CdTe Alloys Research Articles

First-principles study of the strain effect with half-metallic ferromagnetism in Cd 1−x V x Te alloys: supercell approaches

In this work, we study the effect of vanadium (V) dopantson the structural, electronic, and magnetic properties of the CdTe alloys using density functional theory (DFT). The calculations are performed as a function of the concentration of V in the host CdTe semiconductor using a 2 × 2 × 1 tetragonal supercell and a 2 × 2 × 2 cubic supercell. The lattice constants and bulk modulus of Cd 1−x V x Te alloys are determined. The analysis of the density of states shows that the Cd0.75 V0.25Te alloys in both supercells present a total polarization of 100% at the Fermi level, which confirms a stable half-metallic ferromagnetism phase in these materials, in contrast to the Cd 0.9375 V 0.0625 Te and Cd 0.875 V 0.125 Te alloys, which have a polarizability that does not reach 100%. In addition, the electronic band structures are used to estimate the s(p) − d exchange constants mainly resulting from the p − d hybridization between V- 3d and Te-p. Furthermore, the local magnetic moment of V of its free space charge value decreases, leading to the appearance of a small magnetic moment on the Cd and Te sites. Besides, the magnetic stability at different doping concentrations in the ferromagnetic (FM) and anti-ferromagnetic (AFM) states is examined, where the total energy of the systems is considered as a stability criterion. The robustness of the half-metallicity and the curie temperatures under the strain effect are also investigated. Finally, we rely on the binding energy to evaluate the energetic stability of the vanadium in the intrinsic compound CdTe under external deformations.

The effect of different carbon-based CdTe alloys for efficient photocatalytic glucose electrooxidation

The activated carbon (AC), graphene (G), reduced graphene oxide (rGO), carbon nanotube (CNT) supported CdTe photocatalysts at (50–50) atomic molar are synthesized by the sodium borohydride (SBH) method and characterized by the XRD, Micro-Raman, TEM-EDS, XPS, and TPx (TPR, TPO, and TPD) analyses. The CV, CA, and EIS electrochemical analyses are performed to investigate the catalytic activities of catalysts for photocatalytic glucose electrooxidation. Characterization analyses reveal that their electronic structures and surface properties change when carbon materials are doped with metal. The photocatalytic glucose electrooxidation results indicate that the 0.1 % CdTe(50–50)/CNT catalyst exhibited better photocatalytic activity, stability, and resistance than other catalysts both at dark (1.9 mA/cm 2 ) and under UV illumination (2.57 mA/cm 2 ). Therefore, the CNT-supported CdTe catalyst can be said a promising catalyst for direct glucose fuel cells.

Advancing thermoelectrics by vacancy engineering and band manipulation in Sb-doped SnTe–CdTe alloys

A decrease in valence band energy offset can considerably improve the thermoelectric performance of SnTe, and alloying CdTe in SnTe has been confirmed to be efficient for inducing band convergence. However, the low solubility of CdTe in SnTe severely limits the further decrease in the energy offset and the reduction of lattice thermal conductivity. Inspired by the high solubility of Sb in SnTe-based thermoelectric materials, the trivalent Sb is introduced into SnTe–CdTe alloys, aiming at manipulating the thermoelectric transport properties. Combined with the valence band model, it is demonstrated that high concentration of Sb in SnTe–CdTe enables a further optimization in valence band structures, resulting in an improvement in density-of-state effective mass, thus significantly reinforces the power factor in the whole temperature range. Meanwhile, we propose the solid solution mode of Sb in SnTe, which always generates vacancies to balance the valence state, and the introduction of vacancies explains the reduced lattice parameters and almost constant carrier concentration. Particularly, the Debye–Callaway model quantitatively compares the contribution of Sb substitutional defects and vacancy defects. As a result, an enhanced zT of ∼1.1 has been achieved for Sn0.83Cd0.05Sb0.12Te at 823 K. This work clearly shows the critical role of Sb for enhancing the thermoelectric performance of SnTe–CdTe materials.

Manipulation of Solubility and Interstitial Defects for Improving Thermoelectric SnTe Alloys

Much effort has recently been directed at advancing thermoelectric SnTe as an alternative to PbTe. Effective methods include energy band convergence, nanostructures, and substitutional and interstitial defects. Among these strategies, it is shown that CdTe alloying effectively reduces the energy offset between valence bands for an enhanced thermoelectric figure of merit, zT. However, its solubility of only ∼6% might limit this effect on a full zT optimization in SnTe–CdTe alloys. Here, we show an effective approach for increasing CdTe solubility up to ∼20%, with the help of the existence of 5% GeTe. This leads the valence bands to be further converged for improving electronic performance. In addition, Cu2Te alloying is used to create interstitial defects for reducing the lattice thermal conductivity. Eventually, the increase of CdTe solubility and the creation of Cu interstitials enable a significantly enhanced zT, indicating the importance of solubility manipulation for engineering the band structure in ...

CdCl2 passivation of polycrystalline CdMgTe and CdZnTe absorbers for tandem photovoltaic cells

As single-junction silicon solar cells approach their theoretical limits, tandems provide the primary path to higher efficiencies. CdTe alloys can be tuned with magnesium (CdMgTe) or zinc (CdZnTe) for ideal tandem pairing with silicon. A II-VI/Si tandem holds the greatest promise for inexpensive, high-efficiency top cells that can be quickly deployed in the market using existing polycrystalline CdTe manufacturing lines combined with mature silicon production lines. Currently, all high efficiency polycrystalline CdTe cells require a chloride-based passivation process to passivate grain boundaries and bulk defects. This research examines the rich chemistry and physics that has historically limited performance when extending Cl treatments to polycrystalline 1.7-eV CdMgTe and CdZnTe absorbers. A combination of transmittance, quantum efficiency, photoluminescence, transmission electron microscopy, and energy-dispersive X-ray spectroscopy clearly reveals that during passivation, Mg segregates and out-diffuses, initially at the grain boundaries but eventually throughout the bulk. CdZnTe exhibits similar Zn segregation behavior; however, the onset and progression is localized to the back of the device. After passivation, CdMgTe and CdZnTe can render a layer that is reduced to predominantly CdTe electro-optical behavior. Contact instabilities caused by inter-diffusion between the layers create additional complications. The results outline critical issues and paths for these materials to be successfully implemented in Si-based tandems and other applications.

CdTe1−xSx (x ⩽ 0.05) thin films synthesized by aqueous solution deposition and annealing

While CdS thin films are commonly deposited from aqueous solutions, CdTe thin films are extremely difficult to deposit directly from aqueous solution. In this work, we report on polycrystalline CdTe1−xSx thin films synthesized via deposition from aqueous precursor solutions followed by annealing treatments and on their physical properties. The deposition method uses spin-coating of alternating Cd2+ and Te2− aqueous solutions and rinse steps to allow formation of the films but to shear off excess reactants and poorly-bonded solids. Films are then annealed in the presence of CdCl2 as is commonly done for CdTe photovoltaic absorber layers deposited by any means. Scanning electron microscopy (SEM) reveals low void fractions and grain sizes up to 4 µm and x-ray diffraction (XRD) shows that the films are primarily cubic CdTe1−xSx (x ⩽ 0.05) with random crystallographic orientation. Optical transmission yields bandgap absorption consistent with a CdTe1−xSx dilute alloy and low-temperature photoluminescence (PL) consists of an emission band centered at 1.35 eV consistent with donor–acceptor pair (DAP) transitions in CdTe1−xSx. Together, the crystalline quality and PL yield from films produced by this method represent an important step towards electroless, ligand-free solution processed CdTe and related alloy thin films suitable for optoelectronic device applications such as thin film heterojunction or nanodipole-based photovoltaics.

Band Gap Optimization of CdTeSe Thin-Film Solar Cells

Device modeling and simulation studies of a CdTeSe thin film solar cell have been carried out. A variety of band-gap profiles, including ungraded, front graded, back graded, and double graded profiles of the CdTeSe absorber layer are examined and their performance characteristics have been analyzed. The calculation reveals that single junction cells with band-gap at the optimum value of 1.38 eV exhibit the maximum performance; alloys of CdTe and CdSe with a ratio of 1:1 forming CdTe0.5Se0.5 achieve the band-gap of 1.38 eV due to the bowing effect. The benefits of the band-gap grading are evaluated when the minimum band-gap is set at the optimum band-gap of 1.38 eV. It is shown that only few graded band-gap profiles exhibit an increase in efficiency, while most of graded profiles reduce performances.

Formation and Stepwise Self-Assembly of Cadmium Chalcogenide Nanocrystals to Colloidal Supra-Quantum Dots and the Superlattices

Nearly monodisperse colloidal superstructures of cadmium chalcogenide quantum dots (QDs) are reported. The superstructures, which we named as supra quantum dot (SQD), are typically composed of hundreds of a-few-nanometer-sized QDs three-dimensionally (3D) assembled by oriented attachment. The synthesis route for SQD is quite universal and can be extended to CdS, CdSe, CdTe, and CdSeTe alloy. The size of SQD can be tuned from tens of nanometers to over a hundred nanometers. In the case of CdSe SQD, zinc-blende seeds (primary QDs) act as the building block for the formation of the 3D assembled structures, SQDs, with discrete intermediates nanostructures. Primary seeds, 4 nm tetrahedral shaped QDs, assembled into a large tetrahedron of 20 nm. The 20 nm tetrahedrons, in turn, self-assembled into a larger tetrahedron of 40 nm. The discrete-in-size and sequential assemblies were followed by conventional growth from the remaining precursors and ripening within the particles to result in spheroidal SQDs. SQDs all...

Polarization Dependent Reflectivity and Transmission for Cd1-Xznxte/GaAs(001) Epifilms in the Far-Infrared and Near-Infrared to Ultraviolet Region

The results of a comprehensive experimental and theoretical study is reported to empathize the optical properties of binary GaAs, ZnTe, CdTe and ternary Cd1-xZnxTe (CZT) alloys in the two energy regions: (i) far-infrared (FIR), and (ii) near-infrared (NIR) to ultraviolet (UV). A high resolution Fourier transform infrared spectrometer is used to assess the FIR response of GaAs, ZnTe, CdTe and CZT alloys in the entire composition 1.0 ≥ x ≥ 0 range. Accurate model dielectric functions are established appositely to extort the optical constants of the binary materials. The simulated dielectric functions ei€¥(ω) and refractive indices n~(ω) are meticulously appraised in the FIR → NIR → UV energy range by comparing them against the existing spectroscopic FTIR and ellipsometry data. These outcomes are expended eloquently for evaluating the polarization dependent reflectivity R(λ) and transmission T(λ) spectra of ultrathin CZT/GaAs (001) epifilms. A reasonably accurate assessment of the CZT film thickness by reflectivity study has offered a credible testimony for characterizing any semiconducting epitaxially grown nanostructured materials of technological importance.

Enhanced thermopower in rock-salt SnTe–CdTe from band convergence

The rock-salt type SnTe–CdTe alloys have been synthesized by the zone-melting method and show enhanced thermoelectric performance due to the improved band convergence.

Study of half-metallic ferromagnetism in V-doped CdTe alloys by using first-principles calculations

For investigating the structural, electronic and magnetic properties of zincblende (ZB) Cd1−xVxTe (0≤x≤1), we have employed the Wu–Cohen generalized gradient approximation (WC-GGA) and the modified Becke and Johnson local density approximation (mBJLDA) functionals within the frame-work of spin polarized density functional theory (DFT). The former exchange-correlation parameterization scheme has been used for optimizing the equilibrium structural properties, while the electronic band structures, electron density of states (DOS) and charge densities are computed using both functionals and their performances are compared. Our results show that the electron spin polarization in unfilled V-3d orbitals gives rise to spin exchange splittings (Δx(d) and Δx(pd)) which is the cause of half metallic (HM) ferromagnetism in the V-doped CdTe as revealed by the computed DOS. Consequently, the nature of effective potential is more attractive in spin-down case rather than that in spin-up case. The total magnetic moment for each of the compounds under study is 3μB where the main contribution comes from the V atom, while the nonmagnetic sites Cd and Te are also bestowed with minor atomic magnetic moments due to Te-5p–V-3d hybridization. Furthermore, we calculated the exchange constants N0α and N0β to determine the conduction and valence band contributions in exchange splitting process. A comparison of the two functionals, considered in this work, shows that the mBJLDA provides a better description of the electronic structure especially of the ferromagnetic Cd1−xVxTe (0.75≤x≤1). Moreover, as compared to WC-GGA, the mBJLDA predicts high Curie temperatures in V-doped CdTe by providing significantly larger values of Δx(d), Δx(pd), N0α, N0β and, importantly, a quite wide HM gap.

Structural, Optical, and Magnetic Properties of Co Doped CdTe Alloy Powders Prepared by Solid-State Reaction Method

Co doped CdTe powder samples were prepared by solid-state reaction method. In the present work effect of Co doping on structural, optical, and magnetic properties has been studied. X-ray diffraction studies confirm zinc blend structure for all the samples. The lattice parameter showed linear increase with the increase in Co content. The elemental constituents were characterized by EDAX. Optical studies showed the increase in band gap with increase in Co level. The samples were diluted magnetic semiconductors and exhibited clear hysteresis loop showing room temperature ferromagnetism as confirmed by vibrating sample magnetometer.

Accuracy of existing atomic potentials for the CdTe semiconductor compound

CdTe and CdTe-based Cd(1-x)Zn(x)Te (CZT) alloys are important semiconductor compounds that are used in a variety of technologies including solar cells, radiation detectors, and medical imaging devices. Performance of such systems, however, is limited due to the propensity of nano- and micro-scale defects that form during crystal growth and manufacturing processes. Molecular dynamics simulations offer an effective approach to study the formation and interaction of atomic scale defects in these crystals, and provide insight on how to minimize their concentrations. The success of such a modeling effort relies on the accuracy and transferability of the underlying interatomic potential used in simulations. Such a potential must not only predict a correct trend of structures and energies of a variety of elemental and compound lattices, defects, and surfaces but also capture correct melting behavior and should be capable of simulating crystalline growth during vapor deposition as these processes sample a variety of local configurations. In this paper, we perform a detailed evaluation of the performance of two literature potentials for CdTe, one having the Stillinger-Weber form and the other possessing the Tersoff form. We examine simulations of structures and the corresponding energies of a variety of elemental and compound lattices, defects, and surfaces compared to those obtained from ab initio calculations and experiments. We also perform melting temperature calculations and vapor deposition simulations. Our calculations show that the Stillinger-Weber parameterization produces the correct lowest energy structure. This potential, however, is not sufficiently transferrable for defect studies. Origins of the problems of these potentials are discussed and insights leading to the development of a more transferrable potential suitable for molecular dynamics simulations of defects in CdTe crystals are provided.

Cation-site intrinsic defects in Zn-doped CdTe

The properties of the cation vacancy and the Te antisite, two dominant defects in CdTe and Cd1-xZnxTe alloys grown in Te-rich conditions, are examined using first-principles calculations. First, the structure, electronic levels, and migration paths of V-Cd and Te-Cd in CdTe are studied in detail. Additionally, we analyze the evolution of the stability and electronic properties in Cd1-xZnxTe alloys, taking into account both the role of alloying in the position of the ionization levels and its effects on the equilibrium concentration of those two defects. It is shown that the formation of cation vacancies becomes progressively more favorable as x increases, whereas Te antisites become less stable, backing the trend towards p-type conductivity in dilute Cd1-xZnxTe.

Intrinsic defects in CdTe and CdZnTe alloys

The Cd vacancy (VCd) and Te anti-site (TeCd) are two dominant defects in CdTe and CdZnTe alloys grown in Te-rich conditions. We examine the properties of these two intrinsic defects in Cd1-xZnxTe alloys with x<0.5 using first-principles calculations. It is shown that Cd vacancies become progressively more favourable with increasing Zn content, in contrast with Te anti-sites, which show the opposite behaviour, explaining the trend towards p-type conductivity in Cd1-xZnxTe.

Study of alloy disorder in (Hg,Cd)Te with the use of infrared photoluminescence

Infrared photoluminescence (PL) measurements were used to probe compositional disorder in semiconductor (Hg,Cd)Te alloy. PL was studied in Hg 1 −x Cd x Te samples with x =0.38 and 0.57, fabricated by molecular beam epitaxy on GaAs substrates. Both as-grown and annealed in Hg vapor and in He atmosphere samples were studied. In all the samples, the PL peak energy was found to be red-shifted from the position of band-to-band recombination maximum expected from the energy gap of the material. The value of the shift depended on temperature. This effect was attributed to recombination of excitons localized at compositional fluctuations, and an analysis was carried out that provided a way for evaluation of a fluctuation measure. The amplitude of the fluctuations was shown to strongly decrease as a result of post-growth annealing.

Optical absorption and reflectivity spectra of highly chromium doped CdTe alloy and layer

Abstract In the present work, a CdTe alloy doped with a relatively high concentration of chromium (1%), and a CdTe:Cr layer, have been studied. Absorption and reflectivity spectra were measured at room temperature. They indicate the presence of chromium in the divalent state, both in the alloy and in the layer.

Measurement of optical and thermal properties of Hg_1−xCd_xTe

Measurements of optical transmission and several thermal properties of Hg(1-x)Cd(x)Te alloys are reported for a few values of the alloy composition parameter x, which was determined by a microprobe technique. The values of the thermal diffusivity, specific heat, and thermal conductivity were measured using the laser-flash method. These results are reported at four discrete temperatures between 90 and 400 K and compared to those of three well-characterized semiconductor materials: Si, InAs, and InSb.

PDF Analysis on Semiconductive CdTe-ZnTe Alloy

High-energy synchrotron X-ray measurements (HEXRD) and pair-distribution function (PDF) analysis were carried out to investigate the local structure in ferroelectric Cd 1−x Zn x Te alloy. The Cd 1−x Zn x Te crystallizes in the zinc-blende structure and the PDF analysis revealed that the zinc-blende lattice of the Cd 1−x Zn x Te alloy distorts locally to accommodate the distinct Cd-Te and Zn-Te alloy bond lengths. The local disorder correlate in the ⟨111⟩ direction and it is the origin of macroscopic ferroelectricity in the Cd1−x Zn x Te alloy.

Sputtered CdZnTe films for top junctions in tandem solar cells

Cd1−xZnxTe alloy films with 1.6 and 1.7 eV band gaps were deposited by RF magnetron sputtering from targets made either of mixed powders or alloys of CdTe and ZnTe (25% and 40%). High-quality polycrystalline films with the (1 1 1) preferred orientation were obtained. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, resistivity, optical absorption, Raman, and photoluminescence. The EDS, XRD, and optical absorption analysis indicated that the x-value of the as-grown films were typically ∼0.20 and 0.30 for films sputtered from 25% and 40% ZnTe containing targets, respectively. The as-deposited alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. Therefore, we have studied various post-deposition treatments with vapors of chlorine-containing materials, CdCl2 and ZnCl2, in dry air or H2/Ar ambient at ∼390 °C. The best performance of a Cd1−xZnxTe cell (VOC=737mV, JSC=19mA/cm2) was found for treatment with vapors of the mixed CdCl2+0.5%ZnCl2 in an H2/Ar ambient after pre-annealing at ∼520 °C in pure H2/Ar.