Te Thin Films Research Articles

Scanning thermoelectric microscopy of local thermoelectric behaviors in (Bi,Sb)2Te3 films

In this paper we develop scanning thermoelectric microscopy (STeM) on the basis of commercial atomic force microscope. The nanoscale thermoelectric behaviors of (Bi,Sb)2Te3 (BST) thin films were studied. 3ω-technique was used for thermal conductivity imaging and quantitative thermal characterization. By acquiring the unique Seebeck information from 2ω frequency component, nanoscale thermoelectric images were firstly obtained, exhibiting remarkably inhomogeneous distribution of local Seebeck coefficient in the thin films. Positive thermoelectric response is revealed by the modulation of temperature difference between thermal tip and sample, corresponding to p-type conduction within BST sample.

Crystallization kinetics of Se–Te thin films

Differential scanning calorimetry was used to study the non-isothermal crystallization behavior of selenium–tellurium thin films within the 0–30at.% Te compositional range. The non-isothermal crystallization kinetics were described in terms of the Johnson–Mehl–Avrami nucleation-growth model. The apparent activation energy of crystallization was found to exhibit a step-like compositional behavior, with E changing from ~115 to ~145kJ·mol−1 in the 10–15at.% Te range. Two-dimensional growth of crystallites, consistent with the idea of sterically restricted crystallization in a thin layer, was confirmed for all data. However, in the case of the Se70Te30 thin film, indications of three-dimensional crystal growth were found at high heating rates. This corresponds to the previously reported behavior of Se–Te chalcogenide matrices, where the addition of tellurium leads to the formation of smaller, volume-located crystallites.

Facile Hydrothermal Synthesis of Tellurium Nanostructures for Solar Cells

Tellurium (Te) nanostructures have been successfully synthesized via a simple hydrothermal methodfrom the reaction of a TeCl4 aqueous solution with thioglycolic acid (TGA) as a reductant. TGA can be easily oxidized to the corresponding disulfide [SCH2CO2H]2, which in turn can reduce TeCl4 to Te. The obtained Te was characterized by XRD, SEM, EDS, and DRS. The effect of reducing agent on morphology and size of the products were also studied. Additionally, Te thin film was deposited on the FTO-TiO2 by Dr- blading then employed to solar cell application and measured open circuit voltage (Voc), short circuit current (Isc), and fill factor (FF) were determined as well. The studies showed that particle morphology and sizes play crucial role on solar cell efficiencies.

Scalable solution assembly of nanosheets into high-performance flexible Bi0.5Sb1.5Te3 thin films for thermoelectric energy conversion

Two-dimensional nanostructures, such as nanosheets and nanoribbons, represent excellent dimension for efficient transport of electrons and phonons and thus are ideal building blocks for hierarchical assembly of functional microscale electronic and phonic structures. We report a convenient and scalable solution approach for the hierarchical assembly of two-dimensional Bi2Te3&Sb2Te3 nanosheets compounds on polyimide substrates to obtain flexible Bi0.5Sb1.5Te3 thin films. We show that the nanosheets can be assembled into compact Bi0.5Sb1.5Te3 films by a solution-processed method followed by annealing treatment. Flexible thin films from two-dimensional nanosheets assemblies exhibit excellent thermoelectric properties and flexibility. The films of Bi0.5Sb1.5Te3/polyimide can be folded and even bent to a crease without cracking. The simple techniques involved in obtaining these films could help facilitate the rapid fabrication of flexible thermoelectric devices, heralding what promises to be a new approach toward the development of next-generation thermoelectric refrigeration and power harvesting technologies. Image of the flexible high-performance thermoelectric thin film synthesized from the Bi2Te3&Sb2Te3 nanosheets compounds on polyimide substrates. Crease picture of the flexible thin film after folded by a pliers.

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge1−x Mn x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge1−x Mn x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge0.96Mn0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge0.96Mn0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge0.96Mn0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.

Spin-glass behavior and anomalous magnetoresistance in ferromagnetic Ge1-xFexTe epilayer

We report that the Ge1-xFexTe thin film exhibits spin-glass behavior when the Fe concentration increases to 0.08. A large bifurcation between the zero-field cooling and field cooling temperature-dependent magnetization was observed. The hysteresis loops after zero-field cooling and field cooling show an exchange bias effect. A time-dependent thermoremanent magnetization follows power-law decay, which confirms the existence of spin glass. The anomalous magnetotranport properties present a further evidence for spin-glass behavior and give a freezing temperature Tg ∼ 5 K in the Ge0.92Fe0.08Te thin film.

Composition effect on the structure and optical parameters of Ge–Se–Te thin films

The present work reported the influence of Ge content variation on the optical properties of GexSe50Te50-x (x=0, 5, 15, 20, 35 at%). Vacuum thermal evaporation technique was employed to prepare amorphous GexSe50Te50−x thin films. The stoichiometry of the chemical composition was checked by energy dispersive X-ray spectroscopy (EDX), whereas the thin films structure was determined by an X-ray diffraction and a scanning electron microscope (SEM). The optical absorption measurements were performed at room temperature in the wavelength range of 200–900nm. Many optical constants were calculated for the studied thin films utilizing the optical absorption data. It was observed that the optical absorption mechanism follows the rule of the allowed direct transition. The optical band gap was found to increase from 2.31 to 2.60eV as the Ge content increases from 0 to 35 at%. This result was explained in terms of the chemical bond approach.

Optical Properties of Te Thin Films Prepared by Thermal Evaporation Technique

This work describes, Tellurium (Te) films were deposited on glass substrates at room temperature by the method of vacuum evaporation with thickness (0.4μm), with rate of deposition equal to (5.5Å/sec), the samples are annealed in a vacuum for one hour at (373 and 433) K. The optical properties such as absorption coefficient (k) was determined using the absorbance and transmittance measurement from FTIR Shimadzu spectrophotometer, with range of wave length (2800-3900) nm. And also calculated optical constants (refractive index (n), extinction coefficient (k)) for Tellurium films. The tests have been shown that the optical energy gap increases with increasing of annealing temperature for the samples.

Experimental and theoretical assessments of thermal boundary resistance between Bi0.4Sb1.6Te3 thin films and metals

An experimental method for measuring thermal boundary resistances of thermoelectrics/metal interfaces using two specially designed multilayer structures is presented. The thermal boundary resistances of Bi0.4Sb1.6Te3 thin films in contact with respective Ni and Ti metal layers are experimentally determined to be 2.65 ± 0.98 × 10−8 and 2.85 ± 1.06 × 10−8 m2K/W. The results agree reasonably well with the predictions from the diffusive mismatch model for Bi0.4Sb1.6Te3/metal interfaces. The effect of phonon transport property of metals on thermal boundary resistance at thermoelectrics/metal interfaces is investigated.

Peculiarities of Bi doping of Ge–Sb–Te thin films for PCM devices

The influence of Bi doping on the thermal, electrical, and optical properties of Ge2Sb2Te5 thin films was investigated. The existence of two Bi concentration ranges with different influence of dopant on the properties of thin films was established. At low concentrations (0.5–1.0 wt.% of Bi), anomalous deviations of physical properties from monotonous concentration dependences were observed. This effect is explained by the use of percolation theory, where formation of infinite clusters is accompanied by critical phenomena at critical concentrations.

Effect of post deposition annealing on thermal evaporated ZnSe:Te towards a scintillator application

ZnSe having a wide but direct bandgap (bulk band gap 2.7eV) is very efficient for detecting nuclear radiations by using its scintillation property. With a high absolute light output and radiation spectra which match well with Si-photodiode spectral sensitivity, ZnSe scintillators have adequately minimized the gap between “scintillator-photodiode” series for modern radiation detectors. Promising features like long decay time and high values of intrinsic radiation absorption make ZnSe based scintillators a very interesting research filed. Here in this report, we investigate an inexpensive and easy method to realize ZnSe:Te film on a quartz substrate. ZnSe:Te film is deposited using a thermal evaporator using ZnSe and ZnTe powders as raw materials to start with. After the deposition, the samples are exposed to a thermal annealing step for 1h. The temperatures are kept at 624, 674 and 724K. We have confirmed presence of a preferred [111] oriented polycrystalline ZnSe:Te thin films by XRD experiments. Photoluminescence experiment using a He:Cd laser at 324nm at low temperature (8K) reports an emission at 2.54eV due to non-equilibrium carrier plasma.

Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction

Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two most commonly observed magnetoresistance (MR) phenomena in topological insulators (TIs) and often attributed to the Dirac topological surface states (TSS). However, ambiguities exist because these phenomena could also come from bulk states (often carrying significant conduction in many TIs) and are observable even in non-TI materials. Here, we demonstrate back-gated ambipolar TI field-effect transistors in (Bi0.04Sb0.96)2Te3 thin films grown by molecular beam epitaxy on SrTiO3(111), exhibiting a large carrier density tunability (by nearly 2 orders of magnitude) and a metal-insulator transition in the bulk (allowing switching off the bulk conduction). Tuning the Fermi level from bulk band to TSS strongly enhances both the WAL (increasing the number of quantum coherent channels from one to peak around two) and LMR (increasing its slope by up to 10 times). The SS-enhanced LMR is accompanied by a strongly nonlinear Hall effect, suggesting important roles of charge inhomogeneity (and a related classical LMR), although existing models of LMR cannot capture all aspects of our data. Our systematic gate and temperature dependent magnetotransport studies provide deeper insights into the nature of both MR phenomena and reveal differences between bulk and TSS transport in TI related materials.

Influence of Cu on Transport Properties of Thermoelectric Thin Film Fabricated via Magnetron Co-sputtering Method

A simple magnetron co-sputtering method was used to fabricate Cu dispersed Bi0.5 Sb1.5 Te3 thin films, and the co-sputtering method was beneficial to the preferential growth of Bi0.5 Sb1.5 Te3 thin films along c-axis. Cu atoms were well-dispersed in the nano-structured materials. The electrical conductivity sharply increased with the increasing content of Cu due to the effect of Cu on transport property. For Cu target sputtering power of 20 W, a maximum power factor of 20 μW/(cm·K2) with an electrical conductivity of 15 × 104 S/m at 355 K were achieved.

Structural and magnetic characterization of (Zn,Fe)Te thin films grown by MBE

AbstractWe investigate the structural and magnetic properties of Zn1–xFex Te with an aim of discerning the intrinsic magnetism of this ternary compounds. Thin films of Zn1–xFex Te with a high Fe composition up to x = 0.2 were grown by molecular beam epitaxy (MBE) in the excess of Te flux against Zn flux. The structural analyses using X‐ray diffraction (XRD) and X‐ray absorption fine structure (XAFS) measurements reveal that crystals of the pure diluted phase are grown at low Fe compositions less than x = 0.02, while precipitates of extrinsic phases start to be formed at higher Fe compositions. In the magnetization measurements, the films of x ≤ 0.02, which were confirmed to be of the pure diluted phase, exhibit paramagnetic behaviors down to 2 K. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Peculiarities of Ultrathin Amorphous and Nanostructured Te Thin Films by Gas Sensing

Peculiarities of Ultrathin Amorphous and Nanostructured Te Thin Films by Gas Sensing

Screened remote plasma-enhanced atomic vapor deposition of Sb–Te thin film for the improvement of trench-covering ability

A new screened remote plasma-enhanced atomic vapor deposition (SPEAVD) technique was studied for depositing Sb–Te phase-change materials inside trench structures with high aspect ratios. The theoretical model of the screening mechanism included the concepts of a plasma sheath and a sticking coefficient. Plasma’s limitation of poor step coverage was overcome by filtering its high-energy ions and electrons. Cross-sectional observation was conducted using field emission scanning electron microscopy (SEM). The trench-covering ability of SPEAVD was much better than that of direct remote PEAVD under broad chamber conditions. Surface morphology was observed by SEM and atomic force microscopy. The surface chemical state of the deposited film was measured by X-ray photoelectron spectroscopy. The crystallinity in various deposition temperatures was analyzed by X-ray diffraction measurements. The electrical characteristics of the films were measured by a four-point probe. We expect this research to provide a new deposition method that will allow fine control of step coverage and other characteristics.

High field vortex phase diagram of Fe(Se, Te) thin films

We report on the (H, T) vortex phase diagram up to 35 T of Fe(Se, Te) thin films deposited on CaF2 substrates as determined by resistivity, Nernst effect and critical current measurements. We found the presence of a large region where the vortex is firmly pinned, allowing the adoption of chalcogenides for low temperature but extremely high magnetic field applications. The fact that high critical current density values—larger than 1 MA cm−2 in self-field and liquid helium—are reached together with a very weak dependence on the magnetic field and a complete isotropy, joined with the very high rigidity of the vortex lattice at very high field, makes the Fe(Se, Te) phase very promising for low temperature (≤4.2 K) and high field (≥25 T) applications.

Study of Gd-doped Bi2Te3 thin films: Molecular beam epitaxy growth and magnetic properties

Incorporation of magnetic dopants into topological insulators to break time-reversal symmetry is a prerequisite for observing the quantum anomalous Hall (QAHE) effect and other novel magnetoelectric phenomena. GdBiTe3 with a Gd:Bi ratio of 1:1 is a proposed QAHE system, however, the reported solubility limit for Gd doping into Bi2Te3 bulk crystals is between ∼0.01 and 0.05. We present a magnetic study of molecular beam epitaxy grown (GdxBi1–x)2Te3 thin films with a high Gd concentration, up to x ≈ 0.3. Magnetometry reveals that the films are paramagnetic down to 1.5 K. X-ray magnetic circular dichroism at the Gd M4,5 edge at 1.5 K reveals a saturation field of ∼6 T, and a slow decay of the magnetic moment with temperature up to 200 K. The Gd3+ ions, which are substitutional on Bi sites in the Bi2Te3 lattice, exhibit a large atomic moment of ∼7 μB, as determined by bulk-sensitive superconducting quantum interference device magnetometry. Surface oxidation and the formation of Gd2O3 lead to a reduced moment of ∼4 μB as determined by surface-sensitive x-ray magnetic circular dichroism. Their large atomic moment makes these films suitable for incorporation into heterostructures, where interface polarization effects can lead to the formation of magnetic order within the topological insulators.

Characterization of Cu1.4Te Thin Films for CdTe Solar Cells

The copper telluride thin films were prepared by a coevaporation technique. The single-phase Cu1.4Te thin films could be obtained after annealing, and annealing temperature higher than 220°C could induce the presence of cuprous telluride coexisting phase. Cu1.4Te thin films also demonstrate the high carrier concentration and high reflectance for potential photovoltaic applications from the UV-visible-IR transmittance and reflectance spectra, and Hall measurements. With contacts such as Cu1.4Te and Cu1.4Te/CuTe, cell efficiencies comparable to those with conventional back contacts have been achieved. Temperature cycle tests show that the Cu1.4Te contact buffer has also improved cell stability.

Fabrication of NiTe films by transformed electrodeposited Te thin films on Ni foils and their electrical properties

Tellurium (Te) source of compact nickel telluride (NiTe) thin film was prepared by simple electrochemical deposition method.