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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Contrasting SnTe–NaSbTe 2 and SnTe–NaBiTe 2 Thermoelectric Alloys: High Performance Facilitated by Increased Cation Vacancies and Lattice Softening

Defect chemistry is critical to designing high performance thermoelectric materials. In SnTe, the naturally large density of cation vacancies results in excessive hole doping and frustrates the ability to control the thermoelectric properties. Yet, recent work also associates the vacancies with suppressed sound velocities and low lattice thermal conductivity, underscoring the need to understand the interplay between alloying, vacancies, and the transport properties of SnTe. Here, we report solid solutions of SnTe with NaSbTe 2 and NaBiTe 2 (NaSn m SbTe m +2 and NaSn m BiTe m +2 , respectively) and focus on the impact of the ternary alloys on the cation vacancies and thermoelectric properties. We find introduction of NaSbTe 2 , but not NaBiTe 2 , into SnTe nearly doubles the natural concentration of Sn vacancies. Furthermore, DFT calculations suggest that both NaSbTe 2 and NaBiTe 2 facilitate valence band convergence and simultaneously narrow the band gap. These effects improve the power factors but also make the alloys more prone to detrimental bipolar diffusion. Indeed, the performance of NaSn m BiTe m +2 is limited by strong bipolar transport and only exhibits modest maximum ZTs ≈ 0.85 at 900 K. In NaSn m SbTe m +2 however, the doubled vacancy concentration raises the charge carrier density and suppresses bipolar diffusion, resulting in superior power factors than those of the Bi-containing analogues. Lastly, NaSbTe 2 incorporation lowers the sound velocity of SnTe to give glasslike lattice thermal conductivities. Facilitated by the favorable impacts of band convergence, vacancy-augmented hole concentration, and lattice softening, NaSn m SbTe m +2 reaches high ZT ≈ 1.2 at 800–900 K and a competitive average ZT avg of 0.7 over 300–873 K. The difference in ZT between two chemically similar compounds underscores the importance of intrinsic defects in engineering high-performance thermoelectrics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Ferroelectrically switched valley-dependent transmission in SnTe-PbTe-SnTe monolayer lateral heterostructures

A special class of valleytronic two-dimensional (2D) semiconductors possesses carrier pockets (i.e., valleys) along certain directions in the first Brillouin zone, which can be applied as a new degree of freedom for information storage and processing. Here we show that members of this family that are ferroelectric allow the location of these valleys to be switched by rotating the ferroelectric polarization. This makes possible the control of electronic state transmission probability through an energy barrier by ferroelectrically switching the polarization direction, thereby creating or eliminating valley matching in reciprocal space. We apply molecular beam epitaxy to grow lateral sandwich heterostructures with monolayer-thick ferroelectric SnTe separated by nanometer-wide paraelectric PbTe as the barriers. Using scanning tunneling microscopy, we show that the transmission probability of the 2D hole states at the valence band maximum of SnTe monolayer strongly relies on the relative orientation between the polarization directions of the two SnTe electrodes. The transmission can be switched from a suppressed state to a permitted state by rotating the ferroelectric polarization of one SnTe electrode by 90 degrees. Our work demonstrates the electric-field-control of valley locations and its potential for tunnel junction valleytronic devices.

electronic devices↗
DOE OSTI2020

Synthesis of Narrow SnTe Nanowires Using Alloy Nanoparticles

Topological crystalline insulator tin telluride (SnTe) provides a rich playground to examine interactions of correlated electronic states, such as ferroelectricity, topological surface states, and superconductivity. The study of SnTe nanowires may lead to even richer physics owing to the one-dimensional (1D) confinement effect and an increased contribution from the topological surface states. Thus, for transport measurements, SnTe nanowires must be synthesized with reduced diameters and high crystalline quality to ensure 1D confinement and phase coherence of the topological surface electrons. Here, we report a facile growth method to produce narrow SnTe nanowires with a high yield using alloy nanoparticles as growth catalysts. The average diameter of the SnTe nanowires grown using alloy nanoparticles is 85 nm, nearly a factor of three reduction compared to the average diameter of 240 nm when using gold nanoparticles as growth catalysts. Transport measurements reveal the effect of the nanowire diameter on the residual resistance ratio and magnetoresistance. Particularly, the ferroelectric transition temperature for SnTe evolves systematically with the nanowire diameter. In situ cryogenic cooling of narrow SnTe nanowires in a transmission electron microscope directly reveals the cubic to rhombohedral structural transition, which is associated with the ferroelectric transition. Thus, these narrow SnTe nanowires represent a model system to study electronic states arising from 1D confinement, such as 1D topological superconductivity and potential multiband superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Materials Data on SnTe by Materials Project

SnTe is Zincblende, Sphalerite structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. Sn2+ is bonded to four equivalent Te2- atoms to form corner-sharing SnTe4 tetrahedra. All Sn–Te bond lengths are 3.13 Å. Te2- is bonded to four equivalent Sn2+ atoms to form corner-sharing TeSn4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SnTe by Materials Project

SnTe is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Sn2+ is bonded to six equivalent Te2- atoms to form a mixture of corner and edge-sharing SnTe6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Sn–Te bond lengths are 3.21 Å. Te2- is bonded to six equivalent Sn2+ atoms to form a mixture of corner and edge-sharing TeSn6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Co2(SnTe)3 by Materials Project

Co2(SnTe)3 crystallizes in the trigonal R-3 space group. The structure is three-dimensional. there are two inequivalent Co sites. In the first Co site, Co is bonded to three Sn and three Te atoms to form corner-sharing CoSn3Te3 octahedra. The corner-sharing octahedra tilt angles range from 47–55°. There are two shorter (2.54 Å) and one longer (2.56 Å) Co–Sn bond lengths. There are a spread of Co–Te bond distances ranging from 2.55–2.57 Å. In the second Co site, Co is bonded to three equivalent Sn and three equivalent Te atoms to form corner-sharing CoSn3Te3 octahedra. The corner-sharing octahedra tilt angles range from 47–55°. All Co–Sn bond lengths are 2.56 Å. All Co–Te bond lengths are 2.54 Å. There are two inequivalent Sn sites. In the first Sn site, Sn is bonded in a 4-coordinate geometry to two Co and two equivalent Te atoms. There are one shorter (2.96 Å) and one longer (3.14 Å) Sn–Te bond lengths. In the second Sn site, Sn is bonded in a 4-coordinate geometry to two equivalent Co and two equivalent Te atoms. There are one shorter (2.97 Å) and one longer (3.13 Å) Sn–Te bond lengths. There are two inequivalent Te sites. In the first Te site, Te is bonded in a 4-coordinate geometry to two equivalent Co and two equivalent Sn atoms. In the second Te site, Te is bonded in a 4-coordinate geometry to two Co and two equivalent Sn atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SnTe(PbS)4 by Materials Project

SnTe(PbS)4 is Molybdenum Carbide MAX Phase-derived structured and crystallizes in the trigonal R3m space group. The structure is three-dimensional. there are four inequivalent Pb2+ sites. In the first Pb2+ site, Pb2+ is bonded to three equivalent Te2- and three equivalent S2- atoms to form PbTe3S3 octahedra that share corners with three equivalent PbS6 octahedra, corners with three equivalent SnTe3S3 octahedra, edges with three equivalent SnTe3S3 octahedra, and edges with nine PbTe3S3 octahedra. The corner-sharing octahedra tilt angles range from 1–2°. All Pb–Te bond lengths are 3.22 Å. All Pb–S bond lengths are 2.99 Å. In the second Pb2+ site, Pb2+ is bonded to six S2- atoms to form a mixture of edge and corner-sharing PbS6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. There are three shorter (3.01 Å) and three longer (3.02 Å) Pb–S bond lengths. In the third Pb2+ site, Pb2+ is bonded to six S2- atoms to form a mixture of edge and corner-sharing PbS6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. There are three shorter (3.02 Å) and three longer (3.04 Å) Pb–S bond lengths. In the fourth Pb2+ site, Pb2+ is bonded to six S2- atoms to form PbS6 octahedra that share corners with three equivalent PbS6 octahedra, corners with three equivalent SnTe3S3 octahedra, edges with three equivalent SnTe3S3 octahedra, and edges with nine PbS6 octahedra. The corner-sharing octahedra tilt angles range from 1–4°. There are three shorter (3.00 Å) and three longer (3.06 Å) Pb–S bond lengths. Sn2+ is bonded to three equivalent Te2- and three equivalent S2- atoms to form SnTe3S3 octahedra that share corners with six PbS6 octahedra, edges with six PbS6 octahedra, and edges with six equivalent SnTe3S3 octahedra. The corner-sharing octahedra tilt angles range from 2–4°. All Sn–Te bond lengths are 3.14 Å. All Sn–S bond lengths are 2.91 Å. Te2- is bonded to three equivalent Pb2+ and three equivalent Sn2+ atoms to form TeSn3Pb3 octahedra that share corners with six SPb6 octahedra, edges with six equivalent TeSn3Pb3 octahedra, and edges with six SPb6 octahedra. The corner-sharing octahedral tilt angles are 6°. There are four inequivalent S2- sites. In the first S2- site, S2- is bonded to six Pb2+ atoms to form SPb6 octahedra that share corners with three equivalent TeSn3Pb3 octahedra, corners with three equivalent SPb6 octahedra, edges with three equivalent TeSn3Pb3 octahedra, and edges with nine SPb6 octahedra. The corner-sharing octahedra tilt angles range from 0–6°. In the second S2- site, S2- is bonded to six Pb2+ atoms to form a mixture of edge and corner-sharing SPb6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. In the third S2- site, S2- is bonded to six Pb2+ atoms to form a mixture of edge and corner-sharing SPb6 octahedra. The corner-sharing octahedra tilt angles range from 1–2°. In the fourth S2- site, S2- is bonded to three equivalent Pb2+ and three equivalent Sn2+ atoms to form SSn3Pb3 octahedra that share corners with three equivalent TeSn3Pb3 octahedra, corners with three equivalent SPb6 octahedra, edges with three equivalent TeSn3Pb3 octahedra, and edges with nine SPb6 octahedra. The corner-sharing octahedra tilt angles range from 2–6°.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Evaluation of PbTe and SnTe as Ohmic Contact Layers in CdTe Solar Cell Devices

For solar cells based on CdTe, the choice of a suitable back contact material is limited by CdTe's deep work function. Here, we explored p-type PbTe and SnTe as ohmic contacts to CdTe. These contact layers were grown on single crystal CdTe substrates by molecular beam epitaxy, and the valence band offset between film and substrate was measured using X-ray photoemission spectroscopy. Polycrystalline device structures were also grown by sublimation to assess performance improvements. Doping was achieved in PbTe by thallium incorporation. Only the highest Tl doped PbTe resulted in a desirable band alignment with the CdTe, forming an electron reflector and no hole barrier. Time-resolved photoluminescence measurements also revealed significant photocarrier lifetime improvements for only the highest doped PbTe. Consequentially, devices incorporating the highest doped PbTe layers showed increased power conversion efficiency, primarily from increased fill factor. Doping of the PbTe was measured via Hall effect with variable magnetic field, which was required due to the formation of an n-type parasitic interface layer. To properly interpret the variable field Hall measurements, we derived an expression for the magnetic field-dependent conductivity tensor of an L-valley semiconductor.

anisotropic band valley↗
DOE OSTI2021

Josephson detection of time-reversal symmetry broken superconductivity in SnTe nanowires

A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. In this work, we present such a case: we investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in JJs and observe a deviation from the standard CPR. We report on indications of an unexpected breaking of time-reversal symmetry in these devices, detailing the unconventional characteristics that reveal this behavior. These include an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. The analysis examines how multiband effects and the experimentally visualized ferroelectric domain walls give rise to this behavior, giving insight into the Josephson effect in materials that possess ferroelectricity and/or multiband superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Realizing tunable Fermi level in SnTe by defect control

The tuning of the Fermi level in tin telluride, a topological crystalline insulator, is essential for accessing its unique surface states and optimizing its electronic properties for applications such as spintronics and quantum computing. In this study, we demonstrate that the Fermi level in tin telluride can be effectively modulated by controlling the tin concentration during chemical vapor deposition synthesis. By introducing tin-rich conditions, we observed a blue shift in the x-ray photoelectron spectroscopy core-level peaks of both tin and tellurium, indicating an upward shift in the Fermi level. Further, this shift is corroborated by a decrease in work function values measured via ultraviolet photoelectron spectroscopy, confirming the suppression of Sn vacancies. Our findings provide a low-cost, scalable method to achieve tunable Fermi levels in tin telluride, offering a significant advancement in the development of materials with tailored electronic properties for next-generation technological applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1983

Solutal diffusion coefficient for liquid PbTe-SnTe

The solutal diffusion coefficient has been determined for liquid lead telluride-tin telluride using a modified shear cell technique. Postdiffusion concentration profiles are presented for several diffusion couples. The best analytical curve fit to the data gives a composition-dependent diffusion coefficient of (/3/7/ to the C power) x 0.00014 sq cm/sec, where C is the PbTe concentration. In addition, data are presented to show the importance of solutal convection in the lead-tin-telluride system.

Clark, I. O.↗
NASA NTRS1985

Effects of supercooling in the initial solidification of PbTe-SnTe solid solutions

Deviations from compositions anticipated by the thermal equilibrium phase diagram have been observed in Bridgman-grown crystals of Pb(1-x)Sn(x)Te, in the first to freeze region of the boule. A set of experiments were conducted to determine the extent of thermal supercooling of Pb(1-x)Sn(x)Te in a Bridgman-like configuration. The results of the compositional profiles and the supercooling measurements are consistent with a diffusionless transformation occurring at the onset of solidification, and the length of uncontrolled growth is inversely related to the temperature gradient of the furnace.

Fripp, A. L.↗
NASA NTRS1993

Growth experiment of narrow band-gap semiconductor PbSnTe single crystals in space (M-1)

An experiment on crystal growth of Pb(1-x)Sn(x)Te in microgravity is planned. This material is an alloy of the compound semiconductors PbTe and SnTe. It is a promising material for infrared diode lasers and detectors in the wavelength region between 6 and 30 micron. Since the electrical properties of Pb(1-x)Sn(x)Te depend greatly on the Pb/Sn ratio and crystalline defects as well as impurity concentration, homogeneous, defect-free, high-quality crystals are anticipated. Although many growth methods, such as the pulling method, the Bridgman method, the vapor growth method, etc., have been applied to the growth of Pb(1-x)Sn(x)Te, large, homogeneous, low-defect-density crystals have not yet been grown on Earth. The unsuccessful results were caused by buoyancy-driven convection in the fluids induced by the specific gravity difference between heated and cooled fluids on Earth. A crystal is grown by cooling the melt from one end of the ampoule. In crystal growth from the melt, about 30 percent of the SnTe in the melt is rejected at the solid-liquid interface during solidification. On Earth, the rejected SnTe is completely mixed with the remaining melt by convection in the melt. Therefore, SnTe concentration in the melt, and accordingly in the crystal, increases as the crystal grows. In the microgravity environment, buoyancy-driven convection is suppressed because the specific gravity difference is negligible. In that case, the rejected SnTe remains at the solid-liquid interface and its concentration increases only at the interface. If the growth rate is higher than the PbTe-SnTe interdiffusion rate, the amount of SnTe which diffuses from the interface into the melt increases as SnTe piles up at the interface, and finally it balances the amount of rejected SnTe during solidification, resulting in steady-state SnTe transportation at the interface. By using this principle, compositionally homogeneous crystals can be grown. Furthermore, low-defect-density crystals will be grown in microgravity, because convection causes crystalline defects by mising hot and cold fluids and generating temperature fluctuations in them.

Yamada, Tomoaki↗
NASA NTRS1999

Growth of Compound Semiconductors in a Low Gravity Environment: Microgravity Growth of PbSnTe

The growth of the alloy compound semiconductor lead tin telluride (PbSnTe) was chosen for a microgravity flight experiment in the Advanced Automated Directional Solidification Furnace (AADSF), on the United States Microgravity Payload-3 (USNP-3) in February, 1996 and on USNW- 4 in November, 1997. The objective of these experiments was to determine the effect of the reduction in convection, during the growth process, brought about by the microgravity environment. The properties of devices made from PbSnTe, an alloy of PbTe and SnTe, are dependent on the ratio of the elemental components in the starting crystal. Compositional uniformity in the crystal is only obtained if there is no significant mixing in the liquid during growth. The technological importance of PbSnTe lies in its band gap versus composition diagram which has a zero energy crossing at approximately 40% SnTe. This facilitates the construction of long wavelength (greater than 6 gm) infrared detectors and lasers. The properties and utilization of PbSnTe are the subject of other papers. 1,2 PbSnTe is also interesting from a purely scientific point of view. It is, potentially, both solutally and thermally unstable due to the temperature and density gradients present during growth. Density gradients, through thermal expansion, are imposed in directional solidification because temperature gradients are required to extract heat. Solutal gradients occur in directional solidification of alloys due to segregation at the interface. Usually the gradients vary with both experiment design and inherent materials properties. In a simplified one dimensional analysis with the growth axis parallel to the gravity vector, only one of the two instabilities work at a time. During growth, the temperature in the liquid increases ahead of the interface. Therefore the density, due to thermal expansion, is decreasing in that direction. However, the phase diagram shows that the lighter SnTe is preferentially rejected at the interface. This causes the liquid density to increase with distance away from the interface.

Fripp, A. L.↗
DOE OSTI2026

Altermagnetism Induced Surface Chern Insulator

We propose a new pathway to the quantized anomalous Hall effect (QAHE) by coupling an altermagnet to a topological crystalline insulator (TCI). The former gaps the topological surface states of the TCI, thereby realizing the QAHE in a robust and switchable platform with near-vanishing magnetization. We demonstrate the feasibility of this approach by studying a slab of the TCI SnTe coupled to an altermagnetic RuO 2 layer. Our first-principles calculations reveal that the d-wave altermagnetism in RuO 2 induces a 7 meV gap to the Dirac surface states on the (110) surface of SnTe, producing a finite anomalous Hall effect. Our approach generalizes to broader classes of altermagnetic materials and TCIs, thereby providing a family of topological altermagnetic heterostructures with small or vanishing magnetization that support nontrivial Chern numbers. In conclusion, our results highlight a promising new topological platform with great tunability and applications to spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Identifying crystal structures and chemical reactions at the interface of stanene on Bi 2 Te 3

Synthesizing monolayers and heterostructures is an enabling approach to extract new physical phenomena from bulk materials. Among the structures amenable to this approach is stanene, which is a monolayer of tin, similar to graphene, and has been predicted to host one-dimensional topological states at its edges. Stanene can be tuned by decorating with different adatoms, which makes it a promising platform on which to engineer topological devices. Here, we deposit Sn on Bi 2 Te 3 and characterize the growth using anomalous synchrotron x-ray scattering and x-ray photoelectron spectroscopy (XPS). X-ray diffraction data reveal the formation of epitaxial Sn-based structures, along with penetration of Sn into the Bi 2 Te 3 , with Sn intercalating between the upper 10 Bi 2 Te 3 quintuple layers. Additionally, XPS data show deposited Sn reacting to form SnTe and Bi at the Bi 2 Te 3 surface. The calculated heat of reaction for Sn and Bi2Te3 is consistent with an exothermic reaction to SnTe and Bi. Using thermodynamic calculations as a guide, we identify several candidate substrates that can stabilize the stanene phase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗