Electro- and thermoreflectance of Bi2Te3 and Bi2Se3

Electroreflectance measurements in Bi₂Te₃ and Bi₂Se₃ with the electric field vector of the incident light both inclined and perpendicular to the C-axis have been made at room temperature. The structures found by other workers in the reflection measurements are observed in the present experiment, together with new structures at 0.91 eV, 1.18 eV, 1.78 eV, and 2.61 eV in Bi₂Se₃ which are not related to formerly observed transitions. From these measurements, the selection rules for direct optical transitions in Bi₂Te₃ and Bi₂Se₃ are studied. Thermoreflectance measurements are also made at both room and liquid-nitrogen temperatures. The positions of the peaks obtained in the present work are compared with the electroreflectance and reflection data.     

Bi2Te3/Sb超晶格纳米线的外延生长和热电测量

通过脉冲电沉积,外延生长出小单元长度的Bi₂Te₃/Sb超晶格纳米线.借助哈曼方法,测量了超晶格纳米线阵列的热电性能,330 K时的ZT值可达0.15.研究了Bi₂Te₃/Sb超晶格纳米线阵列器件的制冷或者加热能力,发现器件的上下表面的最大温差可以达到6.6 K.     

Pressure-induced phase transition in Sb2Te3

An anomalous change in electrical resistance with pressure was observed in crystalline Sb₂Te₃ around 80 kbar at room temperature. This anomaly was found to be closely related with a pressure-induced structural phase transition which is discovered by the present x-ray diffraction experiment.     

Thermoelectric properties of Bi2Se3/Bi2Te3/Bi2Se3 and Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems

In this work, we develop a theory of thermoelectric transport properties in two-dimensional semiconducting quantum well structures. Calculations are performed for n-type 0.1 wt.% CuBr-doped Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ and p-type 3 wt.% Te-doped Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ quantum well systems in the temperature range 50–600 K. It is found that reducing the well thickness has a pronounced effect on enhancing the thermoelectric figure of merit (ZT). For the n-type Bi₂Se₃/Bi₂Te₃/Bi₂Se₃ with 7 nm well width, the maximum value of ZT is estimated to be 0.97 at 350 K and for the p-type Sb₂Te₃/Bi₂Te₃/Sb₂Te₃ with well width 10 nm the highest value of the ZT is found to be 1.945 at 440 K. An explanation is provided for the resulting higher ZT value of the p-type system compared to the n-type system.     

Ab initio calculation of the growth of Te nanorods and Bi2Te3 nanoplatelets

In this paper the growth mechanism of a Te/Bi$_{2}$Te$_{3}$ novel structure is studied by \textit{ab-initio} calculations. The results show that the growth of Te nanorods is determined by the adsorption energy of Te atoms on different crystalline Te surfaces. The adsorption energy of Te on the Te (001) surface is 3.29 eV, which is about 0.25 eV higher than that of Te on the Te (110). This energy difference makes the preferential growth direction along the $<001>$ direction. In addition, the higher surface energy of Bi$_{2}$Te$_{3}$ (110) and the lattice misfit between crystalline Bi$_{2}$Te$_{3}$ and Te along $<001>$ direction are considered to explain the growth of the Bi$_{2}$Te$_{3}$ nanoplatelets, in which Volmer--Weber model is used. The theoretical results are in agreement with experimental observation.     

A b initio calculation of the growth of Te nanorods and Bi2Te3 nanoplatelets

In this paper the growth mechanism of a Te/Bi2Te3 novel structure is studied by ab-initio calculations. The results show that the growth of Te nanorods is determined by the adsorption energy of Te atoms on different crystalline Te surfaces. The adsorption energy of Te on the Te （001） surface is 3.29 eV, which is about 0.25 eV higher than that of Te on the Te （110）. This energy difference makes the preferential growth direction along the 〈 001 〉 direction. In addition, the higher surface energy of Bi2Te3 （110） and the lattice misfit between crystalline Bi2We3 and Te along 〈 001 〉 direction are considered to explain the growth of the Bi2Te3 nanoplatelets, in which Volmer-Weber model is used. The theoretical results are in agreement with experimental observation.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Pulsed laser deposition of Bi2Te3 thermoelectric films

Bi₂Te₃ is one of the most used materials for thermoelectric applications at ambient temperature. An improvement of thermoelectric performances through a suitable modification of electron and phonon transport mechanisms is predicted for low dimensional or nanostructured systems, but this requires a control of the material structure down to the nanoscale. We show that pulsed laser deposition provides control on film composition, phase and structure, necessary for a comprehension of the relationship between structure and thermoelectric properties. We have explored the role of deposition temperature, background inert gas type and pressure, laser fluence and target-to-substrate distance and we found the experimental condition ranges to obtain crystalline films containing the Bi₂Te₃ phase only, by comparing energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray diffraction analysis. Variations of substrate temperature and deposition gas pressure prove to be crucial also for the control of film morphology and crystallinity. Substrate type has no influence on film stoichiometry and crystallinity, but highly oriented growth can be achieved on mica due to van der Waals epitaxy.     

Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.     

过渡金属掺杂在Bi2Te3体系中诱导产生的磁性

用从头计算方法研究了在Bi₂Te₃体系采用过渡金属掺杂从而诱导出磁性的可能性．用一个过渡金属原子置换一个Bi原子之后,可以在该体系中产生磁矩,这主要是由于过渡金属原子的3d轨道电子的自旋极化导致的．当一个Ti、V、Cr、Mn和Fe原子分别替代一个Bi原子时,体系显示的磁矩分别为0.92、1.97、2.97、4.04和4.98 μ_B．当引入两个过渡金属原子代替两个Bi原子的时候,交换耦合的特性取决于取代Bi原子的分布;代替的Bi原子的位置在Bi1和Bi5的位置时,这两个过渡金属原子之间的距离为11.52 ?, Bi_1.84TM_0.16Te₃体系能量最稳定并且表现出铁磁性耦合．     

Influence of negative pressure on thermoelectric properties of Sb2Te3

We investigated the influence of negative pressure on the electrical conductivity, the Seebeck coefficient, and the power factor of Sb₂Te₃. We performed first-principles calculations with the linearized-augmented plane-wave method considering negative hydrostatic pressure in the range from zero to −2 GPa and doping for electrons and holes up to 10²⁰ cm⁻³. Our results predict a significant increase of the Seebeck coefficient and the power factor under negative pressure for certain doping concentrations.     

基于Si掺杂Sb2Te3薄膜的相变存储器研究

采用磁控三靶(Si，Sb及Te)共溅射法制备了Si掺杂Sb₂Te₃薄膜，作为对比，制备了Ge₂Sb₂Te₅和Sb₂Te₃薄膜，并且采用微加工工艺制备了单元尺寸为10μm×10μm的存储器件原型来研究器件性能.研究表明，Si掺杂提高了Sb₂Te₃薄膜的晶化温度以及薄膜的晶态和非晶态电阻率，使得其非晶态与晶态电阻率之比达到10⁶，提高了器件的电阻开/关比；同Ge₂Sb₂Te₅薄膜相比，16at% Si掺杂Sb₂Te₃薄膜具有较低的熔点和更高的晶态电阻率，这有利于降低器件的RESET电流.研究还表明，采用16at% Si掺杂Sb₂Te₃薄膜作为存储介质的存储器器件原型具有记忆开关特性，可以在脉高3V、脉宽500ns的电脉冲下实现SET操作，在脉高4V、脉宽20ns的电脉冲下实现RESET操作，并能实现反复写/擦，而采用Ge₂Sb₂Te₅薄膜的相同结构的器件不能实现RESET操作. 关键词： 相变存储器 硫系化合物 2Te₃薄膜')" href="#">Si掺杂Sb₂Te₃薄膜 SET/RESET转变     

Structural characterization of thermally evaporated Bi2Te3 thin films

Thermally evaporated Bi₂Te₃ thin films were deposited on glass substrates. X-ray diffraction study confirmed that the growned films are polycrystalline in nature having hexagonal structure. The film exhibits preferential orientation along the [0 1 5] direction for the films of all thickness together with other abundant planes [0 1 1 1] and [1 1 0]. Various structural parameters such as lattice constants, crystallite size, strain, and dislocation density have been calculated and they are found to be thickness dependent. The lattice parameters are found to be a=4.38 Å and c=30.40 Å. The grain size of the films increases with thickness as the dislocation density and the microstrain decreases with thickness. The mean bond energy and the average coordination number of Bi₂Te₃ thin film are found to be 1.72 eV and 2.4, respectively.     

Structural and optical properties of thermally evaporated Bi2Te3 films

Bi₂Te₃ films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 °C were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67–162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5–10 μm for Bi₂Te₃ films with different thicknesses (25–99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi₂Te₃ is a high refractive index material (n has values of 4.7–8.8 in the wavelength range 2.5–10 μm). The allowed optical transitions were found to be direct optical transitions with energy gap  eV. The optical conductivities σ₁ = ƒ(hν) and σ₂ = f(hν) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions.     

Temperature dependence of the Raman spectra of Bi2Te3 and Bi0.5Sb1.5Te3 thermoelectric films

The temperature dependence of the Raman spectra of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ thermoelectric films was investigated. The temperature coefficients of the E_g(2) peak positions were determined as –0.0137 cm^–1/°C and –0.0156 cm^–1/°C, respectively. The thermal expansion of the crystal caused a linear shift of the Raman peak induced by the temperature change. Based on the linear relation, a reliable and noninvasive micro‐Raman scattering method was shown to measure the thermal conductivity of the thermoelectric films. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Performance improvement of Sb2Te3 phase change material by Al doping

Al doped Sb₂Te₃ material was proposed to improve the performance of phase-change memory. Crystallization temperature, activation energy, and electrical resistance of the Al doped Sb₂Te₃ films increase markedly with the increasing of Al concentration. The additional Al-Sb and Al-Te bonds enhance the amorphous thermal stability of the material. Al_0.69Sb₂Te₃ material has a better data retention (10 years at 110 °C) than that of Ge₂Sb₂Te₅ material (10 years at 87 °C). With a 100 ns width voltage pulse, SET and RESET voltages of 1.3 and 3.3 V are achieved for the Al_0.69Sb₂Te₃ based device.     

Study on the delamination of tungsten thin films on Sb2Te3

To investigate the reliability of electrode materials for chalcogenide random access memory （C-RAM） applications, the geometry and time evolution of the worm-like delamination patterns on a tungsten/Sb2Te3 bilayer system surface are observed by field emission scanning electronic microscope （FESEM） and optical microscopy. The tungsten film stress and interface toughness are estimated using a straight-side model. After confirming the instability of this system being due to large compressive stress stored in the tungsten film and relative poor interface adhesion, a preliminary solution as the inset of a TiN adhesion layer is presented to improve the system performances.     

Electrodeposition and characterization of thermoelectric Bi2Te2Se/Te multilayer nanowire arrays

Multilayer nanowire arrays of a new heterogeneous thermoelectric material, Bi₂Te₂Se/Te, were successfully fabricated by a template-assisted pulsed electrodeposition method. The nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy, respectively. The diameter of the nanowires is in the range of 60–85 nm. By adjusting the concentration of Te precursor, the length of the Te segment can be modulated. At sufficient low concentration of Te precursor, the phase composition of the other segment can even be changed from Bi₂Te₂Se to Bi₂Se₂Te. The formation and growth mechanisms of the nanowires were proposed.