Excess Te Research Articles

Enhanced thermoelectric properties of bismuth telluride bulk achieved by telluride-spilling during the spark plasma sintering process

We offered a new and simple routine for significant reducing lattice thermal conductivity and improve the thermoelectric properties of Bi2Te3 materials. Dense dislocations were introduced by means of adding excess Te into Bi2Te3 powder by combining ball milling and spark plasma sintering (SPS). During the high temperature SPS process, excess Te spilled out of the die. The ultra-low lattice thermal conductivity of Bi2Te3 was achieved due to the enhanced phonon scattering by dislocations. The highest ZT value of 0.7 at 398K was obtained for a pure Bi2Te3 bulk sample which is 175% higher than that of pristine Bi2Te3 sample.

Airy pattern on narrow photoluminescence spectrum of band to band recombination in CdTe:Te thin films

Semiconductor CdTe:Te films were deposited by means of rf sputtering on glass substrates. The excess of Te gave place to a high number of Cd-vacancies (VCd) producing p-type CdTe films. The density of carriers produced a high strength surface electric field which allowed obtain the bandgap value employing modulated transmittance spectroscopy. The obtained bandgap value of 1.40 ± 0.01eV was confirmed by absorption spectroscopy measures. The density of holes is so high that bandgap renormalization is observed. Photoluminescence (PL) measurements were carried out with the down-converted 883.2nm (1.403eV) line of the 441.6nm wavelength of a He-Cd laser. This energy allows to produce a resonant excitation of the CdTe:Te films, in such a way that electrons from the conduction band (CB) can be just excited to the valence band (VB). The resonant excitation produced a PL spectrum of band to band electron-hole recombination showing discrete energy emissions that follow the pattern of oscillations corresponding to the Airy model for a quantum triangular potential well. The average width of signals of the higher energy oscillations is 12 ± 3μeV and separation between energy levels is of the order of 12 ± 3μeV.

Synthesis and microstructure of NiTe2

This paper reports on the microstructure of NiTe2 samples grown with different concentrations of Te and thermal gradient. Large crystals were obtained through a self-flux method. Two phases were identified within the Ni-Te system and their stoichiometries determined using scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction. For the sample synthesized without tellurium excess, the compound was found to have a CdI2 -type structure (space group P–3m1) with unit cell parameters a=b=3.8581(2)Å and c=5.2644(3)Å. From SEM and X-ray, it was found that the volume of NiTe2 phase prepared without Te excess and thermal gradient increased in relation to that obtained by employing other methods. Also, the microstructure of the grain boundaries was examined using SEM and crystals rich in Ni were also observed in samples prepared without thermal gradient.

Effects of annealing temperature on the structural, mechanical and electrical properties of flexible bismuth telluride thin films prepared by high-pressure RF magnetron sputtering**Contribution at the 4th Southeast Asia Conference on Thermoelectrics 2016 (SACT 2016), 15–18 December 2016, Da Nang City, Vietnam.

In this work BixTey thin films were deposited on polyimide substrate by a high-pressure RF magnetron sputtering technique. The deposited condition was maintained using a high pressure of 1.3 × 10−2 mbar. The as-deposited films show Bi2Te3 structure with Te excess phase (Te-rich Bi2Te3). After that, as-deposited films were annealed in the vacuum chamber under the N2 flow at temperatures from 250 to 400 °C for one hour. The microstructure, cross-section, [Bi]:[Te] content, and the mechanical, electrical and thermoelectric properties of as-deposited and different annealed films were investigated. It was found that the annealing temperature enhanced the crystallinity and film density for the temperature range 250–300 °C. However, the crystal structure of Bi2Te3 almost changed to the BiTe structure after annealing the films above 350 °C, due to the re-evaporation of Te. Nano-indentation results and cross-section images indicated that the hardness of the films related to the film density. The maximum hardness of 2.30 GPa was observed by annealing the films at 300 °C. As a result of an improvement in crystallinity and phase changes, the highest power factor of 11.45 × 10−4 W m−1K−2 at 300 °C with the carrier concentration and mobility of 6.15 × 1020 cm−3 and 34.03 cm2 V−1 s−1, respectively, was achieved for the films annealed at 400 °C.

High thermoelectric performance in (Bi0.25Sb0.75)2Te3 due to band convergence and improved by carrier concentration control

Bi_2Te_3 has been recognized as an important cooling material for thermoelectric applications. Yet its thermoelectric performance could still be improved. Here we propose a band engineering strategy by optimizing the converging valence bands of Bi_2Te_3 and Sb_2Te_3 in the (Bi_(1−x)Sb_x)_2Te_3 system when x = 0.75. Band convergence successfully explains the sharp increase in density-of-states effective mass yet relatively constant mobility and optical band gap measurement. This band convergence picture guides the carrier concentration tuning for optimum thermoelectric performance. To synthesize homogeneous textured and optimally doped (Bi0.25Sb0.75)2Te3, excess Te was chosen as the dopant. Uniform control of the optimized thermoelectric composition was achieved by zone-melting which utilizes separate solidus and liquidus compositions to obtain zT = 1.05 (at 300 K) without nanostructuring.

Minority Carrier Blocking to Enhance the Thermoelectric Performance of Solution-Processed BixSb2-xTe3 Nanocomposites via a Liquid-Phase Sintering Process.

Minority carrier blocking through heterointerface barriers has been theoretically proposed to enhance the thermoelectric figure of merit (ZT) of bismuth telluride based nanocomposites at elevated temperatures recently (Phys. Rev. B 2016, 93, 165209). Here, to experimentally realize the minority carrier blocking, a liquid-phase sintering process enabled by excess Te is applied to the solution-processed BixSb2-xTe3 nanocomposites to introduce interfacial energy barriers. The controlling parameters in the liquid-phase sintering process such as the amount of excess Te, sintering temperature and holding time, and the Bi composition (x) are systemically tuned and investigated to fully understand the minority carrier blocking mechanism. These interface-engineering parameters are optimized for introducing maximum lattice imperfections and band-bending interfaces that are responsible for blocking the minority carrier and wide-range scattering of the phonons toward enhanced thermoelectric performance. High ZT > 1.4 at 375 K is realized in the Bi0.5Sb1.5Te3 sample, which is much higher than those of the state-of-the-art commercial ingots (ZT ∼ 1) and other solution-processed nanocomposites. The enhanced ZT at elevated temperatures is mostly due to the suppression of bipolar thermal conductivity by minority carrier blocking as well as the reduction of lattice thermal conductivity. Adapting this solution synthesis process to design favorable heterointerfaces for minority carrier blocking in the liquid-phase sintering process holds promise to further enhance the ZT values.

Effect of Zn and Te beam intensity upon the film quality of ZnTe layers on severely lattice mismatched sapphire substrates by molecular beam epitaxy

Zinc telluride layers were grown on highly-lattice-mismatched sapphire substrates by molecular beam epitaxy, and their crystallographic properties were studied by means of X-ray diffraction pole figures. The crystal quality of the ZnTe thin film was further studied by scanning electron microscopy, X-ray rocking curves and low-temperature photoluminescence measurements. These methods show that high-crystallinity (111)-oriented single domain ZnTe layers with the flat surface and good optical properties are realized when the beam intensity ratio of Zn and Te beams is adjusted. The migration of Zn and Te was inhibited by excess surface material and cracks were appeared. In particular, excess Te inhibited the formation of a high-crystallinity ZnTe film. The optical properties of the ZnTe layer revealed that the exciton-related features were dominant, and therefore the film quality was reasonably high even though the lattice constants and the crystal structures were severely mismatched.

Carrier density and lifetime for different dopants in single-crystal and polycrystalline CdTe

CdTe defect chemistry is adjusted by annealing samples with excess Cd or Te vapor with and without extrinsic dopants. We observe that Group I (Cu and Na) elements can increase hole density above 1016 cm−3, but compromise lifetime and stability. By post-deposition incorporation of a Group V dopant (P) in a Cd-rich ambient, lifetimes of 30 ns with 1016 cm−3 hole density are achieved in single-crystal and polycrystalline CdTe without CdCl2 or Cu. Furthermore, phosphorus doping appears to be thermally stable. This combination of long lifetime, high carrier concentration, and improved stability can help overcome historic barriers for CdTe solar cell development.

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

125Te NMR spectra and spin-lattice relaxation times, T1, have been measured for several GeTe-based materials with Te excess. The spectra show inhomogeneous broadening by several thousand ppm and a systematic variation in T1 relaxation time with resonance frequency. The quadratic dependence of the spin-lattice relaxation rate, 1/T1, on the Knight shift in the Korringa relation is found to be valid over a wide range of Knight shifts. This result confirms that T1 relaxation in GeTe-based materials is mostly dominated by hyperfine interaction between nuclei and free charge carriers. In GeTe with 2.5% excess of Te, about 15% of the material exhibits a Knight shift of ≥4500ppm and a T1 of only 0.3ms, indicating a high hole concentration that could correspond to close to 50% vacancies on the Ge sublattice in this component. Our findings provide a basis for determining the charge carrier concentration and its distribution in complex thermoelectric and phase-change tellurides, which should lead to a better understanding of electronic and thermal transport properties as well as chemical bonding in these materials.

Structural properties of Sn-doped CdTe thin films grown by pulsed laser deposition using powder as target

Undoped and Sn-doped CdTe thin films were grown by a pulsed laser deposition on glass substrates at 300 °C using a powder as target. CdTe films were grown using CdTe and Sn powders, varying the tin concentration in the range of 1–7 wt. %. The structural properties were analyzed as a function of the Sn amount in the target. The x-ray diffraction shows that the undoped CdTe film has a cubic phase, while Sn-doped CdTe films have a mixture of cubic and hexagonal phases. The compositional analysis showed that the undoped CdTe film has Te excess, while Sn-doped CdTe films have Te deficiencies.

Growth and structure of thermally evaporated Bi2Te3 thin films

The growth mechanism, microstructure, and crystal structure of the polycrystalline n-Bi2Te3 thin films with thicknesses d=15–350nm, prepared by thermal evaporation in vacuum onto glass substrates, were studied. Bismuth telluride with Te excess was used as the initial material for the thin film preparation. The thin film characterization was performed using X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, scan electron microscopy, and electron force microscopy. It was established that the chemical composition of the prepared films corresponded rather well to the starting material composition and the films did not contain any phases apart from Bi2Te3. It was shown that the grain size and the film roughness increased with increasing film thickness. The preferential growth direction changed from [00l] to [015] under increasing d. The X-ray photoelectron spectroscopy studies showed that the thickness of the oxidized surface layer did not exceed 1.5–2.0nm and practically did not change in the process of aging at room temperature, which is in agreement with the results reported earlier for single crystals. The obtained data show that using simple and inexpensive method of thermal evaporation in vacuum and appropriate technological parameters, one can grow n-Bi2Te3 thin films of a sufficiently high quality.

Formation of Te-rich phase and its effect on microstructure and thermoelectric properties of hot-extruded Bi–Te–Se bulk materials

A mechanical alloying (MA) and hot-extrusion technique was used to prepare n-type Bi–Te–Se bulk materials. The formation and distribution of Te-rich phase as well as its effect on microstructure and thermoelectric properties of the extruded samples have been investigated. The formation of Te-rich phase is mainly associated with sublimation of Te and eutectic reaction. The former occurs at any extrusion temperature and leads to formation of small-sized Te-rich phase, while the latter occurs at ≥400 °C and the resultant Te-rich phase possesses large sizes. The Te-rich phase was distributed in different morphologies depending on the extrusion temperature. Small-sized Te-rich phase was distributed around the grain boundaries in extruded samples. When excessive amount of Te was incorporated, the average grain size and orientation degree were found to be reduced due to formation of more Te-rich phase. Furthermore, the addition of excessive Te led to decrease of the Seebeck coefficient and increase of thermal conductivity, thus resulting in a significant reduction in ZT value.

Oscillations and huge preferences of PbTe crystal surface sputtering under Secondary Neutral Mass Spectrometry conditions

Sputtering of PbTe crystals by Ar+ plasma with ion energies 50–550eV is investigated. A dependence of sputter yields of the Te and Pb on the sputtering ion energy and sputtering time is measured. New phenomena: aperiodical oscillations of Pb and Te sputtering; a huge preference of Te sputtering reaching more than two orders of magnitude at the beginning of sputtering process; and a significant excess of Te integrated sputter yield over that of Pb for prolonged sputtering by low energy plasma at 50–160eV, are observed. It is substantiated that these phenomena can be caused by the peculiarities of the charge states of the interstitial Pb and Te in PbTe crystal matrix and the processes of re-deposition of sputtered atoms on the sputtered surface.

Effects of doping on the positional uniformity of the thermoelectric properties of n-type Bi2Te2.7Se0.3 polycrystalline bulks

The reproducibility problem is one of the central issues in n-type Bi2Te3-based thermoelectric materials. We herein investigate the effects of doping on the positional uniformity of the electronic transport properties of spark-plasma-sintered Bi2Te2.7Se0.3 polycrystalline bulks. Both the electrical conductivity and the Seebeck coefficient showed severe deviations with position in the pristine Bi2Te2.7Se0.3 bulk due to the variation in carrier concentration originating from the formation of Te- and Se-site vacancies. This non-uniformity problem could be significantly improved by the addition of excess elemental Cu and Te. Uniformity enhanced by Cu addition might be considered to be related with the suppression of Te/Se evaporation in the presence of intercalated Cu atoms. A maximum power factor value of ∼2.72 mW/mK2 at 300 K was obtained for Cu0.01Bi2Te2.7Se0.3.

Phase-Engineered Synthesis of Centimeter-Scale 1T'- and 2H-Molybdenum Ditelluride Thin Films.

We report the synthesis of centimeter-scale, uniform 1T'- and 2H-MoTe2 thin films via the tellurization of Mo thin films. 1T'-MoTe2 was initially grown and converted gradually to 2H-MoTe2 over a prolonged growth time under a Te atmosphere. Maintaining excessive Te was essential for obtaining the stable stoichiometric 2H-MoTe2 phase. Further annealing under a lower partial pressure of Te at the same temperature, followed by a rapid quenching, led to the reverse phase transition from 2H-MoTe2 to 1T'-MoTe2. The orientation of the 2H-MoTe2 film was determined by the tellurization rate. Slow tellurization was the key for obtaining a highly oriented 2H-MoTe2 film over the entire area, while fast tellurization led to a 2H-MoTe2 film with a randomly oriented c-axis.

Structural and optical properties of CdTe-nanocrystals thin films grown by chemical synthesis

By mans of a chemical synthesis technique stoichiometric CdTe-nanocrystals thin films were prepared on glass substrates at 70°C. First, Cd(OH)2 films were deposited on glass substrates, then these films were immersed in a growing solution prepared by dissolution of Te in hydroxymethane sulfinic acid to obtain CdTe. The structural analysis indicates that CdTe thin films have a zinc-blende structure. The average nanocrystal size was 19.4nm and the thickness of the films 170nm. The Raman characterization shows the presence of the longitudinal optical mode and their second order mode, which indicates a good crystalline quality. The optical transmittance was less than 5% in the visible region (400–700nm). The compositional characterization indicates that CdTe films grew with Te excess.

Nonstoichiometric composition shift in physical vapor deposition of CdTe thin films

While it is being debated whether Cd vacancy is an effective p-dopant in CdTe, and whether CdTe thin film in solar energy application should be Cd-deficient or Cd-rich, in the theory of CdTe physical vapor deposition (PVD) it has been assumed that both the source material and the thin film product is stoichiometric. To remediate the lack of effective theory, a new PVD model for CdTe photovoltaic (PV) modules is presented in this work, in which the composition of the CdTe thin film under growth is a parameter determined by the source CdTe composition as well as the growth condition. The solid phase Cd1−δTe1+δ compound under deposition temperature is treated as a solid solution with a mole of excess pure Te or Cd as solute and one mole of congruently grown CdTe as solvent. Assuming that the vapor pressure of Te2 can be calculated by using the law of solid solution PTe=H0+aH1+a2H2 round the congruent composition, where the molar number a and the constants H0, H1 and H2 as functions of temperature T are extracted from the experimental data. Thus, the mole fraction of solute in the grown CdTe thin film as well as the growth rate, as a function of the solute mole fraction in the source CdTe can be determined.

Analysis on the structural, vibrational and defect states of chlorine treated polycrystalline cadmium telluride structures grown by e-beam evaporation

Temperature dependent photoluminescence (PL) measurements are performed in order to study the defect states in cadmium chloride treated polycrystalline cadmium telluride (CdTe) thin films grown by e-beam evaporation technique. Three luminescence bands are observed including a double peak emission at 1.577 eV and 1.573 eV corresponding to free electron-to- acceptor transition and a donor–acceptor pair (DAP) transition, respectively, along with a broad peak at 1.45 eV. This broad band emission is related to A-center chlorine based complex and also includes longitudinal (LO) phonon emission lines for CdTe spaced by ∼21 meV. Investigation into grain sizes revealed grains of 0.2 μm for as-grown films and ∼2–3 μm for chlorine activated films shown by atomic force microscopy (AFM). Raman analysis indicates that the films have been grown with excess of Te leading to p-type conductivity in the structure, whereas LO phonon mode of polycrystalline CdTe reveals quasi phonon modes nature.

Te Inclusions in CdMnTe Crystal Grown by the Traveling Heater Method and their Effects on the Electrical Properties

The concentration and size distribution of Te inclusions in CdMnTe (CMT) are key factors in nuclear devices performance. High-concentration and large-size Te inclusions degrade the performance drastically, especially for electrical properties. In this paper, the Te inclusions along the axial and radial directions in CMT crystal grown by the Traveling Heater Method (THM) were revealed by Infrared Microscopic System. The size of Te inclusions is 5-17 μm and the concentration is (2.5-4.6) ×105cm-3. The Current-Voltage measurements were performed and the resistivity of the CdMnTe crystal with excess Te was in the range of 108-109Ωcm, and the correlation between the Te inclusions and resistivity was investigated. The large size (>17 μm) and high concentration (4.5 of ×105cm-3) Te inclusion decrease the resistivity of the CMT crystal seriously.

Microporous gold: Comparison of textures from Nature and experiments

Recent experiments have shown that microporous gold can be obtained via the oxidative dealloying of Au(Ag)-tellurides such as calaverite (AuTe2), krennerite (Au3AgTe8), and sylvanite [(Au,Ag)2Te4] under mild hydrothermal conditions. The same Au textures have been found in natural gold-telluride ores from the Late Miocene epithermal Aginskoe Au-Ag-Te deposit in Kamchatka, Russia. This confirms that natural microporous gold can form via the replacement of telluride minerals. This replacement may take place under hydrothermal conditions, e.g., during the late stage of the ore-depositing event, explaining the wide distribution of “mustard gold” in some deposits. At Aginskoe, the oxidation of Au-tellurides appears to have resulted only in local redistribution of Au and Te, because the associated oxidation of chalcopyrite scavenged the excess Te, inhibiting the crystallization of secondary Te minerals more than a few micrometers in size. Such cryptic mobility may explain the lack of reported secondary Te minerals in many Te-bearing deposits.