Te Sublattice Research Articles

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable ‘ideal glass’ state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Competition between Ta-Ta and Te-Te bonding leading to the commensurate charge density wave in TaTe4

The origin of the charge density wave in ${\mathrm{TaTe}}_{4}$ is discussed on the basis of a first-principles density functional theory analysis of the Fermi surface, electron-hole response function, phonon band structure of the average structure, and structural optimization of the modulated phase. Analysis of the band structure and Fermi surface of the average structure clearly proves that despite the presence of $\mathrm{Ta}{\mathrm{Te}}_{4}$ chains in the crystal structure, $\mathrm{Ta}{\mathrm{Te}}_{4}$ is in fact a 3D material as far as the electronic structure near the Fermi level is concerned. A Fermi surface nesting mechanism is dismissed as the origin of the $2a\ifmmode\times\else\texttimes\fi{}2a\ifmmode\times\else\texttimes\fi{}3c$ structural modulation. The optimized $2a\ifmmode\times\else\texttimes\fi{}2a\ifmmode\times\else\texttimes\fi{}3c$ structure, which is found to be the more stable modulation in agreement with the experimental observations, can be obtained directly from a soft-phonon mode computed for the undistorted structure. Our results suggest that the driving force for the distortion is the maximization of Ta-Ta metal-metal bonding subject to inducing the minimum bonding decrease in the Te sublattice.

Topological Thermoelectric Materials Based on Bismuth Telluride

The main directions of experimental studies of topological insulators based on bismuth and antimony chalcogenides, which are related to the possibility of using the properties of the surface states of Dirac fermions in thermoelectricity, have been considered. The results of studies of the Van der Waals interlayer surface (0001) in single-crystal layered films of solid solutions of n- and p-type conductivity with substitutions of atoms in Bi and Te sublattices performed by micro-Raman spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy have been discussed. The oscillations of galvanomagnetic effects in strong magnetic fields and thermoelectric properties measured under normal conditions and at high pressures have been analyzed. The compositions of solid solutions in which the contribution of the surface states of Dirac fermions increases because of an increase in the surface concentration of fermions and the Fermi velocity depending on the energy of the Dirac point, the value of the Seebeck coefficient, and the power parameter have been determined.

Anisotropic optical properties of single Si2Te3 nanoplates

We report a combined experimental and computational study of the optical properties of individual silicon telluride (Si2Te3) nanoplates. The p-type semiconductor Si2Te3 has a unique layered crystal structure with hexagonal closed-packed Te sublattices and Si–Si dimers occupying octahedral intercalation sites. The orientation of the silicon dimers leads to unique optical and electronic properties. Two-dimensional Si2Te3 nanoplates with thicknesses of hundreds of nanometers and lateral sizes of tens of micrometers are synthesized by a chemical vapor deposition technique. At temperatures below 150 K, the Si2Te3 nanoplates exhibit a direct band structure with a band gap energy of 2.394 eV at 7 K and an estimated free exciton binding energy of 150 meV. Polarized reflection measurements at different temperatures show anisotropy in the absorption coefficient due to an anisotropic orientation of the silicon dimers, which is in excellent agreement with theoretical calculations of the dielectric functions. Polarized Raman measurements of single Si2Te3 nanoplates at different temperatures reveal various vibrational modes, which agree with density functional perturbation theory calculations. The unique structural and optical properties of nanostructured Si2Te3 hold great potential applications in optoelectronics and chemical sensing.

Exotic bonding interactions and coexistence of chemically distinct periodic lattice distortions in the charge density wave compound TaTe2

The superstructure of $\mathrm{TaT}{\mathrm{e}}_{2}$ is examined by total high-energy x-ray scattering and large-scale structure modeling over a broad temperature range. At room temperature, it features double zigzag chains of Ta atoms exhibiting metallic bonding. The chains are sandwiched between planes of weakly interacting Te atoms. Below 170 K, the chains appear broken to ``butterflylike'' clusters and single zigzag chains of covalently bonded Te atoms emerge while Ta-Te interactions remain largely ionic. This is a rare example of a charge density wave compound stabilized by bonding that is a mix of covalent, ionic, and metallic character giving rise to chemically distinct periodic lattice distortions. We argue that the formation of periodic lattice distortions in the Te sublattice in addition to those in the Ta sublattice favors charge delocalization, which may explain the unusually diminished resistivity and increased magnetic susceptibility exhibited by the low-temperature charge density wave phase of $\mathrm{TaT}{\mathrm{e}}_{2}$. The effect may be common for $5d$ transition metal chalcogenides containing Te because of the extended electronic orbitals of the constituent atoms.

Topological Surface States of Multicomponent Thermoelectrics Based on Bismuth Telluride

The surface states of Dirac fermions in p-Bi2Te3, p-Bi2 –xSbxTe3 –ySey, and p-Bi2 –x–ySnxGeyTe3 thermoelectrics were studied and its topological characteristics were obtained by scanning tunneling techniques in dependence on compositions. It was shown that the thermoelectric power factor and the material parameter enhance with the shift of the Dirac point to the top of the valence band with the increasing of atomic substitution in these thermoelectrics. A growth contribution of the surface states is determined by an increase of the Fermi velocity for large atomic substitutions of Bi at x > 1.5 and small substitutions in the Te sublattice (y = 0.06). In compositions with smaller substitutions at x = 1–1.3 and y = 0.06–0.09, similar effect of the surface states is determined due to raising the surface concentration of charge carriers.

On the Power Factor of Bismuth-Telluride-Based Alloys near Topological Phase Transitions at High Pressures

The Seebeck coefficient S and electrical conductivity σ of bismuth-telluride-based alloys with substituents in the Bi and Te sublattices are studied at pressures up to 12 GPa at room temperature. It is shown that the electrical conductivity increases with pressure and, despite a decrease in the Seebeck coefficient, the power factor S2σ increases in p-Bi0.5Sb1.5Te3 and n-Bi2Te1.65Se0.65S0.7 alloys. A maximum increase in the power factor is observed for n-Bi2Te1.65Se0.65S0.7 in the pressure range of 3–4 GPa corresponding to the electronic topological phase transition. The studied alloys are used in modeling the thermoelectric module with adjustable mechanical stress applied to thermoelements.

Фактор мощности твердых растворов на основе теллурида висмута в области топологических фазовых переходов при высоких давлениях

The Seebeck coefficient S and the electrical conductivity σ of solid solutions based on bismuth telluride with substitutions in Bi and Te sublattices were studied at pressures up to 12 GPa at room temperature. It is shown that with increasing pressure, the electrical conductivity increases and, despite the decrease in the Seebeck coefficient, an increase in the power factor (S^2)σ is observed in p-Bi0.5Sb1.5Te3 and n-Bi2Te1.65Se0.65S0.7 solid solutions. The maximum increase in the power factor was found for n-Bi2Te1.65Se0.65S0.7 in the pressure range of 3−4 GPa, corresponding to the electronic topological phase transition. The investigated solid solutions were used in the model of a thermoelectric module with adjustable mechanical stress applied to thermoelements.

Scanning tunneling spectroscopy of the surface states of Dirac fermions in thermoelectrics based on bismuth telluride

The morphology of the interlayer van der Waals surface and differential tunneling conductance in p-Bi2−xSbxTe3−ySey solid solutions were studied by scanning tunneling microscopy and spectroscopy in dependence on compositions. The topological characteristics of the Dirac fermion surface states were determined. It was shown that the thermoelectric power factor and the material parameter enhance with the shift of the Dirac point to the top of the valence band with the increasing of atomic substitution in these thermoelectrics. A correlation between topological characteristics, power factor and material parameters was found. A growth contribution of the surface states is determined by an increase of the Fermi velocity for large atomic substitutions of Bi at x > 1.5 and small substitutions in the Te sublattice (y = 0.06). In compositions with smaller substitutions at x = (1–1.3) and y = (0.06–0.09), similar effect of the surface states is determined by raising the surface concentration of charge carriers.

Sodium Substitution in Lead Telluride

Sodium is widely used as a substituting element in p-type PbTe-based thermoelectric materials. In this work, two series of polycrystalline samples Pb1–yNayTe1–y/2 (total charge balance) and Pb1–xNaxTe (total charge nonbalance) were examined. Na has limited solubility for both of the series. The MAS 23Na NMR analysis of Pb1–xNaxTe series (for Pb0.98Na0.02Te sample after spark-plasma sintering, SPS) reveals only one Na signal, corresponding to Na atoms coordinated by six Te atoms, indicating substitution of Pb by Na without defects in the Te sublattice. In the Pb1–yNayTe1–y/2 series, clustering of Na atoms with reduced coordination by Te was observed. Additional heat treatment of these samples leads to the reorganization of the Na clusters in PbTe and their equilibration with the homogenized distribution of Na in the whole volume. The maximum ZT values of 1.4–1.6 at 760 K are established for both Pb1–xNaxTe and Pb1–yNayTe1–y/2 series. Upon long-time annealing at 873 K, reorganization and redistribution of N...

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate that As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 1016 cm−3 hole density in polycrystalline CdTe films by As and P diffusion.

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT. The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb0.9875Sb0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m−1 K−1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.

Variability of structural and electronic properties of bulk and monolayer Si2Te3

Silicon telluride has diverse properties for potential applications in Si-based devices ranging from fully integrated thermoelectrics to optoelectronics to chemical sensors. This material has a unique layered structure: it has a hexagonal closed-packed Te sublattice, with Si dimers occupying octahedral intercalation sites. Here, we report a theoretical study of this material in both bulk and monolayer form, unveiling an array of diverse properties arising from reorientations of the silicon dimers between planes of Te atoms. The band gap varies up to 30% depending on dimer orientations. The variation of dimer orientations gives rise to thermal contraction, arising from more dimers aligning out of the plane as the material is heated. Strain also affects the dimer orientations and provides a degree of control of the materials properties, making Si2Te3 a promising candidate for nanoscale mechanical, optical, and memristive devices.

AgI alloying in SnTe boosts the thermoelectric performance via simultaneous valence band convergence and carrier concentration optimization

SnTe, a Pb-free analogue of PbTe, was earlier assumed to be a poor thermoelectric material due to excess p-type carrier concentration and large energy separation between light and heavy hole valence bands. Here, we report the enhancement of the thermoelectric performance of p-type SnTe by Ag and I co-doping. AgI (1–6mol%) alloying in SnTe modulates its electronic structure by increasing the band gap of SnTe, which results in decrease in the energy separation between its light and heavy hole valence bands, thereby giving rise to valence band convergence. Additionally, iodine doping in the Te sublattice of SnTe decreases the excess p-type carrier concentration. Due to significant decrease in hole concentration and reduction of the energy separation between light and heavy hole valence bands, significant enhancement in Seebeck coefficient was achieved at the temperature range of 600–900K for Sn1−xAgxTe1−xIx samples. A maximum thermoelectric figure of merit, zT, of ~1.05 was achieved at 860K in high quality crystalline ingot of p-type Sn0.95Ag0.05Te0.95I0.05.

Real-space imaging of atomic arrangement and vacancy layers ordering in laser crystallised Ge2Sb2Te5 phase change thin films

In this work, the local structure of metastable Ge2Sb2Te5 (GST) phase change thin films crystallised by laser irradiation of amorphous GST films is studied by state-of-the-art aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM). By analysing simulated and experimental atomic-resolution Cs-corrected STEM images, a structure model for metastable GST is proposed. The GST lattice is described by a distorted rock-salt like structure with an ordered Te sublattice at the 4(a) site and a disordered sublattice of Ge, Sb and vacancies at the 4(b) site, where only a distorted octahedral atomic arrangement of Ge and Sb atoms exists without layered ordering of intrinsic vacancies. Additionally, no evidence for the presence of either tetrahedral Ge atoms in the GST lattice or an amorphous component at the grain boundaries is found. Moreover, a formation of vacancy layers in metastable GST in <111> direction under the influence of focused electron beam irradiation is observed. These vacancy layers vanish during repeated scanning of the electron beam over these defects. The gained outcomes of this study shed new insight into understanding the atomic arrangement and phase change mechanism in GST thin films as well as the control of disorder in phase change materials.

Low-temperature transport properties of Tl-doped Bi2Te3single crystals

While the bulk, stoichiometric Bi${}_{2}$Te${}_{3}$ single crystals often exhibit $p$-type metallic electrical conduction due to the Bi${}_{\mathrm{Te}}$-type antisite defects, doping by thallium (Bi${}_{2\ensuremath{-}x}$Tl${}_{x}$Te${}_{3}$, $x=0\ensuremath{-}0.30$) progressively changes the electrical conduction of single crystals from $p$ type (0 \ensuremath{\le} $x$ \ensuremath{\le} 0.08) to $n$ type (0.12 \ensuremath{\le} $x$ \ensuremath{\le} 0.30). This is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic (0001) plane in the temperature range of 2--300 K. Since any kind of substitution of Tl on the Bi or Te sublattices would result in an enhancement of the density of holes rather than its decrease, and because simple incorporation of Tl at interstitial sites or in the van der Waals gap is unlikely as it would increase the lattice parameters which is not observed in experiments, incorporation of Tl likely proceeds via the formation of TlBiTe${}_{2}$ fragments coexisting with the quintuple layer structure of Bi${}_{2}$Te${}_{3}$. At low levels of Tl, 0 \ensuremath{\le} $x$ \ensuremath{\le} 0.05, the temperature-dependent in-plane ($I\ensuremath{\perp}c$) electrical resistivity maintains its metallic character as the density of holes decreases. Heavier Tl content with 0.08 \ensuremath{\le} $x$ \ensuremath{\le} 0.12 drives the electrical resistivity into a prominent nonmetallic regime displaying characteristic metal-insulator transitions upon cooling to below \ensuremath{\sim}100 K. At the highest concentrations of Tl, 0.20 \ensuremath{\le} $x$ \ensuremath{\le} 0.30, the samples revert back into the metallic state with low resistivity. Thermal conductivity measurements of Bi${}_{2}$Te${}_{3}$ single crystals containing Tl, as examined by the Debye-Callaway phonon conductivity model, reveal a generally stronger point-defect scattering of phonons with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi${}_{2}$Te${}_{3,}$ which initially lies in the valence band (for $x$ = 0), gradually shifts toward the top of the valence band (for 0.01 \ensuremath{\le} $x$ \ensuremath{\le} 0.05), then moves into the band gap (for 0.08 \ensuremath{\le} $x$ \ensuremath{\le} 0.12), and eventually intersects the conduction band (for 0.20 \ensuremath{\le} $x$ \ensuremath{\le} 0.30).

Determination of Bond Lengths and Electronic Structure of Cd&lt;sub&gt;1-x&lt;/sub&gt;Zn&lt;sub&gt;x&lt;/sub&gt;Te Ternary Alloys by Synchrotron Radiation

High-resolution synchrotron radiation x-ray absorption spectroscopy on Zn K-, Cd L3- and Te L3-edges for Cd1-xZnxTe ternary alloys with x = 0.10, 0.30, 0.50 and 0.90 are presented. A detailed analysis of the extended x-ray absorption fine structure using the IFEFFIT program, and the chemical bonds of Zn-Te are obtained, suggesting distortion of the Te sub-lattice. The x-ray absorption near-edge structure of the Zn K-, Cd L3- and Te L3-edge are investigated, and the electronic structures of Cd1-xZnxTe with various compositions are studied.

Band gap and type of optical transitions at the interband absorption edge in solid solutions based on bismuth telluride

The spectra of optical absorption in multicomponent n- and p-type solid solutions based on bismuth and antimony chalcogenides with substitutions in both sublattices of Bi2Te3 have been investigated. It has been found that, in all the solid solutions studied, just as in the parent compound Bi2Te3, direct allowed transitions occur at the interband absorption edge at T = 300 K. The band gap Eg in the n-Bi2 − xSbxTe3 − y − zSeySz solid solutions weakly increases with increasing number of substituted atoms in the Bi and Te sublattices. These atomic substitutions do not leads to an increase in Eg as compared to that of the n-Bi2Te2.7Se0.3 composition. An analysis of the optical absorption spectra suggests that the solid solutions under consideration are weakly degenerate, a conclusion supported by the earlier studies of the thermoelectric and galvanomagnetic properties. It has been established that, in the conduction band of the Bi1.8Sb0.2Te2.7Se0.15S0.15 solid solution, there is an additional extremum lying above the main extremum at a distance no more than 0.1 eV.

Precipitation of Ag 2Te in the thermoelectric material AgSbTe 2

The microstructure of AgSbTe 2, prepared by solidification, is investigated using electron microscopy. During solidification and thermal treatment, the material separates into a two-phase mixture of a rocksalt phase, which is Ag 22Sb 28Te 50, and silver telluride, Ag 2Te. Ag 2Te formation results either from eutectic solidification (large lamellar structures), or by solid-state precipitation (fine-scale particles). The crystal structure of the AgSbTe 2 phase determined by electron diffraction is consistent with a rocksalt structure that has a disordered cation sublattice. A preferred crystallographic orientation relationship at the interface between the matrix and the low-temperature monoclinic Ag 2Te phase is defined and discussed. This orientation relationship is observed for both second-phase morphologies. In both cases, the orientation relationship originates from a topotactic (cube-on-cube) alignment of the Te sublattices in the initially cubic Ag 2Te and the matrix at elevated temperature. This Te sublattice alignment is retained as the Ag 2Te undergoes a cubic-to-monoclinic transformation during cooling. This orientation relationship is observed for both second-phase morphologies.

Defect structure of Sb2−xCrxTe3 single crystals

Single crystals of Sb2Te3 doped with Cr (cCr=0–6×1020 cm−3) were prepared by the Bridgman method. The measurements of the Hall coefficient reveal a nonmonotonous dependence of hole concentrations on the Cr content in the crystal. The hole concentration decreases at low content of Cr, while at higher content of Cr it increases again. However, according to magnetic measurements, Cr atoms enter the structure and form uncharged substitutional defects CrSb×, which cannot affect the free carrier concentration directly. The observed dependence can be elucidated by means of a point defect model. The model is based on an assumption that defect structure of Sb2Te3 can be treated as hybrid Schottky and antisite defect disorder. Thus, we assume an interaction of CrSb× with the most populated native defects in the structure—antisite defects SbTe−1 and vacancies in the Te sublattice VTe+2.