Phase transition behaviors and thermal conductivity measurements of nitrogen-doped Sb2Te3 thin films

Crystallization temperature of nitrogen-doped Sb₂Te₃ (ST) thin films increased with increasing nitrogen doping concentration, which indicates that the long-term stability of the metastable amorphous state can be improved by nitrogen doping. The root-mean-square (rms) roughness values of the films showed a significant decrease with nitrogen doping. Thermal conductivity of nitrogen-doped ST thin films was measured using a transient thermoreflectance (TTR) technique. It was found that the thermal conductivity decreased with increasing nitrogen doping concentration and increased with increasing annealing temperature. Nitrogen-doped ST thin films are suitable phase-change materials for low programming power consumption applications of phase-change random access memory (PCRAM).     

AC conductivity and dielectric properties of Sb2Te3 thin films

Sb₂Te₃ films of different thicknesses, in the thickness range 300-620 nm, were prepared by thermal evaporation. X-ray analysis showed that the as-deposited Sb₂Te₃ films are amorphous while the source powder and annealed films showed a polycrystalline nature. The AC conductivity and dielectric properties of Sb₂Te₃ films have been investigated in the frequency range 0.4-100 kHz and temperature range 303-373 K. The AC conductivity σ_AC(ω) was found to obey the power law ω^s where s?1 independent of film thickness. The temperature dependence of both AC conductivity and the exponent s can be reasonably well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and the dielectric loss ε₂ are frequency and temperature dependent and thickness independent. The maximum barrier height W_M calculated from dielectric measurements according to the Guintini equation agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The effect of annealing at different temperatures on the AC conductivity and dielectric properties was also investigated. Values of σ_AC, ε₁ and ε₂ were found to increase with annealing treatment due to the increase of the degree of ordering of the investigated films. The Cole-Cole plots for the as-deposited and annealed Sb₂Te₃ films have been used to determined the molecular relaxation time τ. The temperature dependence of τ indicates a thermally activated process.     

Catalyst-free growth of Sb2Te3 nanowires

In this study, a catalyst-free growth method was discovered to prepare the high-quality single crystal Sb₂Te₃ nanowires from the Al:Ge:Sb:Te thin films. The diameters of Sb₂Te₃ nanowires were found to be ~ 100 nm and their lengths were as great as tens of micrometers. The Al content and the annealing temperature play an important role in the growth of Sb₂Te₃ nanowires. When the Al content (> 12.4 at.%) was sufficiently contained in Al:Ge:Sb:Te film, Sb₂Te₃ nanowires were extruded spontaneously on the surface of thin film with increase in annealing temperatures. Compared with the vapor-liquid-solid method, our method has advantages of low temperature (~ 300 °C) and no impurities, such as a metal catalyst.     

Optimization of Bi2Te3 and Sb2Te3 thin films deposited by co-evaporation on polyimide for thermoelectric applications

The optimization of the deposition process of n-type Bismuth Telluride and p-type Antimony Telluride thin films for thermoelectric applications is reported. The films were deposited on a 25 μm-thick flexible polyimide (kapton) substrate by co-evaporation of Bi and Te, for the n-type element, and Sb and Te, for the p-type element. The evaporation rate of each material was monitorized by an oscillating crystal sensor and the power supplied to each evaporation boat was controlled with a PID algorithm in order to achieve a precise user-defined constant evaporation rate.The influence of substrate temperature (in the range 240-300 °C) and evaporation rates of Bi, Te and Sb on the electronic properties of the films was studied and optimized to obtain the highest Seebeck coefficient. The best n-type Bi₂Te₃ films were deposited at 300 °C with a polycrystalline structure, a composition close to stoichiometry, electrical resistivity ∼20 μΩ m and Seebeck coefficient −195 μV/°C. The best p-type Sb₂Te₃ films were deposited at 240 °C, are slightly Te-rich, have electrical resistivity ∼20 μΩ m and Seebeck coefficient +153 μV/°C. These high Seebeck coefficients and low electrical resistivities make these materials suitable for fabrication of Peltier coolers and thermopile devices.     

Crystallization behavior and thermoelectric characteristics of the electrodeposited Sb2Te3 thin films

Crystallization behavior of the electrodeposited Sb₂Te₃ film was characterized and the effect of the amorphous-crystalline transition on the Seebeck coefficient was evaluated. The as-electrodeposited Sb₂Te₃ film was amorphous and exhibited the Seebeck coefficient of 268-322 μV/K, which was much larger than the value of the crystalline Sb₂Te₃ film. When annealed at temperatures above 100 °C, the Seebeck coefficient of the Sb₂Te₃ film dropped significantly to 78-107 μV/K due to the amorphous-crystalline transition at 94 °C. The thermal stability of the electrodeposited Sb₂Te₃ film was improved by the addition of Cu, and the crystallization temperature of the CuSbTe film increased up to 149.5 °C.     

Pulsed laser deposition of Bi2Te3 thin films

A feasibility study of thin stoichiometric Bi₂Te₃ films by pulsed laser deposition (PLD) was performed, the difficulty arising from the differences of vapour pressure between Te and Bi. The films were elaborated using a pulsed Nd:YAG laser under various experimental conditions and were characterized by electron microsprobe, scanning electron microscopy, X-ray diffraction and secondary ion mass spectroscopy analyses. Congruent transfer of stoichiometry occurs from the target to the substrate on several cm² and a good crystallinity can be achieved, even on glass substrates at room temperature, by combining convenient target to substrate facing and distance, respectively. Depletion in Te observed in some films may result from a laser beam-plume interaction that was put forward during elaboration of films on large scale substrates.     

Moire-pattern-modulated electronic structures in Sb2Te3/graphene heterostructure

Moire superlattice has recently been found in topological insulators,which can lead to periodic modulation on the electronic structure.In this work,we report the low-temperature scanning tunneling microscopy study of Sb₂Te₃ films grown on graphitized 4H-SiC.We find that substrate temperature can strongly influence the rotation angles between Sb₂Te₃ film and graphene substrate.Three kinds of moire patterns are observed at the first quintuple layer Sb₂Te₃ film under different substrate temperatures.One shows complicated patterns with a rotation angle of nearly 0°relative to the substrate,another just exhibits simple 1 ×1 structure with a rotation angle of 30°.Other rotation angle like 8.2°is observed at higher substrate temperature as well,which is relatively rare.Comparison of the d//dV curves from Sb₂Te₃ films with different moire patterns indicates that the superstructure can offer degrees of freedom in tailoring electronic structure.This work may stimulate the further study on the moire modulation to the electronic properties of topological insulators.     

Sb2Te3-Ta2O5 nano-composite films for low-power phase-change memory application

Sb₂Te₃-Ta₂O₅ nano-composite films were deposited by the cosputtering of Sb₂Te₃ and Ta₂O₅ targets using radio frequency magnetron sputtering system at room temperature. A phase-change random access memory (PCRAM) device based on the Sb₂Te₃-Ta₂O₅ films was successfully fabricated. Compared to a pure Ge₂Sb₂Te₅ based PCRAM cell, the reset voltage of the Sb₂Te₃-Ta₂O₅ based cell was obviously reduced, which was attributed to the reduced thermal conductivity and lower melting point of the Sb₂Te₃-Ta₂O₅ films. In addition, the device with the Sb₂Te₃-Ta₂O₅ layer could work with much shorter pulse widths for both SET and RESET, suggesting that the Sb₂Te₃-Ta₂O₅ based compounds are promising candidates for low-power and fast-speed PCRAM application.     

Thermal co-evaporation of Sb2Te3 thin-films optimized for thermoelectric applications

Antimony telluride (Sb₂Te₃) is a chalcogenide material used in thermoelectric applications. The deposition of thin films of Sb₂Te₃ requires a precisely controlled process to achieve a desirable high thermoelectric figure-of-merit. The optimization of the thermal co-evaporation process for p-type Sb₂Te₃ thin-film onto plastic substrates (Kapton© polyimide) for thermoelectric applications is reported. The influence of deposition parameters and composition on thermoelectric properties was studied, seeking optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy all confirmed the formation of Sb₂Te₃ thin films. Seebeck coefficient (up to 190 μVK⁻¹), in-plane electrical resistivity (8-15 μΩm), carrier concentration (1 × 10¹⁹-7 × 10¹⁹ cm⁻³) and Hall mobility (120-180 cm²V⁻¹s⁻¹) were measured at room temperature for the best Sb₂Te₃ thin-films.     

Synthesis of Sb2Te3 nanopowders by vacuum arc plasma evaporation

Taking elemental antimony and tellurium as source materials, Sb₂Te₃ nanopowders were prepared by vacuum arc plasma evaporation technique. Microstructure and morphology of the samples were characterized via X-ray diffraction and field emission scanning electron microscope. Compositional analysis was carried out by energy dispersive analysis of X-rays. Lattice constants (a = 4.267 Å, c = 30.469 Å) of Sb₂Te₃ nanopowders are calculated by (015) and (110) diffraction peaks of X-ray diffraction patterns. Field emission scanning electron microscope surface morphology of the nanopowders shows the irregular polyhedron and rhombohedral structure. Atoms percentage of the nanopowders is calculated by quantitative analysis using energy dispersive analysis of X-rays spectrum. The experimental results show that the percentages of Sb and Te atoms are 41.3% and 58.7% respectively.     

Solar cells with Sb2S3 absorber films

CdS/Sb₂S₃/PbS structures were prepared by sequential chemical deposition of CdS, Sb₂S₃ and PbS thin films on TEC-8 (Pilkington) transparent electrically conductive SnO₂ (TCO) coatings. CdS thin films (100 nm) were deposited with hexagonal structure from Cd-citrate bath and of cubic structure from Cd-ammine/triethanolamine bath. Sb₂S₃ thin films were deposited at 40 °C from a solution mixture of potassium antimony tartrate, triethanolamine, ammonia and thioacetamide(TA) or at 1 to 10 °C from a mixture of antimony trichloride and thiosulfate (TS). These films were made photoconductive by heating at temperatures 250 to 300 °C. When heated in the presence of a chemically deposited Se thin film of 300 nm, a solid solution Sb₂S_1.8Se_1.2 resulted. PbS thin films of 100-200 nm thickness were deposited on the TCO/CdS/Sb₂S₃ or TCO/CdS/Sb₂S_1.8Se_1.2 structure. Graphite paint was applied on the PbS film prior to applying a silver epoxy paint. The cell structures were of area 0.4 cm². The best results reported here is for a cell: TCO/CdS(hex-100 nm)/Sb₂S₃(TS-100 nm)/PbS(200 nm) with open circuit voltage (V_oc) 640 mV, short circuit current density 3.73 mA/cm², fill factor 0.29, and conversion efficiency 0.7% under 1000 Wm^− 2 sunlight. Four series-connected cells of area 1 cm² each gave V_oc of 2 V and short circuit current of 1.15 mA.     

Electrical contacts on photoconductive Sb2S3 films

Evaporated metal contacts to layers of deposited Sb₂S₃ 1 μm thick have been investigated. It has been found that Ni, Au and Pt make blocking contacts while contacts of In, Ga, Al and Bi are ohmic. Cu, Ag and Pd contacts show poor stability and have a very low breakdown voltage; they are not considered to be suitable contact materials. If Al is used care must be taken to avoid the formation of a layer of Al₂O₃ between the Al and the Sb₂S₃ which will result in a blocking contact. Au makes a very reproducible and reliable contact capable of withstanding fields up to about 6 x 10⁵ V/ cm.The type of contact made by any of these metals depends on the work function of the metal. The low work function metals In and Ga make the best ohmic contacts. The high work function metals Pt and Au make good blocking contacts. The results can be explained in terms of the conventional theory of contacts if the Sb₂S₃ layer is assumed to be n-type with a low density of surface states. On this basis it is expected that the work function of Sb₂S₃ lies in the range 4.7–4.9 eV.     

Thermoelectric Properties of (Bi2Te3)1 ? x ? y(Sb2Te3)x(Sb2Se3)y Single Crystals

Homogeneous and graded n- and p-type (Bi₂Te₃)_{1 − x − y} (Sb₂Te₃) _x (Sb₂Se₃) _y crystals are grown by the Czochralski technique with melt supply through a floating crucible. The dimensionless thermoelectric figure of merit of the n- and p-type crystals is ZT = 1.1 (350 K) and 1.0 (375 K), respectively. It is shown that (Bi₂Te₃)_{1 − x − y} (Sb₂Te₃) _x (Sb₂Se₃) _y pseudoternary solid solutions can be used to produce monolithic n- and p- type graded and segmented thermoelectric materials by Czochralski growth. The thermoelectric power distribution across the seed-crystal interface is studied using scanning hot microprobe measurements. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 10, 2005, pp. 1186–1193. Original Russian Text Copyright ? 2005 by Svechnikova, Shelimova, Konstantinov, Kretova, Avilov, Zemskov, Stiewe, Zuber, Muller.     

Rapid thermal annealing and conventional furnace effect on SrBi2Ta2O9 thin films crystallization

The effect of rapid thermal annealing and conventional furnace annealing on the crystallization of SrBi₂Ta₂O₉ is modeled using an Arrhenius-type equation. Based on previous experimental results several authors have suspected that different reaction mechanisms occur depending of the heating rate employed in these two treatment methods. Oppositely, the results here presented and the analysis of reported data suggest two concurrent reaction mechanisms 1) direct transformation of the reagents into the perovskite phase and 2) transformation of an intermediate into the perovskite phase. Another important conclusion here obtained confirms the validity of the Arrhenius equation to predict the behavior of the crystallization process by either of the two heating methods mentioned above.     

水热合成Sb_2Se_3纳米线及其对Bi_2Te_3纳米粉末热电性能的影响

以SbCl3和Se粉为原料,水合肼(N2H4·H2O)为还原剂,采用水热法在150℃下,分别保温不同的时间合成Sb2Se3纳米粉末.通过X射线衍射(XRD)、场发射电子扫描电镜(FESEM)、透射电镜(TEM)以及高分辨透射电镜(HRTEM)等分析方法对产物的物相成分和微观形貌等进行了表征,实验结果表明保温时间达到24h时,获得产物为单相Sb2Se3纳米线晶体.根据实验结果还研究了水热合成Sb2Se3纳米线晶体可能的反应及生长机理,结果表明一维纳米线沿[001]方向生长,纳米线的形成与其独特的层状晶体结构有关.最后采用放电等离子体快速热压烧结法将水热合成的Bi2Te3纳米粉末与不同含量Sb2Se3纳米线进行复合,分析了Sb2Se3纳米线对Bi2Te3纳米材料热电性能的影响,发现复合约1at%Sb2Se3纳米线可以使Bi2Te3纳米材料热电性能有一定提高.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

The optical properties of thin films of Sb2O3

The refractive index of Sb₂O₃ thin films deposited on a variety of substrates at temperatures from 15° to 200°C is measured as a function of wavelength in the range 300–4000 nm. The clarity of the films and the values of the refractive index are correlated with the degree of orientation of the crystallites comprising the films.     

Dielectric and photoconducting properties of Ga2Te3 and In2Te3 crystals

Single crystals of defect III–VI semiconductors Ga₂Te₃ and In₂Te₃ have been grown by the Bridgman method. Capacitance vs frequency measurements have been carried out from which the low frequency dielectric constants ?₅ have been determined to be 10.95 ± 0.26 and 12.3 ± 0.13 respectively. These values are compared with the high-frequency dielectric constants ?₆₀ calculated from the Phillips' model. Dark conductivity and photoconductivity have been studied as a function of annealing upto 210°C, maxinum photosensitivity being obtained for both crystals for T_anneal = 80°C. This behaviour has been related to lattice ordering through x-ray diffraction studies. Measurements of photo conductive gain indicate carrier life-times of 2 × 10^?4s and 5 × 10^?4s respectively at room-temperature.     

Electron-microscopic study of glassy As2Te3

Electron-microscopic investigations of glassy As₂Te₃ reveal that this material is a homogeneous glass and does not contain any crystalline phase. In order to verify the detection limit of the employed method with respect to crystalline phase the presence of the latter was provoked by heating the glass in DTA. The crystallization procceeds in two stages: (1) in T_g region the growth of crystallization centres originating probably from the initial melt starts; (2) closely below the recrystallization temperature T_r spontaneous crystallization in the whole glassy bulk is initiated.