Increasing Te Content Research Articles

Theoretical investigation of electronic, optical and thermoelectric properties of tellurium doped barium titanate (BTO) through modified Becke – Johnson exchange potential

The stability, electronic structure, optical and thermoelectric properties of Te -doped BaTiO3 are investigated by first-principal calculation based on the density functional theory and Boltzmann transport theory implemented in WIEN2K and BoltzTraP simulation program. This study is carried out by applying LDA + TB-mBJ potential. Formation energy of each doped structure is calculated to examine the stability and feasibility of the synthesis. Incorporating Te into BaTiO3 efficiently reduces the electronic band gap and the level of band gap reduction can be controlled by varying the amount of dopant (Eg = 2.752 eV for pure BTO and Eg = 0.500 eV for Te doped-BTO: 8.3 %). Hence, the absorption ability is improved in the visible light (380∼790 nm). Our findings suggest that all the doped structures are significantly absorbent and productive with an optical absorption that exceeds 105 cm−1 in the visible range (λ ∼ 500 nm). In addition, BaTiO3 revealed a smaller dielectric constant at zero frequency (ε10 = 3.98) compared to Te doped-BTO: 8.3 % (ε10 = 5.55), while the optical energy gap is reduced from 3.692 eV to 1.619 eV by growing Te concentration. Then, optical conductivity and Urbach's parameters are predicted. The transport properties are assessed as a function of temperature. It is found that the electrical conductivity is considerably enhanced with increasing Te concentration. Other thermoelectric properties such as Seebeck coefficient and figure of merit are also investigated. Our theoretical results can be useful for thermoelectric and visible-light photoelectrical device applications.

A first-principles study of the stability, electronic structure, and optical properties of halide double perovskite Rb2Sn1-xTexI6 for solar cell applications.

Owing to their emerging role in solar cell technology, lead halide perovskites have aroused significant research interest in the recent past. However, due to its obvious toxicity, looking for a potential alternative to lead is becoming one of the most important pursuits in present times. We present our work based on density functional theory (DFT) investigating lead free defect perovskites (Rb2Sn1-xTexI6 (0 ≤x≤ 1)). In particular, we explore the crystal structure, thermodynamic stability, electronic structure, and optical properties of Rb2Sn1-xTexI6 (0 ≤x≤ 1) as a function of increasing Te concentration. Our results show that the Sn-Te alloyed perovskites exhibit considerable stability, a suitable band gap, small effective mass, and excellent light absorption. Especially, Rb2Sn0.75Te0.25I6 and Rb2Sn0.50Te0.50I6 have a direct band gap of 1.35 and 1.44 eV, respectively, which is highly favorable for use in a single-junction photovoltaic cell. We hope that our work will arouse the interest of experimental as well as theoretical scientists for synthesizing new materials and/or exploring the Sn-Te mix as a potential substitute for lead in photovoltaic materials.

Strongly correlated superconductor with polytypic 3D Dirac points

Topological superconductors should be able to provide essential ingredients for quantum computing, but are very challenging to realize. Spin–orbit interaction in iron-based superconductors opens the energy gap between the p-states of pnictogen and d-states of iron very close to the Fermi level, and such p-states have been recently experimentally detected. Density-functional theory predicts existence of topological surface states within this gap in FeTe1−xSex making it an attractive candidate material. Here we use synchrotron-based angle-resolved photoemission spectroscopy and band structure calculations to demonstrate that FeTe1−xSex (x = 0.45) is a superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points and that its electronic structure remains topologically trivial. We show that the inverted band gap in FeTe1−xSex can possibly be realized by further increase of Te content, but strong correlations reduce it to a sub-meV size, making the experimental detection of this gap and corresponding topological surface states very challenging, not to mention exact matching with the Fermi level. On the other hand, the p–d and d–d interactions are responsible for the formation of extremely flat band at the Fermi level pointing to its intimate relation with the mechanism of high-Tc superconductivity in IBS.

STRUCTURAL, OPTICAL CONSTANTS AND ENERGY GAP TUNABILITY WHEN INCORPORATING Te INTO CdSe THIN FILM FOR SOLAR CELLS

In the present work, polycrystalline materials of CdSe1-xTex with (x = 0, 0.2, 0.4, 0.6, 0.8 and 1 at. %) were prepared by a conventional solid-state reaction method. Thin films of CdSe1-xTex of about 0.5 µm have been produced using evaporation method. The prepared thin films were characterized by using EDAX and X-ray diffractometer. The Xray diffraction studies shows that the films are polycrystalline in nature, and well oriented along a preferred direction of (002) for hexagonal and along (111) for cubic crystal structure. Both of optical constants (n, k) and film thickness have been determined precisely in terms of envelop method. Analysis of the optical absorption data showed that the transition mechanism takes place by a direct transition. The band gap decreases from 1.677 eV (CdSe) with an increase in Te concentration passing through a minimum of 1.412 eV for CdSe0.4Te0.6 (x = 0.6) and then for higher Te concentration band gap increases to 1.486 eV corresponds to pure CdTe. The importance of CdSe1-xTex compound is the tunability of band gap when incorporating Te into the CdSe. The dispersion of the refractive index is described using the Wimple–DiDomenico (WDD) single oscillator model. The non-linear refractive index has been discussed.

Correlation between superconducting transition temperatures and carrier densities in Te- and S-substituted FeSe thin films

We comparatively investigated the transport properties for S- and Te-substituted FeSe thin films under magnetic fields to clarify the origin of the contrasting behavior of the superconducting transition temperature in S and Te substitution. A classical two carrier analysis revealed that the carrier densities of the films increased with increasing Te content, while no significant change was observed for the S-substitution. This composition dependence of the carrier density well corresponds to the $T_{\mathrm c}$ behavior. The clear correlation between $T_{\mathrm c}$ and the carrier densities suggests that the structural transition affects the electronic structure in a different manner between Fe(Se,S) and Fe(Se,Te) and that this fact is the direct cause of the difference in the $T_{\mathrm c}$ behaviors at the end point of the structural transition.

Modification of MnS inclusion by tellurium in 38MnVS6 micro-alloyed steel

In 38MnVS6 steel, the morphology of sulfide inclusion has a strong influence on the fatigue life and machinability of the steel. In most cases, the MnS inclusions show strip morphology after rolling, which significantly affects the steel quality. Usually, the MnS inclusion with a spherical morphology is the best morphology for the steel quality. In the present work, tellurium was applied to 38MnVS6 micro-alloyed steel to control the MnS inclusion. Trace tellurium was added into 38MnVS6 steel and the effect of Te on the morphology, composition, size and distribution of MnS inclusions were investigated. Experimental results show that with the increase of Te content, the equivalent diameter and the aspect ratio of inclusion decrease strikingly, and the number of inclusions with small aspect ratio increases. The inclusions are dissociated and spherized. The SEM-EDS analysis indicates that the trace Te mainly dissolves in MnS inclusion. Once the MnS is saturated with Te, MnTe starts to generate and wraps MnS. The critical Te/S value for the formation of MnTe in the 38MnV6 steel is determined to be approximately 0.075. With the increase of Te/S ratio, the aspect ratio of MnS inclusion decreases and gradually reaches a constant level. The Te/S value in the 38MnVS6 steel corresponding to the change of aspect ratio from decreasing to constant ranges from 0.096 to 0.255. This is most likely to be caused by the saturation of Te in the MnS inclusion. After adding Te in the steel, rod-like MnS inclusion is modified to small inclusion and the smaller the MnS inclusion, the lower the aspect ratio.

Electrical Transport and Thermoelectric Properties of SnSe-SnTe Solid Solution.

SnSe is considered as a promising thermoelectric (TE) material since the discovery of the record figure of merit (ZT) of 2.6 at 926 K in single crystal SnSe. It is, however, difficult to use single crystal SnSe for practical applications due to the poor mechanical properties and the difficulty and cost of fabricating a single crystal. It is highly desirable to improve the properties of polycrystalline SnSe whose TE properties are still not near to that of single crystal SnSe. In this study, in order to control the TE properties of polycrystalline SnSe, polycrystalline SnSe–SnTe solid solutions were fabricated, and the effect of the solid solution on the electrical transport and TE properties was investigated. The SnSe1−xTex samples were fabricated using mechanical alloying and spark plasma sintering. X-ray diffraction (XRD) analyses revealed that the solubility limit of Te in SnSe1−xTex is somewhere between x = 0.3 and 0.5. With increasing Te content, the electrical conductivity was increased due to the increase of carrier concentration, while the lattice thermal conductivity was suppressed by the increased amount of phonon scattering. The change of carrier concentration and electrical conductivity is explained using the measured band gap energy and the calculated band structure. The change of thermal conductivity is explained using the change of lattice thermal conductivity from the increased amount of phonon scattering at the point defect sites. A ZT of ~0.78 was obtained at 823 K from SnSe0.7Te0.3, which is an ~11% improvement compared to that of SnSe.

Structural, electronic and thermal properties of TexCo4Sb11.75Te0.25

The aim of this work is the theoretical and experimental study of the influence of Te, introduced in two Wyckoff positions (2a and 24g) of the CoSb3 skutterudite structure, on the electronic and thermoelectric properties of the material. The samples with the nominal composition of TexCo4Sb11.75Te0.25, where x = 0, 0.125, 0.25, 0.5 have been prepared using FAST and HPHT methods. Powder XRD analysis confirmed that Te can be introduced into both considered structural positions at high pressure of 7.8 GPa and can partially substitute Sb, as well as occupy the void at 2a site even if low pressure of 30 MPa is applied. Electrical and thermal conductivities vary with the increase of Te content up to x = 0.25 and are almost invariant with further doping with Te. Changes in transport properties well correspond with alternations of the lattice parameter a. The electronic transport properties are coherently explained through electronic structure calculations results. Two opposing effects of Te doping were found, depending on the doping sites, since tellurium behaves either as a one-electron donor (24g site) or two-electron acceptor (2a site). The filling of structural voids with Te results in a decrease of the lattice thermal conductivity and the improvement of the power factor (α2σ). The combined effects lead to enhancement of thermoelectric figure of merit to zT = 1.02 at 673 K for Te0.25Co4Sb11.75Te0.25.

Tellurium Doping and the Structural, Electronic, and Optical Properties of NaYS2(1-x)Te2x Alloys.

New ternary and quaternary NaYS2(1–x)Te2x alloys (with x = 0, 0.33, 0.67, and 1) are proposed as promising candidates for photon energy conversion in photovoltaic applications. The effects of Te doping on crystal, spectral, and optical properties are studied within the framework of periodic density functional theory. Increasing Te content decreases the band gap (Eg) considerably (from 3.96 (x = 0) to 1.62 eV (x = 0.67)) and fits a quadratic model (Eg(x) = 3.96–6.78x + 4.70x2, (r2 = 0.96, n = 4)). The band gap of 1.62 eV makes the NaYS0.67Te1.33 alloy ideal for photovoltaic applications for their ability to absorb in the visible segment of the sunlight spectrum. The calculated exciton binding energies are 9.78 meV for NaYS1.33Te0.67 and 6.06 meV for NaYS0.67Te1.33. These values of the order of the thermal energy at room temperature suggest an easily dissociable hole–electron pair. The family of NaYS2(1–x)Te2x alloys are, therefore, promising candidates for visible photocatalytic devices and worthy of further experimental and theoretical investigations.

ZnO1−xTex highly mismatched alloys beyond the dilute alloy limit: Synthesis and electronic band structure

We have synthesized ZnO1−xTex highly mismatched alloys (HMAs) with high, up to x = 0.34, Te content using pulsed laser deposition. We have found that the film composition is strongly dependent on the growth temperature and background pressure during growth. Lowering the growth temperature and/or increasing the background Ar or N2 pressure increases the Te content in the film. When grown in O2 atmosphere, however, oxidation of the Te species occurred, resulting in much less Te incorporation in the O sublattice. While a lower substrate temperature is needed for the incorporation of more Te in the ZnO1−xTex film, the crystallinity of the film degrades at low growth temperature. X-ray photoemission, soft x-ray absorption, and x-ray emission measurements reveal that the observed drastic narrowing of the optical bandgap with increasing Te content is primarily due to the modification of the valence band. The experimentally observed evolution of the optical properties of ZnO1−xTex HMAs from dilute to mid-composition range (x > 0.3) is analyzed within the framework of a modified band anticrossing model with composition dependent coupling parameters describing the anticrossing interaction between the valence band of the matrix and Te localized states. Electrically, we found that adding Te in ZnO increases the film resistivity. When doped with N, a drastic drop in n from mid-1019 to 1015 cm−3 is observed for ZnO1−xTex with ∼2%–4% of Te. These electrical behaviors can be understood as the effect of the upward shift of the valence band, which favors the formation of native as well as N acceptors.

Photoluminescene study acceptor defects in lightly doped n type GaSb single crystals

Lightly Te-doped GaSb samples grown by the liquid encapsulated Czochralski (LEC) method have been studied by Hall measurements and low-temperature PL spectroscopy. The results suggest that acceptor-related antisite is the dominant defect in n-type GaSb with low Te-doping concentration. As the Te concentration increases, gallium vacancy related defects become the main acceptor. A new band of around 665 meV is observed in the GaSb sample with the lowest Te-doping concentration. The variation of the acceptor defects and their influence on the electronic and optical property on the n-GaSb single crystal are discussed based on the results.

Conformal deposition of GeTe films with tunable Te composition by atomic layer deposition

Atomic layer deposition (ALD) of chalcogenide amorphous films has attracted attention, thanks to its ability to deposit highly conformal and uniform thin films on three-dimensional structures; these film properties are critical for the development of ultrahigh-density integrated electronic circuits such as nanoscale 3D cross-point memory stacks. Control over the composition of chalcogenide compounds is indispensable for fabricating functional electronic devices. In this work, the authors used trichlorogermane (HGeCl3) and bis(trimethylsilyl)telluride [(Me3Si)2Te] as ALD precursors to develop conformal and uniform germanium telluride (GeTe) films at low temperature (60 °C). The growth of GeTe resulted in moderately Ge rich films independently of the ALD precursor injection time and the use of single or discrete (Me3Si)2Te doses. To achieve control over the GeTe film composition, the authors developed a unique ALD of elemental tellurium (Te) using tellurium ethoxide [Te(OEt)4] and (Me3Si)2Te as precursors. Combining GeTe and elemental Te in an ALD supercycle fashion, they obtained GeTex films with a controllable Te content showing a uniform and conformal morphology on high aspect ratio trench structures. A detailed analysis of the variations in Ge and Te contents upon exposure of GeTex films to Te(OEt)4 revealed that an interfacial replacement of Ge atoms to form volatile Ge(OEt)2 takes place and produces a decrease in Ge and an increase in Te content. The ALD studies of GeTe and Te films described in this work provide a facile route for deposition of GeTe with controllable composition. This material system is important for the development of ovonic threshold switching selectors and phase change memories.

High Thermoelectric Figure of Merit Achieved in Cu2S1- xTe x Alloys Synthesized by Mechanical Alloying and Spark Plasma Sintering.

Chalcogenides have been considered as promising thermoelectric materials because of their low cost, nontoxicity, and environmental benignity. In this work, we synthesized a series of Cu2S1- xTe x (0 ≤ x ≤ 1) alloys by a facile, rapid method of mechanical alloying combined with spark plasma sintering process. The Cu2S1- xTe x system provides an excellent vision of the competition between pure phase and phase transformation, entropy-driven solid solution, and enthalpy-driven phase separation. When the Te concentration increases, the Cu2S1- xTe x system changed from the pure monoclinic Cu2S at x = 0 to monoclinic Cu2S1- xTe x solid solution at 0.02 ≤ x ≤ 0.06 and then transforms to hexagonal Cu2S1- xTe x solid solution at 0.08 ≤ x ≤ 0.1. The phase separation of hexagonal Cu2Te in the hexagonal Cu2S matrix occurs at 0.3 ≤ x ≤ 0.7 and finally forms the hexagonal Cu2Te at x = 1. Owing to the changed band structure and the coexisted Cu2S and Cu2Te phases, greatly enhanced power factor was achieved in all Cu2S1- xTe x (0 < x < 1) alloys. Meanwhile, the point defect introduced by the substitution of Te/S atoms strengthened the phonon scattering, resulting in a lowered lattice thermal conductivity in most of these solid solutions. As a consequence, Cu2S0.94Te0.06 exhibits a maximum ZT value of 1.18 at 723 K, which is about 3.7 and 14.8 times as compared to the values of pristine Cu2S (0.32) and Cu2Te (0.08), respectively.

Extended Infrared Photoresponse inTe-HyperdopedSiat Room Temperature

Presently, silicon photonics requires photodetectors that are sensitive in a broad infrared range, can operate at room temperature, and are suitable for integration with the existing Si-technology process. Here, we demonstrate strong room-temperature sub-band-gap photoresponse of photodiodes based on Si hyperdoped with tellurium. The epitaxially recrystallized Te-hyperdoped Si layers are developed by ion implantation combined with pulsed-laser melting and incorporate Te-dopant concentrations several orders of magnitude above the solid solubility limit. With increasing Te concentration, the Te-hyperdoped layer changes from insulating to quasi-metallic behavior with a finite conductivity as the temperature tends to zero. The optical absorptance is found to increase monotonically with increasing Te concentration and extends well into the mid-infrared range. Temperature-dependent optoelectronic photoresponse unambiguously demonstrates that the extended infrared photoresponsivity from Te-hyperdoped Si p-n photodiodes is mediated by a Te intermediate band within the upper half of the Si band gap. This work contributes to pave the way toward establishing a Si-based broadband infrared photonic system operating at room temperature.

The Effect of Tellurium on the Formation of MnTe-MnS Composite Inclusions in Non-Quenched and Tempered Steel

Te is a seldom-used alloying element, which is mainly used in free-machining steel. The Te content in this kind of steel is relatively low, and only a small amount of MnTe is generated. In previous studies, the effect of Te on the formation of inclusions was not fully studied. In order to clarify the mechanism, different amounts of tellurium were added into 38MnVS6 steel to form a certain amount of MnTe. The MnTe wrapped the MnS, forming a composite inclusion. With the increase of Te content in the steel, the diameter of inclusion increased, while the aspect ratio of inclusion varied little. The aspect ratio of most inclusions was in the range of 1~3. Besides, the MnS in the MnTe-MnS composite inclusions gradually changed from one big particle to several small particles. The solidification of the steel, as well as the precipitation of MnS and MnTe, was the main factor that affecting the formation of the composite inclusion. The mechanism of the process was systematically expressed in the current study.

Molecular Beam Epitaxy and p-type Doping of ZnMgSTe Quaternary Alloys

ZnS-based ZnMgSTe quaternary alloy layers have been grown by molecular beam epitaxy. The bandgap of ZnMgSTe has been estimated from the reflectance spectra, and it was found that it increases with increasing Mg content, while it decreases with increasing Te content. Nitrogen acceptor doping to $$\hbox {Zn}_{1-x}\hbox {Mg}_x\hbox {S}_{1-y}\hbox {Te}_y$$ layers has also been investigated. The layers with Te content $$y > 0.1$$ were found to be p-type, and the layer with the larger Te content exhibited lower resistivity. From these results, it seems that the ZnMgSTe quaternary alloy with appropriate composition possesses both a wide bandgap and p-type conductivity.

Structure and thermoelectric property of Te doped paracostibite CoSb1-xTexS compounds

Paracostibite (CoSbS), a newly developed thermoelectric material, has aroused lots of interest due to its highly earth abundant and inexpensive constituent elements and potential application for thermoelectric power generation in the intermediate temperature range. Herein, a series of CoSb1-xTexS (x = 0–0.09) compounds were prepared by vacuum melting and annealing followed by SPS processing, and the effects of Te doping on the structure and thermoelectric properties were systematically investigated. Doping Te on the Sb site increases the carrier concentration up to 7.24 × 1020 cm−3 for CoSb0.93Te0.07S compound which is several orders of magnitude higher than that of un-doped CoSbS, and enhances the power factor. The maximum power factor of 14.07 μW cm−1 K−2 is attained at 900 K. Concomitantly, doping with Te on the Sb site leads to effective scattering of heat carrying phonon, accompanying with a strong suppression of the thermal conductivity with the increase of Te content, resulting in an increase of the ZT. A maximum ZT of 0.43 at 900 K is attained for CoSb0.93Te0.07S compound, which is 139% higher than that of un-doped CoSbS compound.

Hydrogen-assisted post-growth substitution of tellurium into molybdenum disulfide monolayers with tunable compositions

Herein we report the successful doping of tellurium (Te) into molybdenum disulfide (MoS2) monolayers to form MoS2xTe2(1−x) alloy with variable compositions via a hydrogen-assisted post-growth chemical vapor deposition process. It is confirmed that H2 plays an indispensable role in the Te substitution into as-grown MoS2 monolayers. Atomic-resolution transmission electron microscopy allows us to determine the lattice sites and the concentration of introduced Te atoms. At a relatively low concentration, tellurium is only substituted in the sulfur sublattice to form monolayer MoS2(1−x)Te2x alloy, while with increasing Te concentration (up to ∼27.6% achieved in this study), local regions with enriched tellurium, large structural distortions, and obvious sulfur deficiency are observed. Statistical analysis of the Te distribution indicates the random substitution. Density functional theory calculations are used to investigate the stability of the alloy structures and their electronic properties. Comparison with experimental results indicate that the samples are unstrained and the Te atoms are predominantly substituted in the top S sublattice. Importantly, such ultimately thin Janus structure of MoS2(1−x)Te2x exhibits properties that are distinct from their constituents. We believe our results will inspire further exploration of the versatile properties of asymmetric 2D TMD alloys.

Physical and optical characterizations of Ge10Se90 − xTex thin films in view of their spectroscopic ellipsometry data

Ge10Se90−xTex (x=10, 20, 30) alloys have been prepared by the conventional melt quenching technique. Several important physical parameters have been calculated to understand the structural changes in the prepared compositions. Spectroscopic ellipsometry technique has been used to investigate the optical properties of the deposited thin films. The extracted optical constants have been utilized to evaluate several interesting optoelectrical parameters. The prepared films have revealed indirect energy gap transition that varied from 1.4 to 1.02eV. Increasing Te content leads to the increase in the joint density of states (JDOS) in the investigated films which is accompanied by the decrease in the energy gap. Profound analysis has been made to study the variation of physical, optical, and optoelectrical parameters of Ge10Se90−xTex as a function of Te content. Moreover, it was fruitful to estimate the changes in the nonlinear parameters of the studied films. An obvious increase in the nonlinear parameters has been observed with the increase in Te percent. This refers to an increase in the nonlinear response of bound electrons in the glassy system to the applied electric field. Believing this we may expect the usage of the samples under investigation in non-linear devices. The studied parameters have enriched our current knowledge regards to the nature of the studied compositions.

ZnO1-xTex thin films deposited by reactive magnetron co-sputtering: composition, structure and optical properties

ABSTRACTZnO1-xTexthin films were deposited by DC reactive magnetron co-sputtering using pure Zn and Te targets. Te atomic concentration in the films ranged from x=0 to 0.33 by adjusting the applied power on the targets or varying the cathode-substrate distance. Chemical composition and crystalline structure were determined by RBS experiments and X-ray Diffraction, respectively. For low Te atomic concentrations (x≤0.04) the deposited ZnO1-xTexfilms showed a crystalline structure ZnO wurtzite type however, for increasing Te concentration significant broadening and decreasing intensities of the main peaks belonging to pure ZnO films together with some weak peaks characteristic of crystalline Trizinc Tellurate salt have appeared. For the highest x values some non-identified weak peaks beside to some others peaks corresponding to the crystalline phases mentioned above as well as, a broad band probably associated to amorphous TeO2phase were observed. A preliminary optical characterization of the samples point out the possibility of different electronic transitions within the ZnO band gap.