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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↗
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↗
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↗
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↗
DOE OSTI2025

ZnGa 2 Te 4 thin-film absorbers for photoelectrochemical CO 2 reduction

Photoelectrochemical (PEC) carbon dioxide reduction reaction (CO 2 RR) has been considered as a promising route to convert and store solar energy into chemical fuels. It is crucial to find suitable photoelectrode materials that are photo-catalytically active and exhibit excellent photochemical stability. One of the promising contenders is ZnTe with the ∼2.26 eV band gap and prolonged stability under CO 2 RR PEC conditions. Herein, a new telluride based thin-film ZnGa 2 Te 4 photocathode with lower band gap and stronger visible light absorption compared to ZnTe is synthesized and characterized using a combinatorial sputtering technique. A two-step annealing method with excess Te supply is implemented to synthesize nearly stoichiometric ZnGa 2 Te 4 absorber material with a zincblende-derived tetragonal crystal structure confirmed by synchrotron X-ray and electron diffraction. Theoretical calculations show that ZnGa 2 Te 4 has suitable direct bandgap (∼1.86 eV) and high absorption coefficient ∼10 5 cm −1 , in agreement with experimentally prepared films. Transient absorption spectroscopy reveals the biexponential decay dynamics, with time constants, τ 1 ∼ 0.04, and τ 2 ∼ 0.65 μs in microsecond time scales and provides the optical transition pathways for this semiconductor thin film. PEC measurements show that the ZnGa 2 Te 4 photocurrent densities are comparable to the widely investigated ZnTe photocathodes or even surpass it under simulated sunlight condition. ZnGa 2 Te 4 samples demonstrate promising photoelectrochemical stability, maintaining consistent performance under illumination. The inclusion of diaryliodonium additive substantially increases its CO 2 RR selectivity to ∼60%. These findings open a new avenue for the synthesis of telluride-based thin-film photocathodes for further exploration and will motivate future research to integrate this potential photocathode material into PEC devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Distribution of Copper States, Phases, and Defects across the Depth of a Cu-Doped CdTe Solar Cell

Copper has been used as a p-type dopant in cadmium telluride (CdTe) for decades. However, the density of Cu atoms in the finished device is much higher than that of holes, which means that most Cu atoms are not activated as acceptors during incorporation. Furthermore, studies have demonstrated that the distribution of copper (Cu) atoms across the device is highly inhomogeneous, with reports citing Cu substitution on Cd sites and segregation to grain boundaries. Fast diffusion along these boundaries and Cu accumulation at the CdTe/CdS interface have also been observed and validated computationally. These levels of inhomogeneity make it difficult to accurately characterize and correlate the performance with the nature of the Cu atomic species present. To address this challenge, we utilize X-ray microscopy and, specifically, nanoscale fluorescence-mode X-ray absorption near-edge structure to resolve the atomic Cu environment throughout the depth of the CdTe layer. Our results suggest that the majority of Cu atoms are in the form of Cu x Te phases (or similar local environments) near the ZnTe|CdTe interface, Cu x O phases in the CdTe absorber, and present in various oxidation states, including Cu 1+ and Cu 2+ , near the CdS/CdTe junction. Here this work also provides experimental evidence for the first time of the presence of CuS around the ZnTe|CdTe interface and the hypothesized Cu Cd -Cl i complex in the CdTe absorber.

14 SOLAR ENERGY↗
DOE OSTI2020

Two-dimensional forms of robust CO 2 reduction photocatalysts

Photoelectrocatalysts that use sunlight to power the CO 2 reduction reaction will be crucial for carbon-neutral power and energy-efficient industrial processes. Scalable photoelectrocatalysts must satisfy a stringent set of criteria, such as stability under operating conditions, product selectivity, and efficient light absorption. Two-dimensional materials can offer high specific surface area, tunability, and potential for heterostructuring, providing a fresh landscape of candidate catalysts. From a set of promising bulk CO 2 reduction photoelectrocatalysts, we screen for candidate monolayers of these materials, then study their catalytic feasibility and suitability. For stable monolayer candidates, we verify the presence of visible-light band gaps, check that band edges can support CO 2 reduction, determine exciton binding energies, and compute surface reactivity. We find visible light absorption for SiAs, ZnTe, and ZnSe monolayers, and that due to a lack of binding, CO selectivity is possible. We thus identify SiAs, ZnTe, and ZnSe monolayers as targets for further investigation, expanding the chemical space for CO 2 photoreduction candidates.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Low-Temperature Gold Deposition Improves CdTe Back Contacts

Improved back contacts can benefit CdTe photovoltaics (PV). In this work, Cd(Se,Te) PV absorbers are cooled during Au evaporation to thermally quench a chemical reaction occurring between gold and CdTe and the generation of a reaction product that lowers device efficiency. Reducing substrate temperature enhances PV power conversion efficiency via open-circuit voltage and fill factor increases. X-ray photoelectron spectroscopy (XPS) reveals that lower temperature also reduces chemical perturbations of the CdTe, potentially linking back contact formation to a CdTe degradation product that hinders PV performance. Comparing reaction enthalpy and substrate heating energy shows that back contact formation by sputtering elemental metals onto ZnTe may exhibit a degradation pathway analogous to that of CdTe/Au reported here. Degradation-diminishing contact formation processes are therefore of general interest for optoelectronic devices, and the reduced substrate temperature in this study is one example.

14 SOLAR ENERGY↗