Electrical and optical properties of (Sb2Se3)2 (Sb2Te3)1 thin films

A study of the synthesized (Sb₂Se₃)₂ (Sb₂Te₃)₁ glassy system has been carried out, X-ray diffraction (XRD) patterns and differential thermal analysis (DTA) of the system studied were used to obtain an insight into the structural information. An investigation of the electrical and optical properties of (Sb₂Se₃)₂ (Sb₂Te₃)₁ thin films prepared by thermal evaporation having different thicknesses (89.2, 214, 223 nm) and annealing temperatures ranging from 300 to 473 K has been carried out. The effect of the thickness and heat treatment on the activation energy E for d.c. conductivity and the density of localized states at the Fermi level N(E_F) were carried out. The electrical conductivity measurements depend on the thickness and annealing temperature, and exhibit two types of conduction mechanisms. Optical absorption measurements have been made on as-deposited and annealed films for the investigated system. The optical transition was found to be indirect. The optical energy gap (E_opt) decreases with increasing thickness and annealing temperatures (below T_g). The corresponding band is approximately twice the conduction activation energy. This effect is interpreted in terms of the density of states model proposed by Mott and Davis. © 2002 Kluwer Academic Publisher     

In Situ TEM Investigations on Thermoelectric Bi2Te3/Sb2Te3 Multilayers

In this work, the impact of heat treatment on the real structure of Bi₂Te₃/Sb₂Te₃ multilayers is investigated. The material was heated in situ in the transmission electron microscope (TEM) and ex situ inside a furnace after preparing these layers with the so‐called nanoalloying deposition technique via molecular beam epitaxy (MBE) equipment. The samples were prepared as a lamella for TEM studies using focused ion beam technique. EDX elemental mapping and high angle annular dark field mode‐STEM were performed to monitor changes of the morphology and interdiffusion phenomena after heating up to 250 °C. A grain growth started during heating and the chemical layer structure was smeared out partly but remained in several grains and was found to be adjusted parallel to a major lattice plane in a crystallite. High resolution TEM shows polysynthetic twinning in a number of crystals.     

Metal organic chemical vapor deposition of phase change Ge1Sb2Te4 nanowires

The self-assembly of Ge(1)Sb(2)Te(4) nanowires (NWs) for phase change memories application was achieved by metal organic chemical vapor deposition, catalyzed by Au nanoislands in a narrow range of temperatures and deposition pressures. In the optimized conditions of 400 °C, 50 mbar, the NWs are Ge(1)Sb(2)Te(4) single hexagonal crystals. Phase change memory switching was reversibly induced by nanosecond current pulses through metal-contacted NWs with threshold voltage of about 1.35 V.     

Thermoelectric properties of fine grained (75% Sb2Te3-25% Bi2Te3)p-type and (90% Bi2Te3-5% Sb2Te3-5% Sb2Se3)n-type alloys

The thermoelectric properties of fine-grained alloys prepared by either cold pressing and sintering or hot pressing in the range 5–50 m are compared with single-crystal best-direction values. It is shown that for thep-type alloy, almost the entire thermoelectric properties are recovered, i.e. the figure of merit for the finest grain size is almost the same as the best single-crystal value. The same trend is observed in then-type alloy except that 90% of the single-crystal figure of merit is recovered. These results are discussed in terms of a model which suggests that degradation of favourable thermoelectric properties by powdering the alloys is compensated by (1) decrease of thermal conductivity due to scattering of phonons at grain boundaries for grain sizes that are comparable to the mean free path of phonons; and (2) retention of some of the anisotropic properties of the single crystal in the fine-grained compacts.     

Crystal growth and thermoelectric properties of tetradymite-like layered chalcogenides and (Bi2Te3)1−x−y (Sb2Te3) x (Sb2Se3) y solid solutions

Thermoelectric materials for segmented n-and p-legs of thermoelectric generators have been prepared by Czochralski growth with melt supply through a floating crucible. Two-segment ingots have been obtained using melt compositions corresponding to ternary layered compounds in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems, with (Bi₂Te₃)_1?x?y (Sb₂Te₃) _x (Sb₂Se₃) _y solid solutions as seed materials. Seeded growth of the ternary compounds makes it possible to fabricate legs without joining segments by soldering. Using scanning hot point microprobe measurements, we have studied the thermoelectric power distribution across the seed-crystal interface. The results attest to a steep thermoelectric power gradient across the seed-crystal interface in a narrow region. Quantitative analysis of the distribution of the number of measurements with respect to thermoelectric power has revealed peaks corresponding to individual segments.     

基于Sn掺杂Ge2Sb2Te5的相变存储器器件单元存储性能

采用0.18μm标准工艺制备出基于Sn掺杂Ge2Sb2Te5相变材料的相变存储器器件单元,利用自行设计搭建的电学测试系统研究了其存储性能.结果表明:Sn的掺杂没有改变Ge2Sb2Te5的相变特性,其相变阚值电压和阈值电流分别为1.6V和25μA;实现了器件单元的非晶态(高阻)与晶态(低阻)之间的可逆相变过程;器件单元中相变材料结晶所需电流最低为1.78mA(电流宽度固定为100ns)、结晶时间大于80ns(电流高度固定为3mA);相变材料非晶化脉冲电流宽度为30ns时,所需电流大于3.3mA;与Ge2Sb2Te5相比,Sn的掺杂降低了SET操作的脉冲电流宽度,提高了结晶速度,有利于提高相变存储器的存储速度.     

Thermoelectric properties of fine-grained sintered (Bi2Te3)25-(Sb2Te3)75 p-type solid solution

P-type semiconductor alloy compacts of the composition (Bi₂Te₃)₂₅-(Sb₂Te₃)₇₅ with grain size (L) in the range 30 > L > 20 m, 20 > L > 15 m, 15 > L > 10 m, 10 > L > 5 and L < 5 m were prepared by cold press at a pressure of 77 × 10⁷ Nm^–2. The samples were sintered at 673 K. Measurements of the Seebeck coefficient, electrical resistivity and thermal conductivity were carried out. The experimental results show that the Seebeck coefficient increases, but not much from the single crystal. The electrical resistivity increases in particular for the size L < 5 m with a reduction in grain size. The total thermal conductivity seriously decreases as grain size decreases. It is concluded that the figure of merit of the compacted alloy would be significantly improved through the use of fine-grained powders of size 30–10 m.     

Information reliability evaluation of a Ge2Sb2Te5-based phase change memory cell

The information reliability of phase change memory cells at different ambient temperatures has been assessed for the first time. The statistical approach chosen is based on A.N. Kolmogorov’s heuristic theory, which describes the kinetics of isothermal crystallization. We have analyzed the influence of the phase change memory cell size and critical size of crystalline nuclei, which depends on the physicochemical parameters of the material and temperature. The results demonstrate that the information reliability of phase change memory cells of typical dimensions can be sufficiently high up to 100°C. Calculation results are compared to available experimental data.     

Fabrication and characterization of superhydrophobic Sb(2)O(3) films

Superhydrophobic Sb(2)O(3) films with micro-nanoscale hierarchical structures have been successfully synthesized for the first time. The resultant materials were characterized in detail by x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, transmission electron microscopy and water contact angle measurements. The water static contact angle of the obtained Sb(2)O(3) film is about 159° ± 2° and the sliding angle is less than 5°. Reasonable mechanisms for the formation of micro-nanoscale hierarchical structures and for the superhydrophobic properties with a small sliding angle of the obtained Sb(2)O(3) film are also presented in this work.     

Electrical, optical, and thermal properties of Sn-doped phase change material Ge2Sb2Te5

In this article, effect of Sn on the electrical, optical, and thermal properties of Ge₂Sb₂Te₅ is studied. Ge₂Sb₂Te₅, Ge_1.55Sb₂Te₅Sn_0.45, and Ge_1.1Sb₂Te₅Sn_0.9 alloys are prepared by melt quenching technique and their thin films are prepared by thermal evaporation on glass substrates. These materials are then characterized by differential scanning calorimetry, X-ray diffraction, optical method, and impedance measurements. Doping with Sn maintains the NaCl-type crystalline structure of Ge₂Sb₂Te₅. Activation energy (E _a) for crystallization is calculated by Kissinger’s method. E _a decreases slightly from 2.56 eV for Ge₂Sb₂Te₅ to 2.24 eV for Ge_1.1Sb₂Te₅Sn_0.9. The distinct change in extinction coefficient (k) of Ge₂Sb₂Te₅ and Sn-doped amorphous films is found in the visible region. A large increase in optical contrast (C) is observed in the Sn-doped phase change materials. The phase change transition is studied using impedance measurements as a function of temperature. Impedance measurements show the appearance of nucleation centers in samples heated at temperatures below crystallization temperature (T _c) and above glass transition temperature (T _g).     

Mechanical properties of Bi(x)Sb(2-x)Te3 nanostructured thermoelectric material

Research on thermoelectric (TE) materials has been focused on their transport properties in order to maximize their overall performance. Mechanical properties, which are crucial for system reliability, are often overlooked. The recent development of a new class of high-performance, low-dimension thermoelectric materials calls for a better understanding of their mechanical behavior to achieve the desired system reliability. In the present study we investigate the mechanical behavior of nanostructure bulk TE material p-type Bi(x)Sb(2-x)Te(3) by means of nanoindentation and 3D finite element analysis. The Young's modulus of the material was estimated by the Oliver-Pharr (OP) method and by means of numerically assisted nanoindentation analysis yielding comparable values about 40 GPa. Enhanced hardness and yield strength can be predicted for this nanostructured material. Microstructure is studied and correlation with mechanical properties is discussed.     

Cu-intercalated (Bi0.5Sb1.5Te3)0.96(Bi2Se3)0.04 single crystals

A procedure is described for converting the conductivity type (p → n) of single crystals of the Bi_0.5Sb_1.5Te₃-4 mol % Bi₂Se₃ solid solution via copper intercalation. Using this procedure, we have produced high-performance single-crystal n-legs with a room-temperature thermoelectric power α = ?200 μV/K. This procedure facilitates the fabrication of thermoelectric coolers because the same solid solution can be used to produce p-and n-legs, the only difference being the presence of copper in the n-legs. We have fabricated thermolements and determined their characteristics in the range 100–300 K.     

MOCVD of Bi2Te3 and Sb2Te3 on GaAs substrates for thin-film thermoelectric applications

Metal organic chemical vapour deposition (MOCVD) has been investigated for growth of Bi2Te3 and Sb2Te3 films on (001) GaAs substrates using trimethylbismuth, triethylantimony and diisopropyltelluride as metal organic sources. The surface morphologies of Bi2Te3 and Sb2Te3 films were strongly dependent on the deposition temperatures as it varies from a step-flow growth mode to island coalescence structures depending on deposition temperature. In-plane carrier concentration and electrical Hall mobility were highly dependent on precursor ratio of VI/V and deposition temperature. By optimizing growth parameters, we could clearly observe an electrically intrinsic region of the carrier concentration over the 240 K in Bi2Te3 films. The high Seebeck coefficient (of -160 microVK(-1) for Bi2Te3 and +110 microVK(-1) for Sb2Te3 films, respectively) and good surface morphologies of these materials are promising for the fabrication of a few nm thick periodic Bi2Te3/Sb2Te3 super lattice structures for thin film thermoelectric device applications.     

Electrical properties of polycrystalline Sb2Te3 films

Single-phase polycrystalline stoichiometric films of Sb₂Te₃ with different thicknesses were prepared on glass substrates by a flash evaporation technique at constant substrate temperature of 423 K. The electrical properties of these films, such as resistivity, Hall mobility, carrier concentration and activation energy, were determined for different film thicknesses. The optical absorption of these films was also studied. The implications are discussed.