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Growth and transport properties of Sb 2− x V x Te 3 thin films on sapphire substrates
Abstract
Thin films of Sb2−xVxTe3 with x=0, 0.15, 0.32 and 0.35 have been grown on sapphire (0001) substrates using molecular-beam epitaxy. With the increasing concentration of vanadium, the in-plane lattice constant of Sb2−xVxTe3 films decreases while the c-axis lattice constant shows a very small increase. The electrical resistivity of Sb2−xVxTe3 films decreases by an order of magnitude when V is substituted on the Sb sublattice. The Hall effect measurements show an increase in the concentration of holes as the content of vanadium increases.
... nm using the formula: 46 l ¼ p hl e 3p p À Á 1=3 (here p is the hole concentration calculated above, 1.73 9 10 20 cm À3 ). The electrical conductivity is approximately calculated to be 323 X À1 cm À1 , remarkably higher than that (200 X À1 cm À1 ) of pure system at 300 K. 47 Also, we calculate another important parameter essential for evaluating the thermoelectric materials, the Seebeck coefficient S, using the following equation: [48][49][50] ...
... In another investigation, 47 a high doping with V (7.5 at.%) caused Sb 2 Te 3 to have low hole mobility (7.61 cm 2 V À1 s À1 ) and electrical conductivity (793 X À1 cm À1 ). 47 In contrast, the same concentration of Bi doping 52 was found to greatly enhance the electrical conductivity of the system (2500 X À1 cm À1 ), at the same time the Seebeck coefficient of 100 lVK À1 was obtained, thus achieving a high thermoelectric power factor (25 lWcm À1 K À2 ) far higher than that (2.21 lWcm À1 K À2 ) in pure Sb 2 Te 3 . 44 Comparatively, Cu doping is not as effective in improving the transport properties of Sb 2 Te 3 . ...
... The detailed exploration for formation energies of the substitutional Sn in different charges has demonstrated the high favorability for forming such doping in Sb 2 Te 3 . The incorporation of Sn substitution has been found to induce significant improvements in hole mobility, electrical conductivity, Seebeck coefficient and power factor of Sb 2 Te 3 , thus leading to the enhanced thermoelectric 51 87 3000 --V (7.5 at.%) 47 7.61 793 --Bi (7.5 at.%) 52 -2500 100 25 Cu (2.2 at.%) 53 ...
The electronic and transport properties of Sn-doped Sb2Te3 have been investigated by using the Heyd–Scuseria–Ernzerhof hybrid functional with spin orbit coupling. Results show that Sn preferentially substitute Sb to induce the p-type characteristics in Sb2Te3. Detailed thermodynamic examinations have been conducted to elaborate the favorability for Sn to act as a shallow acceptor in Sb2Te3. Extensive calculations of transport properties reveal that Sn doping gives rise to remarkable enhancements in hole mobility, electrical conductivity, Seebeck coefficient and power factor of Sb2Te3, thus significantly improving the thermoelectric performance. The present work offers a valuable insight on how Sn doping strongly influences the electronic and transport properties of Sb2Te3.
... The difficulty in using many of the more closely lattice-matched substrates is the presence of dangling bonds and surface oxide layers. InP (111), for example, is closely lattice-matched to Bi 2 Se 3 . However, InP has a surface oxide that is normally thermally desorbed at a relatively high temperature. ...
... Thus, theoretically, using In2Se3 as a buffer layer could enhance the crystal quality of Bi2Se3 films. Systematic optimization of MBE growth parameters of In2Se3 on Si (111) substrate was reported by Rathi et al. [43]. The authors concluded that the In2Se3 layers that are most suitable for Bi2Se3 growth resulted from a three-step growth sequence: Se passivation of the Si substrate at 100℃, In2Se3 codeposition with a temperature ramp from 100℃ to 400℃, followed by Se anneal at 400℃. ...
... While this does help to decrease twin defects, it also increases antiphase domain boundaries since not all of the hollows will be of equal depth. Xie et al. also did studies on InP (111) that support this finding [67]. When substrates were prepared to have large flat terraces, the Bi 2 Se 3 film grew via island nucleation and twin domains were observed. ...
In this article, we will review recent progress in the growth of topological insulator (TI) thin films by molecular beam epitaxy (MBE). The materials we focus on are the V2-VI3 family of TIs. These materials are ideally bulk insulating with surface states housing Dirac excitations which are spin-momentum locked. These surface states are interesting for fundamental physics studies (such as the search for Majorana fermions) as well as applications in spintronics and other fields. However, the majority of TI films and bulk crystals exhibit significant bulk conductivity, which obscures these states. In addition, many TI films have a high defect density. This review will discuss progress in reducing the bulk conductivity while increasing the crystal quality. We will describe in detail how growth parameters, substrate choice, and growth technique influence the resulting TI film properties for binary and ternary TIs. We then give an overview of progress in the growth of TI heterostructures. We close by discussing the bright future for TI film growth by MBE.
... Similar to conventional diluted magnetic semiconductors (DMSs) [46,47], impurity doping using transition metal (TM) elements (e.g. Cr, V, Mn) is a convenient approach to induce long-range FM order in TIs [48][49][50][51]. Many recent experiments, from angle-resolved photoemission spectroscopy (ARPES) to electrical transport measurements, have been devoted to the study of magnetically doped TIs of the Bi 2 Se 3 family, including Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 [6,7,[51][52][53][54][55][56][57][58]. ...
... Before the conceptual realization of TIs, magnetically doped Sb 2 Te 3 and Bi 2 Te 3 have been studied extensively as important thermoelectric materials. Long-range FM order in these materials can be easily realized via Cr and V doping [48][49][50]. First-principle calculations originally predicted there is no QAH state in V-doped TI system due to the existence of d electron impurity band at Fermi level [19]. In addition, compared to Cr-doped Bi 2 Te 3 (figure 11(a)), Cr-doped Sb 2 Te 3 has much better FM order as seen by a sharper magnetization reversal (figure 12), inspiring us to choose Cr-doped Sb 2 Te 3 as the parent material for realizing the QAHE at the beginning, as discussed in section 4. Quite strikingly, two years after the first observation of QAHE in Cr-doped TI [6], we discovered a much better QAHE in a V-doped TI system [7]. ...
... For nearly two years since 2013, most QAHE experiments were dominated by Cr-doped TI thin films, since Fe-doped TIs do not exhibit any long-range FM order, while the high growth temperature of V (~2000 °C) causes many feasibility issues. However, there is one thing that caught our attention: among the various TM atoms (Cr, V and Mn) [48][49][50][51], V-doped Sb 2 Te 3 exhibits the most stable FM order [49], as seen by a larger hysteresis loop at fixed temperature as well as a higher T C at fixed dopant concentration ( figure 22). This fact motivated us to explore the QAHE in V-doped TI thin films. ...
The quantum anomalous Hall effect (QAHE), the last member of Hall family, was
predicted to exhibit quantized Hall conductivity e2/h without any external
magnetic field. The QAHE shares a similar physical phenomenon with the integer
quantum Hall effect (QHE), whereas its physical origin relies on the intrinsic
topological inverted band structure and ferromagnetism. Since the QAHE does not
require external energy input in the form of magnetic field, it is believed
that this effect has unique potential for applications in future electronic
devices with low-power consumption. More recently, the QAHE has been
experimentally observed in thin films of the time-reversal symmetry breaking
ferromagnetic (FM) topological insulators (TI), Cr- and V- doped (Bi,Sb)2Te3.
In this Topical Review , we review the history of TI based QAHE, the
route to the experimental observation of the QAHE in the above two systems, the
current status of the research of the QAHE, and finally the prospects for
future studies.
... It has also been shown that if CdS is doped with Cu, this element need not be used for the formation of the back contact [68,69]. Antimony telluride (Sb 2 Te 3 ) deposited at low temperature is amorphous and resistive, but at higher temperatures, it crystallizes, and carrier densities increase with 77 substrate temperature to p ≈ 1 x 10 20 cm -3 [70][71][72]. Romeo et al. [45,62] ...
... The resistivities obtained here are significantly lower than those reported by Romeo et al. [76] for films deposited by sputtering at a substrate temperature of 300°C and a film thickness of 300 nm. Crystals of Sb 2 Te 3 prepared from stoichiometric amounts of Sb and Te typically contain an overstoichiometric amount of Sb [72]. ...
Cadmium telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal band gap of 1.45 eV, its high optical absorption coefficient and availability of various device fabrication methods. Superstrate CdTe solar cells fabricated on glass have to-date exhibited efficiencies of 16.5%. Work on substrate devices has been limited due to difficulties associated with the formation of an ohmic back contact with CdTe. The most promising approach used to-date is based on the use of an interlayer between the CdTe and a metal electrode, an approach that is believed to yield a pseudo-ohmic contact. This research investigates the use of ZnTe and Sb2Te3 as the interlayer, in the development of efficient back contacts. Excellent adhesion and minimum stress are also required of a CdTe thin film solar cell device on a flexible stainless steel (SS) foil substrate.
Foil substrate curvature, flaking, delamination and adhesion as a result of compressive strain due to the coefficient of thermal expansion (CTE) mismatch between the flexible SS foil substrate and the solar cell films have been studied. A potential problem with the use of a SS foil as the substrate is the diffusion of iron (Fe), chromium (Cr) and other elemental impurities into the layers of the solar cell device structure during high temperature processing. A diffusion barrier limiting the out diffusion of these substrate elements is being investigated in this study. Silicon nitride (Si3N4) films deposited on SS foils are being investigated as the barrier layer, to reduce or inhibit the diffusion of substrate impurities into the solar cell. Thin film CdTe solar cells have been fabricated and characterized by AFM, XRD, SEM, ASTM D3359-08 tape test, current-voltage (I-V) and spectral measurements.
My individual contributions to this work include the Molybdenum (Mo) development, the adhesion studies, the silicon nitride (Si3N4) barrier studies, and EDS and SEM lines measurements and analysis of substrate out-diffused impurities. The rest of my colleagues focused on the development of CdTe, CdS, ZnTe, the CdCl2 heat treatment, and other back contact interlayer materials.
... The QAHE has been observed in Cr-and V-doped (Bi,Sb) 2 Te 3 [24][25][26][27][28], completing the quantum Hall effect trio [24]. Thin films of undoped and doped Sb 2 Te 3 have been prepared on a number of substrates, such as Si(111) [29], c-plane sapphire [30,31], and GaAs(100) [32], in part in the context of thermoelectrics research [33,34]. The topological surface state in Sb 2 Te 3 bulk crystals as well as molecular beam epitaxy (MBE) grown thin films [29] has been predicted [7] and experimentally confirmed by ARPES [35]. ...
Chromium-doped Sb2Te3 is a magnetic topological insulator (MTI), which belongs to the (Sb,Bi)2(Se,Te)3 family. When doped with the transition metals V, Cr, and Mn this family displays long-range ferromagnetic order above liquid nitrogen temperature and is currently intensely explored for quantum device applications. Despite the large magnetic ordering temperature, the experimental observation of dissipationless electrical transport channels, i.e., the quantum anomalous Hall effect, is limited in these materials to temperatures below ≈2 K. Inhomogeneities in the MTI have been identified as a major concern, affecting the coupling between the Dirac states and the magnetic dopants. Nevertheless, details on the local magnetic order in these materials are not well understood. Here, we report the study of the magnetic correlations in thin films using a combination of muon spin relaxation (μSR), and magnetic soft x-ray spectroscopy and imaging. μSR provides two key quantities for understanding the microscopic magnetic behavior: The magnetic volume fraction, i.e., the percentage of the material that is ferromagnetically ordered, and the relaxation rate, which is sensitive to the magnetic static (≈μs) and dynamic disorder. By choosing different implantation depths for the muons, one can further discriminate between near-surface and bulk properties. No evidence for a surface enhancement of the magnetic ordering is observed, but, instead, we find evidence of small magnetically ordered clusters in a paramagnetic background, which are coupled. The significant magnetic field shift that is present in all samples indicates a percolation transition that proceeds through the formation and growth of magnetically ordered spin clusters. We further find that fluctuations are present even at low temperatures, and that there appears to be a transition between superparamagnetism and superferromagnetism.
... Claims range from increased to decreased lattice constants, from additional n-type over charge neutral to p-type doping caused by V and widely fluctuating magnetization values from 0.6 µ B up to 2.6 µ B per V ion. A value of 2.0 µ B is predicted by theoretical calculations with an induced moment for the next-nearest Te neighbors of -0.27 µ B .[ICP + 18] Consequently, there is no agreement on the valence or magnetic state of V, which was suggested to incorporate as V 3+ , V 4+ or V 5+ ion.[CZK + 15b, MMC + 14, CZU05,DHLcvU02] In order to investigate the influence of V systematically, a series of about 50 nm thick V z (Bi 0.2 Sb 0.8 ) 2−z Te 3 layers of varying V content z (z = 5 × [V at.%]) was grown by MBE. Growth conditions and Sb content x were kept similar to layers exhibiting the QAHE. ...
The subject of this thesis is the fabrication and characterization of magnetic topological insulator layers of (V,Bi,Sb)\(_2\)Te\(_3\) exhibiting the quantum anomalous Hall effect. A major task was the experimental realization of the quantum anomalous Hall effect, which is only observed in layers with very specific structural, electronic and magnetic properties. These properties and their influence on the quantum anomalous Hall effect are analyzed in detail. First, the optimal conditions for the growth of pure Bi\(_2\)Te\(_3\) and Sb\(_2\)Te\(_3\) crystal layers and the resulting structural quality are studied. The crystalline quality of Bi\(_2\)Te\(_3\) improves significantly at higher growth temperatures resulting in a small mosaicity-tilt and reduced twinning defects. The optimal growth temperature is determined as 260\(^{\circ}\)C, low enough to avoid desorption while maintaining a high crystalline quality. The crystalline quality of Sb\(_2\)Te\(_3\) is less dependent on the growth temperature. Temperatures below 230\(^{\circ}\)C are necessary to avoid significant material desorption, though. Especially for the nucleation on Si(111)-H, a low sticking coefficient is observed preventing the coalescence of islands into a homogeneous layer. The influence of the substrate type, miscut and annealing sequence on the growth of Bi\(_2\)Te\(_3\) layers is investigated. The alignment of the layer changes depending on the miscut angle and annealing sequence: Typically, layer planes align parallel to the Si(111) planes. This can enhance the twin suppression due to transfer of the stacking order from the substrate to the layer at step edges, but results in a step bunched layer morphology. For specific substrate preparations, however, the layer planes are observed to align parallel to the surface plane. This alignment avoids displacement at the step edges, which would cause anti-phase domains. This results in narrow Bragg peaks in XRD rocking curve scans due to long-range order in the absence of anti-phase domains. Furthermore, the use of rough Fe:InP(111):B substrates leads to a strong reduction of twinning defects and a significantly reduced mosaicity-twist due to the smaller lattice mismatch. Next, the magnetically doped mixed compound V\(_z\)(Bi\(_{1−x}\)Sb\(_x\))\(_{2−z}\)Te\(_3\) is studied in order to realize the quantum anomalous Hall effect. The addition of V and Bi to Sb\(_2\)Te\(_3\) leads to efficient nucleation on the Si(111)-H surface and a closed, homogeneous layer. Magneto-transport measurements of layers reveal a finite anomalous Hall resistivity significantly below the von Klitzing constant. The observation of the quantum anomalous Hall effect requires the complete suppression of parasitic bulklike conduction due to defect induced carriers. This can be achieved by optimizing the thickness, composition and growth conditions of the layers. The growth temperature is observed to strongly influence the structural quality. Elevated temperatures result in bigger islands, improved crystallographic orientation and reduced twinning. On the other hand, desorption of primarily Sb is observed, affecting the thickness, composition and reproducibility of the layers. At 190\(^{\circ}\)C, desorption is avoided enabling precise control of layer thickness and composition of the quaternary compound while maintaining a high structural quality. It is especially important to optimize the Bi/Sb ratio in the (V,Bi,Sb)\(_2\)Te\(_3\) layers, since by alloying n-type Bi\(_2\)Te\(_3\) and p-type Sb\(_2\)Te\(_3\) charge neutrality is achieved at a specific mixing ratio. This is necessary to shift the Fermi level into the magnetic exchange gap and fully suppress the bulk conduction. The Sb content x furthermore influences the in-plane lattice constant a significantly. This is utilized to accurately determine x even for thin films below 10 nm thickness required for the quantum anomalous Hall effect. Furthermore, x strongly influences the surface morphology: with increasing x the island size decreases and the RMS roughness increases by up to a factor of 4 between x = 0 and x = 1. A series of samples with x varied between 0.56-0.95 is grown, while carefully maintaining a constant thickness of 9 nm and a doping concentration of 2 at.% V. Magneto-transport measurements reveal the charge neutral point around x = 0.86 at 4.2 K. The maximum of the anomalous Hall resistivity of 0.44 h/e\(^2\) is observed at x = 0.77 close to charge neutrality. Reducing the measurement temperature to 50 mK significantly increases the anomalous Hall resistivity. Several samples in a narrow range of x between 0.76-0.79 show the quantum anomalous Hall effect with the Hall resistivity reaching the von Klitzing constant and a vanishing longitudinal resistivity. Having realized the quantum anomalous Hall effect as the first group in Europe, this breakthrough enabled us to study the electronic and magnetic properties of the samples in close collaborations with other groups. In collaboration with the Physikalisch-Technische Bundesanstalt high-precision measurements were conducted with detailed error analysis yielding a relative de- viation from the von Klitzing constant of (0.17 \(\pm\) 0.25) * 10\(^{−6}\). This is published as the smallest, most precise value at that time, proving the high quality of the provided samples. This result paves the way for the application of magnetic topological insulators as zero-field resistance standards. Non-local magneto-transport measurements were conducted at 15 mK in close collaboration with the transport group in EP3. The results prove that transport happens through chiral edge channels. The detailed analysis of small anomalies in transport measurements reveals instabilities in the magnetic phase even at 15 mK. Their time dependent nature indicates the presence of superparamagnetic contributions in the nominally ferromagnetic phase. Next, the influence of the capping layer and the substrate type on structural properties and the impact on the quantum anomalous Hall effect is investigated. To this end, a layer was grown on a semi-insulating Fe:InP(111)B substrate using the previously optimized growth conditions. The crystalline quality is improved significantly with the mosaicity twist reduced from 5.4\(^{\circ}\) to 1.0\(^{\circ}\). Furthermore, a layer without protective capping layer was grown on Si and studied after providing sufficient time for degradation. The uncapped layer on Si shows perfect quantization, while the layer on InP deviates by about 5%. This may be caused by the higher crystalline quality, but variations in e.g. Sb content cannot be ruled out as the cause. Overall, the quantum anomalous Hall effect seems robust against changes in substrate and capping layer with only little deviations. Furthermore, the dependence of the quantum anomalous Hall effect on the thickness of the layers is investigated. Between 5-8 nm thickness the material typically transitions from a 2D topological insulator with hybridized top and bottom surface states to a 3D topological insulator. A set of samples with 6 nm, 8 nm, and 9 nm thickness exhibits the quantum anomalous Hall effect, while 5 nm and 15 nm thick layers show significant bulk contributions. The analysis of the longitudinal and Hall conductivity during the reversal of magnetization reveals distinct differences between different thicknesses. The 6 nm thick layer shows scaling consistent with the integer quantum Hall effect, while the 9 nm thick layer shows scaling expected for the topological surface states of a 3D topological insulator. The unique scaling of the 9 nm thick layer is of particular interest as it may be a result of axion electrodynamics in a 3D topological insulator. Subsequently, the influence of V doping on the structural and magnetic properties of the host material is studied systematically. Similarly to Bi alloying, increased V doping seems to flatten the layer surface significantly. With increasing V content, Te bonding partners are observed to increase simultaneously in a 2:3 ratio as expected for V incorporation on group-V sites. The linear contraction of the in-plane and out-of-plane lattice constants with increasing V doping is quantitatively consistent with the incorporation of V\(^{3+}\) ions, possibly mixed with V\(^{4+}\) ions, at the group-V sites. This is consistent with SQUID measurements showing a magnetization of 1.3 \(\mu_B\) per V ion. Finally, magnetically doped topological insulator heterostructures are fabricated and studied in magneto-transport. Trilayer heterostructures with a non-magnetic (Bi,Sb)\(_2\)Te\(_3\) layer sandwiched between two magnetically doped layers are predicted to host the axion insulator state if the two magnetic layers are decoupled and in antiparallel configuration. Magneto-transport measurements of such a trilayer heterostructure with 7 nm undoped (Bi,Sb)\(_2\)Te\(_3\) between 2 nm thick layers doped with 1.5 at.% V exhibit a zero Hall plateau representing an insulating state. Similar results in the literature were interpreted as axion insulator state, but in the absence of a measurement showing the antiparallel magnetic orientation other explanations for the insulating state cannot be ruled out. Furthermore, heterostructures including a 2 nm thin, highly V doped layer region show an anomalous Hall effect of opposite sign compared to previous samples. A dependency on the thickness and position of the doped layer region is observed, which indicates that scattering at the interfaces causes contributions to the anomalous Hall effect of opposite sign compared to bulk scattering effects. Many interesting phenomena in quantum anomalous Hall insulators as well as axion insulators are still not unambiguously observed. This includes Majorana bound states in quantum anomalous Hall insulator/superconductor hybrid systems and the topological magneto-electric effect in axion insulators. The limited observation temperature of the quantum anomalous Hall effect of below 1 K could be increased in 3D topological insulator/magnetic insulator heterostructures which utilize the magnetic proximity effect. The main achievement of this thesis is the reproducible growth and characterization of (V,Bi,Sb)2Te3 layers exhibiting the quantum anomalous Hall effect. The detailed study of the structural requirements of the quantum anomalous Hall effect and the observation of the unique axionic scaling behavior in 3D magnetic topological insulator layers leads to a better understanding of the nature of this new quantum state. The high-precision measurements of the quantum anomalous Hall effect reporting the smallest deviation from the von Klitzing constant are an important step towards the realization of a zero-field quantum resistance standard.
... Various substrate materials and surfaces such as graphene/ SiC [17], Si [18,19], SiO 2 /BaF 2 [20], GaAs [21], and sapphire [22] have been reported for the epitaxial growths of Sb 2 Te 3 , but the growth of Sb 2 Te 3 /Ge heterojunctions by molecular beam epitaxy has not been found to the best of our knowledge. Furthermore, the band alignment properties play a dominant role in understanding spin transport behaviours of heterostructure barrier tunnelling [23]. ...
Sb2Te3/Ge heterojunctions were grown on deoxidized GaAs (001) substrates by molecular beam epitaxy to explore a new type of spin torque device. Despite the large lattice mismatch between Ge and Sb2Te3, the films display highly uniform fabrication and good crystallinity, which have been confirmed by structural characterization. The band structures of Sb2Te3/Ge heterojunctions were investigated by x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Small chemical shift of Sb 3d5/2 indicates that TI conducting surface is not destroyed, and Ge valence band bending contributes to Fermi level depinning. The band offset of Sb2Te3/Ge heterojunctions is different from common gate dielectric/Ge heterojunctions. The integral quality of the heterostructure reveals the potential of combining topological insulators with semiconductors for the advancement of spintronic devices.
... This indicates an improved crystallinity of the epitaxial BiSbTe 3 films. Figs. 5 (a-d) display RHEED patterns of annealed BiSbTe 3 films along the azimuthal direction ½1100 [21]. For any annealing temperature (540 K-620 K) or any duration of annealing, the intensity of Kikuchi lines is remarkably increased compared to films not subjected to an annealing treatment. ...
We report on the growth of high-quality topological insulator BiSbTe3 films (thickness of 30 nm) on sapphire (0001) substrates through the molecular beam epitaxy (MBE) technique and discuss the possibility to improve the film quality and surface flatness through annealing. Instead of using elemental Sb, the choice of Sb2Te3 as well as the use of solid sources of Bi and To assures excellent stability during thermal evaporation enabling layer-by-layer epitaxial growth of high-quality films of BiSbTe3. The crystallinity and the terrace size of the BiSbTe3 films are found to be improved through increasing the deposition temperature and/or after annealing at 540 K-620 K for 1-4 hours. The films grown at 485 K, 500 K, and 515 K exhibit root-mean-square (RMS) roughness of 2.9 nm, 2.3 nm, and 2.3 nm, respectively, whereas the RMS roughness is reduced to 0.6 nm or less when the films are annealed at 580 K for 2-4 hours. Annealing the film at too high temperature, such as 620 K, introduces a rougher surface due to the loss of material during annealing. A relatively low electron density of similar to 22 x 1018 cm(-3) (at 2 K) is achieved for the as-grown films deposited at 485 K and 500 K. Significantly enhanced electron density is found in the case of either increasing growth temperature or increasing annealing temperature. An exception is the film annealed at 620 K, which became a p-type conductor. In addition, weak antilocalization effects are evident for the n-type films, but they nearly vanish for p-type conducting films. The significant influence of temperature on the crystallinity, the surface roughness, and the electronic transport in BiSbTe3 films will be instructive for further investigations of the transport behavior of surface states in BiSbTe3 films. (C) 2014 Elsevier B.V. All rights reserved
... The phenomenon is often observed in substitution alloys. 25,26 To measure the T C and T m of TST, DSC measurement was carried out, the results of which are shown in Figure 2. The heating speed was also set to 10 C/min. From the DCS curve, we can find one sharp exothermic peak along 210.33 C, which corresponds to T C . ...
As a growth-dominated phase change material, Sb 4 Te (ST) has fast crystallization speed while thermal stability is very poor, which makes it unsuitable for application in phase change random access memory (PCRAM). After doping Ti, the crystallization temperature is greatly improved to 210.33 C, which is much higher than that of conventional Ge 2 Sb 2 Te 5 ($150 C), and the melting point is reduced to 540.27 C. In addition, grain size of crystalline Ti-doped Sb 4 Te (TST) film is significantly decreased to nanoscale. Ti atom is believed to occupy the lattice site of Sb atom in TST. With good thermal stability, TST-based PCRAM cell also has fast crystallization rate of 6 ns. Furthermore, the energy consumption is also lower than that of Ge 2 Sb 2 Te 5 -based one. Endurance of exceeding 2E5 cycles is obtained with a resistance ratio of one order of magnitude. Therefore, Ti doping seems to be a good way to solve the contradiction between thermal stability and fast crystallization speed of Sb-Te alloys. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4821769]
... Such extraordinary states feature spin-momentum locked helical Dirac fermions [11], giving TIs great promise in spintronics [12] and fault-tolerant quantum computing [13]. Sb 2 Te 3 has been extensively studied in the context of thermoelectric material research [10,14,15] and the growth of Sb 2 Te 3 films by molecular beam epitaxy and other techniques has been investigated [16][17][18][19]. It is generally known that the surface states of TIs are robust against external perturbations because they are protected by time-reversal symmetry. ...
The effects of S atom surface adsorption and substitution on the helical surface states of Sb2Te3 are studied by the density-functional theory with spin-orbit coupling being taken into account self-consistently. It is found that S atoms play the role of surface passivation when adsorbed on both surfaces of a 6QL Sb2Te3 film in symmetrical configuration. For symmetrical surfaces with both the top and bottom surfaces of a thin film with adsorbed S atoms, the linear dispersion of the surface states is found to be preserved and the topological surface states survive. The spatial distribution of charge density of the surface state at the [Formula: see text] point is also symmetric. For a film with asymmetric S atom adsorption, i.e., only one of the surfaces has adsorbed S atoms, the surface band structure is found to be very different. The degeneracy of the surface states from the two sides of a film is broken. The gap opens slightly at [Formula: see text] and the spatial distribution of charge density of the surface state at the [Formula: see text] point is also modified greatly. The Fermi level is robust against S impurity adsorption on the surface of Sb2Te3. Compared with S substitution, the effect of single surface S adsorption on electron structures is more prominent. This supports the idea that the topological insulator surface electronic states are dominated by its structural symmetry and the effect of the asymmetric environment of topological insulator Sb2Te3 films should thus be considered.
... In these materials, doping had to be used to move the Fermi energy back to the bulk gap [13, 14, 16]. Sb 2 Te 3 has been extensively studied in the context of thermoelectric material research [30, 31] and the growth of Sb 2 Te 3 films by MBE and other techniques has been investigated323334. In fact, p-type Bi 2 Te 3 /Sb 2 Te 3 superlattices were found to possess the highest thermoelectric figure of merit (ZT) of about 2.4 at 300 K [35]. ...
The growth and characterization of single-crystalline thin films of topological insulators (TIs) is an important step towards
their possible applications. Using in situ scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we show that moderately thick
Sb2Te3 films grown layer-by-layer by molecular beam epitaxy (MBE) on Si(111) are atomically smooth, single-crystalline, and intrinsically
insulating. Furthermore, these films were found to exhibit a robust TI electronic structure with their Fermi energy lying
within the energy gap of the bulk that intersects only the Dirac cone of the surface states. Depositing Cs in situ moves the Fermi energy of the Sb2Te3 films without changing the electronic band structure, as predicted by theory. We found that the TI behavior is preserved
in Sb2Te3 films down to five quintuple layers (QLs).
KeywordsTopological insulator-electronic structure-scanning tunneling microscopy-angle-resolved photoemission spectroscopy-molecular beam epitaxy
The quantum Hall (QH) effect, quantized Hall resistance combined with zero longitudinal resistance, is the characteristic experimental fingerprint of Chern insulators—topologically nontrivial states of two-dimensional matter with broken time-reversal symmetry. In Chern insulators, nontrivial bulk band topology is expressed by chiral states that carry current along sample edges without dissipation. The quantum anomalous Hall (QAH) effect refers to QH effects that occur in the absence of external magnetic fields due to spontaneously broken time-reversal symmetry. The QAH effect has now been realized in four different classes of two-dimensional materials: (i) thin films of magnetically (Cr- and/or V-) doped topological insulators in the (Bi,Sb)2Te3 family, (ii) thin films of the intrinsic magnetic topological insulator MnBi2Te4, (iii) moiré materials formed from graphene, and (iv) moiré materials formed from transition-metal dichalcogenides. In this Colloquium, the physical mechanisms responsible for each class of QAH insulator are reviewed, with both differences and commonalities highlighted, and potential applications of the QAH effect are commented upon.
We report on the molecular beam epitaxy and properties of a magnetic topological insulator (TI): Cr-doped Sb2Te3. Composition analysis reveals that Cr replaces Sb, and X-ray diffraction confirms that a single-phase textured crystal structure can be obtained for (CrxSb1-x)2Te3 with x up to 0.44. A further increase in x results in phase separation or precipitates in the material. The Curie temperature TC increases with x up to 0.44 and
reaches 250 K, which is the highest TC observed thus far in magnetically doped TIs.
Phase change memory has great potential for numerous attractive applications, especially in storage class memory, on the premise of its high-device performances, which still need to be improved by employing a material with good overall phase change properties. This chapter presents Ti-doped Sb-Te phase change materials in the hope of balancing the thermal stability and the operation rate of phase change memory. The component of Sb-Te is optimized. Compared to Ti-doped Sb2Te and Sb4Te alloy, Ti-doped Sb2Te3 has been proved to be the best candidate in respect of resistance ratio and device lifetime.
This chapter focuses on the interplay of transport and structural properties of thin films of Bi2Te3 grown on different substrates using the technique of molecular beam epitaxy (MBE). By observing the growth process with reflection high energy electron diffraction (RHEED), one can monitor the influence of lattice mismatch and different substrate symmetries on the quality of films. Films of Bi2Te3, Sb2Te3, and Bi2Se3 can be grown by a variety of deposition techniques including co-evaporation, co-sputtering, electrochemical deposition, pulsed laser deposition, metalorganic chemical vapor deposition, and MBE. It is found that the van der Waals growth mode facilitates the growth of thin films on substrates with vastly different lattice constants and even with different crystal symmetries. Most of the films are c-oriented when the substrate surface is very flat and clean.
We demonstrate the growth of highly oriented CdSe and ZnTe films at ∼300 °C on amorphous substrates such as glass and flexible polyimide using ultrathin tetradymite buffer layers composed of SbxBi2-xTe3 alloys lattice-matched to the film overgrowth. This leads to significant improvement of the crystallinity, roughness, grain size, and pit density of the II-VI overlayer along with enhancement of the optoelectronic properties. For example, photoluminescence emission is observed at ∼1.74 eV for optimized CdSe films, the same as in a single crystal reference. An in-plane carrier diffusion length of ∼500 nm is inferred from transient optical data. The use of tetradymite buffer layers to control II-VI compound deposition on non-crystalline substrates is a promising route for large area optoelectronic applications such as photovoltaic, light-emission, or infrared detector devices.
Within the past year, transistors have been used in increasing numbers in the oscillator stages of uhf television tuners. This application not only represents the introduction of the first transistors in television receivers; it also indicates that transistors capable of good uhf and vhf performance can be mass-produced at consumer prices. Both germanium and silicon devices which are suitable for use in the oscillator function are now commercially available. The question arises as to the relative merits of amplifier versions of these devices for uhf and vhf amplifier service. Until recently, silicon transistors had not been considered for use in rf amplifier stages in consumer applications because of high noise figures or high cost, the former not being particularly significant in oscillator service. (Although silicon n-p-n uhf low-noise amplifiers have been available for some time, their use in consumer products has been restricted by their high cost.) This paper describes a new n-p-n silicon uhf-vhf transistor, RCA Dev. No. TA2503, having electrical characteristics tailored for consumer amplifier applications at frequencies up to 900 megacycles. Gain and noise through the useful frequency range are discussed, and relative performance is given for various vhf and uhf applications.
We report the selective and catalyst-free growth of Sb2Te3 and Te nanowires by thermal annealing of sputter-deposited Al–Sb–Te thin films. Sb2Te3 and Te nanowires were grown at low temperature (250 °C) under N2 and O2 gas atmospheres, respectively, and they were extruded spontaneously on the surfaces of Al–Sb–Te films with increasing annealing temperature.
Using magnetron sputtering method the Ge2Sb2Te5 films were deposited at different substrate temperatures (from room temperature to 300 °C) on Si substrate. The structure and the crystallization temperature of the films were determined by X-ray diffraction and Differential Scanning Calorimeter, respectively. The electrical resistance and the reflectivity of the films were measured with a four-point probe and ultraviolet photo-spectrometer, respectively. Based on the reflectivity of the films, it is found that the reflectivity contrasts of the Ge2Sb2Te5 films at the wavelengths of 405 and 650 nm change with the substrate temperature. The films prepared at room temperature are amorphous, and crystalline (fcc) at 140 °C, and a little hexagonal (hex) structure comes forth at 300 °C. At 140 °C the phase separation may take place, and exhibits significant influence on the electrical and optical properties.
Thin film semiconductors Sb2−xCrxTe3 (0⩽x⩽0.59) based on tetradymite-type structure of Sb2Te3 have been prepared on the (0001) sapphire substrates by low-temperature molecular beam epitaxy. The films display ferromagnetism with the Curie temperature increasing nearly linearly with the content of Cr incorporated in the lattice. The highest Curie temperature reached so far is 190 K in a Sb1.41Cr0.59Te3 film. Structural studies, magnetic characterization, and transport measurements indicate the robust nature of the magnetic state that has its easy axis of magnetization perpendicular to the plane of the film. The structures thus represent diluted magnetic semiconductors with a high Curie temperature and highly anisotropic properties.
Thin-film ferromagnetic semiconductors Sb2−xVxTe3 with the Curie temperature as high as 177 K were prepared on sapphire (0001) substrates by molecular-beam epitaxy. Films of Sb2−xVxTe3 with x up to 0.35 display robust, out-of-plane ferromagnetic ordering that depends on the concentration of vanadium in the structure. The Curie temperature was determined from magnetization measurements and Arrott plots. Ferromagnetic order is manifested by hysteresis loops observed in magnetization, magnetoresistivity, and the anomalous Hall effect.
The effect of thickness and heat treatment on the structural and optical
properties of Sb2Te3 thin films fabricated by using flash
evaporation technique were investigated. The as-deposited films were found
to be stoichiometric. For the as-deposited and annealed samples of thickness
in the range of 20.7–120.5 nm, large variations in transmittance and
reflectance were observed for wavelength below 2000 nm due to strong
absorption, while slight variations were observed above 2000 nm. The optical
constants, n and k, measured in the range of 500–4000 nm, increased slightly
by annealing the samples at 573 K for 2 h in air and a red shift was
observed. Allowed indirect transitions were observed for the as-deposited
and the annealed samples. The results showed two energy gaps for these
transitions varied from 0.28 to 0.31 eV for the as-deposited samples and
from 0.56 to 0.54 eV for the annealed films. The spectral distribution of
the real part of the dielectric constant ε′ showed a maximum at
0.73 eV for the as-deposited films decreased to 0.65 eV by annealing.
Semiconductor trilayer structures with ferromagnetic Sb <sub>2-x</sub> Cr <sub>x</sub> Te <sub>3</sub> layers separated by a nonmagnetic Sb <sub>2</sub> Te <sub>3</sub> layer of different thickness have been fabricated by molecular beam epitaxy. Ferromagnetic out-of-plane exchange coupling between the SbCrTe layers was found and the coupling strength, which can be represented by a saturation field H<sub>S</sub> , depends on both the Sb <sub>2</sub> Te <sub>3</sub> spacer thickness and temperature.
Ferromagnetism in tetradymite-type diluted magnetic semiconductors ( Sb <sub>1-x</sub> Bi <sub>x</sub>)<sub>1.98</sub> V <sub>0.02</sub> Te <sub>3</sub> (0≤x≤1) is revealed to be of hole-mediated nature. The increasing replacement of antimony with bismuth results in a monotonous decrease of the hole concentration and the Curie temperature while the electrical resistivity increases. The value of the Curie temperature shows a linear dependence of Np<sup>1/3</sup> , where N is the vanadium concentration and p is the concentration of hole. This trend agrees with the mean-field theory predictions.
Motivated by the discovery of the ferromagnetic order in V-doped Sb2Te3 and Fe-doped Bi2Te3 bulk crystals, the study of the transition metal (TM) doped tetradymite-type semiconductors was extended to their thin film forms with the hope of significantly increasing the content of TM and thus enhancing the Curie temperature. High quality Sb2-xVxTe3 (x up to 0.35) and Sb2-xCrxTe3 (x up to 0.59) epitaxial films have been successfully prepared on sapphire (0001) substrates by non-equilibrium MBE growth technique, and their structural, transport and magnetic properties have been determined. Magnetization studies, Arrott plot analyses, and anomalous Hall effect measurements indicate that long range magnetic order persists to temperatures of at least 177K (in the case of Sb2-xVxTe3) and 190K (for Sb2-xCrxTe3). The observed carrier mediated, p-type favored ferromagnetic behavior supports a scenario of RKKY interaction with mean-field approximation. Bi2-xFexTe3 samples have also been prepared (for x up to 0.46), and they displayed n-type behavior and no signature of spontaneous magnetic ordering down to 2K. Trilayer structures with a normal Sb2Te3 layer sandwiched between two ferromagnetic Sb2-xCrxTe3 layers were fabricated in order to explore the influence of the interlayer exchange coupling mechanism (IEC). Such coupling was found to be present for small thickness of the spacer layer at low temperatures. Transition metal-doped tetradymite-type semiconductors represent a new class of diluted magnetic semiconductors that are octahedrally coordinated and that possess highly anisotropic structural and magnetic properties. Ph.D. Applied Physics University of Michigan, Horace H. Rackham School of Graduate Studies http://deepblue.lib.umich.edu/bitstream/2027.42/57593/2/yjchien_1.pdf
Iron-doped Sb2Te3 single crystals (C-Fe = (0-9.5) x 10(19) atoms/cm(-3)) were prepared from elements of semiconductor purity using a modified Bridgman method. The samples of these crystals were characterized by means of X-ray diffraction analysis, measurements of the reflectance in the plasma resonance frequency range omega (p), of the Hall constant R-H(B parallel to c) and electrical conductivity sigma (perpendicular toc). It was found that the incorporation of Fe atoms into the Sb2Te3 crystal lattice reduces the volume of the unit cell; the values of omega (p) and RH(B parallel to c) (i.e. the concentration of holes) increase and the values of sigma (perpendicular toc) decrease with increasing iron content. The observed increase in the hole concentration is explained as due to the incorporation of Fe atoms into the Sb-sublattice creating thus substitutional defects of Fe'(Sb). The decrease in the electric conductivity is associated with a decrease in the hole mobility.
We have studied the structural and transport properties of Sb 2 Te 3 thin films prepared by molecular beam epitaxy as a function of the Te/Sb flux ratio during deposition. Both the crystallinity and the transport properties are found to be strongly affected by nonstoichiometry. The most stoichiometric sample prepared with a Te/Sb ratio of 3.6 had a high degree of crystallinity, high thermopower, and high carrier mobility. However, Sb 2 Te 3 films with excess Sb or Te had poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility as a result of the formation of antisite defects. These antisite defects were able to be reduced by controlling the relative flow rate ratio of Te to Sb during growth. © 2002 American Institute of Physics.
We have measured electrical and magnetic properties of single crystals of Sb2−xMnxTe3Sb2−xMnxTe3 with x=0–0.045x=0–0.045 at temperatures of 2 K to 300 K. Hall effect measurements indicate that each manganese atom donates approximately one hole to the valence band. The magnetic susceptibility is paramagnetic down to 2 K, and both Curie–Weiss and Brillouin analyses show that manganese substitutes for Sb and takes the Mn2+Mn2+ state with S=5/2.S=5/2. Contrary to the case of III–V host matrices, manganese does not stimulate ferromagnetic order in the family of bulk layered V2–VI3V2–VI3 diluted magnetic semiconductors, at least in the range of magnetic impurity and carrier concentrations studied here. © 2003 American Institute of Physics. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/70033/2/JAPIAU-94-12-7631-1.pdf
The electrical conductivity, the Hall-constant, the Seebeck coefficient and the position of the reflectivity minima in the range of plasma resonance frequency of Sb//2Te//3 and the Bi//2Te//3 crystals doped with iodine have been measured. From the concentration dependence of iodine atoms incorporated in the crystal lattice on the mentioned physical properties the authors conclude that the transition from the p-type of Bi//2Te//3 to the n-type of Bi//2Te//3 is correlated with the decrease of the concentration of antistructural defects Bi prime //T//e and simultaneous increase of the I** multiplied by (times) //T//e defects. With respect to the similarity in changes of the physical properties of Sb//2Te//3(I) crystals, the Bi//2Te//3(I) crystals contain similar antistructural defects. The hole conductivity of p-Bi//2Te//3 crystals is based on the existence of antistructural defects Bi prime //T//e and that the model assuming the tellurium vacancies V prime //T//e in the lattice of p-Bi//2Te//3 is not acceptable.
For the first time the temperature dependences are presented of all of the twelve galvanomagnetic low field transport coefficients and the two Seebeck coefficients of Sb//2Te//3 in the temperature range from 100 to 360 K. Furthermore the change of some of these effects by Pb doping is observed. Starting from qualitative differences in the transport behavior of Sb//2Te//3 and the analogous substance Bi//2Te//3 some models for a consistent description of transport properties of Sb//2Te//3 are discussed.
Metal organic chemical vapor deposition (MOCVD) has been investigated for elaboration of Bi2Te3 and Sb2Te3 using TMBi (Trimethylbismuth), TESb (Triethylantimony) and DETe (Diethyltellurium) as metal–organic sources. Their thermoelectric and physical properties were studied versus growth conditions. The MOCVD elaboration of Bi2Te3 and Sb2Te3 was carried out in an horizontal reactor for a temperature varying from 400 to 500°C, a total hydrogen flow rate DT varying from 3 to 6 l mm−1 and (RVI/V) ratio ranging from 1.5 to 15. The thin films were deposited on pyrex and silicon substrates. The partial pressure of the V element varied between 0.5 10−4 to 2 10−4 atm to obtain high growth rate for micro-peltier applications. The cristallinity was investigated by X-ray diffraction and we observed a typical preferential c-orientation. The SEM micrographs show the layers quality and confirms the hexagonal structure. The microprobe data indicate that the stoichiometry of Bi2Te3 and Sb2Te3 is constant for all thickness of the epitaxial films (0.3–7 μm). The films are always n-type conduction for Bi2Te3 and p-type for Sb2Te3. Seebeck coefficient and the minimum values of the resistivity were found close to −210 and +110 μV K−1, 9 and 3.5 μΩ.m for Bi2Te3 and Sb2Te3, respectively. Electrical measurements (mobility and carrier density) were performed by Van der Pauw method. For the two materials, the best values of thermoelectrical properties were obtained at a growth temperature closed to 450°C and a VI/V ratio varying from 2 to 8. The thermoelectric properties of the two materials stay constant when the growth rate is increasing to value higher than 1.5 μm h−1. This result is very interesting for thick film applications. The previous objective of these experimental results has been to perform the thermoelectric properties of n- and p-type films by establishing first suitable deposition conditions and the elaboration of ternary alloys is now possible.
The free carrier concentration of the Sb2−xInxTe3, Bi2−xInxTe3 and Bi2Te3−xSx crystals has been determined from the values of the Hall constants and the free carrier concentration of the Sb2−xTlxTe3 has been calculated from the plasma resonance frequency; with increasing value of x, the hole concentration decreases. As the incorporation of the elements In, Tl and S into the lattice Sb2Te3 or Bi2Te3, respectively, gives rise to the uncharged defects InxSb, TlxSb, InxBi and SxTe, the x causes the decrease of the antisite defects concentration. The proven effect is explained in the following way: the antisite defects can be created only in crystals whose atoms are bound by weakly polarized bonds. The incorporation of In, Tl and S atoms into the crystal lattice of Sb2Te3 or Bi2Te3 increases the bond polarity, the ionicity of ternary crystals increases. This unfavorably affects the increase of antisite defects whose concentration decreases. The change of the bond polarity is considered from the changes discovered in the formation energy of antisite defects of the above mentioned ternary crystals.
The variation of in situ reflection high-energy electron diffraction (RHEED) patterns was observed for the study of crystal quality of Bi2Te3/Sb2Te3 multilayer films on sapphire (0001) substrates during molecular beam epitaxy (MBE) growth. At the initial stage of growth, the RHEED pattern shows a superposition of spots and streaks which indicates the existence of three-dimensional island growth. Moreover, weak streaks between fundamental streaks, which indicate rotation around the c-axis, are observed. It is found that a clear streaky pattern (1×1) gradually appears with the growth of larger grains. The existence of heterointerfaces in Bi2Te3/Sb2Te3 multilayer films is confirmed from the X-ray diffraction (XRD) measurements and the depth profile of Auger electron spectroscopy (AES) measurements.
The reflectance and the transmittance spectra in the IR region are measured on Ti-doped Sb2Te3 single crystal samples grown by a modified Bridgman technique. The plasma resonance frequency, the optical relaxation time, and the high-frequency dielectric constant are determined by fitting the Drude-Zener formulas to the reflectance spectra. It is found that Ti impurities in Sb2Te3 behave as donors. The transmittance spectra are used for the determination of the dependence of the absorption coefficient K on the energy of incident photons. The optical width of the energy gap is found to decrease with increasing Ti content. The values of the exponent α from the relation of K ∼ λα for the long-wavelength absorption edge range within the interval of 1.8 to 2.2, i.e. the dominant scattering mechanism of free current carriers in Sb2Te3 crystals doped with Ti atoms is the scattering by acoustic phonons.
An Sb2Te3-Einkristallen mit Beimengungen von Ti-Atomen, die nach der modifizierten Bridgman-Methode hergestellt wurden, werden Reflexionsvermögen und Transmission im IR-Bereich ermittelt. Mit einer numerischen Analyse der Reflexionsspektren werden die Werte der Plasma-Resonanzfrequenz, der optischen Relaxationszeit und der Hochfrequenz-Dielektrizitätskonstante bestimmt. Es wird gefunden, daß Ti-Beimengungen im Sb2Te3 Donatoren bilden. Die Transmissionsspektren werden benutzt, um die Abhängigkeit des Absorptionskoeffizienten K von der Energie der einfallenden Photonen zu bestimmen. Es wird gefunden, daß die optische Breite der Bandlücke mit wachsenden Beimengungen von Ti-Atomen abnimmt. Die Werte des Exponenten α in der Beziehung K ∼ λα für die langwellige Absorptionskante liegen im Interval 1,8 bis 2,2, d. h. der dominierende Streumechanismus für freie Ladungsträger in Sb2Te3 Kristallen mit Beimengungen von Ti-Atomen ist die Streuung an akustischen Phononen.
Es werden erstmalig alle zwölf galvanomagnetischen Schwachfeldtransportkoeffizienten sowie die beiden Seebeck-Koeffizienten von Sb2Te3 im Temperaturbereich von 100 bis 360 K angegeben. Ferner wird die Änderung einiger dieser Effekte bei Pb-Dotierung beobachtet. Ausgehend von qualitativen Unterschieden im Transportverhalten von Sb2Te3 und der Analogiesubstanz Bi2Te3 werden Modelle zur konsistenten Beschreibung der Transporteigenschaften von Sb2Te3 diskutiert.
For the first time the temperature dependences are presented of all of the twelve galvanomagnetic low field transport coefficients and the two Seebeck coefficients of Sb2Te3 in the temperature range from 100 to 360 K. Furthermore the change of some of these effects by Pb doping is observed. Starting from qualitative differences in the transport behaviour of Sb2Te3 and the analogous substance Bi2Te3 some models for a consistent description of transport properties of Sb2Te3 are discussed.
We report on a diluted magnetic semiconductor based on the Sb2Te3 tetradymite structure doped with very low concentrations of vanadium (1–3 at. %). The anomalous transport behavior and robust magnetic hysteresis loops observed in magnetotransport and magnetic measurements are experimental manifestations of the ferromagnetic state in these materials. The p-d exchange between holes and vanadium 3d spins is estimated from the behavior of the magnetoresistance. A Curie temperature of at least 22 K is observed for Sb1.97V0.03Te3. This discovery offers possibilities for exploring magnetic properties of other tetradymite structure semiconductors doped with a wide range of 3d transition metals.
It is succeeded for the first time to interpret the thermoelectric and galvanomagnetic effects, in terms of a nonparabolic six-valley one-valence band model and anisotropic mixed scattering on acoustical phonons and ionized impurities and also the magnitude of thermomagnetic effects and the Lorenz numbers for Sb2Te3 single crystals in dependence on temperature between 100 to 300 K using physical relevant microscopic transport data. By a numerical analysis the effective main axis masses mHii, the tilt angle of Fermi ellipsoid ϑ, the masses m ⟂ c, m ‖ c, the density of state effective mass md, the mobilities u ⟂ c, u ‖ c, and the carrier density ptot are calculated. Also the Fermi level EF, the relaxation time factor τac0, the mixture parameters BHi and theparameter of nonparabolicity β are given. The calculated values are ingood agreement with the corresponding data found by using optical effects at 80 K and 300 K.Im Rahmen eines nichtparabolischen Sechstal-Ein-Valenzbandmodells und anisotrop-gemischter Streuung an akustischen Phononen und ionisierten Störstellen ist es erstmals gelungen, die Temperaturabhngigkeit zwischen 100 und 300 K der thermoelektrischen, galvanomagnetischen sowie der Größe der thermomagnetischen Effekte und der Lorenzzahlen von Sb2Te3−Einkristallen mit physikalisch relevanten mikroskopischen Transportdaten zu erklren. Mittels einer numerischen Analyse werden die effektiven Hauptachsenmassen mHii, der Neigungswinkel des Fermiellipsoids ϑ, die Massen m ⟂ c, m ‖ c, die effektive Zustandsdichtemasse md, die Beweglichkeiten u ⟂ c, u ‖ c und die Trgerdichte ptot berechnet. Außerdem werden das Ferminiveau EF, der Relaxationszeitfaktor τac0, die Mischungsparameter BHi und der Parameter der Nichtparabolizitt β angegeben. Die berechneten Werte sind in guter Übereinstimmung mit entsprechenden bei 80 und 300 K unter Nutzung von optischen Effekten gefolgerten Daten.
On Sb//2Te//3 crystals, both pure and doped with manganese, the IR reflectivity, the temperature dependence of the magnetic susceptibility, and the EPR spectrum at 77 K were investigated. The shift of the reflectivity minimum towards shorter wavelengths for samples with increasing concentration of Mn atoms shows that the introduction of Mn atoms into the Sb//2Te//3 lattice results in an increase of the hole concentration, with approximately one hole per one Mn atom. The temperature dependence of the magnetic susceptibility and the EPR spectrum lead to the conclusion that each Mn atom corresponds to an unpaired electron. Hence, one can argue that the d-orbits of a Mn atom in the octahedral field of six tellurium atoms are split into a triply degenerate t//2//g-orbit and a doubly degenerate e//g-orbit, and occupied in such a way that the d-electrons of Mn realize a low-spin arrangement (t//2//g)**5 (e//g)**0. The discussion, based on a bonding model due to Krebs, revealed that the most probable defects in Sb//2Te//3 lattice are the substitutional defects, where manganese atoms occupy vacant antimony sites. This defect is evidently accompanied by a single negative charge which is compensated by a hole.
A thin film Sb Te -Bi Te based thermocouple was fabricated by co-evaporation. The conditions for deposition have been 2 3 2 3 investigated as a function of substrate temperature (T) and flux ratio wF sF(Te)yF(Sb,Bi)x and optimised to achieve a high s r thermoelectric power factor. It has been observed that the Seebeck coefficient and electrical conductivity of p-type Sb Te thin 2 3 films (a , r) and n-type Bi Te thin films (a , r) deposited by co-evaporation were found to be approximately 160 mVyK, p p 2 3 n n 3.12=10 Vcm and y200 mVyK, 1.29=10 Vcm, respectively. From optimal deposition parameters, a thin film thermocouple y3 y3 was fabricated and operated in Peltier mode. The observed maximum value for temperature difference between hot and cold end is approximately 15.5 K for a current of 55 mA. The figure of merit and coefficient of performance of the thermocouple are estimated. 2002 Elsevier Science B.V. All rights reserved.
An electron diffraction study has been made of vacuum deposited Sb <sub>2</sub> Te <sub>3</sub> on (100) and (111) faces of rocksalt as well as on (0001) face of mica at substrate temperatures varying from room temperature to about 350° C. The deposits developed two-degree orientations generally above 100° C. On a (100) face of rocksalt the deposits grew with 2-d {30.4} (both normal and anti) orientations along with {00.1}, but the crystallites with the latter orientation were rotated by 30°. At higher temperature only the former orientations were predominant. On the other faces only {00.1} orientation was observed, though on a mica substrate, the crystallites were rotated by 30°. In all cases Sb <sub>2</sub> Te <sub>3</sub> retained its normal rhombohedral structure.
p -type antimony telluride thin films and n -type bismuth telluride thin films have been deposited by coevaporation on glass substrates. The conditions for deposition have been investigated as a function of substrate temperature (T<sub>s</sub>), flux ratio (F<sub>r</sub>=F( Te )/F( Sb, Bi )) and optimized to achieve a high thermoelectric power factor. The quality of deposited films, e.g. structure, composition and morphology, has been examined by x-ray diffraction, energy dispersive x-ray analysis, flame atomic absorption spectroscopy, and with an atomic force microscope. The thermoelectric properties of the thin films have been evaluated by room temperature measurement of the Seebeck coefficient, Hall coefficient, and electrical resistivity. Both the crystallinity and the transport properties are strongly affected by nonstoichiometry with the highly stoichiometric samples exhibiting a high crystallinity and high thermoelectric power factor. The Seebeck coefficient and electrical conductivity of p -type Sb <sub> 2 </sub> Te <sub> 3 </sub> thin film (α<sub>p</sub>, σ<sub>p</sub>) and n -type Bi <sub> 2 </sub> Te <sub> 3 </sub> thin films (α<sub>n</sub>, σ<sub>n</sub>) were found to be about 185 μV/K, 0.32×10<sup>3</sup> Ω<sup>-1</sup> cm <sup> -1 </sup> and -228 μV/K, 0.77×10<sup>3</sup> Ω<sup>-1</sup> cm <sup> -1 </sup>, respectively. The results indicate that good quality antimony telluride and bismuth telluride thin fi-
-
lms grown by coevaporation are promising candidate materials for use in the fabrication of micro-Peltier modules. © 2001 American Vacuum Society.
Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.