Crystallophysical properties of the In1 ? yGayAs1 ? xNx/GaAs heterostructures

Some properties of the In_{1 − y} Ga _y As_{1 − x} N _x unordered alloys and physical prerequisites of their use in science and technology are considered. The results of studying the intermolecular interaction in the systems under study and the features of their application to the In_{1 − y} Ga _y As_{1 − x} N _x /GaAs functional hetero-structures are presented.     

The influence of hydrogenation on the electrical properties of the CdxHg1 ? xTe epitaxial structures

The phenomenon of hydrogenation of the Cd _x Hg_{1 − x} Te films is studied. Hydrogenation was performed via either boiling the Cd _x Hg_{1 − x} Te films in deionized water or using the electrochemical treatment. It is established that, during contacts with aqueous media, the acceptor centers are introduced into the films; their concentration can exceed 10¹⁷ cm⁻³. It is shown that two types of hydrogen-based acceptors are introduced, namely, fast and slow acceptors, and their diffusivities are evaluated. It is found that hydrogen partially exists after treatment in an electrically inactive form and can be activated with further storage or during heating. After activation, the hole concentration can become as high as 10¹⁸ cm⁻³. The influence of pH of the medium on the rate of introduction of hydrogen into the material is discussed.     

Design of GaAs/AlxGa1?xAs asymmetric quantum wells for THz-wave by difference frequency generation

The energy levels, wave functions and the second-order nonlinear susceptibilities are calculated in GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As asymmetric quantum well (AQW) by using an asymmetric model based on the parabolic and non-parabolic band. The influence of non-parabolicity can not be neglected when analyzing the phenomena in narrow quantum wells and in higher lying subband edges in wider wells. The numerical results show that under double resonance (DR) conditions, the second-order difference frequency generation (DFG) and optical rectification (OR) generation susceptibilities in the AQW reach 2.5019 mm/V and 13.208 mm/V, respectively, which are much larger than those of the bulk GaAs. Besides, we calculate the absorption coefficient of AQW and find out the two pump wavelengths correspond to the maximum absorption, so appropriate pump beams must be selected to generate terahertz (THz) radiation by DFG.     

Transport and Thermoelectric Properties of the Ca1?xSrxRu1?yMnyO3 System

The magnetic, transport, and thermoelectric properties of Ca_1−x Sr _x Ru_1−y Mn _y O₃ have been investigated. Ferromagnetism with relatively high T _C (>200 K) was introduced by Mn doping. In particular, ferromagnetism appeared in the Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system at y > 0.2. The maximum T _C (=270 K) was recorded for a specimen of Ca_0.5Sr_0.5Ru_0.4Mn_0.6O₃. The ferromagnetism seems to be due to the mixed-valence states of Mn³⁺, Mn⁴⁺, Ru⁴⁺, and Ru⁵⁺ ions. The metallic character of Ru-rich specimens was suppressed by Mn substitution, and the system was transformed into a semiconductor at relatively low Mn content near y = 0.1. Specimens with higher Mn content (y > 0.8) had large thermoelectric power (50 μV K⁻¹ to 130 μV K⁻¹ at 280 K) accompanied by relatively low resistivity (0.03 Ω cm to 1 Ω cm). The Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system seems to have good potential as a thermoelectric material for use above 300 K.     

Spinodal Decomposition of GaxIn1?xAsyP1?yQuaternary Alloys

Using X-ray structural analysis, scanning electron microscopy, atomic force microscopy, and photoluminescent spectroscopy, it is shown that it is possible to obtain a small-scale domain structure on the surface of liquid-phase epitaxial heterostructures. The domain structure emerges as a result of spinodal decomposition of the Ga _x In_{1 − x} As _y P_{1 − y} quaternary alloy due to immiscibility of its components and relaxation of its lattice parameter to surrounding layers.     

Electrical and Thermal Properties of the Ca3?xGdxCo4O9+δ System

The effects of Gd substitution on the thermoelectric (TE) properties of Ca₃Co₄O_9+δ have been systematically investigated from 25 K to 335 K. Partial substitution of Gd in Ca₃Co₄O_9+δ results in an increase of thermopower and resistivity, and a decrease of thermal conductivity. A maximum dimensionless figure of merit (ZT) of 0.028 was achieved at 335 K for Ca_2.4Gd_0.6Co₄O_9+δ , which is about one order of magnitude larger than that for Ca₃Co₄O_9+δ . The investigation demonstrates that the TE performance of the Ca₃Co₄O_9+δ system can be improved through Gd doping.     

Low-Temperature Transport Properties of InxFeyCo4?ySb12

Interesting results for cobalt triantimonide partially filled with indium have encouraged us to explore skutterudites filled with higher indium fractions. For pure In _x Co₄Sb₁₂, the fraction of voids filled is limited to about x = 0.25. To enable the insertion of more indium atoms, charge compensation is necessary. In this work, we studied the skutterudite compound In _x Fe _y Co_4−y Sb₁₂ partially filled with indium, where iron substitution for cobalt was employed for charge compensation. Polycrystalline samples were prepared by direct reaction of constituents. Structural and chemical characterization were accomplished by x-ray diffraction and energy-dispersive x-ray spectroscopy. Electrical resistivity, thermoelectric power, and thermal conductivity were measured between 2 K and 350 K. The influence of indium and iron on the charge-carrier transport properties and thermal conductivity in In _x Fe _y Co_4−y Sb₁₂ compounds is presented and discussed.     

Analysis of Si/GaAs Bonding Stresses with the FiniteElement Method

在考虑材料热膨胀系数随温度变化后，采用有限元方法结合ANSYS软件对Si/GaAs键合热应力进行了分析，研究了普通应力、轴向应力和剪切力的分布云图和沿界面的分布. 同时提出了新的键合结构以减小热应力的影响，计算结果证明了该结构的有效性.     

Electronic and Optical Properties of MgxZn1?xO and BexZn1?xO Quantum Wells

As a preparatory step toward establishing reliable numerical design tools for ZnO-based optoelectronic devices, we have reassessed the available information on material parameters relevant for the simulation of light-emitting diodes (LEDs) with active regions including ZnO, MgZnO, and BeZnO layers. The impact of different approximations for the electronic structure and the interface polarization charge on the optical properties of bulk ZnO and ZnO/MgZnO quantum wells has been evaluated, and a consistent set of parameters has been used not only for systematic comparison of ZnO/MgZnO and ZnO/BeZnO single quantum well structures but also for the first simulation of a realistic ZnO/BeZnO multiple quantum well LED.     

Dispersion of the refractive index of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 1) alloy layers below the absorption edge

The transmittance spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤x ≤ 0.1) alloy layers are exploited to study the dispersion of their refractive index in the spectral range from 650 to 8000 cm⁻¹ (below the absorption edge). The refractive index and the position of the absorption edge as functions of the composition parameter of the alloys are determined at two temperatures, 80 and 295 K. The refractive index is calculated in the context of the classic wave concepts of propagation of electromagnetic radiation. The experimentally determined dispersion dependences are described by the empiric Sellmeier expression of the second order. From analysis of the transmittance of the layers, it follows that the band gap of the epitaxial Pb_{1 − x} Eu _x Te alloys increases with increasing temperature at x < 0.5 and decreases at x > 0.5.     

Optical properties of quaternary GaNxAsyP1 ? x ? y semiconductor alloys

The optical properties of quaternary GaN _x As _y P_{1 − x − y} semiconductor alloys grown on a GaP (100) substrate surface are studied by photoluminescence spectroscopy in the temperature range 20–300 K and by photoluminescence excitation spectroscopy at liquid-nitrogen temperature. The measurements are carried out for the GaN _x As _y P_{1 − x − y} alloys, for which the nitrogen and arsenic molar fractions x and y range from 0.006 to 0.012 and from 0.00 to 0.18, respectively. A comparative analysis of the data is conducted, and the dependences of the energy position of the photoluminescence peak on the composition of the quaternary alloy are established. From the studies of photoluminescence in the range 20–300 K, it is found that the temperature dependence of the position of the photoluminescence peak substantially differs from the behavior described by Varshni’s expression.     

The Influence of Sintering Temperature on the Microstructure and Thermoelectric Properties of n-Type Bi2Te3?xSex Nanomaterials

Pure Bi₂Te₃ and Bi₂Se₃ nanopowders were hydrothermally synthesized, and n-type Bi₂Te_3−x Se _x bulk samples were prepared by hot pressing a mixture of Bi₂Te₃ and Bi₂Se₃ nanopowders at 623 K, 648 K or 673 K and 80 MPa in vacuum. The phase composition of the powders and bulk samples were characterized by x-ray diffraction. The morphology of the powders was examined by transmission electron microscopy. The microstructure and composition of the bulk samples were characterized by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy, respectively. The density of the samples increased with sintering temperature. The samples were somewhat oxidized, and the amount of oxide (Bi₂TeO₅) present increased with sintering temperature. The samples consisted of sheet-like grains with a thickness less than 100 nm. Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples were measured from room temperature up to 573 K. Throughout the temperature range investigated, the sample sintered at 623 K had a higher power factor than the samples sintered at 648 K or 673 K.     

Optical properties of quantum-confined heterostructures based on GaPxNyAs1 ? x ? y alloys

The results of calculations of the band gap in GaP _x N _y As_{1 − x − y} alloys and the estimated parameter of hybridization of the conduction band in GaP and the localized level of nitrogen are reported. The optical properties of quantum-confined heterostructures based on GaP _x N _y As_{1 − x − y} alloys synthesized on the GaP (100) substrate surface are studied by photoluminescence measurements in the temperature range of 15–300 K. The heterostructures consist of GaP_0.814N_0.006As_0.18 quantum wells separated by GaP barrier layers. The well width and the barrier thickness are 5 nm. Heterostructures with different numbers of periods are considered. On optical excitation of the structures, an intense photoluminescence line is observed in the spectral range 620–650 nm. The photoluminescence spectra of the GaP_0.814N_0.006As_0.18/GaP quantum wells are profoundly broadened because of the inhomogeneity of the quaternary alloy in composition. It is established that the increase in the number of quantum well layers from 10 to 25 does not results in degradation of the photoluminescence properties of the heterostructures. The results of the study support the view that it is possible to produce efficient optoelectronic devices on the basis of GaP _x N _y As_{1 − x − y} alloys.     

Structure and High-Temperature Thermoelectric Properties of the n-Type Layered Oxide Ca2?xBix?δMnO4?γ

We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca₂MnO_4−γ . The electrical conductivity σ and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity κ of doped Ca₂MnO_4−γ is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = σS ² T/κ, increases with Bi doping. The maximum ZT is 0.023 for Ca_1.6Bi_0.18MnO_4−γ at 877°C, which is ten times higher than that of the end member, Ca₂MnO_4−γ . The increase of ZT mainly results from the considerable increase of σ, which can be explained in terms of structural change. The␣Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO₃ layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.     

Effect of Substrate on the Structure and Thermoelectric Properties of n-Type Bi2Te3?ySey Thin Films Prepared by Electrodeposition

n-Type Bi₂Te_3−y Se _y thin films were prepared by potentiodynamic electrodeposition onto Au, Bi, and Bi₂Te_3−y Se _y substrates at room temperature. The electrochemical behaviors of Bi³⁺, HTeO₂ ⁺, and H₂SeO₃ on different substrates were investigated by cyclic voltammetry. The morphology, composition, and structure of the films were studied by using environmental scanning electron microscopy (ESEM), energy-dispersive spectroscopy (EDS), and x-ray diffraction (XRD), respectively. The thermoelectric properties of n-type Bi₂Te_3−y Se _y films were determined by measuring the Seebeck coefficient (α) and electrical resistivity (ρ). The results showed that the composition and morphology of the films were sensitive to the substrate material. X-ray diffraction (XRD) analysis indicated that the preferred orientation of annealed films was affected by the substrate and that the film prepared on the Bi₂Te_3−y Se _y substrate exhibited the strongest (015) orientation, with rhombohedral structure. It was proved that the properties of the annealed films could be affected by the substrate and that the film with the highest power factor (P = α ²/ρ) was obtained on the Bi₂Te_3−y Se _y substrate.     

Thermoelectric Properties of Tix(HfyZr1?y)1?xNiSn0.998Sb0.002 Half-Heusler Ribbons

Ribbons of Ti _x (Hf _y Zr_1−y )_1−x NiSn_1−z Sb _z (x = 0.1 to 1, y = 0.1 to 0.9, z = 0, 0.002, 0.004) were prepared by spin casting and annealed for 1 h at T _a = 1000 K, 1050 K, 1073 K, and 1100 K. The crystal phase of the ribbons was investigated by x-ray diffraction analysis and transmission electron microscopy. All the ribbons consisted of a phase with a half-Heusler structure. The Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit ZT at room temperature were clarified experimentally as a function of x, y, z, and T _a. Despite the large thermal conductivity, the power factor and figure of merit were remarkably large at x = 0.5, y = 0.5, z = 0.002, and T _a = 1073 K, because the Seebeck coefficient and electrical conductivity were large.     

Characterization of Zn1?xMgxO Films Prepared by the Sol–Gel Process and Their Application for Thin-Film Transistors

Transparent semiconductor thin films of Zn_1−x Mg _x O (0 ≤ x ≤ 0.36) were prepared using a sol–gel process; the crystallinity levels, microstructures, and optical properties affected by Mg content were studied. The experimental results showed that addition of Mg species in ZnO films markedly decreased the surface roughness and improved transparency in the visible range. A Zn_1−x Mg _x O film with an x-value of 0.2 exhibited the best average transmittance, namely 93.7%, and a root-mean-square (RMS) roughness of 1.63 nm. Therefore, thin-film transistors (TFTs) with a Zn_0.8Mg_0.2O active channel layer were fabricated and found to have n-type enhancement mode. The Zn_0.8Mg_0.2O TFT had a field-effect mobility of 0.1 cm²/V s, threshold voltage of 6.0 V, and drain current on/off ratio of more than 10⁷.     

Influence of temperature on Auger recombination lifetime in In1?xGaxAs materials

The influence of temperature and Ga composition on Auger recombination lifetime in n-type and p-type In1-xGaxAs materials is investigated through the simulation, assuming the concentrations of electrons and holes are 1017 cm-3 and 1018 cm-3, respectively. The results show that the temperature has little influence on Auger recombination lifetime of In1-xGaxAs materials at x<0.3. However, it has a great impact when x>0.3 and the effect is more obvious at a lower temperature. Moreover, Auger ...     

Improved Thermoelectric Properties of (GeTe)90(AgySb2?yTe3?y)10 by Tuning the Ag-to-Sb Ratio

(GeTe)₉₀(Ag _y Sb_2−y Te_3−y )₁₀ (y = 0.6, 0.7, 0.8, 1.0) compounds were prepared by combining melting and hot pressing, and the thermoelectric properties were studied over the temperature range of 300 K to 770 K. Powder x-ray diffraction results revealed that all the samples were the rhombohedral phase with space group R3m. The electrical conductivity of samples decreased with temperature, while the Seebeck coefficient increased. The thermal conductivity of all the samples was very low, especially for those with the lower y values. High ZT values above 1.6 were obtained for the samples with y = 0.6, 0.7, and 0.8.