Microstrip U-shaped resonators

Influence analysis of U-shaped microstrip resonators?? parameters and parameters of dielectric substrate on resonant frequencies and unloaded quality-factor of resonators is conducted. Three kinds of U-shaped resonators are considered which differ in operation modes at the resonator??s ends: short-circuit or no-load. It is revealed that in cases of small spacing between the arms of U-shaped resonators distribution of their resonant frequencies significantly differs from the corresponding quarter-wavelength or half-wavelength resonators and leads to decreased stop-band in pass-band filters based on them. Experimental research shows that decreasing the spacing between the arms of U-shaped resonators leads to decrease in quality-factor by 20?C30%. Experimental samples were manufactured on substrates with ? _r = 92 and ? _r = 38.     

Dependence of carrier mobility on an electric field in gallium selenide crystals

The dependence of the mobility of charge carriers on voltage has been studied in undoped GaSe single crystals and crystals doped with gadolinium; the latter crystals have exhibited various values of dark resistivity (??_d.r ?? 10⁴?10⁸ ?? cm at 77 K) and of the doping level (N = 10^?5, 10^?4, 10^?3, 10^?2, and 10^?1 at %). It is established that the dependence of the charge-carrier mobility on the electric field applied to the sample E ?? 10² V/cm is observed in undoped high-resistivity GaSe crystals (??_d.r ?? 10⁴ ?? cm) and in lightly doped GaSe crystals (N ?? 10^?2 at %) in the region of T ?? 150 K. It is found that this dependence is not related to heating of the charge carriers by an electric field; rather, it is caused by elimination of drift barriers as a result of injection.     

Two-frequency resonance scattering by finite thin metamaterial plates

A finite thin plate made from a metamaterial with negative values of permittivity ? and permeability ?? is considered. The 2D problem of excitation of the plate by a cylindrical wave is numerically investigated. It is found that the frequency dependence of the excited near field has two high-Q resonances closely spaced in frequency and that the constitutive parameters have values close to those that lead to triple (double) degeneration of a surface wave in an infinite plate. The near and far fields at the resonance frequencies are calculated.     

S-Graded Buffer Layers for Lattice-Mismatched Heteroepitaxial Devices

We have conducted a theoretical study of the equilibrium strain and misfit dislocation density profiles for ??S-graded?? buffer layers of In _x Ga_1?x As on GaAs (001) substrates in which the compositional profile follows a normal cumulative distribution function. On the basis of this modeling work we show that the S-graded layer exhibits misfit dislocation-free regions near the substrate interface and the free surface (or device interface). The equilibrium peak misfit dislocation density as well as the thicknesses of the dislocation-free regions may be tailored by design of the compositional profile; this in turn should enable minimization of the density of electronically active threading dislocations at the top surface. S-graded buffer layers may therefore facilitate the achievement of metamorphic device structures with improved performance compared with similar structures having uniform or linearly graded buffers.     

Small-signal field effect in GaAs/InAs quantum-dot heterostructures

InAs quantum dots (QDs), incorporated into the space-charge region of an epitaxial n-GaAs film at different distances (5?C300 nm) from the surface, decrease the potential barrier for the electrons located in n-GaAs. For tunnel-thin coating layers this decrease is related to tunneling through QD energy levels. For thick layers this decrease is caused by negative charging of the QDlevels and defects located near QDs. The decrease in the barrier increases the efficiency of electron capture by surface states and shifts the frequency dispersion of mobility under the field effect, related to this capture, toward higher frequencies. When QDs are incorporated near the barrier??s base, they manifest themselves in the relaxation of the small-signal field effect. Some parameters of the QD levels are determined. Defect formation is revealed in the layers adjacent to QDs.     

High-temperature ferromagnetism of Si1 ? x Mn x films fabricated by laser deposition using the droplet velocity separation technique

The transport and magnetic properties of Si_{1 ? x} Mn _x films of thickness 55?C70 nm with various Mn content (x = 0.44?C0.6) are studied in the temperature range of 5?C400 K and in magnetic fields up to 2 T. The films are grown by pulsed laser deposition on Al₂O₃ (0001) substrates at a temperature of 340°C using velocity separation of deposited particles. The films exhibit metal conductivity and the resistivity ?? = (2?8) × 10^?4 ?? cm, typical of highly degenerate semiconductors. It is found that the anomalous component of the Hall effect dominates over the normal component at T = 300 K for the Si_{1 ? x} Mn _x alloy with x ?? 0.5, and that the Curie temperature significantly exceeds room temperature and is estimated as ??500 K from magnetization measurements (for MnSi silicide the Curie temperature is T _C = 30 K). It is shown that the anomalous component of the Hall conductivity at low temperatures is controlled by ??side-jump?? and (or) ??intrinsic?? mechanisms independent on the carrier scattering time. The results are explained by features of the formation of defects with localized magnetic moments in the case of Si_{1 ? x} Mn _x films with x ?? 0.5 and by the significant role of matrix spin fluctuations in the exchange between these defects.     

Pure shear backward waves in the <Emphasis Type="Italic">X</Emphasis>-cut and <Emphasis Type="Italic">Y</Emphasis>-cut piezoelectric plates of potassium niobate

The problem of the existence of pure shear backward waves and waves with zero group velocity is investigated for X-cut and Y-cut plates of orthorhombic crystal of potassium niobate, which distinguishes from the other crystals by exceptionally strong piezoelectric effect. The secular equations of the problem are derived in an explicit analytical form and they are studied numerically in the case of crystal-vacuum interface. Two factors contributing to the appearance of backward waves are identified: the negative displacement of the rays of bulk shear waves at oblique reflection from the surface in piezoelectric crystals and the concavity in cross section of the slowness surface for bulk shear waves near the X axis of the potassium niobate plate. The wide frequency ranges of the existence of symmetric and antisymmetric backward waves of the first and second orders are numerically found in the X-cut plate, for which both of these factors affect. To study the dispersion spreading of backward shear wave pulses, it is suggested to apply the parabolic equation corresponding to the second approximation in the dispersion theory. It is demonstrated that the “diffusion” coefficient in this equation vanishes at certain frequencies, leading to significant suppression of the dispersion distortions in the pulses of the waves under study.     

Duplexer designed on the basis of microstrip filters using high dielectric constant substrates

A microstrip duplexer for a modification of a PCS communication system operating at frequencies of 1.84?C1.87 and 1.75?C1.78 GHz is described. The duplexer containing microstrip bandpass filters on high dielectric constant substrates (? _r = 92) are compared with duplexers manufactured on microwave ceramic materials with high values of ? _r . The microstrip bandpass filters use stepped-impedance resonators placed one near another with a gap of 0.1?C0.2 mm. It is shown that the microstrip duplexer has slightly lower insertion loss and occupies smaller volume than a duplexer using coaxial dielectric resonators having a rectangular cross section of 3 × 3 mm and ? _r = 92.     

Symmetric waves in a circular metal waveguide containing an anisotropically conducting coaxial cylindrical surface

Symmetric waves propagating in a hollow metallic waveguide that contains a coaxial cylindrical surface which conducts perfectly along helical lines with a small twist angle are investigated. It is found that, at certain ratios between the structure’s geometric parameters, there exist two modes with close propagation constants. It is shown that the superposition of these modes results in a field that, during propagation along the waveguide, periodically coincides either with the field of the E ₀₁ mode of the inner cylindrical channel (in this case, the field is zero in the coaxial area) or with the field of the H ₀₁ mode of the outer channel. A circular-waveguide E ₀₁ ? H ₀₁ mode converter has been developed and experimentally investigated. It is shown that the conversion loss does not exceed 0.8 dB.     

Amplification and nonlinear interaction of space charge waves of microwave band in heterogeneous gallium nitride films

Theoretical study of amplification and nonlinear interaction of space charge waves caused by negative differential conductivity in gallium nitride n-GaN films placed over a semi-infinite substrate is presented. A general case of transversally heterogeneous films is considered. Hydrodynamic diffusion-drift equations for bulk electron concentration together with Poisson equation are used. Transversal heterogeneity leads to decreased electronic mobility near the film??s surface and to decreased space amplification increments of space charge waves. Heterogeneous alloying may compensate the influence of surfaces on space amplification increments. Amplification of space charge waves in n-GaN films of sub-micron thickness on higher frequencies f ?? 100 GHz is possible in comparison to n-GaAs films. High output electric fields ??10 kV/cm can be obtained in the short-wave range of the millimeter band. Nonlinear interaction of space charge waves in n-GaN films is also considered using the diffusion-drift equation. The possibility of generating second and third multiples of input signal as well as amplification of combination frequencies is proven.     

IR photodetectors in the range of λ = 1.5–8 μm, based on silicon with multicharged nanoclusters of manganese atoms

It is demonstrated that infrared photodetectors based on silicon with multicharged nanoclusters of manganese atoms can be designed that operate in the wavelength range of ?? = 1.55?C8 ??m. Photodetectors fabricated on such materials have the following parameters: a spectral sensitivity range of ?? = 1.55?C8 ??m, an operating temperature range of T = 77?C250 K, an optimal electric field of E = 5 V/cm, an optimal size of V = 3 × 2 × 1 mm³, a sensitivity threshold of S = 10^?9 W/cm², and a response time of ?? < 10^?6 s.     

Features of the field of a cylindrical wave scattered and focused by an isolated perfect Veselago lens with finite dimensions or by a system of three lenses

The results of solving a scattering problem for the field of a cylindrical wave incident onto an isolated Veselago lens with finite dimensions or onto a system of three lenses are presented for the lens refractive index n _r = ?1 and loss parameter ν. The influence of geometric dimensions, the loss in the lens medium, and the position of the source of cylindrical wave on the field structure near the focusing region are studied. It is shown that the resolution of this system cannot be increased via an increase in the geometric dimensions of the lens or the number of lenses.     

Triply degenerate surface waves in the metamaterial plate

A triply degenerate surface wave has been shown to exist in the metamaterial plate with the electrodynamic parameters ɛ₀= −1.036… and μ₀ > −0.964… The behavioral features of the field excited by a point source located near the plate have been revealed under the assumption that the plate parameters tend to resonant quantities and ɛ₀ and μ₀.     

Power Generation Characteristics of Mg2Si Uni-Leg Thermoelectric Generator

Mg₂Si thermoelectric (TE) elements were fabricated by a plasma-activated sintering method using a commercial polycrystalline n-type Mg₂Si source produced by the Union Material Co., Ltd. This material typically has a ZT value of ??0.6. A monobloc plasma-activated sintering technique was used to form Ni electrodes on the TE elements. The dimensions of a single element were 4.0?mm?×?4.0?mm?×?10?mm, and these were used to construct a TE module comprising nine elements connected in series. To reduce the electrical and thermal contact resistance of the module, each part of the module, i.e., the elements, terminals, and insulating plates, was joined using a Ag-based brazing alloy. In addition, to maintain the temperature difference between the top and bottom of the module, a thermal insulation board was installed in it. The observed values of open-circuit voltage (V _OC) and output power (P) of a uni-leg structure module were 594?mV and 543?mW, respectively, at a maximum ??T?=?500?K.     

Vanadium deep impurity level in diluted magnetic semiconductors Pb1 ? x ? y Sn x V y Te

The crystal structure, Sn and V distribution over the length of single-crystal ingots, and galvanomagnetic effects in low magnetic fields (4.2 K ?? T ?? 300 K, B ?? 0.07 T) in Pb_1?x?y Sn _x V _y Te alloys (x = 0.05?0.21, y ?? 0.015) are studied. It is shown that all the samples are single-phase, while the Sn and V concentrations exponentially increase from the beginning to the end of the ingots. Upon doping with V, a decrease in the concentration of free holes and a metal-insulator transition are found. They are related to the appearance of a deep impurity level of V in the band gap, electron redistribution between the level and the valence band, and pinning of the Fermi-level to the impurity level. The shift rate of the V level relative to the conduction band bottom is determined and a diagram of the reconstruction of the electronic structure of the Pb_{1 ? x ? y} Sn _x V _y Te alloy upon varying the host composition is suggested.     

A Power-Generation Test for Oxide-Based Thermoelectric Modules Using p-Type Ca3Co4O9 and n-Type Ca0.9Nd0.1MnO3 Legs

Metal oxides are considered to be promising thermoelectric (TE) materials, especially for high-temperature power-generation applications, because they have many advantages such as low price, light weight, thermal stability, nontoxicity, and high oxidation resistance. For these reasons, oxide-based TE modules were fabricated using p-type pure Ca₃Co₄O₉ and n-type Ca_0.9Nd_0.1MnO₃ legs for power generation at temperatures in excess of 1000?K. This study involved the use of Ag sheets with a Ag paste as electrode materials and alumina plates as a substrate for the modules. The p-type pure Ca₃Co₄O₉ legs were manufactured by spark plasma sintering, and the n-type Ca_0.9Nd_0.1MnO₃ legs were sintered by a conventional process at atmospheric pressure. From a unicouple, a power density as high as 93.2?mW/cm² under a temperature condition of ??T?=?727?K (T _hot?=?1175?K) was obtained. This high power density is believed to be a result of the modified contact of the electrode (notch process) and the optimized material properties (the SPS process and a dopant effect) along with the high ??T obtained in this study (reduced thermal losses because of good packing of thermal insulation). Areas of concern for future research include the following: (1) the measured open-circuit voltage from the present unicouples was only 94.3% of the theoretical voltage, and (2) the internal resistance value was as high as 490% of the theoretical resistance.     

Dynamic characteristics of double-barrier nanostructures with asymmetric barriers of finite height and widths in a strong ac electric field

The theory of the interaction of a monoenergetic flow of injected electrons with a strong high-frequency ac electric field in resonant-tunneling diode (RTD) structures with asymmetric barriers of finite height and width is generalized. In the quasi-classical approximation, electron wavefunctions and tunneling functions in the quantum well and barriers are found. Analytical expressions for polarization currents in RTDs are derived in both the general case and in a number of limiting cases. It is shown that the polarization currents and radiation power in RTDs with asymmetric barriers strongly depend on the ratio of the probabilities of electron tunneling through the emitter and collector barriers. In the quantum mode, when δ = ? ? ? _r = ?ω ? Γ (? is the energy of electrons injected in the RTD, ? is Planck’s constant, ω is the ac field frequency, ? _r and Γ are the energy and width of the resonance level, respectively), the active polarization current in a field of E ≈ 2.8?ω/ea (e is the electron charge and a is the quantum-well width) reaches a maximum equal in magnitude to 84% of the direct resonant current, if the probability of electron tunneling through the emitter barrier is much higher than that through the collector barrier. The radiation-generation power at frequencies of ω = 10¹²–10¹³ s^?1 can reach 10⁵–10⁶ W/cm² in this case.     

Low-Temperature Thermoelectric Properties of Fe2VAl with Partial Cobalt Doping

Ternary metallic alloy Fe₂VAl with a pseudogap in its energy band structure has received intensive scrutiny for potential thermoelectric applications. Due to the sharp change in the density of states profile near the Fermi level, interesting transport properties can be triggered to render possible enhancement in the overall thermoelectric performance. Previously, this full-Heusler-type alloy was partially doped with cobalt at the iron sites to produce a series of compounds with n-type conductivity. Their thermoelectric properties in the temperature range of 300?K to 850?K were reported. In this research, efforts were made to extend the investigation on (Fe_1?x Co _x )₂VAl to the low-temperature range. Alloy samples were prepared by arc-melting and annealing. Seebeck coefficient, electrical resistivity, and thermal conductivity measurements were performed from 80?K to room temperature. The effects of cobalt doping on the material??s electronic and thermal properties are discussed.     

Spontaneous formation of the periodic composition-modulated nanostructure in CdxHg1−x Te films

Anomalous electrical properties have been observed in Cd_xHg_1?x Te films grown on GaAs substrates by MBE at elevated temperature. The anomalies are manifested in the conductivity anisotropy, modifications of the transmission and photoconductivity spectra upon low-temperature annealing, and the existence of a periodic undulatory surface microprofile. The temperature dependence of conductivity along and across the microprofile waves has been studied. It is suggested that a Cd_xHg_1?x Te film with anomalous electrical properties is a spontaneously formed periodic structure in the form of vertical nanowalls of different compositions. Possible mechanisms for the formation of such a structure are discussed.     

Energy levels of a quantum ring in a lateral electric field

The electronic states of a semiconductor quantum ring (QR) under an applied lateral electric field are theoretically investigated and compared with those of a quantum disk of the same size. The eigenstates and eigenvalues of the Hamiltonian are obtained from a direct matrix diagonalization scheme. Numerical calculations are performed for a hard-wall confinement potential and the electronic states are obtained as a function of the electric field and the ratio r₂/r₁, where r₂ (r₁) is the outer (inner) radius of the ring. The effects of decreasing symmetry and mixing on the energy levels and wave functions due to the applied electric field are also studied. The direct optical absorption are reported as a function of the electric field.