Thermoelectric Properties of CdTe1−x Cl x Material Prepared by Spark Plasma Sintering Method

CdTe compound is a prospective thermoelectric material due to its high Seebeck coefficient and low thermal conductivity. In the present study, we optimized its carrier concentration by substituting Cl on the Te site in order to improve the electrical conductivity and decrease the lattice thermal conductivity. The polycrystalline CdTe_1?x Cl _x (x = 0.005, 0.01, 0.03, 0.05) samples were fabricated by solid state reaction followed with spark plasma sintering, and the relative densities of the sintered samples were higher than 98%. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ). and thermal conductivity (κ), were measured in the temperature range of 300–700 K. The increase of Cl content (x) caused an increase of σ, and the maximum ZT value of 0.2 was obtained at about 630 K for the CdTe_0.97Cl_0.03 sample.     

Inelastic electron scattering cross-section spectroscopy of Ge x Si1 − x nanoheterostructures

Two-component Ge _x Si_{1 ? x} (0 ≤ x ≤ 1) structures are studied by electron spectroscopy. The atomic composition of the structures is determined from X-ray photoelectron spectroscopy data. The reflection electron-energy-loss spectra for a series of samples with different x at primary-electron energies from 200 to 3000 eV are recorded. Using the experimental spectra, the electron energy loss dependences of the product of the electron inelastic mean free path and the differential inelastic electron scattering cross section are calculated. It is shown that the quantitative characteristics of these dependences can be used to determine the atomic concentrations of elements in the investigated system.     

Si1 − x Ge x /Si heterostructures grown by molecular-beam epitaxy on silicon-on-sapphire substrates

The growth of heterostructures with Si_{1 ? x} Ge _x layers on $\left( {1\bar 102} \right)$ sapphire substrates by molecular-beam epitaxy with a silicon sublimation source and a germanium gas source (GeH₄) is reported. The systematic study of the influence of substrate temperature and thickness of the silicon buffer layer shows that the optimal conditions for growing epitaxial Si_{1 ? x} Ge _x layers are provided at a temperature of T _S = 375–400°C. There are significant differences in the orientations of Si_{1 ? x} Ge _x layers, depending on the thickness d of the Si buffer layer: the preferred orientations are (100) at d ≥ 100 nm and (110) for thinner layers. Heterostructures with thick (～1 μm) Si_{1 ? x} Ge _x layers, doped with erbium atoms, exhibit intense photoluminescence at λ = 1.54 μm.     

Solid-State Synthesis and Thermoelectric Properties of Mg2Si1−x Sn x

Mg₂Si_1?x Sn _x (0 ≤ x ≤ 1) solid solutions have been successfully prepared by mechanical alloying and hot pressing as a solid-state synthesis route. All specimens were identified as phases with antifluorite structure. The electrical conduction changed from n-type to p-type at room temperature for x ≥ 0.5 due to the intrinsic properties of Mg₂Sn. The absolute value of the Seebeck coefficient decreased with increasing temperature, and the electrical conductivity increased with increasing temperature; this is indicative of nondegenerate semiconducting behavior. The thermal conductivity was reduced by Mg₂Si-Mg₂Sn solid solution due to phonon scattering by the alloying effect.     

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_{1 ? x} Ge _x heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si_{1 ? x} Ge _x crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.     

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.     

Growth and Characterization of In x Ga1−x N Multiple Quantum Wells Without Phase Separation

Efficient conversion of photon energy into electricity is a crucial step toward a sustainable solar-energy economy. Likewise, solid-state lighting devices are gaining prominence because of benefits such as reduced energy consumption and reduced toxicity. Among the various semiconductors investigated, In _x Ga_1–x N alloys or superlattices are fervently pursued because of their large range of bandgaps between 0.65 eV and 3.4 eV. This paper reports on the fabrication of multiple quantum wells on LiGaO₂ (001) substrates by plasma-assisted molecular beam epitaxy. Metal modulated epitaxy was utilized to prevent formation of metal droplets during the growth. Streaky patterns, seen in reflection high-energy electron diffraction, indicate two-dimensional growth throughout the device. Postdeposition characterization using scanning electron microscopy also showed smooth surfaces, while high-resolution x-ray diffraction and high-resolution transmission electron microscopy confirm the epitaxial nature of the overall quantum well structure.     

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.     

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.     

Structural and electronic properties of Si1 − x Ge x binary semiconducting alloys under the effect of temperature and pressure

Based on the empirical pseudo-potential method which incorporates compositional disorder as an effective potential, the band structure of Si_{1 ? x} Ge _x alloy are calculated for different alloy composition x. The effect of temperature and pressure on the electronic band structure of the considered alloy has been studied. Monotonic decreasing and increasing functions are obtained for the temperature and pressure dependent form factors respectively. Some physical quantities as band gaps, bowing parameters, and the refractive index of the considered alloy with different Ge concentration and under the effect of temperature and pressure are calculated. The results obtained are found in good agreement with the experimental and published data.     

Mechanism of local amorphization of a heavily doped Ti1 − x V x CoSb intermetallic semiconductor

Structural and electron transport characteristics of a TiCoSb intermetallic semiconductor heavily doped with the V donor impurity (dopant concentration ～9.5 × 10¹⁹ ? 1.9 × 10²¹ cm^?3, temperatures 80–380 K) have been investigated and the distribution of the electron density of states in this material has been calculated. Different occupancies have been established for Co and (Ti, V) atomic positions in the Ti_{1 ? x} V _x CoSb lattice; this difference is equivalent to introduction of two types of acceptors into the semiconductor. Suppression of the metallic conductivity in the n-type semiconductor with an increase in the donor concentration has been found, which is explained by simultaneous generation of acceptors. It is shown that the methods of numerical calculation adequately describe the physical processes if the occupancy of unit-cell positions is taken into account in construction of the Wigner-Seitz lattice. The results obtained are discussed within the model of a heavily doped and compensated Shklovskii-Efros semiconductor.     

Thermoelectric Performance of p-Type (Bi85Sb15)1?x Sn x Materials Prepared by a Pressureless Sintering Technique

In this study, a series of Sn-doped (Bi₈₅Sb₁₅)_1?x Sn _x (x?=?0, 0.025, 0.05, 0.1, 0.2, 0.3) thermoelectric materials was fabricated through mechanical alloying followed by pressureless sintering. The crystal structure was characterized by x-ray diffraction. The electrical transport properties and thermal properties were measured in the temperature range from 77?K to 300?K. The electrical transport as a function of temperature appeared to be characteristic of a semimetal. The Seebeck coefficient gradually changed from negative to positive with increasing Sn doping, showing p-type electrical transport properties. It is found that the Seebeck coefficients of the p-type Bi-Sb alloys decrease with increasing dopant concentration of Sn, which may be due to increasing carrier concentration. Among the p-type alloys, the power factor of (Bi₈₅Sb₁₅)_0.975Sn_0.025 reached a maximum value of 1.3?×?10^?3?W/mK² at 265?K, and the optimum figure of merit value of 0.13 was obtained at 240?K. The results indicate that good p-type Bi-Sb alloys can be prepared by this synthesis procedure.     

Characterization of ternary MgxZn1−xO thin films deposited by electron beam evaporation

Mg_xZn_1−xO (0≤x≤1) thin films were deposited on glass and quartz substrates by electron beam evaporation and effect of the Mg content of the film on its structural, optical and electrical properties were investigated. The structure, surface morphology, optical transmittance, band gap, refractive index and electrical resistivity were found to depend on the Mg content of the film. XRD data revealed that films were polycrystalline in nature. The structure of the films having Mg content in the range of 1–0.74 was cubic, mixed cubic-hexagonal phases for x=0.47 and hexagonal phase for x=0. The composition analysis showed that Mg content in Mg_xZn_1−xO film is high as compared to the corresponding target alloy. It was observed that the optical band gap increases from 3.3 to 6.09 eV, refractive index at 550 nm decreases from 1.99 to 1.75, transmittance increases from about 70% to 90% and electrical resistivity increases from 0.5 to 1.48×10⁶ Ω cm with the increase of Mg concentration in the film from 0 to 1. The results reported in this work are useful for window layer of solar cells and other optoelectronic devices.     

Comparison of GexSi1—x Grown by UHV／CVD from Si2H6／GeH4 and SiH4／GeH4

Using double crystal X-rays diffraction (DCXRD) and atomic force microscopy (AFM), the results of Ge x Si 1- x grown by UHV/CVD from Si 2H 6 and SiH 4 are analyzed and compared. Adsorbates can migrate to the energy-favoring position due to the slow growth rate from SiH 4. In this case, a Si buffer that isolates the effect of substrate on epilayer could not be grown, which results in a pit penetrating into epilayer and buffer. The FWHM is 0.055° in DCXRD from SiH 4. The presence of diffraction fringes is an indication of an excellent crystalline quality. The roughness of the surface is improved if grown by Si 2H 6; however, the crystal quality of the Ge x Si 1- x material became worse than that from SiH 4 due to much larger growth rate from Si 2H 6. The content of Ge is obtained from DCXRD, which indicates the growth rate from Si 2H 6 is largest, then GeH 4, and that from SiH 4 is least.     

Nanostructuring and Thermoelectric Characterization of (GaSb)3(1−x)(Ga2Te3) x

GaSb is a promising thermoelectric material that exhibits good electrical properties. However, it has a high lattice thermal conductivity (κ _lat). Nanostructured bulk materials have been attracting interest because they effectively scatter phonons, significantly reducing κ _lat. AgPb _m SbTe _m+2 (LAST-m) compounds have recently been reported to have low κ _lat. These compounds have a NaCl structure, similar to that of binary PbTe, where Ag and Sb occupy the Pb site. In these compounds, two divalent Pb atoms are replaced with a monovalent Ag atom and a trivalent Sb atom to maintain charge compensation. In the present study, we reduced κ _lat of GaSb by applying the same principle as in LAST-m. Specifically, we substituted Te for Sb and generated vacancies at the Ga site to maintain charge compensation. This produced compounds with chemical compositions of (GaSb)_3(1?x)(Ga₂Te₃) _x (x = 0, 0.05, 0.10, and 0.25), where GaSb and Ga₂Te₃ both have the zincblende crystal structure. We employed two different annealing conditions: annealing at 833 K followed by quenching, and annealing at 833 K followed by cooling to room temperature over 3 days. The former annealed samples with compositions of x = 0.05 and 0.10 had nanoscale Ga-rich precipitates and exhibited a large reduction in κ _lat.     

Specifics of MOCVD formation of InxGa1−x N inclusions in a GaN matrix

MOCVD-grown heterostructures with one or several In_xGa_1?x N layers in a GaN matrix have been studied by transmission electron microscopy. In heterostructures with thick InGaN layers, a noncoherent system of domains with lateral dimensions (～50 nm) on the order of the layer thickness (～40 nm) is formed. In the case of ultrathin InGaN inclusions, nanodomains coherent with the GaN matrix are formed. The content of indium in nanodomains, determined by the DALI method, is as high as x≈0.6 or more, substantially exceeding the average In concentration. The density of the nanodomains formed in the structures studied is n≈(2–5)×10¹¹ cm^?2. In the structures with ultrathin InGaN inclusions, two characteristic nanodomain sizes are observed (3–6 and 8–15 nm).     

Layered Growth of Lattice-Mismatched Ga x In1−x P on GaP Substrates by Liquid Phase Epitaxy

We report layered growth of Ga _x In_1?x P on GaP substrates using single-step liquid phase epitaxy (LPE) with a Sn-based melt when the lattice mismatch is greater than 0.4 % (x < 0.95). Compositional control was observed by (1) varying the cooling rate and (2) changing the melt-back temperature at the beginning of the growth. Possible growth mechanisms are proposed to explain the principles of both approaches of compositional control. Smooth epilayers have been observed. High resolution x-ray diffraction was used to characterize the composition of the epilayers, and room temperature photoluminescence was reported for one of the samples with the composition of x = 0.11. Plan-view TEM measurements revealed threading dislocation densities on the order of 10⁸ cm^?2 in the upper regions of the Ga _x In_1?x P epilayers. In contrast, when using In-based melts, LPE of Ga _x In_1?x P on GaP (100) substrates exhibited island growth at large misfits, whereas edge growth dominated when using GaP (111B) substrates under equivalent growth conditions.