Telluride Films Research Articles

Structural, Electrical, and Optical Properties of PbTe Thin Films Prepared by Simple Flash Evaporation Method

Lead telluride (PbTe) films have been prepared on glass substrates by flash evaporation method. Structure of the film is found to possess stable face-centered cubic (fcc) NaCl phase in which the grains predominantly grow in the direction of (200) plane. The calculated grain size was in the range 19 nm. The electrical resistivity as a function of temperature was measured using four-probe technique. The electrical conductivity was calculated, and the value is found to be from 67.11 to 344.82 S/cm in the temperature range 303–453 K. The value of mobility was evaluated, and it is found to be0.329×10-3 cm2·V−1 S−1. An optical study reveals that the PbTe thin films exhibit large blue shift. The optical constants such as absorption coefficient and refractive index have been estimated, and the results are discussed.

Injection photodiode based on p-CdTe film

Photosensitive large-block p-type cadmium telluride (p-CdTe) film with a resistivity of ρ ≈ 106−107 Ω cm has been used to create an Al-p-CdTe-Mo structure with a Schottky barrier, which exhibits the properties of an injection photodiode. Being switched in the forward current passage direction (with “+” on the Mo contact) at high illumination levels, the proposed structure exhibits a responsivity of Sλ ≈ 2.6 A/W at a wavelength of λ = 0.625 μm, which is about five times the spectral response of the ideal photodetector for this wavelength range.

A hybridized graphene carrier highway for enhanced thermoelectric power generation

The decoupling and enhancement of both Seebeck coefficient and electrical conductivity were achieved by constructing the c-axis preferentially oriented nanoscale Sb(2)Te(3) film on monolayer graphene. The external graphene layer provided a highway for charge carriers, which were stored in the thicker binary telluride film, due to the extremely high mobility.

Feasibility study of direct-conversion x-ray detection using cadmium zinc telluride films

Polycrystalline Cd(Zn)Te films deposited by vacuum thermal evaporation are investigated as a direct-conversion medium for x-ray detection. The use of evaporation techniques allows for the preparation of large-area films with the potential for large-area x-ray imaging with high spatial resolution. Films with compositions of Cd1−xZnxTe (x = 0.15,0.25,0.3) were prepared to a thickness of 20 μm on slide glass substrates with an aluminum or indium tin oxide (ITO) bottom electrode and a silver top electrode, with and without additional charge-blocking layers (CeO2 and parylene dielectric) between the electrode and CdZnTe film to reduce leakage current. Composition and structural analyses of the as-deposited films confirm the development of polycrystalline CdZnTe. Leakage current in the CdZnTe film is approximately ten times lower than for a comparable CdTe film, and the lowest leakage is obtained for a film with composition of Cd0.7Zn0.3Te The use of ITO rather than aluminum as the bottom electrode provides a further improvement in leakage current and improved stability. The Cd0.7Zn0.3Te-based device with ITO electrode and charge-blocking layers achieves the highest total output charge (180.44 pC/cm2 or 1.1 × 109 e−/cm2) and signal-to-noise ratio (6.19 at applied bias of 30 V). The present experiments show that Cd1−xZnxTe in a multilayer structure with charge-blocking layers is feasible as a good radiation conversion layer for flatpanel radiation imaging systems.

B132 耐熱性ユニットを有するブロックコポリマーを用いたナノ構造体の作製と熱電材料の高性能化(OS-6: マイクロ・ナノ熱工学(3))

We investigated the effects of nano-structure (pore size and depth) on the thermoelectric properties of porous p-Bi_<0.4>Te_3Sb_<1.6> and n-Bi_<2.0>Te_<2.7>Se_<0.3> thin films.The cross-plane thermal conductivity of the porous p-Bi_<0.4>Te_3Sb_<1.6> and n-Bi_<2.0>Te_<2.7>Se_<0.3> thin films is 0.26 and 0.22 W m^<-1> K^<-1>, respectively. The value was significantly reduced from that of standard p- and n-type flush evaporated bismuth telluride films. The figure of merit (ZT) of the porous p-Bi_<0.4>Te_3Sb_<1.6> and n-Bi_<2.0>Te_<2.7>Se_<0.3> thin films is estimated to be ZT = 1.30, 1.47, respectively. We evaluated the thermoelectric power generation of porous and flat p-Bi_<0.4>Te_3Sb_<1.6> single cell. The output power is improved 25 % by nano porous structure.

Impact of CdTe surface composition on doping and device performance in close Space sublimation deposited CdTe solar cells

ABSTRACTCool‐down after close‐space sublimation growth of cadmium telluride films has been shown to lead to the formation of a Cd‐rich surface that can block the subsequent in‐diffusion of cadmium chloride that is used in post‐growth doping of solar cell devices. Pre‐etching of the as‐grown surfaces using a nitric–phosphoric acid etch prior to doping leads to increase in chlorine and oxygen in‐diffusion, and an associated improvement in device efficiency from &lt;3% to 12% in controlled trials. Capacitance‐voltage analysis of such devices revealed an increase in the acceptor concentration from 3.76 × 1013 cm−3 to 5.6 × 1014 cm−3, whereas thermal admittance spectroscopy revealed fewer deep states at energies &gt;0.4 eV for etched devices. Impedance analysis showed the improved cadmium chloride treatment—resulting from the additional etching step—removed the grain boundary component from the equivalent circuit, indicating a grain boundary passivation effect. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Study of molecular beam epitaxy of cadmium telluride on sapphire

The effect of thermal treatment of (0001) sapphire substrates on the structure of cadmium telluride films grown by molecular beam epitaxy was investigated. The growth process was carried out on a laboratory facility equipped with a mass spectrometer and electron diffractometer. It was established that no annealing or annealing in vacuum (P 1000°C. Electron diffraction patterns showed that they have a mosaic structure and contain twins with the 〈111〉 twinning axis. Atomic-force microscopic images revealed CdTe crystallites with lateral sizes of ∼50 nm arranged along the steps on the sapphire substrate surface. These results allowed the conclusion to be drawn that the growth of CdTe on sapphire substrates can occur by the formation of the three-dimensional nucleation centers according to the Volmer-Weber mechanism.

Telluride films and waveguides for IR integrated optics

AbstractThe fabrication of micro‐components for far infrared applications such as spatial interferometry requires the realization of single‐mode channel waveguides being able to work in the infrared region. One of the key issues in case of channel waveguides is the selection of materials for the core layer. Amorphous telluride films are particularly attractive for their transparency in a large spectral domain in the infrared region. A second key issue is the selection of an appropriate method for film deposition. Indeed, waveguides for far infrared applications are characterized by a thick core layer (10‐15 µm, typically). The challenge is thus to select a deposition method which ensures the deposition of thick films of optical quality. In this paper, it is shown that thermal co‐evaporation meets this challenge. In particular, it allows varying the composition of the films very easily and thus adjusting their optical properties (refractive index, optical band gap). The example of thermally co‐evaporated Te‐Ge films is given. Films with typical thickness of 7‐15 µm were elaborated. Their morphological, structural, thermal and optical properties were measured. A particular attention was paid to the checking of the film homogeneity. The realized waveguiding structures and their optical testing are then described. In particular, the first transmission measurements at 10.6 µm are presented. In conclusion, the feasibility of micro‐components based on the stacking and etching of chalcogenide films is demonstrated, opening the door to applications related to detection in the mid‐ and thermal infrared spectral domains (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of hot-press treatment on electrochemically deposited antimony telluride film

Antimony telluride thin film electrochemically deposited in a triethanol based alkaline electrolyte features amorphous structure, high electrical resistance, as well as fine morphology, minor impurity incorporation and anti-corrosivity. To further improve film thermoelectric performance, this film was subjected to hot-uniaxial-press (HUP) treatment at 170–250 °C. HUP treated films revealed crystallized structures, and exhibited 2–3 orders of magnitude improvement of electrical conductance. The [TeO 3 2−]/[SbO 2 −] of the deposition electrolyte was utilized to fine tune film composition and thermal electrical performance. Ni diffusion from the substrate into the film was also studied, and it can be reduced by using lower temperature and shorter time of HUP treatment. Film Seebeck coefficient and power factor reached 138 μV/K and 337 μW/K 2∙m, respectively, at elaborated deposition and HUP conditions.

개조된 MOCVD법으로 성장한 Bi2Te3박막의 기판온도에 따른 열전 특성

Thermoelectric bismuth telluride (<TEX>$Bi_2Te_3$</TEX>) films were deposited on <TEX>$4^{\circ}$</TEX> off oriented (001) GaAs substrates using a modified metal organic chemical vapor deposition (MOCVD) system. The effects of substrate temperature on surface morphologies, crystallinity, electrical properties and thermoelctric properties were investigated. Two dimensional growth mode (2D) was observed at substrate temperature lower than <TEX>$400^{\circ}C$</TEX>. However, three dimensional growth mode (3D) was observed at substrate temperature higher than <TEX>$400^{\circ}C$</TEX>. Change of growth mechanism from 2D to 3D was confirmed with environmental scanning electron microscope (E-SEM) and X-ray diffraction analysis. Seebeck coefficients of all samples have negative values. This result indicates that <TEX>$Bi_2Te_3$</TEX> films grown by modified MOCVD are n-type. The maximum value of Seebeck coefficient was -225 <TEX>${\mu}V/K$</TEX> and the power factor was <TEX>$1.86{\times}10^{-3}\;W/mK^2$</TEX> at the substrate temperature of <TEX>$400^{\circ}C$</TEX>. <TEX>$Bi_2Te_3$</TEX> films deposited using modified MOCVD can be used to fabricate high-performance thermoelectric devices.

Investigation of Dimethyl Sulfoxide Electrolytes for Electrodepositing Thermoelectric Bismuth Telluride Films

Dimethyl sulfoxide (DMSO) electrolytes were investigated for the electrodeposition of bismuth telluride films for thermoelectric applications. The investigation revealed the influences of electrolyte ion concentration, deposition potential, and temperature on the alloy composition. Full composition range of bismuth telluride BixTey materials could be prepared by varying either electrolyte ion concentration or deposition potential. The film morphology was found smooth at the stoichiometric composition and affected by the film composition, rather than deposition conditions. The highest Seebeck coefficient observed for n-type films was -134 uV/K (unannealed) and +150 uV/K for p-type (annealed).

Fabrication and study of porous PbTe layers on silicon substrates

AbstractFabrication of porous PbTe layers on Si substrates using a developed technique based on anodic electrochemical etching of PbTe/CaF2/Si(111) heteroepitaxial structures in a Norr electrolyte with anodizing current density 2‐4 mA/cm2 is reported. The structural and morphological parameters of such layers are investigated with x‐ray reflectometry and diffractometry methods and with high‐resolution scanning electron microscopy. The influence of anodizing conditions on the parameters of the layers is discussed, as well as the morphological peculiarities of the anodized structures. The presence of cylindrical mesopores with diameter of 7‐26 nm inclined at the angle of ∼35° to the surface normal is shown. The value of the porosity for the near‐surface layer of lead telluride films is in the interval of 41‐68% and depends on the anodic electrochemical treatment conditions. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth and transport properties of oriented bismuth telluride films

Oriented n-type bismuth telluride thin films with various layered nanostructures have been fabricated by radio-frequency (RF) magnetron sputtering. The crystal structures and microstructures of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The transport properties including carrier concentration, mobility, Seebeck coefficient and in-plane electrical conductivity were measured, which showed strong microstructure-dependent behaviors. The relationship between morphologies and transport properties of the films was explored. The optimal morphology and transport properties of films were obtained at the substrate temperature of 350 °C under the pressure of 1.0 Pa with oriented layered structure. Based on these results, a formation mechanism of these nanostructures is proposed and discussed. The interfaces and grain boundaries formed in these layered structures are beneficial to the reduction in thermal conductivity, which could result in potential TE films with high ZT value.

The rapid prototyping of textured amorphous surfaces for the graphoepitaxial deposition of CdTe films using focused ion beam lithography

Cadmium telluride films deposited on amorphous substrates exhibit a grain structure characterized by [111]-oriented grains, but where the in-plane grain orientation is randomized due to the absence of epitaxy. Here, we explore the viability of promoting an in-plane grain alignment through graphoepitaxy. Fifteen different substrate surface textures were fabricated using focused ion beam lithography. This approach allows for the side-by-side deposition of surface textures where both the areal extent and depth of the surface features are varied in a systematic manner. CdTe films deposited overtop these textures show grain structures with dramatic variations, revealing that particular length scales have the most pronounced effect on the grain structure.

Self-assembled bismuth telluride films with well-aligned zero- to three-dimensional nanoblocks for thermoelectric applications

The dimension and size of the building blocks as well as the preferential orientation, geometry and regularity of their assemblies are the most important key factors for fabricating thermoelectric materials with a high figure-of-merit (ZT) which is governed by the efficiencies in transporting electrical and thermal energies along the measurement direction. A one-step and large-area growth approach has been successfully developed employing pulsed laser deposition (PLD) for producing physically self-assembled and well-aligned Bi2Te3 nanostructures on SiO2/Si substrates without pre-built templates or catalysts. The precisely parameter-controlled growth provides four highly reproducible and significant Bi2Te3 assemblies, comprising 0-dimensional (0-D) nanoparticles, 1-D nanorods, 2-D nanoflakes, and 3-D nanocanyons, respectively exhibiting an overall (006), (015), (110), and (006) preferential orientation normal to the substrate surface. The nanoparticle assisted crystal growth is proposed, mainly involving the condensation of the plasma species in the gas phase at higher ambient pressures and the following diffusion and reorganization of the deposited nanoparticle atoms on the substrates. The well-aligned 0-D to 3-D Bi2Te3 nanostructures show more excellent in-plane power factors than most of the randomly aligned Bi2Te3 nanostructures at room temperature mainly due to significantly reducing inter resistance. The thermoelectric properties of these well-aligned Bi2Te3 nanostructures are comparable to any other intrinsic Bi2Te3 nanostructures that have ever been reported. The present data are valuable for further improving and designing advanced thermoelectric materials and confirm that precise control of nanostructural aggregation is an effective strategy for enhancing the thermoelectric performance.

Electrochemical Deposition and Optimization of Thermoelectric Nanostructured Bismuth Telluride Thick Films

Bismuth telluride thick films are suitable for thermoelectric (TE) devices covering large areas and operating at small-to-moderate temperature differences (20 - 200 K). High efficiency and high coefficient of performance (COP) are expected to be achieved by using thick films in some cooling applications. Bismuth telluride thick films fabrication have been achieved with Galvanostatic and Potentionstatic deposition. Stoichiometric bismuth telluride thick film was obtained by Galvanostatic deposition at current density of 3.1 mAcm-2. Bismuth telluride films with average growth rate of 10 µmh-1 and different composition were obtained. Effects of current density and composition of electrolyte in Galvanostatic deposition were studied. The current density affected the film compactness, where films deposited at lower current density were more compact than those deposited at higher current density. The morphology of the films did not depend on the current density, but chemical composition was observed when different composition of electrolyte was used. Effects of distance between electrodes, composition of electrolyte solution, and stirring in Potentionstatic deposition were studied. The shorter the distance between electrodes, the higher the electric field, thus the higher current density was applied and the deposited film was less compact. The current density increased more rapidly with stirring during electrodeposition which leads to less compact film. Through this study, films electrode-posited from solution containing 0.013 M Bi(NO3)3.5 H2O, 0.01 M TeO2 and 1 M HNO3 at 3.1 mA cm-2 for 6 hours without stirring and with interelectrode distance of 4.5 cm were free-standing with average film thickness of 60 µm and optimum film composition of Bi2Te3. The crystallite size of the later films was found to be around 4.3 nm using Scherrer’s equation from XRD patterns. Also, negative Seebeck coefficient for the same samples was revealed with an average value of -82 μV.K-1.

Nanostructured Telluride Films on Macroporous Silicon for High Efficiency Thermoelectric Devices

ABSTRACTWe propose using macroporous silicon as an ultra-high aspect ratio scaffolding for epitaxially grown thermoelectric materials, so that thin films can be shaped into materials thick enough for practical devices. The self-limiting nature of atomic layer deposition (ALD) makes it an ideal growth technique for this substrate, as uniform thickness can be obtained at all points inside the macroporous structure, and we demonstrate successful deposition of antimony telluride on pore walls using ALD. Extension of this work to telluride superlattices should enable fabrication of thermoelectric devices with figures of merit (ZT) in excess of 2. Characterization of the thermoelectric and other properties of ALD grown telluride on silicon is ongoing.

Inflation (1943) and the Blacklist: The Disrupted Film Career of Cy Endfield

Inflation (1943) and the Blacklist: The Disrupted Film Career of Cy Endfield

Quantification of multiple twinning in face centred cubic materials

Grain boundary engineering (GBE) aims at optimizing the properties of face centred cubic materials with low stacking fault energy by creating a high content of special twin boundaries. Quantifying twinning and its parameters, such as the twin related domains (TRDs) and the related ∑3 n special grain boundaries, is thus of prime importance for GBE. A method is presented in detail to treat the electron backscatter diffraction (EBSD) maps. The TRDs are automatically reconstructed, the twinning chain trees of the TRDs are determined and represented with fractal graphs, and the ∑3 n grain boundaries are identified up to order n = 12. New parameters, such as the numbers of grains ( N g ), the lengths of the longest chain (LLC), the twinning polysynthesism ( p) and twinning anisotropy ( a) factors, are also calculated. Examples have been given for two nickel superalloys, a silicon ingot, a cadmium telluride film, and a copper thin film.

"Graphene-Like" Exfoliation of Atomically-Thin Films of Bi2Te3 and Related Materials: Applications in Thermoelectrics and Topological Insulators

In this paper we overview our results on "graphene-inspired" exfoliation of crystalline bismuth telluride films with a thickness of just a few atoms. The atomically thin films were suspended across trenches in Si/SiO2 substrates, and subjected to detail material characterization. The presence of the van der Waals gaps allowed us to disassemble Bi2Te3 crystal into its quintuple building blocks - five mono-atomic sheets - consisting of Te(1)-Bi-Te(2)-Bi-Te(1). By altering the thickness and sequence of atomic planes we were able to create "designer" quasi-2D crystalline films, change their composition, the type of charge carriers as well as other properties. The exfoliated quintuples and ultra-thin films have low thermal conductivity, relatively high electrical conductivity and enhanced thermoelectric properties. The obtained results pave the way for producing stacks of crystalline bismuth telluride quantum wells with nearly infinite potential barriers, strong spatial confinement of charge carriers and acoustic phonons, beneficial for thermoelectric devices.