Thermoelectric Performance of Multiple-Doped Co4Sb12−x−y−z Ge x Te y S z Skutterudite Compounds

CoSb₃ skutterudites multiply doped with Ge, Te, and S were synthesized by solid-state reaction and spark plasma sintering. x-Ray diffraction studies revealed that Ge, Te, and S entered the lattice of the CoSb₃ compounds, and while Te increased the lattice volume, Ge and S decreased it. Compared with the undoped and single-doped CoSb₃ compounds, the thermal conductivity and lattice thermal conductivity are significantly suppressed due to greatly increased point defect scattering. It is found that S is more effective for decreasing the lattice thermal conductivity than Te and Ge. The highest thermoelectric figure of merit, ZT, exceeds 1.1 for the Co₄Sb_11.25Ge_0.05Te_0.63S_0.07 compound at 800 K.     

Properties of epitaxial (Al x Ga1 − x As)1 − y C y alloys grown by MOCVD autoepitaxy

The growth of epitaxial Al _x Ga_{1 ? x} As:C alloys by metal-organic chemical vapor deposition (MOCVD) at low temperatures results in the formation of quaternary (Al _x Ga_{1 ? x} As)_{1 ? y} C _y alloys, in which carbon atoms can be concentrated at lattice defects in the epitaxial alloy with the formation of impurity nanoclusters.     

Features of ecological control in nanotechnologies in view of features of Ca y La1 − y F3 − y and La x Ca1 − x F2 + x structuring

General features of ecological control in nanotechnologies were found. Based on the study of structural-morphological characteristics of nanoprecipitates with ordered structure in Ca _y La_{1 ? y} F_{3 ? y} and La _x Ca_{1 ? x} F_{2 + x} , it was shown that it is necessary to additionally control the formation of ordered states, the presence of antiphase domains, and the degree of order in ecological control of solution-based materials.

     

Structural and spectral features of MOCVD Al x Ga y In1 ? x ? y As z P1 ? z /GaAs (100) alloys

The study is concerned with MOCVD epitaxial heterostructures grown on the basis of Al _x Ga _y In_{1 ? x ? y} As _z P_{1 ? z} quinary alloys in the region of alloy compositions isoperiodic to GaAs. By the X-ray diffraction technique and atomic force microscopy, it is shown that, on the surface of the heterostructures, there are nanometric objects capable of lining up along a certain direction. From calculations of the crystal lattice parameters with consideration for internal strains, it can be inferred that the new compound is a phase based on the Al _x Ga _y In_{1 ? x ? y} As _z P_{1 ? z} alloy.     

On the resonant level of chromium in the rhombohedral and cubic phases of Pb1 − x − y Ge x Cr y Te alloys

The temperature dependences of the Hall coefficient (4.2 K ≤ T ≤ 300 K, B ≤ 0.07 T) in Pb_{1 ? x ? y} Ge _x Cr _y Te alloys (x = 0.03–0.08, y ≤ 0.01) are studied. An increase in the absolute value of the Hall coefficient with an increase in temperature is found. This fact is indicative of a decrease in the concentration of free electrons as a result of the motion of the resonant level of chromium stabilizing the Fermi level relative to the conduction-band bottom. The temperature dependences of the Hall coefficient, in satisfactory agreement with the experimental ones, are calculated in the context of the two-band Kane dispersion law allowing for the structural phase transition upon increasing temperature. The energy position and temperature coefficients of the motion of the resonant level of chromium relative to the middle of the band gap in the rhombohedral and cubic phases are determined.     

Long-wavelength photodiodes based on Ga1−x InxAsySb1−y wit composition near the miscibility boundary

The possibility of using liquid-phase epitaxy to obtain Ga_1−x In_xAs_ySb_1−y solid solutions isoperiodic with GaSb near the miscibility boundary is investigated. The effect of crystallographic orientation of the substrate on the composition of the solid solutions grown in this way is examined, and the indium concentration is observed to grow from 0.215 to 0.238 in the Ga_1−x In_xAs_ySb_1−y solid phase in the series of substrate orientations (100), (111)A, (111)B. A change in the composition of the solid solution leads to a shift of the long-wavelength edge of the spectral distribution of the photosensitivity. The use of a GaSb (111)B substrate made it possible, without lowering the epitaxy temperature, to increase the indium content in the solid phase to 23.8% and to create long-wavelength photodiodes with spectral photosensitivity threshold λ _th=2.55 μm. The primary characteristics of such photodiodes are described, along with aspects of their fabrication. The proposed fabrication technique shows potential for building optoelectronic devices (lasers, LED’s, photodiodes) based on Ga_1−x In_xAs_ySb_1−y solid solutions with red boundary as high as 2.7 μm. Fiz. Tekh. Poluprovodn. 33, 249–253 (February 1999)     

Superstructured ordering in Al x Ga1 − x As and Ga x In1 − x P alloys

Epitaxial heterostructures produced on the basis of Al _x Ga_{1 ? x} As and Ga _x In_{1 ? x} P ternary alloys by metal-organic chemical vapor deposition are studied. The composition parameter x of the alloys was ～0.50. By X-ray diffraction studies, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy, it is shown that superstructured ordered phases with the stoichiometry composition III_{1 ? η}III_{1 + η}V₂ can be formed. As a consequence of this effect, not only does the cubic crystal symmetry change to the tetragonal type in the new compound, but also the optical properties become different from those of disordered alloy with the same composition.     

Reinvestigation of Thermoelectric Properties of n- and p-Type Ba8−d Au x Si46−x−y Clathrate

We have synthesized n- and p-type clathrates Ba_8?d Au _x Si_46?x?y with various Au contents (4.6 < x < 6.0) by arc-melting, annealing at 1173 K, and spark plasma sintering at 1073 K. The Au compositions found by wavelength-dispersive x-ray spectrometry for the synthesized samples were slightly lower than the nominal compositions. Ba_7.8Au_4.6Si_41.4 and Ba_7.7Au_4.9Si_41.1 samples showed n- and p-type conduction, respectively. According to the electron count (Ba²⁺)₈Au(^3?)_5.33Si_40.67, the clathrate composition with x = 5.33 is expected to be an intrinsic semiconductor. Our experimental results show that increase of the Au composition causes a transition from n-type to p-type conduction between x = 4.6 and 4.9. We have also calculated the band structures of the Ba₈Au _x Si_46?x clathrate including a vacancy by ab initio calculation based on density functional theory with structure optimization. It was found that the vacancy behaves like an electron acceptor and the numbers of vacancies at 24k sites for the synthesized Ba₈Au _x Si_46?x?y clathrates can be estimated as ～0.4 in a unit cell.     

关于4＼n=1／x＋1／y＋1／z问题的解

对４／ｎ＝１／ｘ＋１／ｙ＋１／ｚ问题进行分析，建立了若干张筛网，从而改进了已有献的结果。     

Technology and electronic properties of PHEMT AlGaAs/In y(z)Ga1 − y(z)As/GaAs compositionally graded quantum wells

Graded In _y Ga_{1 ? y} As quantum well epitaxial technology is developed for engineering the band potential profile. The crystal structure of the samples is clarified by high-resolution X-ray diffraction. The influence of quantum-well bending on the crystal and electron transport properties is studied on one- and two-side δ-doped Al_0.23Ga_0.77As/In _y Ga_{1 ? y} As/Al_0.23Ga_0.77As PHEMT heterostructures. The highest InAs content gradient reached is 1.2%/nm for the mean InAs content y = 0. 2. Optimization of the InAs content grading leads to an increase in the electron mobility and concentration. This effect is related to the straightening and deepening of the quantum-well potential profile. In addition, the electron wavefunction shifts toward the quantum-well center, thus reducing electron scattering.     

Band-edge photoluminescence of heavily doped InxGa1−x As1−y Py(λ=1.2 µm)

Band-edge photoluminescence spectra of heavily donor-doped samples of In_xGa_1−x As_1−y P_y (x=0.77, y=0.53) were investigated in the temperature range (77–300) K. A theory of luminescence that takes into account fluctuations in the band-edge potentials due to nonuniform distribution of impurities is used to calculate temperature dependences of the positions and half-widths of peaks in these spectra. Good agreement is obtained between experimental and calculated curves. For heavily doped In_xGa_1−x As_1−y P_y samples with either p-or n-type conductivity, the peak energy of the band-edge PL is observed to shift towards lower frequencies at low temperatures. This shift is accompanied by broadening of the spectra and a decrease in the photoluminescence intensity compared to the analogous parameters for the spectra of undoped material. Possible mechanisms for radiative recombination are analyzed. Fiz. Tekh. Poluprovodn. 33, 907–912 (August 1999)     

Transparent and Flexible Mn1−x−y(CexLay)O2−δ Ultrathin-Film Device for Highly-Stable Pseudocapacitance Application

Control over the fabrication of state-of-the-art portable pseudocapacitors with the desired transparency, mechanical flexibility, capacitance, and durability is challenging, but if resolved will have fundamental implications. Here, defect-rich Mn_1−x−y(Ce_xLa_y)O_2−δ ultrathin films with controllable thicknesses (5–627 nm) and transmittance (≈29–100%) are fabricated via an electrochemical chronoamperometric deposition using a aqueous precursor derived from end-of-life nickel-metal hydride batteries. Due to percolation impacts on the optoelectronic properties of ultrathin films, a representative Mn_1−x−y(Ce_xLa_y)O_2−δ film with 86% transmittance exhibits an outstanding areal capacitance of 3.4 mF cm⁻², mainly attributed to the intercalation/de-intercalation of anionic O²⁻ through the atomic tunnels of the stratified Mn_1−x−y(Ce_xLa_y)O_2−δ crystallites. Furthermore, the Mn_1−x−y(Ce_xLa_y)O_2−δ thin-film device exhibits excellent capacitance retention of ≈90% after 16 000 cycles. Such stability is associated with intervalence charge transfer occurring among interstitial Ce/La cations and Mn oxidation states within the Mn_1−x−y(Ce_xLa_y)O_2−δ structure. The energy and power densities of the transparent flexible Mn_1−x−y(Ce_xLa_y)O_2−δ full-cell pseudocapacitor device, is measured to be 0.088 μWh cm⁻² and 843 µW cm⁻², respectively. These values show insignificant changes under vigorous twisting and bending to 45–180° confirming these value-added materials are intriguing alternatives for size-sensitive energy storage devices.     

Optical band gap of Cd1−x MnxTe and Zn1−x MnxTe semiconductors

The dependence of the optical band gap for Zn_1?x Mn_xTe and Cd_1?x Mn_xTe semiconductor compounds was investigated by the methods of cathodoluminescence and optical reflection. It was found that, for Zn_1?x Mn_xTe compounds in the region x?0.2, the band gap is additionally broadened by a magnitude of about 0.08 eV, which is related to the high density of interstitial-type defects in single crystals. For x?0.3, the probability of the existence of these defects decreases substantially, which is related to the distortion of tetrahedra of the crystal lattice of Zn_1?x Mn_xTe by Mn atoms, which are incorporated into each tetrahedron.     

Room-Temperature Quantum Cascade Laser: ZnO/Zn1−x Mg x O Versus GaN/Al x Ga1−x N

A ZnO/Zn_1?x Mg _x O-based quantum cascade laser (QCL) is proposed as a candidate for generation of THz radiation at room temperature. The structural and material properties, field dependence of the THz lasing frequency, and generated power are reported for a resonant phonon ZnO/Zn_0.95Mg_0.05O QCL emitting at 5.27 THz. The theoretical results are compared with those from GaN/Al _x Ga_1?x N QCLs of similar geometry. Higher calculated optical output powers [ $ {P}_{\rm{ZnMgO}} $  = 2.89 mW (nonpolar) at 5.27 THz and 2.75 mW (polar) at 4.93 THz] are obtained with the ZnO/Zn_0.95Mg_0.05O structure as compared with GaN/Al_0.05Ga_0.95N QCLs [ $ {P}_{\rm{AlGaN}} $  = 2.37 mW (nonpolar) at 4.67 THz and 2.29 mW (polar) at 4.52 THz]. Furthermore, a higher wall-plug efficiency (WPE) is obtained for ZnO/ZnMgO QCLs [24.61% (nonpolar) and 23.12% (polar)] when compared with GaN/AlGaN structures [14.11% (nonpolar) and 13.87% (polar)]. These results show that ZnO/ZnMgO material is optimally suited for THz QCLs.     

Thermal Conductivity in Bi1−x Sb x Solid Solutions

Bi_1?x Sb _x solid solutions have attracted much attention as promising low-temperature thermoelectric materials. Previously, we observed distinct extrema in the isotherms of the transport and mechanical properties of polycrystalline Bi_1?x Sb _x and attributed their presence to the transition from diluted to concentrated solid solutions and to the reconstruction of the energy band structure under increasing Sb concentration. The goal of the present work is a detailed study of the concentration dependences of the thermal conductivity λ for Bi_1?x Sb _x polycrystalline solid solutions (x = 0 to 0.09) in the temperature range of 170 K to 300 K. It is established that the λ(x) dependences exhibit a nonmonotonic behavior: in certain concentration ranges an anomalous increase in λ with increasing x is observed. It is shown that the concentration dependences of the thermoelectric figure of merit calculated on the basis of the measured λ values are also nonmonotonic. The obtained data represent additional evidence in favor of our assumptions stated earlier about a significant effect of electronic phase transitions observed in Bi_1?x Sb _x solid solutions on the concentration dependences of their thermoelectric properties. These results should be taken into account when developing new Bi_1?x Sb _x -based materials.     

Magnetotransport in a semimetal channel in p-Ga1−x InxAsySb1−y /p-InAs heterostructures with various compositions of the solid solution

Magnetotransport properties of an electron channel at the heteroboundary in type II separated p-Ga_1?x In_xAs_ySb_1?y /p-InAs heterostructures grown by LPE (x=0.09–0.22) were studied in the temperature range of 77–300 K. It is shown that an electron channel, which is formed at the heteroboundary and has high mobility μ=(3–5)×10⁴ cm² V^?1 s^?1, exists throughout the whole composition range. The band diagram of the heterostructures under study is discussed, and some parameters of the electron channel are evaluated. It is found that the electron channel with high mobility persists up to room temperature. Type II GaInAsSb/p-InAs heterostructures can find application in new Hall sensor devices with an electron channel at the heteroboundary.     

Conduction- and Valence-Band Energies in Bulk InAs1−x Sb x and Type II InAs1−x Sb x /InAs Strained-Layer Superlattices

The energy gaps were studied in two types of structures: unrelaxed bulk InAs_1?x Sb _x layers with x = 0.2 to 0.46 grown on metamorphic buffers and type II InAs_1?x Sb _x /InAs strained-layer superlattices (SLS) with x = 0.225 to 0.296 in the temperature range from T = 13 K to 300 K. All structures were grown on GaSb substrates. The longest wavelength of photoluminescence (PL) at low temperatures was observed from bulk InAs_0.56Sb_0.44 with a peak at 10.3 μm and full-width at half-maximum (FWHM) of 11 meV. The PL data for the bulk InAs_1?x Sb _x materials of various compositions imply an energy gap bowing parameter of 0.87 eV. A low-temperature PL peak at 9.1 μm with FWHM of 13 meV was observed for InAs_0.704Sb_0.296/InAs SLS. The PL spectrum of InAs_0.775Sb_0.225/InAs SLS under pulsed excitation revealed a second peak associated with recombination of electrons in the three-dimensional (3D) continuum with holes in the InAs_0.775Sb_0.225. This experiment determined the conduction-band offset in the InAs_0.775Sb_0.225/InAs SLS. The energies of the conduction and valence bands in unstrained InAs_1?x Sb _x and their bowing with respect to the Sb composition are discussed.     

Photoluminescence spectra of Cd x Hg1 − x Te quantum-well heterostructures

Photoluminescence spectra are measured for a nanoscale Cd _x Hg_{1 ? x} Te heterostructure with a single quantum well about 12.5 nm thick in the case x = 0.24. The confined energy levels and the respective rates of different optical transitions are calculated on this basis. Major radiative processes are identified.     

Structure and properties of Cd x Hg1−x Te-metal contacts

A metal-semiconductor contact is a composite structure consisting of several nanodimensional layers. The contact properties depend strongly on the technique of metal deposition. A metal forms chemical compounds with the components of Cd _x Hg_1?x Te (CMT), thus changing the properties of the surface layer. Mercury is accumulated at the interface with the metal, while tellurium is accumulated on the metal surface. The CMT compounds with metals, heats of their formation, and the Fermi level shifts are reported. The structure and properties of the interfaces between CMT and gold, silver, indium, aluminum, copper, and other metals, as well as the effect of sublayers of other metals and insulators, are described.     

Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.