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.

     

Preparation, Characterization, and Microwave Dielectric Properties of Sr2La3Nb1−x Ta x Ti4O17 (0 ≤ x ≤ 1) Ceramics

Sr₂La₃Nb_1?x Ta _x Ti₄O₁₇ (0 ≤ x ≤ 1) ceramics were processed via a solid-state mixed oxide route. Sr₂La₃Nb_1?x Ta _x Ti₄O₁₇ (0 ≤ x ≤ 1) solid solutions were single phase in the whole range of x values within the x-ray diffraction (XRD) detection limit. The microstructure comprised elongated and needle-shaped grains. The ceramics exhibit relative permittivity (ε _r) of 73 to 68.6, product of unloaded quality factor and resonant frequency (Q _u f ₀) of 7100 GHz to 9500 GHz, and temperature coefficient of resonant frequency (τ _f) of 78.6 ppm/°C to 56.6 ppm/°C.     

Optical and Electrical Properties of Polyimide Thin Films Doped-Cu-phthalocyanine Polymerized by Vapor Phase Deposition

Using Cu-phthalocyanine(CuPc),4,4’-diaminodiphenyl ether and pyromellitic dianhydride as monomer materials, polyimide(PI) thin films doped-CuPc have been prepared onto glass substrate by vapor phase co-deposition polymerization under a vacuum of 2×10~(-3)Pa and thermal curing of polyamic acid film in at temperature of 150-200℃ for 60min. In this process, the polymerization can be carried out through controlling the stoichiometric ratio, heating time and deposition rates of the three monomers. IR spectrum identifies the designed chemical structure of the polymer. The absorption of polyimide doped-CuPc is very intense in vis-range and near-infrared by UV-Vis spectrum. And, the PI films doped-CuPc polymerized by vapor phase deposition have uniformity, fine thermal stability and good nonlinear optical properties, and the third-order optical nonlinear susceptibility χ~((3)) with degenerate four-wave mixing can be 1.984×10~(-9)ESU.     

Effect of Se Substitution on Structural and Electrical Transport Properties of Bi0.4Sb1.6Se3x Te3(1−x) Hexagonal Rods

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ ingots dispersed with x amount of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3x Te_3(1?x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field-emission scanning electron microscopy (FESEM). The crystal structure of the powders was examined by x-ray diffraction (XRD) analysis, which showed that powders of the 0.0 ≤ x ≤ 0.8 samples could be indexed to the rhombohedral phase, whereas the sample with x = 1.0 had an orthorhombic phase structure. The influence of variations in the Se content on the thermoelectric properties was studied in the temperature range from 300 K to 523 K. Alloying of Se into Bi_0.4Sb_1.6Te₃ effectively caused a decrease in the hole concentration and, thus, a decrease in the electrical conductivity and an increase in the Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/mK² at 373 K for the x = 0.8 sample.     

Structural and Thermoelectric Properties of Bi1−x Sb x Nanoalloys Prepared by Mechanical Alloying

Bi_1?x Sb _x nanoparticles were prepared by mechanical alloying and compacted using different techniques. The influence of the composition as well as the pressing conditions on the thermoelectric performance was investigated. A strong dependence of the thermoelectric properties on the composition was found, which deviates from the behavior of single crystals. The results indicate a significant change in the band structure of the material induced by the reduced size. The influence of the pressing conditions on the thermoelectric properties also showed composition dependence. The results show that the compacting method has to be chosen carefully.     

Electrochemical Deposition and Characterization of Thermoelectric Ternary (Bi x Sb1?x )2Te3 and Bi2(Te1?y Se y )3 Thin Films

Thermoelectric thin films of the ternary compounds (Bi _x Sb_1?x )₂Te₃ and Bi₂(Te_1?y Se _y )₃ were synthesized using potentiostatic electrochemical deposition on gold-coated silicon substrates from aqueous acidic solutions at room temperature. The surface morphology, elemental composition, and crystal structure of the deposited films were studied and correlated with preparation conditions. The thermoelectric properties of (Bi _x Sb_1?x )₂Te₃ and Bi₂(Te_1?y Se _y )₃ films, i.e., Seebeck coefficient and electrical resistivity, were measured after transferring the films to a nonconductive epoxy support. (Bi _x Sb_1?x )₂Te₃ thin films showed p-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively large negative potential with composition close to Bi_0.5Sb_1.5Te₃. In addition, Bi₂(Te_1?y Se _y )₃ thin films showed n-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively small negative potential, having composition close to Bi₂Te_2.7Se_0.3. In contrast to Bi₂Te_2.7Se_0.3 thin films, an annealing treatment was required for Bi_0.5Sb_1.5Te₃ thin films to achieve the same magnitude of power factor as Bi₂Te_2.7Se_0.3. Therefore, Bi₂Te_2.7Se_0.3 thin films appear to be good candidates for multilayer preparation using electrochemical deposition, but the morphology of the films must be further improved.     

Preparation, Structure, and Electrical Properties of Ca1−x Er x MnO3 Powders

To improve the electrical properties of modified CaMnO₃ powders, we synthesized Ca_1?x Er _x MnO₃ (CEM) (0 ≤ x ≤ 0.3) powders by sol–gel autocombustion technology. The effects of the Er-doping concentration on the structure and electrical properties of the powders were studied. The results show that Er-doping can decrease the resistivity of the CEM powders effectively and the variation of the resistivity presents a typical V-type with increasing Er-doping concentration. The resistivity reaches the lowest point of 0.5258 Ω m at x = 0.25. Meanwhile, the Ca_0.75Er_0.25MnO₃ powder showed good frequency stability and higher conductivity at high temperatures. x-Ray diffraction (XRD) patterns and valence analysis illustrate that the grain size and average valence of the Mn ions, which relate to the resistivity of the CEM powders, are affected by the Er-doping concentration. When x = 0.25, the average valence of the Mn ions reaches the lowest point of 3.576. Scanning electron microscopy (SEM) analysis demonstrates that higher Er-doping concentration leads to smaller grain size in the CEM powders. Er-doping restrains the growth of the powder particles while decreasing the porosity and increasing the unit cell volume, resulting in improvement of the electrical properties of the modified powders.     

Terahertz-Pulse Radiation Properties of Oxygen-Deficient YBa2Cu3O7−δ Thin Films

By measuring the detailed characteristics of terahertz-pulse (THz-pulse) radiation and applying THz time-domain spectroscopy, we investigate the generation mechanism of THz-pulse radiation from biased YBa₂Cu₃O_7?δ (YBCO) thin films with various carrier concentrations. To vary the carrier concentration, the oxygen deficiency of the YBCO samples was systematically varied. With decreasing carrier concentration, the THz-pulse width increases and the response speed of supercarriers to illumination by femtosecond optical pulses tends to decrease. By examining the radiated waveforms, we clarify the influence of carrier doping not only on the relaxation process of quasiparticles but also on the excitation process. A suitably underdoped YBCO film results in maximum efficiency for generating THz radiation. This tendency is difficult to explain if we only consider the variation in transmittance, which has been the approach till now. Instead, we suggest that a variation in quasiparticle excitation efficiency due to carrier doping should also be considered.     

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.     

Exploring the Optical Properties of Hg1−x Cd x Se Films Using IR-Spectroscopic Ellipsometry

We have used infrared spectroscopic ellipsometry to interrogate the dielectric response of a series of Hg_1?x Cd _x Se samples in a spectral range between 2000 and 40000 nm. Using a standard inversion technique, the experimental data obtained at multiple angles of incidence were modeled to deduce the dielectric function of each sample. The dielectric functions obtained for Hg_1?x Cd _x Se samples allowed us to predict the band gaps for the samples, which increase as a function of the Cd concentration. Next, we modeled the dielectric function as a collection of oscillators, each of which represented a particular transition manifested in the spectrum. The most significant result obtained from this work is the recovery of their doping characteristics from the ellipsometric data. Specifically, two Hg_1?x Cd _x Se samples with x = 0.21 and 0.28, grown on GaSb substrates, show a carrier concentration of 1.6 × 10¹⁷ and 1.8 × 10¹⁸ cm^?3, respectively. These results are particularly helpful because conventional Hall measurements cannot be used for these specific samples due to the substrates (i.e., GaSb) used to grow them being highly conductive.     

Structural and Electrical Behaviors of Ag Films Deposited on Liquid Substrates

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Structural Un-uniformity and Electrical Anisotropy of μc-Si:H Films

Structural un-uniformity and electrical anisotropy of μc-Si∶H film are investigated in this paper. It is found that the structure of μc-Si∶H film along the direction perpendicular to the substrate is not uniform, which is modulated by film thickness. In addition, there is a dark conductivity anisotropy along the direction parallel(σ∥) and perpendicular(σ⊥)to the substrate in μc-Si∶H film. The reasons for such an property of μc-Si∶H film and the effect of oxygen contamination are analyzed.     

Electrical and Thermal Transport Properties of Ge1–xPbxCuySbyTeSe2y

Balancing the contradictory relationship between thermoelectric parameters, such as effective mass and carrier mobility, is a challenge to optimize thermoelectric performance. Herein, the exceptional thermoelectric performance is realized in GeTe through collaboratively optimizing the carrier and phonon transport via stepwise alloying Pb and CuSbSe₂. The formation energy of Ge vacancy is efficiently bolstered by alloying Pb, which reduces carrier density and carrier scattering to maintain superior carrier mobility in GeTe. Additionally, CuSbSe₂, acting as an n-type dopant, further modulates carrier density and validly equilibrates carrier mobility and effective mass. Accordingly, the promising power factor of 45 µW cm⁻¹ K⁻² is achieved at 723 K. Meanwhile, point defects are found to significantly suppress phonons transport to descend lattice thermal conductivity by Pb and CuSbSe₂ alloying, which barely impacts the carrier mobility. A combination with superior carrier mobility and lower lattice thermal conductivity, a maximum ZT of 2.2 is attained in Ge_0.925Pb_0.075Cu_0.005Sb_0.005TeSe_0.01, which corresponds to a 100% promotion compared with that of intrinsic GeTe. This study provides a new indicator for optimizing carrier and phonon transport properties by balancing interrelated thermoelectric parameters.     

Growth and Characterization of In x Ga1−x As/GaAs1−y P y Strained-Layer Superlattices with High Values of y (~80%)

Strained-layer superlattice (SLS) structures, such as InGaAs/GaAsP lattice matched to GaAs, have shown great potential in absorption devices such as photodetectors and triple-junction photovoltaic cells. However, until recently they have been somewhat hindered by their usage of low-phosphorus GaAsP barriers. High-P-composition GaAsP was developed as the barrier for InGaAs/GaAsP strained-layer superlattice (SLS) structures, and the merits of using such a high composition of phosphorus are discussed. It is believed that these barriers represent the highest phosphorus content to date in such a structure. By using high-composition GaAsP the carriers are collected via tunneling (for barriers ≤30 Å) as opposed to thermionic emission. Thus, by utilizing thin, high-content GaAsP barriers one can increase the percentage of the intrinsic in a p-i-n structure that is composed of InGaAs wells in addition to increasing the number of periods that can be grown for given depletion width. However, standard SLSs of this type inherently possess undesirable compressive strain and quantum size effects (QSEs) that cause the optical absorption of the thin InGaAs SLS wells to shift to higher energies relative to that of bulk InGaAs of the same composition. To circumvent these deleterious QSEs, stress-balanced, pseudomorphic InGaAs/GaAsP staggered SLSs were grown. Staggering was achieved by removing a portion of one well and adding it to an adjacent well. The spectral response obtained from device characterization indicated that staggering resulted in thicker InGaAs films with reduced cutoff energy. Additionally, these data confirm that tunneling is a very effective means for carrier transport in the SLS.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.