Excess Tellurium Research Articles

Modified Bridgman assisted ZnTe crystal growth for THz device applications

Zinc Telluride (ZnTe) single crystals have enormous potential for generating and detecting broadband terahertz (THz) radiation, making them suitable for a wide range of applications in communication, defence security and biomedical applications. The quality and homogeneities of the crystal plays an important role in obtaining THz response. This paper elaborates on the steps involved in the crystal growth of ZnTe and the characterization technique utilized for the determination of quality of the grown crystal, identifications of defects and qualifications of THz signals. Single crystal was grown by the Bridgman technique in vacuum sealed carbon-coated quartz ampoule with excess tellurium (Zn:Te/3:7). To obtain maximum THz response, crystals were oriented along (110) direction using a Laue diffraction pattern and sliced accordingly. Three different crystals along the length (bottom-middle-top) of size ∼ 10 mm × 10 mm are taken for analysis. The crystallinity of ZnTe was confirmed with X-ray diffraction pattern and lattice parameters were obtained using Rietveld refinement. Raman analysis was carried out at three different regions of the grown crystal which revealed the presence of same high-intensity mode at 413 cm−1 in all the regions confirming the quality of the crystal. The EDS compositional analysis confirms the stoichiometry along the grown crystal length. Optical bandgap of 2.216 eV was measured by UV-Vis-NIR spectra and >55 % transmission was obtained across the grown crystal length. The size and distribution of Te inclusions were measured using IR microscopy. The crystals were subjected to terahertz investigation in the range of 0.1–4 THz using a time domain spectrometer. All the crystals exhibited negligible absorption coefficients in the 0.1–3.75 THz range, confirming the maximum transmission in the THz domain. The crystals also exhibit excellent electro-optic detection which is confirmed by signal-to-noise ratio in the measured range.

Understanding palladium-tellurium cluster formation on WTe2: From a kinetically hindered distribution to thermodynamically controlled monodispersity.

A fundamental understanding of the transition metal dichalcogenide (TMDC)-metal interface is critical for their utilization in a broad range of applications. We investigate how the deposition of palladium (Pd), as a model metal, on WTe2(001), leads to the assembly of Pd into clusters and nanoparticles. Using X-ray photoemission spectroscopy, scanning tunneling microscopy imaging, and ab initio simulations, we find that Pd nucleation is driven by the interaction with and the availability of mobile excess tellurium (Te) leading to the formation of Pd-Te clusters at room temperature. Surprisingly, the nucleation of Pd-Te clusters is not affected by intrinsic surface defects, even at elevated temperatures. Upon annealing, the Pd-Te nanoclusters adopt an identical nanostructure and are stable up to ∼523 K. Density functional theory calculations provide a foundation for our understanding of the mobility of Pd and Te atoms, preferential nucleation of Pd-Te clusters, and the origin of their annealing-induced monodispersity. These results highlight the role the excess chalcogenide atoms may play in the metal deposition process. More broadly, the discoveries of synthetic pathways yielding thermally robust monodispersed nanostructures on TMDCs are critical to the manufacturing of novel quantum and microelectronics devices and catalytically active nano-alloy centers.

Cadmium telluride for high-efficiency solar cells

Problems of the synthesis of cadmium telluride powders having required purity and grain size distribution for high-efficiency solar cells have been analyzed. A test batch of powders has been synthesized and used for the manufacture and study of thin-film solar cell specimens exhibiting parameters compliant with the best worldwide standards. The phase composition of the powders has been studied using X-ray diffraction. Structural analysis and elemental composition measurements have been carried out using electron microscopy. The effect of free tellurium phase in the powders on the endurance of devices manufactured from the powder has been described. We show that excess tellurium in the film specimens whose atoms are predominantly localized along grain boundaries may cause temporal degradation of the electrical properties of the manufactured solar cells due to changes in the parameters of the crystalline structure of the cadmium telluride phase which are caused in turn by changes in the stoichiometric composition of the material. Structural studies of the film specimens have not revealed differences in the film structure before and after endurance tests. A new cadmium telluride powder process route has been developed, proven and tested taking into account the advantages and drawbacks of the previously used process and experiments confirming the correctness of the technical solutions chosen have been conducted.

Thermodynamic constraints and substrate influences on the growth of high-quality Bi2Te3 thin films by pulsed laser deposition

We investigated the growth of Bi2Te3 thin films on (111) CaF2 and (111) BaF2 substrates by pulsed laser deposition. Stoichiometric Bi2Te3 thin films were grown using targets with excess tellurium. We adopted a combinatorial temperature variation principle, where the growth temperature is monotonically varied during the growth. This growth method is supportive to effectively minimize Te loss in the Bi2Te3 films. We found large differences in growth conditions between Bi2Te3 thin films on the CaF2 and BaF2 substrates. The lattice matched (111) BaF2 substrate is preferred to grow Bi2Te3 for the further development of topological electronics.

Development of a method for producing effective CdS/CdTe/Cu/Au solar elements on a flexible substrate designed for backup supplying systems prevention of emergency situations

The technology of forming film solar cells based on CdS / CdTe configuration of the "superstrat" type on a flexible substrate has been improved. To increase the efficiency of the developed solar cells on a flexible substrate, a chemical etching procedure in a nitrogen-phosphorus mixture was added to the traditional "chemical treatment". The conducted studies of the output parameters of the developed device structures showed that the highest values are observed in the case of chemical etching, both before the "chloride treatment" and after it. In the course of the study, it was found that a mandatory procedure in the formation of effective device structures is chemical etching in a nitrogen-phosphorus mixture both before the "chloride treatment" and after it. Carrying out the described procedures made it possible to obtain solar cells on a flexible substrate with an efficiency of 13.1 %. The increase in the efficiency of solar cells with two-stage chemical etching can be explained by the formation of excess tellurium on the surface, which leads to a decrease in resistance and, therefore, to a more efficient penetration of chlorine during the subsequent chloride treatment. Analysis of the transverse cleavage of the investigated device structures demonstrates significant grain growth and surface smoothness of the base layer, which ensures good adhesion with back contact. A study of the degradation resistance of the developed device structures during operation has been carried out. It was found that the obtained solar cells based on CdTe on a flexible substrate have a high degradation resistance and after 10 bending cycles there is no decrease in the output parameters. Thus, it has been established that chemical etching in a nitrogen-phosphorus mixture is a mandatory procedure for the formation of efficient solar cells on a flexible substrate.

Compositional Fluctuations Mediated by Excess Tellurium in Bismuth Antimony Telluride Nanocomposites Yield High Thermoelectric Performance

Compositional Fluctuations Mediated by Excess Tellurium in Bismuth Antimony Telluride Nanocomposites Yield High Thermoelectric Performance

Thermoelectric properties of BiSbTe-type alloys prepared by chill-casting and cryo-milling

The thermoelectric properties of BixSb1-xTe3 processed through milling have been extensively studied with varied results that needs to be rationalized. Since cryo-milling is a relatively cleaner milling process compared to other milling processes, we explore this technique to reduce the particle size prior to compaction to get an insight into the thermoelectric properties. We first report the results of cryomilled BiSbTe3 compacted by spark plasmas sintering (SPS) process and show that milling leads to a change in transport behavior from ‘p’ to ‘n’ type similar to that reported for Bi2Te3. It is shown that the defects induced by milling is responsible for this change in materials behavior. We further study the effect of change in Bi to Sb ratio and excess tellurium on the thermoelectric properties when processed through cryo-milling as well as casting using a chilled copper block. Excess tellurium leads to a microstructure containing a fraction of layered eutectic that gets further fragmented during cryo-milling. Although the Seebeck coefficient does not change significantly, reflecting the cleanliness of the process, the thermal conductivity changes during hot compaction leading to an improvement of ZT. However, a comparison of all the reported results does not reveal any systematics in thermoelectric properties reflecting the difficulties with these materials.

High power factor due to multi-scale engineering in ultra-thin bismuth telluride films

High thermoelectric (TE) power factors were obtained for bismuth telluride by deploying confinement and multi-scale engineering in synergy. The thickness of the film was kept in the ultra-thin range (41 nm) following which a high magnitude of 1.9 × 104 S m−1 was obtained at room temperature (RT). Films were deposited at an elevated substrate temperature to enhance the grain quality and high mobility bearing (00l) grain growth. Thus, relatively large crystallite sizes (∼26 nm) with less grain boundaries and directional growth with a low defect profile were the prime reasons for highly enhanced electrical conductivity. Apart from the multiple effects that were deployed, ultra-thin dimensions of the films proved to be effective in further enhancing Seebeck coefficient values. The co-alloyed In minimized the hole concentration through reducing antisite defects and also preserved the reduced bipolar effect at elevated temperatures. The inclusion of excess tellurium induced Te segregates in the film that helped in energy dependent scattering of carriers in addition to its donor-like effect. Hot carrier filtering, induced by excess Te along with ultra-thin dimensions resulted in a Seebeck coefficient (S) of −223.6 μV K−1 at RT. A soaring value of −338.1 μV K−1 was obtained at 90 °C. Following the synergetic employment of multiple enhancement strategies, a high power factor of 959.9 μW m−1 K−2 was obtained at room temperature with a towering magnitude of 2537.7 μW m−1 K−2 at 90 °C.

Role of excess tellurium on the electrical and thermal properties in Te-doped paracostibite

The effect of excess Te in the preparation of synthetic mineral paracostibite, CoSbS, was investigated in an attempt to produce a nano–micro-porous architecture.

Hexabromobenzene-Doped Bismuth Chalcogenide Solid Solutions Prepared through Melt Solidification in a Liquid

n-Type Bi2Te3–Bi2Se3 solid solutions containing 6, 8, and 10 mol % Bi2Se3 and doped with hexabromobenzene (C6Br6) and excess tellurium have been prepared by extruding granules produced by melt solidification in a liquid. The solid solutions have been characterized by X-ray microanalysis and microstructural analysis using a scanning electron and an optical microscope, and their mechanical and thermoelectric properties have been studied. Their ultimate strength, fracture strain, and elastic coefficient have been evaluated from room-temperature compression test results. The electrical conductivity, Seebeck coefficient, and thermal conductivity of the solid solutions have been measured in the temperature range 100–700 K. The temperature behavior of the Seebeck coefficient and electrical conductivity in the temperature range 150–300 K has been shown to differ from that predicted theoretically. The highest thermoelectric figure of merit of the solid solutions, (ZT)max, is ~0.9 at temperatures from 320 to 390 K.

Reduction of the Cd0.9Zn0.1Te surface conductivity by chemical treatment

The influence of Cd0.9Zn0.1Te crystals pretreatment on the effectiveness of the passivation was studied. The optimal conditions of passivation process are determined. Samples of Cd0.9Zn0.1Te were treated with Br2-DMF solution before the investigation. After treatment the surface of the sample was smooth without visible irregularities, which indicates the good polishing properties of this etching solution and the possibility of using the samples for further study. Passivation of samples surface was carried by the NH4F-H2O2 solution. The qualitative and quantitative composition of the Cd0.9Zn0.1Te sample surface was investigated by means of energy-dispersive X-ray spectroscopy (EDX). The formation of a uniform oxide surface layer was confirmed. The electrical characteristics of the passivated samples were investigated by measurent the current-voltage dependences. The expediency of applying of the Cd0.9Zn0.1Te sample treatment in 50% KOH solution after their chemical treatment in the NH4F-H2O2 solution was proved. It was shown that after etching, excess tellurium appears on the surface of the specimens and causes increasing surface conductivity. Measurement of current-voltage dependencies for pre-treated specimens with and without KOH solution showed that the application of this additional chemical treatment step leads to a significant increase in the surface resistance of the specimen. Visual changes in the surface are also observed. It was found that the optimal conditions for Cd0.9Zn0.1Te crystals surface passivation are chemical treatment with NH4F-H2O2 solution for 2 min at a temperature of 18 °C. The current-voltage characteristics of the sample Cd0.9Zn0.1Te were measured at different stages of surface treatment: after polishing, after etching and after passivation. It was done for compares the overall effect of the semiconductor's surface quality on their electrical properties.

Mechanical and Thermoelectric Properties of Bismuth Chalcogenide Solid Solutions Prepared through Melt Solidification in a Liquid

We have studied the mechanical and thermoelectric properties of samples of n-type Bi2Te3–Bi2Se3 solid solutions containing 8 and 20 mol % Bi2Se3; doped with excess tellurium, cadmium chloride, and antimony iodide; and prepared by hot-pressing and extruding granules produced through melt solidification in a liquid. The materials have been tested in compression at 300, 423, 523, and 623 K and in bending at 300 K. Their electrical conductivity, Seebeck coefficient, and thermal conductivity have been measured in the temperature range 100–700 K. We have obtained materials with a compressive strength of ~260 MPa and maximum thermoelectric figure of merit ZT ~ 0.9 in the range 320–390 K.

Enhanced thermoelectric performance of solution-derived bismuth telluride based nanocomposites via liquid-phase Sintering

Abstract Bismuth telluride based thermoelectric materials show great promise in electricity generation from waste heat and solid-state refrigeration, but improving their conversion efficiency with economical approaches for widespread use remains a challenge. An economical facile bottom-up approach has been developed to obtain nanostructured powders, which are used to build bulk thermoelectric materials. Using excess tellurium as sacrificial additive to enable liquid-phase sintering in the spark plasma sintering process, the lattice and bipolar contributions to the thermal conductivity are both greatly reduced without compromising too much the power factor, which leads to the achievement of high figure of merit (ZT) in both n-type and p-type bismuth telluride based nanocomposites. The ZT values are 1.59±0.16 for p-type Bi 0.5 Sb 1.5 Te 3 and 0.98±0.07 for n-type Bi 2 Te 2.7 Se 0.3 at 370 K, which are significantly high for bottom-up approaches. These results demonstrate that solution-chemistry approaches as facile, scalable and low-energy-intensive ways to achieve nanopowders, combined with liquid-phase sintering process, can open up great possibilities in developing high-performance low-price thermoelectric bulk nanocomposites.

Synthesis, structure, and luminescence properties of Cd1–x Mg x Te (0 < x < 0.20) solid solutions

We have synthesized CdTe–MgTe samples in the composition range 0–30 mol % MgTe. The composition dependence of the unit-cell parameter of the Cd1–xMxTe solid solutions has been obtained for the first time. The lattice parameter of cadmium telluride (sphalerite structure) has been shown to decrease almost linearly in the range 0 ≤ x ≤ 0.20. The 78-K cathodoluminescence spectra of the Cd1–xMgxTe (0 < x ≤ 0.15) solid solutions heat-treated under vacuum in order to remove the excess tellurium contain strong lines in the edge emission region, due to a transition from the conduction band to a shallow acceptor level.

Application of abnormally high sputtering rate of PbTe(Te) single crystals during inductively coupled argon plasma treatment for fabrication of nanostructures

This paper reports the investigations of the sputtering process of (111) oriented single crystals of PbTe with excess tellurium (4 at.%) in RF high-density low-pressure inductively coupled argon plasma. An increase of 1.6 times the sputtering rate of lead telluride in comparison with the classical case of single-crystal state with a slight deviation from stoichiometry is shown and the explanation of the results is carried out based on the analysis of the crystal point defects. The active sputtering is used in a new approach to form lead telluride nanostructures on oxidized Si substrates via the vapour–liquid–solid (VLS) redeposition mechanism, and the fabrication of PbTe nanocones, nanocubes and nanowires with various geometrical parameters is demonstrated.

Self-formation of lead telluride nanostructures during argon plasma etching of single-crystal wafers

In this work we report a phenomenon of PbTe nano- and microstructure selfformation that takes place during dry etching of lead telluride (111)-oriented single-crystal wafers with 4 at.% excess tellurium in RF high-density low-pressure inductively coupled argon plasma. Using scanning electron microscopy and energy dispersive X-ray spectroscopy investigations it is established that during the plasma treatment high sputtering rates are accompanied by active redeposition processes on crystal surface and on the inner side of the Si etching mask resulting in a nanostructure arrays self-formation via vapour-liquid-solid mechanism.

Raman and interband optical spectra of epitaxial layers of the topological insulators Bi2Te3 and Bi2Se3 on BaF2 substrates

We report results of Raman and ellipsometric spectroscopy of the topological insulators Bi2Te3 and Bi2Se3 grown by molecular beam epitaxy on BaF2 (111) substrates. Surfaces and interfaces of the films are probed by Raman scattering from the front and back sides of the samples, which is possible owing to the transparent substrate. Surface modifications induced by intense illumination with exciting laser light have been detected, with excess tellurium at the surface during and after exposure. We also report data for thin epilayers containing a fractional number of unit cells and/or incomplete Bi2Te3 and Bi2Se3 quintuples. We have used spectroellipsometric measurements to obtain response functions and have derived the penetration depth of light in the 1.0–6.5 eV range.

Effect of annealing on the electrical properties of Pb1 − x Mn x Te single crystals with excess tellurium

The electrical conductivity σ, Hall coefficients R, and thermopower α of Pb0.96Mn0.04Te (Te) single crystals annealed at 573 K for 120 h are investigated. It is shown that, in contrast to unannealed samples, the investigated samples exhibit n-type conductivity and metal-type dependences σ(T) in the temperature range 77–300 K. It is suggested that, upon annealing, a portion of the excess tellurium atoms occupy vacancies in the lead sublattice with the formation of new vacancies in the tellurium sublattice of the samples.

Correlations of Bridgman-Grown Cd0.9Zn0.1Te Properties with Different Ampoule Rotation Schemes

A unique ampoule rotation system was developed at the Center for Materials Research at Washington State University for enhancing convection in the cadmium zinc telluride (CZT) melt by applying different ampoule rotation schemes (RS). Experiments were performed with different initial charge material concentrations and rotation parameters (acceleration, speed, rotation time, etc.). The applied speed and acceleration ranged from 30 rpm to 50 rpm and 30 rpm2 to 200 rpm2, respectively. Zinc (Zn) distribution profiles of radial and axial slices from the same regions in the grown ingot were determined by room-temperature photoluminescence mapping. The results demonstrate the effects of ampoule rotation on Zn segregation and growth interface evolution. The most stable interface propagation was obtained when 0.2 atomic percent (at.%) excess tellurium (Te) was used in the initial charge material along with a trapezoidal RS. Uniform radial Zn distribution was achieved using triangular RS, which is because of the interface flatness near the axis. Comparison of secondary phase (SP) generation for different RS and initial excess Te was performed. Closed-container CZT growth was performed using the trapezoidal RS, which resulted in high single-crystal yield with lower-diameter SP near the last-to-freeze region. High-resistivity (on the order of 1010 Ω-cm) crystals were obtained from all the RS. The mobility–lifetime product (μτ)e of electrons for planar detectors was found to be on the order of 3 × 10−3 cm2/V to 5 × 10−3 cm2/V for all the RS with 3.5 at.% excess Te growths.

Thermal conductivity of tellurium-enriched Pb1 − x Mn x Te single crystals

We have studied the effect of excess tellurium atoms (up to 0.5 at %) on the thermal conductivity of single-crystal Pb1 − xMnxTe (x = 0.04) samples annealed at 570K for 120 h. We have evaluated the lattice and electron thermal conductivities of the samples, their additional thermal resistance due to structural defects, and the coefficient in the expression for the effective phonon scattering cross section. We assume that some of the excess Te atoms fill lead vacancies, thereby reducing the effective phonon scattering cross section.