Tellurium Phase Research Articles

Phase-engineered synthesis of atomically thin te single crystals with high on-state currents

Multiple structural phases of tellurium (Te) have opened up various opportunities for the development of two-dimensional (2D) electronics and optoelectronics. However, the phase-engineered synthesis of 2D Te at the atomic level remains a substantial challenge. Herein, we design an atomic cluster density and interface-guided multiple control strategy for phase- and thickness-controlled synthesis of α-Te nanosheets and β-Te nanoribbons (from monolayer to tens of μm) on WS2 substrates. As the thickness decreases, the α-Te nanosheets exhibit a transition from metallic to n-type semiconducting properties. On the other hand, the β-Te nanoribbons remain p-type semiconductors with an ON-state current density (ION) up to ~ 1527 μA μm−1 and a mobility as high as ~ 690.7 cm2 V−1 s−1 at room temperature. Both Te phases exhibit good air stability after several months. Furthermore, short-channel (down to 46 nm) β-Te nanoribbon transistors exhibit remarkable electrical properties (ION = ~ 1270 μA μm−1 and ON-state resistance down to 0.63 kΩ μm) at Vds = 1 V.

Effect of the roughness on the photoinduced growth of crystalline tellurium on MoTe2 matrix

In this work, a series of 5 nm thick films of molybdenum oxide (MoO3) were synthesized on SiO2/Si substrates by magnetron sputtering of molybdenum in low vacuum conditions. We used closed-space sublimation (CSS) to tellurize the Mo oxide and to produce pure-phase 1 T′-MoTe2 films by controlling the temperature inside the CSS reactor. Surface analyses showed that tellurium spreads over the surface of the as-grown films, forming localized crystalline precipitates whose distribution depends on the tellularization time. From Raman spectroscopy investigations, we observed that the exposure of these films to visible light leads to a growth of a highly-ordered trigonal phase of tellurium. The intensity evolution of the vibrational modes related to the crystalline phase of tellurium allows us to perform a quantitative analysis of the crystallization. Our results indicate that this process is consistent with the Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory for the overall crystallization process. By the characteristic growth exponent, we introduce a more specific model which takes into account the diffusion and the dissociation of tellurium chains, which is used to probe how the effective growth rate is affected by the roughness of the samples.

Photoinduced growth of the crystalline phase of tellurium on a 1T′-MoTe2 matrix

Due to the growing demand for miniaturization and energy efficiency in modern electronic devices, there is a renewed interest for optoelectronic memories and sensors based on 2D materials. In particular, the molybdenum ditelluride (MoTe2) is one of the most promising materials for applications in nonvolatile phase-change memory devices, as its properties can be controlled by visible-light illumination. Among the several ways to synthesize MoTe2, the molybdenum oxide tellurization through isothermal close space sublimation (CSS) annealing in gas atmosphere is a simple and low-cost effective method for large-scale production of devices based on this layered material. Therefore, the understanding of the physical properties of MoTe2 thin films produced by this technique is crucial for future applications. Surprisingly, our results indicate that there is a photoinduced growth of the crystalline phase of tellurium on the 1T′-MoTe2 matrix even when the power density of the laser is low. From Raman spectroscopy investigations, we were able to show that nanometer-sized tellurium crystallites work as seed sites for the photocrystallization of tellurium. By assuming that the overall crystallization process is described by a kinetic approach that is based on the Kolmogorov–Johnson–Mehl–Avrami theory, our results indicate that the process is governed by an anisotropic organization of the tellurium atoms in helical structures during the crystal growth.

Ferroelectricity and Large Rashba Splitting in Two-Dimensional Tellurium

Two-dimensional (2D) ferroelectric (FE) systems are promising candidates for non-volatile nanodevices. Previous studies mainly focused on 2D compounds. Though counter-intuitive, here we propose several new phases of tellurium with (anti)ferroelectricity. Two-dimensional films can be viewed as a collection of one-dimensional chains, and lone-pair instability is responsible for the (anti)ferroelectricity. The total polarization is determined to be 0.34 × 10−10 C/m for the FE ground state. Due to the local polarization field in the FE film, we show a large Rashba splitting (α R ∼ 2 eV⋅Å) with nonzero spin Hall conductivity for experimental detection. Furthermore, a dipole-like distribution of Berry curvature is verified, which may facilitate a nonlinear Hall effect. Because Rashba-splitting/Berry-curvature distributions are fully coupled with a polarization field, they can be reversed through FE phase transition. Our results not only broaden the elemental FE materials, but also shed light on their intriguing transport phenomena.

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.

Морфология поверхности и структурные свойства кристаллов GaTe после ионно-плазменной обработки

The effect of ion-plasma treatment on the physical properties of the surface of GaTe crystals is investigated. Gallium telluride crystals were grown by vertical zone melting under the pressure of an inert argon gas of 10.0 MPa at a temperature of 1000 °C and a zone displacement velocity of 9 mm/hr. The treatment was carried out in argon plasma in a high-density low-pressure radio frequency (RF) inductively coupled plasma reactor at an argon ion energy of 100-200 eV for 15-120 s. Using scanning electron microscopy methods, it was shown that the formation of nano- and submicron structures of various architectures (nanohillocks, nanocones, droplet structures) occurred on the surface during processing. It is shown that the sputtering processes are accompanied by enrichment of the near-surface layer with metal atoms and a decrease in oxygen content. The formation of nano- and submicron gallium droplets on the surface has been proved by X-ray diffractometry. The analysis of the raman scattering spectra showed a decrease in the oxide phases of tellurium after plasma treatment. It is established that modification of the GaTe surface leads to suppression of specular optical reflection in the range of 0.4-6.2 eV.

Controlling Stoichiometry in Ultrathin van der Waals Films: PtTe2, Pt2Te3, Pt3Te4, and Pt2Te2

The platinum-tellurium phase diagram exhibits various (meta)stable van der Waals (vdW) materials that can be constructed by stacking PtTe2 and Pt2Te2 layers. Monophase PtTe2, being the thermodynamically most stable compound, can readily be grown as thin films. Obtaining the other phases (Pt2Te3, Pt3Te4, Pt2Te2), especially in their ultimate thin form, is significantly more challenging. We show that PtTe2 thin films can be transformed by vacuum annealing-induced Te-loss into Pt3Te4- and Pt2Te2-bilayers. These transformations are characterized by scanning tunneling microscopy and X-ray and angle resolved photoemission spectroscopy. Once Pt3Te4 is formed, it is thermally stable up to 350°C. To transform Pt3Te4 into Pt2Te2, a higher annealing temperature of 400°C is required. The experiments combined with density functional theory calculations provide insights into these transformation mechanisms and show that a combination of the thermodynamic preference of Pt3Te4 over a phase segregation into PtTe2 and Pt2Te2 and an increase in the Te-vacancy formation energy for Pt3Te4 compared to the starting PtTe2 material is critical to stabilize the Pt3Te4 bilayer. To desorb more tellurium from Pt3Te4 and transform the material into Pt2Te2, a higher Te-vacancy formation energy has to be overcome by raising the temperature. Interestingly, bilayer Pt2Te2 can be retellurized by exposure to Te-vapor. This causes the selective transformation of the topmost Pt2Te2 layer into two layers of PtTe2, and consequently the synthesis of e Pt2Te3. Thus, all known Pt-telluride vdW compounds can be obtained in their ultrathin form by carefully controlling the stoichiometry of the material.

Lithium–Tellurium System: Thermodynamic and Electrochemical Studies and Prospects for Use in Chemical Current Sources

Published data on the lithium–tellurium phase diagram and on thermodynamic properties of liquid alloys and solid phases formed are summarized and comprehensively analyzed in connection with prospects for using tellurium and its compounds and composites in various energy conversion systems involving lithium. The results of studying the thermodynamic properties of lithium tellurides by different methods are compared. The results of the main studies dealing with the use of tellurium in chemical current sources (CCSs) of various designs are briefly considered.

The new tellurium compounds: Na1.5Te2Mo0.5O6.25 and K6(Te94+Te6+)Mo6O42

The new ternary compounds Na1.5Te2Mo0.5O6.25 and K6(Te94+Te6+)Mo6O42 have been revealed by the study of the phase relation in the systems Na2O–TeO2–MoO3 and K2O–TeO2–MoO3. The structure of the compounds has been investigated by X-ray Single-Crystal Analysis. The Na1.5Te2Mo0.5O6.25 compound belongs to anti-glass tellurium phase with fluorite-type structure and space group Fm3‾m, Z = 1, a = 5.4087 Å, where the oxygen atoms are strongly disordered near the 8a site. The K6(Te94+Te6+)Mo6O42 mixed-valence tellurium compound possesses trigonal space group P3‾, Z = 1, a = 12.0074 Å, c = 7.8979 Å. The K6(Te94+Te6+)Mo6O42 structure contains rare units of two [TeO6] octahedron sharing three common oxygen. The decomposition temperatures are 378 °C and 410 °C for Na1.5Te2Mo0.5O6.25 and K6(Te94+Te6+)Mo6O42, respectively.

Defect Levels in Nuclear Detector Grade Cd0.9Zn0.1Te Crystals

Nuclear detector grade semi-insulating Cd0.9Zn0.1Te (CZT) crystals were grown by a low temperature solution method from in-house zone refined precursor materials. The extended defects due to secondary phases of tellurium inclusions/precipitates in the grown crystals were characterized by a non-destructive electron beam induced current (EBIC) imaging method. The EBIC results were correlated with the infrared (IR) transmittance mapping, which clearly shows the variation of contrast is due to non-uniform distribution of tellurium inclusions/precipitates in CZT crystals. Electrical characteristics of defect levels in the fabricated detectors were further investigated by thermally stimulated current (TSC) measurements. Pulse height spectra (PHS) measurements were carried out using nuclear radiation sources of 241Am (59.6 keV) and 137Cs (662 keV) and energy resolutions of 6.2% and 1.6% respectively were achieved.

Effect of oxygen on structural stability of nitrogen-doped germanium telluride films with and without silicon nitride layer

Nitrogen-doped germanium telluride (N-GeTe) films with and without silicon nitride (SiN) layer were thermally annealed in an air atmosphere. The SiN layer prevented the oxidation of GeTe films despite the massive in-diffusion of oxygen atoms. The phase transition from cubic to rhombohedral phase occurred only in the air-annealed samples, not in the samples annealed at 2.0mPa. The in-diffused oxygen is probably the leading cause of this phase transition. N-GeTe films without SiN layer showed an increase in sheet resistance after 1000min of air annealing; this could be attributable to a phase transition from the cubic GeTe phase to the amorphous germanium oxide and metallic tellurium phases.

Intensity enhancement of Te Raman modes by laser damage in ZnTe epilayers

Damage caused by laser irradiation on the surface of ZnTe epilayers was studied by micro-Raman and atomic force microscopy (AFM). ZnTe LO-phonon overtones up to four order and TO + (n − 1)LO zone-center phonons were observed in the resonant micro-Raman spectra at room temperature. Discrepancies in the literature regarding the origin of two features observed at low frequencies around 120 and 140 cm−1 in the Raman spectrum of ZnTe are discussed and resolved. These Raman peaks were not detected by using a low excitation laser power density on a Zn-terminated ZnTe surface; however, with the increase of the laser power density they were found to arise irreversibly. The correspondence of these peaks in a wave number with the strongest Raman peaks of the crystalline tellurium phase and the intensity enhancement behavior with the laser power in a similar way as for CdTe strongly suggests the formation of crystalline tellurium aggregates on the layer surface due to laser irradiation damage. AFM data reveal the occurrence of laser ablation on the ZnTe surface even though the surface temperature of the sample is below the melting point.

Synthesis of tellurium nanorods via spontaneous oxidation of NaHTe at room temperature

Trigonal phase of tellurium (t-Te) nanorods with tapered ends have been synthesized through spontaneous oxidation of NaHTe by dissolved oxygen at room temperature. Utilization of sodium dodecyl benzenesulfonate was found to help to obtain high-quality nanorods. The product was characterized by X-ray diffraction and Transmission electron microscopy. In addition, the possible nucleation and growth mechanism of the t-Te nanorods was discussed.

Tellurium and precious-metal ore minerals at Mina Capillitas, Northwestern Argentina

Mina Capillitas is an epithermal-type deposit located in Catamarca province of NW Argentina. Six separate stages of mineralization were identified in this complex Cu-Au-S-As-Sb-Pb-Zn association; the native elements (Au and Te) and Te minerals are concentrated in the fourth stage. Gold occurs as small grains with an average fineness of 920, hosted mainly by quartz and coexists with hubnerite and Bi minerals. Tellurium occurs locally in grains up to 10 μm in diameter, and is associated with the Te-bearing minerals of Au, Ag, Bi, Cu, and Ni. These Te minerals include krennerite, calaverite, sylvanite, petzite, hessite, stutzite, goldfieldite, melonite, tetradymite, and possibly volynskite. The grains are generally arranged in larger polycrystalline aggregates. These minerals occur in quartz, generally accompanied by hubnerite, pyrite, chalcopyrite, and Bi- and Sn-bearing minerals. The mineralizing fluids cooled gradually with the typical temperatures ranges characteristic of epithermal systems. Based on the preponderance of the different gangue minerals, we suggest that the pH of the environment became progressively more basic after the third stage of mineralization. This mineral assemblage together with the fluid inclusion homogenization temperature data indicate that the log fS 2 values declined as the fluids deposited the Te-bearing minerals. The absence of tellurium phases in the first three stages suggests that either the source of the tellurium had not yet been tapped or that the log fTe 2 lay below -10 to -17, the minimum activity for the Te phases present in stage IV to form.

Large structural modulations in incommensurate Te-III and Se-IV.

The high-pressure phase of tellurium, Te-III, is found to have an incommensurate monoclinic structure, superspace group I(')2/m(0q0)s0, of a type previously unknown in the elements. Te-III is stable from 4.5(2) to 29.2(7) GPa; the previously reported transition to a distinct Te-IV phase at 10.6 GPa is not observed. The incommensurate wave vector of Te-III is strongly pressure dependent and varies in a strongly nonlinear way. Se-IV is found to be isostructural with Te-III.

Electronic structure of tellurium under high pressure

AbstractWe have performed band structure calculations of high‐pressure phases of tellurium within the local density‐functional formalism and the norm‐conserving pseudo‐potential method. We have calculated the band structure for the newly proposed model of Te‐III phase, and the result suggests that the monoclinic distortion lowers the band structure energy and contributes to the structural stability.

X-ray structural analysis of the high-pressure phase III of tellurium

A structure of a high-pressure phase of tellurium (Te-III) has been re-examined byan x-ray diffraction method. This phase appears at a pressure between7 and 27 GPa, where a second-order phase transition to Te-IV occurs.The newly obtained crystal lattice of Te-III at 8 GPa is monoclinic witha = 8.4682(14)Å, b = 4.7424(8) Å,c = 3.9595(7) Å, β = 88.112(11)°.The space group is C2/m,with six atoms in the unit cell, two in positions 2a at (0, 0, 0) and four in positions 4i at(x, 0, z) withx = 0.324(11),z = 0.675(3).

Processing and characterisation of sol-gel glasses and powders in the system TeO2-TiO2

Sol-gel processing routes have been developed for the production of thin films and powders in the system TeO2-TiO2 from tellurium and titanium alkoxides. The structure and properties of the resultant materials have been characterised as a function of heat treatment temperature. Pure sol-gel derived TeO2 thin films are difficult to prepare with good optical transparency due to the presence of organic impurities and/or a highly dispersed metallic tellurium phase when heated at temperatures up to approximately 340°C, with crystallisation to α-TeO2 occurring when the heat treatment temperature is further increased. Additions of TiO2 were found to retard the crystallisation of the α-TeO2 but promote the formation of other TiO2 or TiTe3O8 phases. However, an optimum composition in the range 0.9TeO2 0.1TiO2 was identified, which allows optically transparent thin films to be prepared with high refractive index and offers the potential for practical device manufacture.

Tellurium-rich phase in n-type bismuth telluride crystals grown by the Bridgman technique

A detailed study of the presence of the segregated tellurium phase in n-type bismuth telluride grown by the Bridgman technique was done, using two different furnace systems. The results of the crystals grown under different conditions (temperature gradient, thermal coupling) are discussed. It is shown that, depending on the growing conditions, the Te-rich phase can be eliminated and, when this is done, variations of the Seebeck coefficient along the crystal depend on the melt volume rather than on crystal length. Values higher than 250 μV/K were reached for the Seebeck coefficient in these crystals.

Phonon Softening and Superconductivity in Tellurium under Pressure.

The phonon dispersion and the electron-phonon interaction for the $\beta$-Po and the bcc high pressure phases of tellurium are computed with density-functional perturbation theory. Our calculations reproduce and explain the experimentally observed pressure dependence of the superconducting critical temperature (T$_{\rm c}$) and confirm the connection between the jump in T$_{\rm c}$ and the structural phase transition. The phonon contribution to the free energy is shown to be responsible for the difference in the structural transition pressure observed in low and room temperature experiments.