Te Precipitation Research Articles

Tailoring the defects and resistivity in CdZnTe single crystal via one-step annealing with CdTe compound

Defects deteriorate the optical and electronic properties of the cadmium zinc telluride (CZT) single crystal. However, the growth of defects, such as vacancies and precipitation, could not be avoided in the industrial-grade preparation of CZT single crystal. Therefore, it is crucial to explore ways to eliminate defects after growth. This paper employs one-step heat treatment with the CdTe compound as an annealing atmosphere source. After annealing, some large-size Te inclusions are eliminated in CZT crystal, but nano Te precipitates still exist. At the same time, the crystal surface becomes rougher, caused by the thermal migration of Te droplets, dislocation proliferation, and aggregation. Increasing annealing time, dislocations migrate, decompose, and interact with each other to form dislocation walls, stacking faults or terminating at the surface, reducing dislocation etch pit density. In addition, a large number of ordered phases are still found in the crystal. Single annealing with CdTe significantly improved the crystal's optical and electrical properties. After 180 h annealing, the infrared (IR) transmittance of the as-grown wafer increased from 40% to 65%, and resistivity increased from 2.2 × 109 Ω cm to 1.7 × 1011 Ω cm. Compared to reducing the Te precipitation phase, these increasements were mainly dominated by replacing point defects, like Cd vacancies.

Segregation-induced Ge precipitation in Ge2Sb2Te5 and N-doped Ge2Sb2Te5 line cells

Electromigration in Ge2Sb2Te5 and N-doped Ge2Sb2Te5 line cell structures has been studied by mapping out electric field/current-induced composition changes using x-ray energy dispersive spectroscopy. Both materials exhibit pronounced segregation in a molten state, with Te moving toward the anode and Ge and Sb toward the cathode. The width of the transition region from a composition of over 90% Te to over 90% Ge–Sb was 500 nm for an electric field of 1.1 × 107 V/m. In the Ge–Sb-rich end of the cell, Ge precipitates out of the melt, forming almost pure Ge inclusions with a size up to 100 nm. The Ge–Sb–Te segregation and precipitation do not appear to be affected by doping with nitrogen.

Impact of atomic vacancy on phase change and structure in GexTe1−x films

Binary compound Ge–Te, which displays intriguing functionalities, has been intensively studied from both fundamental and technological perspectives. In Ge–Te compound, a deviation from the Ge–Te stoichiometry will lead it thermodynamically unstable as well as to the change in the phase change characters. In this study, a series of non-stoichiometric Ge–Te films were prepared and a detailed study on the impact of Ge vacancy was carried out. The phase change characters can be tuned by adjusting the composition. Although the Ge vacancy does not lead to phase separation and Te precipitation in these films, Raman spectroscopy analysis reveals a dramatic change in the bonding environment. The microstructure has been modified by the induced Ge vacancy, especially the threefold Te unit and Ge–Ge bond in the crystalline GeTe.

Pyrite chemistry: A new window into Au-Te ore-forming processes in alkaline epithermal districts, Cripple Creek, Colorado

Tellurium has a wide variety of applications, most importantly in the solar energy industry and is eco-toxicologically significant; however, the magmatic-hydrothermal processes causing the pronounced Te enrichment together with Au in some epithermal districts are still poorly constrained. Hydrothermal and alkaline magmatic activity in post-subduction environments are suggested to be a critical component in the evolution of this Te-rich sub-class of low-sulfidation epithermal deposits. Cripple Creek represents an example for a world-class low-sulfidation epithermal Au-Te anomaly in the continental crust. This area represents a natural laboratory to investigate the processes of ore-formation and Au-Te enrichment in alkaline igneous rock-hosted epithermal systems.Here, we present the first micro-analytical approach that combines petrographic observations with in situ LA-ICP-MS analyses and trace element mapping to define the key ore-forming processes of Au and Te in the Cripple Creek epithermal complex. Two main styles of mineralization can be distinguished: (1) low-grade Au disseminated pyrite-rich ores in the permeable brecciated host rocks and (2) high-grade quartz-fluorite veins rich in calaverite (AuTe2), coloradoite (HgTe), petzite (Ag3AuTe2), altaite (PbTe) and native Au.Pyrite trace element mapping revealed distinct variations and decoupling of elements (Au-Te) and element pairs (Au-As vs. Co-Ni) within and between different sites in the epithermal district. This is reflected by the concentric/growth zoning of these elements in pyrite that were interpreted to be caused by fluid boiling associated with the deposition of the low-grade disseminated host rock ores at temperatures between 220 and 350 °C leading to the precipitation of Au, As, Co and Ni from the liquid phase and the preferential partitioning of Te into the vapor phase. The subsequent condensation of the Te-rich vapors in metal-bearing meteoric waters led to the Te precipitation being decoupled from Au in the low-grade ores.The high-grade Au-Te mineralization was likely initiated by the influx of magmatically derived oxidized fluids. Subsequently, fluid temperatures dropped due to mixing with meteoric waters resulting in fluid boiling under low pressure conditions at shallower crustal levels (<1000 m) compared to the low-grade ores. Elevated fO2 minimized the loss of Te to the vapor phase and the strong boiling conditions at lower fluid temperatures (105–200 °C) caused the contemporaneous precipitation of Au and Te in the high-grade veins.

Selective recovery of Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag by drop-wise H2O2 addition followed by Na2S–Na2SO3 precipitation

Sb and Te in Te-bearing alkaline skimming slag were extracted into a leach solution using a two-stage sodium sulfide leaching process. In this study, a possible method for selective recovery of Sb and Te from the sodium sulfide leach solution by drop-wise H2O2 addition followed by Na2S–Na2SO3 precipitation was developed. 99% of Sb was precipitated selectively at 50 °C for 100 min with addition of 90 mL H2O2 while Te precipitation was near-zero. Te in the oxidized solution was converted to the form of TeS42− with Na2S, and recovered by Na2SO3 reduction. The results indicated that 95% of Te was precipitated under the optimum conditions: 1.0 of Na2S excess coefficient, 1.75 of Na2SO3 excess coefficient, 50 °C of temperature and 30 min of time. XRD pattern and chemical analysis indicated that the H2O2 oxidation precipitate was NaSb(OH)6 with Sb content of 46.92% and the Na2S–Na2SO3 precipitate was elemental Te with a purity of 98.61%.

Thermo-kinetic behaviour of Ge20Te75I5 glass for infrared optics

ABSTRACTThe iodine-doped GeTe4 infrared chalcogenide glass was studied by means of differential scanning calorimetry DSC, X-ray diffraction XRD, Raman spectroscopy and infrared microscopy. Extensive non-isothermal thermo-kinetic characterisation of the glass transition, crystallisation and melting phenomena was performed in dependence on the particle size. The Tool–Narayanaswamy–Moynihan model was applied to describe the enthalpy relaxation processes: the compositional evolution of the relaxation parameters was then explained in terms of the structural changes and movements of the characteristic structural units. Mathematic deconvolution was applied to treat the complex crystallization kinetics – two crystal growth sub-processes were identified and described in terms of the autocatalytic Šesták-Berggren model. Based on the XRD and microscopic analyses the following crystallization mechanisms were revealed: initial precipitation of tellurium (surface-located) followed by a combined surface- and bulk-located formation of GeTe and GeI4 phases. Based on the DSC results obtained for fine powders, the presence of mechanically induced defects was found to accelerate the Te precipitation, the consequences of which are discussed with regard to the performance of nowadays glass stability criteria.

The effect of powder coarseness on crystallization kinetics of Ge11Ga11Te78 infrared glass

The effect of powder particle size on non-isothermal crystallization kinetics of Ge11Ga11Te78 glass was examined by differential scanning calorimetry. Multivariate kinetic analysis was used to deconvolute the complex crystallization data. The overall process was found to be composed of initial tellurium precipitation followed by simultaneous formation of Ga2Te5 and GeTe phases. While the Te precipitation followed the nucleation growth Johnson–Mehl–Avrami kinetics with the growth originating mainly from surface and mechanically induced defects, the formation of gallium and germanium tellurides proceeded autocatalytically with a significantly lower involvement of the defects-based crystal growth. The model-based part of the kinetic behavior was found to be uninfluenced by the change in powder coarseness—both crystallization sub-processes were described in terms of similar models with similar kinetic exponents for all particle size fractions. On the other hand, the model-free kinetic parameters (activation energies, pre-exponential factors and crystallization enthalpies) were found to be significantly changing with increasing particle size; the observed trends were explained in terms of the changing amounts of the two involved types of crystallization centers.

A strategy for fabrication of controllable 3D pattern containing clusters and nanoparticles inside a solid material.

Directly controlling the growth process of clusters and nanoparticles is an effective way to tune their specific properties, which has been considered as a significant issue lying at the heart of nanotechnology. For technological applications, great strides have been made in the assembly of clusters and nanoparticles. However, controllable synthesis of clusters and nanoparticles inside a bulk solid-state medium remains a tremendous challenge, which is important for integrated devices. Here we report a strategy for space-selective control of elemental tellurium (Te) precipitation as clusters or nanoparticles in glass by femtosecond (fs) laser irradiation. After irradiation by a 1 kHz fs laser at 800 nm, Te2 clusters, which emit near-infrared (NIR) light, are observed at the focal point of the laser beam inside the glass sample. By shifting the repetition rate to 250 kHz, a temperature field forms around the focal area that facilitates transformation of Te clusters into nanoparticles. Raman mapping shows that the clusters are localized in the center of the laser-induced microstructure, while the nanoparticles exhibit an annular distribution. The possible mechanisms of generation and distribution of different species are discussed. We have also demonstrated optical data storage and embedded micro-grating by using this technique.

Se-doped GeTe4 glasses for far-infrared optical fibers

Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were used to study thermokinetic behavior of Ge21SeyTe79−y infrared glasses (for y up to 8 at.% of Se). Glass transition kinetics was described in terms of the Tool-Narayanaswamy-Moynihan model and the relaxation motions were interpreted using Raman spectroscopy data. Both enthalpy and volume relaxation kinetics were found to be uninfluenced by Se addition. On the other hand, DSC crystallization behavior changed significantly with increasing Se content: initial addition of Se partially delays and decelerates both the initial Te precipitation and the consequent volume-located crystallization of GeTe and Te; additional increase of Se content then further delays and separates the volume-located crystallization processes but the initial surface Te precipitation is no longer affected. From this point of view the fine compositional tuning of basic thermal stability of GeTeSe infrared glasses appears meaningless. On the contrary, glass workability window (determined by means of the TMA crystallization experiments) was found to be significantly affected by Se addition, emphasizing the importance of this type of measurements for the glass technology and material science. The 4 at.% Se content was found optimal with respect to fiber-drawing procedure.

Correlation of structural, thermo-kinetic and thermo-mechanical properties of the Ge11Ga11Te78 glass

Cold crystallization and structural relaxation kinetic processes occurring in the Ge11Ga11Te78 infrared bulk glass were studied using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The complex crystallization behaviour was described as overlapping surface nucleation-growth Te precipitation and volume-located autocatalytic formation of GeTe and Ga2Te5 crystallites. Similar crystallization activation energies were obtained by DSC and TMA. Direct comparison of the DSC and TMA data reveals that only a narrow temperature window of approx. 20°C exists between the start of the glass softening and first formation of Te crystallites, which makes knowledge of crystallization kinetics crucial for the industrial glass processing and real-life commercial applications.Structural relaxation kinetics was described in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Good agreement between the relaxation activation energies determined by DSC and TMA was obtained. Values of the parameters of the TNM model were interpreted with respect to the molecular structures determined by Raman spectroscopy and reverse Monte Carlo simulations. The stabilizing role of Ga was explained by its preferred bonding to the non-bonding p electrons of Te.

Thermal behavior of Ge20SeyTe80−y infrared glasses (for y up to 8 at.%)

The Te-rich Ge20SeyTe80−y infrared chalcogenide glasses (for y up to 8 at.%) were studied by means of DSC, XRD, Raman spectroscopy and infrared microscopy. Thorough non-isothermal thermo-kinetic characterization of the glass transition, crystallization and melting phenomena was performed in dependence on the particle size. The Tool–Narayanaswamy–Moynihan model was successfully used to describe the structural relaxation processes; the compositional evolution of the relaxation parameters was then explained in terms of the structural changes and movements of the characteristic structural units detected by Raman spectroscopy. The nucleation-growth Johnson-Mehl-Avrami model and empirical Autocatalytic model were used to describe the complex kinetics of the DSC crystallization data. Based on the XRD and microscopic analyses the following crystallization mechanisms were revealed: initial nucleation-growth precipitation of hexagonal Te (surface-located) followed by a bulk-located autocatalytic growth of rhombohedral GeTe; in case of higher Se contents an additional formation of the Ge-Te-Se crystalline phase occurred at high temperatures. Higher glass-stability determined for increasing Se content can be associated with the partial inhibition of the crystallization processes, the initial Te precipitation remains, however, further unaffected once Se content reaches ∼4 at.%.

Influence of particle size on crystallization and relaxation behavior of Ge20Se4Te76 glass for infrared optics

Influence of particle size on relaxation and crystallization behavior of the Ge20Se4Te76 chalcogenide glass was studied by differential scanning calorimetry (DSC). The structural relaxation kinetics was described by means of the Tool–Narayaswamy–Moynihan (TNM) model; it was found that the structural relaxation processes are not influenced by powder particle size or by the mechanically induced defects and heterogeneities originating from the grinding procedures — the evaluated TNM parameters were constant for all tested powders. On the other hand, the crystallization kinetics of the Ge20Se4Te76 glass was found to be significantly influenced by the powder size. While the coarser powders exhibited a single-peak behavior corresponding to the nucleation-growth Johnson–Mehl–Avrami (JMA) kinetics, the crystallization of finest powders proceeded in two distinct steps. XRD analysis identified the two DSC signals as crystallization of hexagonal Te and rhombohedral GeTe, indicating that large amounts of mechanically induced defects accelerate the Te precipitation. Regarding the overall thermal characterization, the evaluated glass-stability criteria have shown that although the studied chalcogenide glass is a very good glass-former, its stability largely depends on the sample form.

Recovery of dissolved platinum group metals from copper sulphate leach solutions by precipitation

The pregnant leach solution produced in the final leaching stage of base metal refineries (BMRs) operated by platinum producers contains impurities such as selenium and tellurium as well as other precious metals (OPMs, which include Rh, Ru and Ir). The aim of this project was to propose operating conditions for a thio-urea precipitation process that would allow maximum OPM recovery and impurity precipitation from the leach solution with minimal copper and nickel co-precipitation. Experimental results illustrating the effects that operating temperature (80°C and 160°C), pressure (atmospheric pressure and seven bar), stirring rate (250rpm and 500rpm) and thio-urea quantity (200% and 320% excess) have on the precipitation behaviour are presented.Virtually all of the Rh contained in the solution was precipitated irrespective of the values of the process variables studied. The maximum percentage Ru and Ir precipitation achieved were 87% and 60%, respectively. Complete Se precipitation was observed at all process conditions, while Te precipitation increased as the operating temperature was increased. Increasing the reagent quantity and temperature did, however, also result in increased copper and nickel co-precipitation.Regression models were used to perform numerical analyses to determine suitable operating conditions. Predictions with this numerical approach suggested that precipitation with 200% excess thio-urea at a temperature of 80°C and a pressure of 7bar would yield 98% Rh, 75% Ru, and 48% Ir precipitation with less than 5% Cu and Ni co-precipitation; these results could be experimentally validated.

Aeration Controls the Reduction and Methylation of Tellurium by the Aerobic, Tellurite-Resistant Marine Yeast Rhodotorula mucilaginosa

We previously described a marine, tellurite-resistant strain of the yeast Rhodotorula mucilaginosa that both precipitates intracellular Te0 and volatilizes methylated Te compounds when grown in the presence of the oxyanion tellurite. The uses of microbes as a "green" route for the production of Te0-containing nanostructures and for the remediation of Te-oxyanion wastes have great potential, and so a more thorough understanding of this process is required. Here, Te precipitation and volatilization catalyzed by R. mucilaginosa were examined in continuously aerated and sealed (low oxygen concentration) batch cultures. Continuous aeration was found to strongly promote Te volatilization while inhibiting Te0 precipitation. This differs from the results in sealed batch cultures, for which tellurite reduction to Te0 was found to be very efficient. We show also that volatile Te species may be degraded rapidly in medium and converted to the particulate form by biological activity. Further experiments revealed that Te0 precipitates produced by R. mucilaginosa can be further transformed to volatile and dissolved Te species. However, it was not clearly determined whether Te0 is a required intermediate for Te volatilization. Based on these results, we conclude that low oxygen concentrations will be the most efficient for production of Te0 nanoparticles while limiting the production of toxic volatile Te species, although the production of these compounds may never be completely eliminated.

Electric current enhanced defect elimination in thermally annealed Bi–Sb–Te and Bi–Se–Te thermoelectric thin films

An electric-current assisted thermal treatment is demonstrated to be an effective process for eliminating crystal lattice defects and improving thermoelectric properties of both Bi–Sb–Te and Bi–Se–Te nanocrystalline thin films. A model based on electromigration-induced preferential Sb and Te diffusion is proposed to explain the observed Sb-rich and Te precipitation as well as the enhancement of Seebeck coefficient and electrical conductivity of the electrically stressed thin films. Owing to anisotropic diffusion and electrical transport properties, charged lattice defects are preferentially eliminated in the direction parallel to the basal plane of bismuth telluride crystal under electric current stressing. The presented current assisted annealing approach can be an efficient postdeposition treatment that prevents from gross grain growth and evaporation of volatile constituents in Bi–Te based nanocrystalline thin films during high-temperature annealing process.

HgCdTe Research at FFI: Molecular Beam Epitaxy Growth and Characterization

This paper presents results from recent work on molecular beam epitaxy growth of HgCdTe at the Norwegian Defence Research Establishment (FFI), including studies of material properties and fabrication of photodiodes and nanostructures. Systematic studies of defect morphology in HgTe and Hg1� xCdxTe have revealed that there is a minimum in the area covered by defects just below the onset of Te precipitation. The shape and density of microvoids in HgTe can be used to determine the deviation from the optimal growth temperature. This can be further related to the optimal growth temperature of Hg1� xCdxTe with any Cd mole fraction by thermodynamic calculations. A mechanism for the formation of microvoids and needles has been presented. Photoluminescence (PL) has been used to study layers without doping and with Hg vacancy, Ag, and In doping. Planar photodiodes with high dynamic resistance and good quantum efficiency were fabricated by ion-milling vacancy-doped mid-wave and long-wave infrared layers. Quantum wells (QWs) with good crystallinity and high PL light output have been grown. Surface patterning has been found to enhance light emission from HgCdTe thin-film and QW samples by � 30%. Single-crystal HgTe and segmented HgTe/Te nanowires have been grown, and the resistivity of the nanowires has been measured by conductive atomic force microscopy (AFM), where the AFM tip has been used as a mobile electrode.

Local Order and Crystallization of Laser Quenched and Ion Implanted Amorphous Ge[sub 1−x]Te[sub x] Thin Films

Amorphous films of Ge 1-x Te x (x = 0.37 and 0.64) prepared by sputtering, melt quenching, or ion irradiation were annealed up to 450°C. The crystallized films consist of stoichiometric GeTe and precipitates of the excess element. The laser and ion irradiated Te-rich amorphous alloy exhibits higher stability with respect to the sputtered film, and an enhancement of the relative abundance of edge-sharing GeTe 4 tetrahedra at the expense of Ge-rich tetrahedra occurs with respect to the as-deposited amorphous layers. The crystallization temperature increases with the density of Ge-Te bonds because this local rearrangement delays Te precipitation and the subsequent GeTe crystallization. Irradiation does not affect the stability of the Ge-rich alloy in which crystallization is limited by Ge mobility, and the induced local rearrangements are probably prevented by the low atomic diffusivity.

Local Order and Crystallization of Laser Quenched and Ion Implanted Amorphous Ge1-xTex Thin Films

AbstractAmorphous films of Ge1-xTex (x=0.37, 0.51, 0.64) prepared by sputtering, by melt quenching or by ion irradiation were annealed up to 450°C. Different phase stability, i.e. crystallization temperature, was observed varying the amorphous status in stoichiometric and Te-rich alloys while no variation was obtained in the Ge-rich alloy. Laser and ion irradiated stoichiometric alloy exhibits lower stability with respect to the sputtered film while irradiated amorphous Te-rich samples are more stable than the as deposited amorphous sample. An enhancement of edge-sharing GeTe4 tetrahedra Raman signal at the expenses of Ge-rich tetrahedra signal occurs in the irradiated samples with respect to the as-deposited amorphous layers both in the stoichiometric and in the Te-rich alloy. The crystallization temperature decreases in GeTe since the system during irradiation is promoted to a state closer to the crystalline phase while in Te-rich alloy the stability increases with the density of Ge-Te bonds since this local rearrangement delays Te precipitation and the subsequent GeTe crystallization. Irradiation does not affect the stability of Ge rich alloy in which crystallization is limited by the Ge mobility and the induced local rearrangements is probably prevented by the low atomic diffusivity.

Defects in CdHgTe grown by molecular beam epitaxy on (211)B-oriented CdZnTe substrates

A systematic study of the evolution of the defect morphology and crystalline quality in molecular beam epitaxially grown CdxHg1−xTe epilayers with growth temperature is presented. The layers were characterized with optical microscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and high-resolution x-ray diffraction. Four types of defects (microvoids, hillocks, high-temperature voids, and needles) were characterized on epilayers grown in the growth temperature range 188.9−209.9 °C. There is a minimum in the area covered by defects at a temperature just below the onset of Te precipitation, which is defined as the optimal growth temperature. Microvoids with various shapes, and at various stages of growth, were observed side-by-side in many of the CdxHg1−xTe layers, along with hillocks and needles. The defect density of microvoids changes by several orders of magnitude in the studied temperature range. A mechanism for the formation of microvoids and needles is ...

Determination of low concentrations of platinum group elements in geological samples by ID-ICP-MS

Accurate and precise determination of platinum group elements (PGEs) at ppb concentrations in geological samples is important but problematic. This paper reports an improved analytical method for the determination of Pt, Pd, Ru, Ir and Rh at sub-ppb levels by isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP-MS). Prior to experimentation, all reagents were carfully purified: HBO3, HCl and SnCl2 by Te coprecipitation and HNO3 and HF by subboiling distillation. HF, HNO3 and HCl were used to decompose 10 gram sample. The fluoride residue from the acid digestion is minimized by using H3BO3 for complexation, thus mini-fusion of sodium peroxide can be performed in corundum crucibles instead of bulk Na2O2 fusion to lower blank level. The solution of acid digestion and Na2O2 fusion are combined and PGEs are then pre-concentrated by Te coprecipitation. Cu, Ni, Zr and Hf are removed using cation exchange resin and P507 extraction chromatography resin combined in the same column to minimize their interference. Pt, Pd, Ru and Ir are determined using ID-ICP-MS, whereas the mono-isotopic element, Rh, is determined by external calibration using highly enriched 194Pt as the internal standard. The enriched 194Pt spike behaves similar to Rh during the Te precipitation procedure and acts as an ideal internal standard. The determination limits for PGE range from 0.01–0.19 ng g−1. The results obtained using this new method for the CCRMP (CANMET, Ottawa, Canada) certified reference materials WGB-1 (gabbro), TDB-1 (diabase) and UMT-1 (ultramafic ore tailings), show good agreement with the reported values, but have discrepancies compared with the certified values when the concentration of Ru, Rh and Ir below 0.5 ng g−1.