X-Ray photoelectron valence band spectra from semiconductors Bi2Te3 and Sb2Te3

Angular resolved X-ray photoelectron spectra (ARXPS) are measured for valence bands of the layer semiconductors Bi₂Te₃ and Sb₂Te₃. Starting from simple models taking into consideration the atomic construction, photoionization cross sections and the crystal structure the spectra can be interpreted. Important conclusions are drawn on the electronic structure of these semiconducting compounds.     

Bi2Te3纳米晶溶剂热合成及表征

采用溶剂热方法,在水合肼溶剂中通过添加适量表面活性剂十二烷基硫酸钠(SDS),在180 ℃下反应10 h后成功地制备了不同形貌的纯相Bi2Te3纳米晶,包括纳米颗粒、纳米棒和花瓣状纳米片.用X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、能量色散谱仪(EDS)对产物进行了表征,探讨了表面活性剂用量和温度对产物形貌的影响,提出了可能的形成机理.     

水热法合成碲化铋粉体及其热稳定特性

通过改变实验条件,在130 ～200℃、NaOH溶液浓度为2～8mol/L的较宽水热条件下合成出单相Bi2Te3粉体.利用X射线衍射(XRD),透射电镜(TEM),高分辨透射电镜(HRTEM),能谱色散仪(EDAX),热分析系统(TG-DTA)对产物的物相组成,形貌特征和晶体结构,热稳定性进行了研究.结果表明当不添加表面活性剂时,在一定反应温度下,NaOH溶液浓度不仅是合成纯Bi2Te3粉体的重要因素,而且影响所得Bi2Te3粉体的晶体形貌和晶粒尺寸.当NaOH溶液浓度较低、反应温度较高时,以原子结合方式生成Bi2Te3,反之,以离子结合方式生成Bi2Te3.一般形成机理的提出对今后采用水热法或溶剂热法合成碲化物有一定帮助,而热稳定性的研究对实验结果提出新的要求.     

Transport Coefficient of Gallium-doped Bi2Te3 Single Crystals

Gallium-doped Bi₂Te₃ single crystals were prepared by means of a modified Bridgman method. Temperature dependences of the Hall constant R_H( B ∥ c ), electrical conductivity σ⊥c and Seebeck coefficient S(Δ T ⊥ c) were measured on the samples of these crystals in the temperature interval of 100–400 K. The variations of the investigated transport coefficients with increasing Ga content in the samples showed that Ga atoms in the Bi₂Te₃ crystal lattice behave as donors. This effect is qualitatively explained on the basis of a model of point defects in Bi₂Te₃(Ga) crystals; singly ionised gallium atoms in interstitial sites; Ga_i, are considered to be the most probable defects.     

Optical Properties of Pb-Doped Bi2Te3 Single Crystals

The reflectance and the transmittance spectra in the IR region were measured on Pb-doped Bi₂Te₃ single crystal samples grown by a modified Bridgman technique. The plasma resonance frequency, the optical relaxation time, and the high-frequency permittivity were determined by fitting the Drude-Zener formulas to the reflectance spectra. It was found that Pb impurities in Bi₂Te₃ behave as acceptors. A part of incorporated Pb atoms behaves as electrically inactive. This effect is explained as due to the fact that electrically active Pb-atoms form substitutional defects, Pb′_Bi, and the others form inactive two-dimensional defects — seven-layer lamellae Te—Bi—Te—Pb—Te—Bi—Te. The transmittance spectra were used for the determination of the dependence of the absorption coefficient K on the energy of incident photons. The optical width of the energy gap increases with increasing Pb content. The values of the exponent α from the relation of K ∼ λ^α for the long-wavelength absorption edge range within the interval of 1.6 to 2.8, i.e. the dominant scattering mechanism of free current carriers in Pb-doped Bi₂Te₃ crystals is the scattering by acoustic phonons. By comparison of the effect of doping atoms of the IV.B group of the periodic table on the concentration of holes in Bi₂Te₃ crystal lattice was concluded that the tendency to form substitutional defects Me′_Bi (Me = Ge, Sn, Pb) in these crystals increases from Ge to Pb, whereas the tendency to the formation of seven-layer lamellae Te—Bi—Te—Me—Te—Bi—Te decreases.     

The effect of divalent impurity state on the microhardness of NaCl crystals

The microhardness and bulk density variations with annealing temperature have been measured in NaCl crystals doped with Ca²⁺, Cd²⁺, Ni²⁺, and Pb²⁺. Both characteristics are not affected by coherent metastable precipitation while incoherent precipitation produces either hardening or softening depending on the impurity phase type (stable or metastable). The results support the suggestion that aggregates and coherent precipitates are cut through by dislocations whereas incoherent precipitates are by-passed via the Orowan looping mechanism.     

Bi2Te3 Crystals Heavily Doped with Germanium Atoms

Elemental Bi, Te, and Ge of 5N purity were used to prepare Bi₂Te₃(GeTe) single crystals with germanium content varying from 0 up to 2.3 × 10²¹ cm⁻³. The samples were characterized by reflectivity measurements in the plasma resonance frequency range and by measurements of the electrical conductivity. Germanium content in the samples was determined by means of energy dispersive analysis. The reflectivity spectra were interpreted on the basis of the Drude-Zener theory in the aim to obtain information on the concentration of the free carriers in the samples. It was found that Ge atoms in the Be₂Te₃ crystal lattice behave as acceptors. A comparison of the hole concentration with the amount of germanium built into the crystal lattice revealed that only about 1/100 of the total number of Ge atoms act as acceptors. This effect is explained by two different ways of incorporation of Ge atoms into the Bi₂Te₃ lattice, viz: the formation of substitutional Ge'_Bi defects acting as acceptors and the formation of seven-layer-lamellae of the Te Bi Te Ge Te Bi Te composition, which corresponds to the structure of GeBi₂Te₄.     

不同表面活性剂制备n型Bi2Te3热电材料

分别使用P123(聚环氧乙烷-聚环氧丙烷-聚环氧乙烷三嵌段共聚物)、PVP(聚乙烯吡咯烷酮)和EDTA(乙二胺四乙酸)为模板剂采用水热法合成n型Bi2 Te3热电纳米粉体,通过放电等离子烧结技术(简称SPS)将粉体烧结成块体样品.利用XRD、SEM、ZEM-3以及激光导热仪等对制备的样品进行物相、形貌及热电性能表征.结果显示:三种模板剂制备的Bi2 Te3纳米颗粒大部分呈片状,其中PVP制备的纳米片最为规整,EDTA制备的纳米片大小不均一,P123制备的纳米片夹杂有棒状和团聚饼状的形貌;XRD表征显示所制备粉体均为纯Bi2 Te3相,没有其它杂质.对块体样品的热电性能研究发现:由于Bi2 Te3具有独特的层状结构,会对载流子和声子的传输产生影响,造成所制备块体样品垂直于压力方向的ZT值要大于平行于压力方向的ZT值;采用PVP模板剂制备Bi2 Te3样品的热电性能最高,在温度为480 K时,ZT值达到0.33.     

Enhancement of thermoelectric efficiency in vapor deposited Sb2Te3 and Sb1.8In0.2Te3 crystals

Pure and indium doped antimony telluride (Sb₂Te₃) crystals find applications in high performance room temperature thermoelectric devices. Owing to the meagre physical properties exhibited on the cleavage faces of melt grown samples, an attempt was made to explore the thermoelectric parameters of p‐type crystals grown by the physical vapor deposition (PVD) method. The crystal structure of the grown platelets (9 mm× 8 mm× 2 mm) was identified as rhombohedral by x‐ray powder diffraction method. The energy dispersive analysis confirmed the elemental composition of the crystals. The electron microscopic and scanning probe image studies revealed that the crystals were grown by layer growth mechanism with low surface roughness. At room temperature (300 K), the values of Seebeck coefficient S (⊥ c) and power factor were observed to be higher for Sb_1.8In_0.2Te₃ crystals (155 μVK⁻¹, 2.669 × 10⁻³ W/mK²) than those of pure ones. Upon doping, the thermal conductivity κ (⊥ c) was decreased by 37.14% and thus thermoelectric efficiency was improved. The increased figure of merit, Z = 1.23 × 10⁻³ K⁻¹ for vapour grown Sb_1.8In_0.2Te₃ platelets indicates that it could be used as a potential thermoelectric candidate.     

Morphology and growth mechanism of vapor grown Cd crystals as affected by Bi impurity

The effects of Bi impurity on the morphology and growth mechanism of (physical) vapor-deposited Cd crystals were investigated. Scanning electron microscopic observation revealed, for the first time, hexagonal prismatic Cd crystals which were grown on the Bi contaminated pyrex glass tube wall. This kind of morphology does not appear in the case of pure Cd without the impurity Bi. The top of the hexagonal prismatic crystal was usually rounded hemispherically. X-ray microprobe investigations showed that the tip of the crystals contained about 57 wt% Bi. It was concluded that the Cd hexagonal prismatic crystals grow by the mechanism of vapor-liquid-solid with the impurity Bi.     

Growth of CuInS2 single crystals from Sb2S3 and Bi2S3 solutions

The phase diagrams of the CuInS₂‐Sb₂S₃ and CuInS₂‐Bi₂S₃ systems were investigated using X‐ray powder diffraction and differential thermal analysis. Based on these results, the compositions for the growth of the CuInS₂ single crystals from CuInS₂‐Sb₂S₃ and CuInS₂‐Bi₂S₃ melts were selected and Bridgman crystal growth process was performed. The investigation of the obtained single crystals using X‐ray powder diffraction and optical absorption spectra indicates that the incorporation of the solvent atoms into the crystal lattice is absent. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoluminescence studies of Al2O3 crystals doped with Cobalt impurity

Thermoluminescence (TL) studies of cobalt-doped Al₂O₃ crystals irradiated with X-rays have shown that the TL glow curve consists of three peaks at 105, 195 and 260°C. Bleaching, annealing and quanching studies have been performed in order to understand the nature of colour centers responsible for the formation of the three peaks. The values of the fundamental parameters like trap depth and frequency factor are estimated from TL data and the results obtained are discussed.     

(Bi2Te3)0.90(Sb2Te3)0.05(Sb2Se3)0.05熔炼冷压烧结材料的制备及其热电性能的研究

本文采用熔炼冷压烧结法制备了n型赝三元(Bi2Te3)0.90(Sb2Te3)0.05(Sb2Se3)0.05熔炼冷压烧结热电材料,分析了材料的微观结构并测试了材料的热电性能.研究表明:烧结有利于提高材料的热电性能;n型赝三元熔炼冷压烧结热电材料的热电性能沿垂直于冷压压力方向存在优化取向.     

Growth and Morphology of Hollow Bi2Te3 Whiskers by Physical Vapour Deposition Method

Hollow crystals of bismuth telluride have been grown by the physical vapour deposition (PVD) method. Whisker crystals as long as 10 mm have been grown and were identified by X-ray analysis. The possible mechanism for the growth of hollow whiskers has also been suggested.     

Complex formation and background impurity of Oxygen in the PbTe:Eu doped crystals grown from melt by the Bridgman method

The magnetic field dependence of magnetization M(B) at the temperature 1.72 K in magnetic fields up to 5 T and the temperature dependence of magnetic susceptibility (M_S) χ(T) in the temperature range 1.7–400 K of six PbTe:Eu samples with the concentration of Eu impurity of the order of 1×10¹⁹–1×10²⁰ cm^?3, prepared from the doped crystals grown from the melt by the Bridgman method, have been investigated. It is shown that the dependence of M(B) and χ(T) can be quantitatively explained by the contribution of the single centers of Eu ions, their pairs, and the matrix of the doped crystals using the same set of parameters for each sample. This is true provided we use in our analysis the values of the exchange integrals between Eu ions in EuO normalized with the lattice constant of PbTe, i.e., J₁/k_B=0.056 K for the ferromagnetic interaction of the NN (nearest neighbor) pairs and J₂/k_B=?0.13 K for the antiferromagnetic interaction of the NNN (next nearest neighbor) pairs, as well as different values of the M_S of crystal χ_matrix. It is revealed that the probability of the formation of complexes based on the magnetic impurity pairs is higher in the incipient section of a doped ingot, and it decreases towards the ingot end where the single centers of Eu ions become the only centers of the impurity. We conclude that the pairs of Eu²⁺ ions, which are formed during the growth of the PbTe:Eu ingots from melt by the Bridgman method, are the constituents of the complexes of the magnetic impurities with the background Oxygen impurities in the crystal matrix of the doped lead telluride. It is shown that the formation of the complexes leads to an increase of the M_S of crystal matrix χ_matrix and can even cause the change of its sign from minus to plus, i.e., it can convert the crystal matrix from the diamagnetic to paramagnetic state. The possible causes of this effect are analyzed.     

Precipitation and dissolution of KCaCl3 impurity in NaCl crystals

Impurity precipitation in NaCl crystals containing CaCl₂ and KCl in various proportions is studied by means of flotation density measurements. The paper is concerned with the determination and dissolution, the kinetics and activation energy, and the mechanism of nucleation. Two temperature regions for solid solution decomposition were revealed. The characteristic stages in density variation upon isochronal annealing are shown to be correlated with radical changes in the X-ray diffraction patterns. Evidence is given that the nucleation is heterogeneous for the stable phase and homogeneous for the metastable one. – The activation energy in the low-temperature region has been found to be 0.8 ± 0.1 eV, which suggests precipitation to be diffusion-limited. From the kinetics studies it follows that the composition of the metastable phase corresponds to KCaCl₃ and the rate-controlling stage in stoichiometric crystals is a K⁺ diffusion.     

Investigations on the structural perfection of bridgman-grown PbTe single crystals

It is reported on the possibility of improving the structural perfection by the Bridgman method, shown with the example of the influence on the low-angle grain boundary substructures in PbTe arranged in growth direction. In the employed crystal growth apparatus temperature gradients from 15 to 100°C/cm and solidification rates from 0.33 to 4.2 mm/hr could be achieved. The low-angle grain boundary substructure was characterized by X-ray-topographic and etch investigations on (100)-orientated ingots. The thermal conditions during the growth affect the structural perfection decisively. An axial spot dependence of etch pit density and substructure abundance was observed. By reducing the cooling rate during growth — expressed by the product of temperature gradient G and solidification rate V — it is possible to obtain more perfect PbTe-crystals. It seems that the diameter l of the substructure cells which was lying between 0.5 and 2 mm, is related to the cooling rate G · V by the functional coherency l ≈ 1/√G · V. The choice of different initial melt combinations up to 1 at.% Te-surplus exerted no influence on the abundance of the low-angle grain boundaries. The in literature suggested connection of low-angle grain boundary substructure with phenomena resulting from a constitutional supercooling could not be established.     

LPE of Isolated Pb0,85Sn0.15Te Islands on PbTe

In recent years the importance of selective epitaxy has continuously increased. It is a tool for controlling the production of semiconductor structures as required. This method has been demonstrated and investigated by LPE as well as VPE and MBE. Usually substrates are used having either preferentially etched channels or windows in masks to predetermine the site. In the present paper epitaxy of Pb_0.85Sn_0.15 Te deposits growing in isolated islands on PbTe substrates by means of LPE is investigated.     

Dislocation and microindentation analysis of vapour grown Bi2Te3‐xSex whiskers

The structural defects and microhardness of Bi₂Te_3‐xSe_x whiskers (x = 0, 0.2 and 0.4 at% Se) grown by physical vapour deposition (PVD) method have been investigated. Concentric pairs of dislocation loops were observed on the as‐grown surfaces of short hexagonal prisms. A systematic study of dislocations in these crystals was carried out by chemical etching technique. The effects of Se doping, annealing and quenching on the mechanical properties have also been studied on the prism faces of Bi₂Te_3‐xSe_x whiskers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)