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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

First-Principles Assessment of ZnTe and CdSe as Prospective Tunnel Barriers at the InAs/Al Interface

Majorana zero modes are predicted to emerge in semiconductor/ superconductor interfaces, such as InAs/Al. Majorana modes could be utilized for fault tolerant topological qubits. However, their realization is hindered by materials challenges. The coupling between the superconductor and the semiconductor may be too strong for Majorana modes to emerge, due to effective doping of the semiconductor by the metallic contact. This could be mediated by adding a tunnel barrier of controlled thickness. We use density functional theory (DFT) with Hubbard U corrections, whose values are machine-learned via Bayesian optimization (BO), to assess ZnTe and CdSe as prospective tunnel barriers for the InAs/Al interface. The results of DFT +U(BO) for ZnTe are validated by comparison to angle resolved photoemission spectroscopy (ARPES). We then study bilayer interfaces of the three semiconductors with each other and with Al, as well as trilayer interfaces with a varying number of ZnTe or CdSe layers inserted between InAs and Al. We find that 16 atomic layers of either material completely insulate the InAs from metal induced gap states (MIGS). However, ZnTe and CdSe differ significantly in their band alignment, such that ZnTe forms an effective barrier for electrons, whereas CdSe forms a barrier for holes. Because of Fermi level pinning in the conduction band at the interface, only electron transport is relevant for InAs-based Majorana devices. Therefore, ZnTe is the better choice. Based on the results of our simulations, we suggest conducting experiments with ZnTe barriers in the thickness range of 6–18 atomic layers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1993

Growth of ZnTe by Physical Vapor Transport and Traveling Heater Method

ZnTe crystals were grown by horizontal physical vapor transport (PVT) and a Te-solution vertical traveling heater method (THM). The grown crystals were examined by X-ray Laue diffraction technique and Hall measurements to determine the growth orientation and the electrical properties of the crystals. They were also characterized by low temperature infrared (IR) absorption measurements. Several sets of distinct peaks were observed in the IR absorption spectra for the THM samples and were identified as resulting from CU(2+) impurities. Similar measurements on vapor grown ZnTe showed featureless absorption spectra. Chemical analyses were carried out to measure the impurity content in various ZnTe samples and synchrotron radiation topography was used to Study crystalline microstructure of the (111) ZnTe single crystals grown by PVT.

Su, Ching-Hua↗
DOE OSTI2022

High Thermoelectric Performance in Chalcopyrite Cu 1-x Ag x GaTe 2 –ZnTe: Nontrivial Band Structure and Dynamic Doping Effect

The understanding of thermoelectric properties of ternary I–III–VI 2 type (I = Cu, Ag; III = Ga, In; and VI = Te) chalcopyrites is less well developed. Although their thermal transport properties are relatively well studied, the relationship between the electronic band structure and charge transport properties of chalcopyrites has been rarely discussed. In this study, we reveal the unusual electronic band structure and the dynamic doping effect that could underpin the promising thermoelectric properties of Cu 1–x Ag x GaTe 2 compounds. Density functional theory (DFT) calculations and electronic transport measurements suggest that the Cu 1–x Ag x GaTe 2 compounds possess an unusual non-parabolic band structure, which is important for obtaining a high Seebeck coefficient. Moreover, a mid-gap impurity level was also observed in Cu 1–x Ag x GaTe 2 , which leads to a strong temperature-dependent carrier concentration and is able to regulate the carrier density at the optimized value for a wide temperature region and thus is beneficial to obtaining the high power factor and high average ZT of Cu 1–x Ag x GaTe 2 compounds. We also demonstrate a great improvement in the thermoelectric performance of Cu 1–x Ag x GaTe 2 by introducing Cu vacancies and ZnTe alloying. The Cu vacancies are effective in increasing the hole density and the electrical conductivity, while ZnTe alloying reduces the thermal conductivity. As a result, a maximum ZT of 1.43 at 850 K and a record-high average ZT of 0.81 for the Cu 0.68 Ag 0.3 GaTe 2 –0.5%ZnTe compound are achieved.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on ZnTe(H2N)4 by Materials Project

ZnTe(NH2)2(NH2)2 crystallizes in the monoclinic P2_1 space group. The structure is one-dimensional and consists of eight ammonia molecules and two ZnTe(NH2)2 ribbons oriented in the (1, 0, 0) direction. In each ZnTe(NH2)2 ribbon, there are two inequivalent Zn2+ sites. In the first Zn2+ site, Zn2+ is bonded in a distorted L-shaped geometry to two N3- and two Te4+ atoms. There are one shorter (2.13 Å) and one longer (2.14 Å) Zn–N bond lengths. There are one shorter (2.62 Å) and one longer (2.64 Å) Zn–Te bond lengths. In the second Zn2+ site, Zn2+ is bonded in a 4-coordinate geometry to two N3- and two Te4+ atoms. There are one shorter (2.13 Å) and one longer (2.15 Å) Zn–N bond lengths. There are one shorter (2.59 Å) and one longer (2.62 Å) Zn–Te bond lengths. There are four inequivalent N3- sites. In the first N3- site, N3- is bonded in a distorted water-like geometry to one Zn2+ and two H+0.75+ atoms. There is one shorter (1.03 Å) and one longer (1.04 Å) N–H bond length. In the second N3- site, N3- is bonded in a distorted water-like geometry to one Zn2+ and two H+0.75+ atoms. Both N–H bond lengths are 1.03 Å. In the third N3- site, N3- is bonded in a distorted water-like geometry to one Zn2+ and two H+0.75+ atoms. There is one shorter (1.03 Å) and one longer (1.04 Å) N–H bond length. In the fourth N3- site, N3- is bonded in a distorted water-like geometry to one Zn2+ and two H+0.75+ atoms. There is one shorter (1.03 Å) and one longer (1.04 Å) N–H bond length. There are eight inequivalent H+0.75+ sites. In the first H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the second H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the third H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the fourth H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the fifth H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the sixth H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the seventh H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. In the eighth H+0.75+ site, H+0.75+ is bonded in a single-bond geometry to one N3- atom. There are two inequivalent Te4+ sites. In the first Te4+ site, Te4+ is bonded in a water-like geometry to two Zn2+ atoms. In the second Te4+ site, Te4+ is bonded in a water-like geometry to two Zn2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Phase transitions of β-ZnTe(en) 0.5 under hydrostatic pressure

Organic–inorganic hybrid semiconductors have enhanced and distinctive material properties. β-ZnTe(en) 0.5 , which consists of alternating layers of two-monolayer-thick zinc telluride (ZnTe) and ethylenediamine (en), exhibits high crystallinity, stability, and tunable optical properties. Using x-ray diffraction (XRD) and Fourier transform infrared spectroscopy, this study investigated the structural response of β-ZnTe(en) 0.5 to applied hydrostatic pressure. Pressure-induced phase transitions were observed at 2.1 and 3.3 GPa. Shifts in the XRD peaks indicate substantial anisotropy in the pressure response, with the layer stacking direction ( b axis) exhibiting high compressibility. The a and b lattice parameters showed −0.55% strain/GPa and −2.26% strain/GPa, respectively, contradicting theoretical calculations that predicted a more isotropic response. IR spectroscopy revealed abrupt changes in NH 2 and CH 2 vibrational modes corresponding to the phase transitions.

Chemical compounds↗
DOE OSTI2023

Symmetry-driven persistent spin texture for the two-dimensional nonsymmorphic CdTe and ZnTe crystal structures

In this paper, two nonsymmorphic two-dimensional structures of CdTe and ZnTe are modeled, and using state-of-the-art density functional theory with the group theory of solids, their symmetry-enforced electronic properties are studied. The in-plane ferroelectricity coupled with strong spin-orbit coupling induces a unidirectional out-of-plane Rashba spin-orbit field that can host a momentum-independent uniform spin configuration known as persistent spin texture (PST) at the Brillouin zone center. PST in these structures is found to be robust against external perturbations such as strain, structural distortion, and independent of layer thickness. These unprecedented intrinsic spin transport properties hold utmost importance in spintronics, as the experimental stringent condition of equal Rashba and Dresselhaus constants [Phys. Rev. Lett. 90, 146801 (2003)] is eliminated. The calculated persistent spin helix wavelength of <~5 nm paves the way for developing next-generation nanosized nonballistic spin field-effect transistors compared with micrometer-sized GaAs/AlGaAs quantum wells. Further, these materials exhibit finite spin Hall conductivity at the band edges and hence can be used in ferromagnet-free spin Hall transistors. Although CdTe and ZnTe systems have been widely studied for photocatalysis and solar cell applications over the past few decades, their potential application in spintronic devices has not been explored. Mono/few layers of CdTe and ZnTe synthesized from (110) facets of bulk zinc-blende crystals [Nat. Commun. 3, 1057 (2012)] satisfy all symmetry operations of the nonsymmorphic space group and hence can be considered ideal materials to verify our theoretical results experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1990

Band bending at Al, In, Ag, and Pt interfaces with CdTe and ZnTe (110)

UV and X-ray photoelectron spectroscopic methods are presently used to study the band-bending behavior and interfacial chemistry of Al, In, Ag, and Pt overlayers on vacuum-cleaved p-CdTe and p-ZnTe (110). All four metals are found to yield Schottky barriers on CdTe and ZnTe. The metal-induced gap states model prediction of a difference in barrier heights for two semiconductors which is dependent on their band lineup is borne out by the results for Ag, Pt, and Al, but not for In. Reaction and intermixing for Al, Ag, and Pt overlayers on CdTe and ZnTe indicate that these interfaces are not ideal.

Wahi, A. K.↗
DOE OSTI2023

Measuring Photoexcited Electron and Hole Dynamics in ZnTe and Modeling Excited State Core-Valence Effects in Transient Extreme Ultraviolet Reflection Spectroscopy

Transient extreme ultraviolet (XUV) spectroscopy is becoming a valuable tool for characterizing solar energy materials because it can separate photoexcited electron and hole dynamics with element specificity. We use surface-sensitive femtosecond XUV reflection spectroscopy to separately measure photoexcited electron, hole, and band gap dynamics of ZnTe, a promising photocathode for CO 2 reduction. We develop an ab initio theoretical framework based on density functional theory and the Bethe-Salpeter equation to robustly assign the complex transient XUV spectra to the material's electronic states. Applying this framework, we identify the relaxation pathways and quantify their time scales in photoexcited ZnTe, including subpicosecond hot electron and hole thermalization, surface carrier diffusion, ultrafast band gap renormalization, and evidence of acoustic phonon oscillations.

14 SOLAR ENERGY↗
DOE OSTI2024

Surface Composition Impacts Selectivity of ZnTe Photocathodes in Photoelectrochemical CO 2 Reduction Reaction

Light-driven reduction of CO 2 into chemicals using a photoelectrochemical (PEC) approach is considered as a promising way to meet the carbon neutral target. The very top surface of the photoelectrode and semiconductor/electrolyte interface plays a pivotal role in defining the performance for PEC CO 2 reduction. However, such impact remains poorly understood. Here, we report an electrodeposition-annealing route for tailoring surface composition of ZnTe photocathodes. Our work demonstrates that a Zn-rich surface on the ZnTe photocathode is essential to impact the CO 2 reduction activity and selectivity. In particular, the Zn-rich surface not only facilitated the interfacial charge carrier transfer, but also acted as electrocatalyst for boosting carbon product selectivity and suppressing the hydrogen evolution reaction. This work provides a new avenue to optimize the photocathode, as well as improvement of the CO 2 RR performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1989

CdTe and ZnTe metal interface formation and Fermi-level pinning

Interfacial morphology and Fermi-level pinning behavior at the interfaces of Al, Ag, and Pt with UHV-cleaved CdTe and ZnTe are studied using X-ray photoelectron and ultraviolet photoemission spectroscopies. Results are compared to metal/HgCdTe interface formation. For Al/CdTe, a case is found where significantly greater intermixing occurs in CdTe than seen on HgCdTe. The Al/ZnTe interface is also more abrupt than Al/CdTe. Band bending results for interfaces of all three metals with p-CdTe and p-ZnTe are presented and implications for metal/HgZnTe interface formation are considered.

Wahi, A. K.↗
DOE OSTI2020

Materials Data on ZnTe by Materials Project

ZnTe is Zincblende, Sphalerite structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. Zn2+ is bonded to four equivalent Te2- atoms to form corner-sharing ZnTe4 tetrahedra. All Zn–Te bond lengths are 2.68 Å. Te2- is bonded to four equivalent Zn2+ atoms to form corner-sharing TeZn4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ZnTe by Materials Project

ZnTe is Millerite-like structured and crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. Zn2+ is bonded to five equivalent Te2- atoms to form a mixture of distorted edge and corner-sharing ZnTe5 trigonal bipyramids. There are a spread of Zn–Te bond distances ranging from 2.72–2.89 Å. Te2- is bonded in a distorted pentagonal planar geometry to five equivalent Zn2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ZnTe by Materials Project

ZnTe is Moissanite-4H-like structured and crystallizes in the trigonal P3_1 space group. The structure is three-dimensional. Zn2+ is bonded to four equivalent Te2- atoms to form corner-sharing ZnTe4 tetrahedra. There are a spread of Zn–Te bond distances ranging from 2.67–2.69 Å. Te2- is bonded to four equivalent Zn2+ atoms to form corner-sharing TeZn4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ZnTe by Materials Project

ZnTe is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Zn2+ is bonded to six equivalent Te2- atoms to form a mixture of corner and edge-sharing ZnTe6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Zn–Te bond lengths are 2.89 Å. Te2- is bonded to six equivalent Zn2+ atoms to form a mixture of corner and edge-sharing TeZn6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ZnTe by Materials Project

ZnTe is Wurtzite structured and crystallizes in the hexagonal P6_3mc space group. The structure is three-dimensional. Zn2+ is bonded to four equivalent Te2- atoms to form corner-sharing ZnTe4 tetrahedra. There are three shorter (2.68 Å) and one longer (2.69 Å) Zn–Te bond lengths. Te2- is bonded to four equivalent Zn2+ atoms to form corner-sharing TeZn4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ZnTe by Materials Project

ZnTe crystallizes in the hexagonal P6_422 space group. The structure is three-dimensional. Zn2+ is bonded in a distorted rectangular see-saw-like geometry to four equivalent Te2- atoms. All Zn–Te bond lengths are 2.72 Å. Te2- is bonded in a distorted rectangular see-saw-like geometry to four equivalent Zn2+ atoms.

36 MATERIALS SCIENCE↗
NASA NTRS1968

GaSb-ZnTe heterojunction.

GaSb-ZnTe heterojunction fabrication method, noting photoresponse, fluorescence and light transmission

Fischler, S.↗