies the carrier relaxation time on “hot spots” in momentum space, causing a strong band filling dependence of μ. We quantitatively derive electronic band parameters from a novel two-band analysis of the Nernst effect Sxy, in good agreement with quantum oscillation measurements and band calculations. While the Nernst response of NdAlSi behaves much like conventional semimetals at high temperatures, an additional contribution ΔSxy from electronic correlations appears just above the magnetic transition. Our work demonstrates the engineering of the relaxation time, or the momentum-dependent self-energy, to generate a large Nernst response independent of a material’s carrier density, i.e., for metals, semimetals, and semiconductors with large μ.
Spintronics is an innovative field that exploits the intrinsic spin property of electrons instead of their charge, holding the promise of revolutionizing conventional electronic devices. Over the past decade, researchers have been actively exploring new materials as potential replacements for traditional spintronic materials. This endeavor is driven by the aspiration to create spintronic devices with ultralow power consumption, ultrahigh storage density, and remarkable stability. In recent years, topological quantum materials (TQMs) have attracted considerable interest due to their unique band structure and exceptional properties. These materials carry the potential to pave the way for breakthroughs in the design of spintronic devices, offering promising solutions to solve challenges currently faced in the field of spintronics. In this review, we first introduce the properties of various TQMs, including band structure and crucial transport properties. Subsequently, we focus on the diverse applications of TQMs in spintronics. Delving further, we discuss the current challenges and the potential directions for advancing and exploring TQMs.
The sum of relative ratios of peak widths of A g and E g modes of BSTS film grown on Si substrate was lower which indicated more ordered structure with lower contribution of localized defects compared to SiC/graphene substrate.
Detailed transport studies of single crystals of Bi2Se3 were made in the temperature range of 2–300 K, and the data were analyzed in terms of a model consisting of two groups of electrons—a centrosymmetrical lower conduction band and an upper conduction band located away from the Γ-point. Very good agreement with the experimental data is obtained assuming the electrons are scattered on acoustic phonons and ionized impurities. A rather strong influence of the latter mechanism is attributed to a large number of charged selenium vacancies in Bi2Se3. The fitted transport parameters were used to calculate the electronic portion of the thermal conductivity that, in turn, allowed for the determination of the lattice thermal conductivity. The Debye model provides a good approximation to the temperature dependence of the lattice thermal conductivity.
Scanning tunneling spectroscopy images of Bi2Se3 doped with excess Bi reveal electronic defect states with a striking shape resembling clover leaves. With a simple tight-binding model, we show that the geometry of the defect states in Bi2Se3 can be directly related to the position of the originating impurities. Only the Bi defects at the Se sites five atomic layers below the surface are experimentally observed. We show that this effect can be explained by the interplay of defect and surface electronic structure.
We study the proximity effect between an s-wave superconductor and the surface states of a strong topological insulator. The resulting two dimensional state resembles a spinless p_x+ip_y superconductor, but does not break time reversal symmetry. This state supports Majorana bound states at vortices. We show that linear junctions between superconductors mediated by the topological insulator form a non chiral 1 dimensional wire for Majorana fermions, and that circuits formed from these junctions provide a method for creating, manipulating and fusing Majorana bound states.
Crystals of n- and p-type conductivity have been produced showing concentrations down to 4 multiplied by 10**1**6 cm** minus **3. The temperature and magnetic field dependences of their galvanomagnetic properties are investigated. At low magnetic fields and low temperatures negative magnetoresistance occurs at high magnetic fields no saturation of the positive magnetoresistance is observed which is assumed to originate from the influence of an impurity conduction band. The temperature dependence of the Hall constant the energy gap for thermal excitations to be evaluated. The temperature dependence of the mobility between about 50 and 200 K supports the assumption that scattering of the free carriers by acoustical phonons dominates in this temperature range.
Bi2Se3 thin films were deposited by metal organic chemical vapour deposition using trimethylbismuth (TMBi) and diethylselenium (DESe) as metal organic sources. Structural properties, electric and thermoelectric results are studied as functions of VI/V ratio (VI/V ratio=DESe partial pressure/TMBi partial pressure). These films always displayed n-type conduction for a VI/V ratio between 14 and 35. The Seebeck coefficient and the carrier concentration vary slightly with the VI/V ratio: −108 to −120 μV/K and 2×1019–4×1019/cm3, respectively. In contrast, the mobility and the resistivity highly depend on the VI/V ratio. Their values vary from 25 to 140 cm2/V s and 18.2 to 98.5 μΩ m, respectively. These studies have allowed us to find the optimal range for the VI/V ratio at growth temperature of 480 °C.
Low-field galvanomagnetic coefficients have been measured on single crystals of Bi2Te3 and Bi2Se3 at 76 °K in fields to 9 kG. Using a six-valley ellipsoid model in the isotropic relaxation-time approximation, the mass parameters of the ellipsoids are calculated for both compounds. The discrepancy between previously reported galvanomagnetic data and de Haas-van Alphen data for Bi2Te3 can be minimized by recalculating the mass parameters from the galvanomagnetic data and by not assuming complete degeneracy. The experimental data on Bi2Te3 are in agreement with those reported earlier. There is also very good evidence of second-band effects at high electron concentrations (> 1019 cm-3), as has been previously suggested. The constant-energy surfaces undergo an apparent change in shape between low- and high-concentration samples. Data on Bi2Se3 indicate that the constant-energy surfaces are more spherical than in the case of Bi2Te3.
ReflectivityR(λ) in infrared spectral region was measured on bismuth selenide crystals Bi2Se3(Cu) doped with copper and on the crystals Bi2Se3(2Cu+3Se) doped with copper and selenium in atomic ratio 2:3. Bi2Se3(Cu) crystals exhibit a pronounced shift of the reflectivity minimum towards shorter wavelengths, as compared with “pure”
Bi2Se3, whereas in the Bi2Se3(2Cu+3Se) crystals the reflectivity minimum is shifted towards longer wavelengths, as compared with “pure” Bi2Se3 crystals.
Hall constant and theN/m
χ ratio, calculated from the reflectivity curves in the plasma resonance frequency region, were determined for “pure” Bi2Se3. Assuming the “single valley” model to hold for the conduction band of Bi2Se3, we calculated the effective mass of free current carriers:m
χ=m
⊥c
=0.13m
0. On the assumption thatm
⊥c
is constant in a moderately broad range of free-carrier concentrations we then calculate the free carrier concentrations
in Bi2Se3(Cu) and Bi2Se3(2Cu+3Se) and correlate them with the concentrations of copper, as determined by a chemical analysis. Discussion of possible
point defects introduced by building of Cu atoms into the Bi2Se3 lattice leads to the conclusion that in Bi2Se3(Cu) crystals copper exist in the form of singly ionized interstitial atoms Cu
i
+
which act as donors, whereas in Bi2Se3(2Cu+3Se) crystals there arise most probably substitutional defects of copper on bismuth sites, CuBi″, which carry double negative charge and act as acceptors.
Single crystals of Bi2Se3 doped with Mn (cMn=0–3.0×1019atoms/cm3) were grown using a modified Bridgman method. The prepared samples were characterized by X-ray diffraction and chemical analysis, and subsequently investigated by measurements of the reflectivity in the plasma-resonance-frequency region and by measurements of the temperature and magnetic field dependencies of the magnetization, specific heat and transport properties. The results show that incorporation of Mn atoms in the crystal structure leads to an increase of the free-electron concentration, as well as to an increase of the mobility of the free electrons. The Mn2+ valence state of the Mn ions has been established by magnetic measurements, which show a different behavior than the scenario proposed by Choi et al. [J. Choi, H.-W. Lee, B.-S. Kim, S. Choi, J. Choi, J. H. Song and S. Cho, J. Appl. Phys. 97, 10D324 (2005)]. The observations are explained in terms of the point-defect model, which is based on the idea that the Mn impurities create substitutional defects of the Mn′Bi-type and reduce the concentration of antistructure Bi′Se defects.
The influence of magnetic impurity Fe on single crystals Bi2Se3 was investigated in high magnetic fields up to 40T by Shubnikov–de Haas (SdH) effect. The samples were characterized by measurements of the Hall coefficient and the temperature dependence of resistance. From the SdH effect and the Hall effect it was found that the doping of Bi2Se3 single crystals by Fe leads to an increase in the concentration of free electrons. The effect is ascribed to the presence of point defects formed by ionized iron atoms occupying interstitial positions in the crystal lattice. We have not observed any noticeable effect of Fe as a magnetic impurity.
Bi2Se3 single crystals were prepared in the electron concentration range from 1.8 × 10¹⁹ to 5.6 × 10¹⁹ cm–3. At room temperature experimental values are given for the specific electrical resistivity, for the Hall and Seebeck coefficients, and the ir-plasma reflectivity spectra were measured in the wave number range from 300 to 4000 cm–1. For the first time it was found that not only the resistivity and the plasma reflectivity but also the thermopower show a distinct anisotropy perpendicular and parallel to the trigonal c-axis, whereas the Hall effect is isotropic. In terms of a one-carrier, one-valley conduction band model with elliptical isoenergy surfaces and a nonparabolic dispersion the anisotropy of the thermopower can be explained in the extrinsic range by an anisotropic mixed scattering mechanism on acoustic phonons and ionized impurities in the relaxation time approximation. In this frame a quantitative, consistent interpretation is reached for all experimental data. The calculated values for the high-frequency dielectric constant, the carrier density, the Fermi level, the parameter of nonparabolicity, the mean optical relaxation times, the effective masses and the drift mobilities are given.