Concentration Of Conduction Electrons Research Articles

Gas sensing with Nb(V) doped nanocrystalline TiO2: Sensitivity and long-term stability study

The work addresses the problem of long- term stability of metal oxide materials gas sensor performance and deals with Nb(V) doped nanocrystalline TiO2 which is characterized by reasonable response values for practical application alongside with the absence of response decay during continuous operation. The TiO2 nanomaterials containing up to 4 at% Nb were synthesized using flame spray pyrolysis. Materials are investigated by XRD and EPR methods. The sensor properties were studied by in situ electrical conductivity measurements. Nb(V) reduces the electrical resistance of TiO2 due to the formation of oxygen vacancies and increase in conduction electron concentration, but this effect is limited by the appearance of a significant number of charge traps. Pure TiO2 tended to transform into rutile phase during annealing in the analyte gas mixture, while Nb stabilized the anatase phase. Both pure and doped TiO2 demonstrate stable sensor performance in long-term, however slow growth of baseline resistance and sensor response are observed which is, according to EPR data, connected with slow processes of defect annealing and intergrain contact formation.

Anisotropic scattering of charge carriers in double continuous solid solutions of the niobium-vanadium system

The article is devoted to the study of anisotropic scattering of charge carriers in double continuous solid solutions of the niobium-vanadium system. It has experimentally investigated the temperature dependence of the Hall coefficient of V, Nb and five continuous solid solutions of V-Nb system at the temperature range 77-1000K. The authors have constructed the concentration dependence of R for the temperature 77K, 300K, 600K and 900K and calculated electronic zone parameters of such samples as concentration of conduction electrons n, Fermi wave vector kf, Fermi velocity vf and Fermi energy Ef using the obtained experimental data on R(T) dependence.

The Modeling of Self-Consistent Electron–Deformation–Diffusion Effects in Thin Films with Lattice Parameter Mismatch

In our work, the model of self-consistent electron–deformation–diffusion effects in thin films grown on substrate with the mismatch of lattice parameters of the contacting materials is constructed. The proposed theory self-consistently takes into account the interaction of the elastic field (created by the mismatch of lattice parameters of the film and the substrate, and point defects) with the diffusion processes of point defects and the electron subsystem of semiconductor film. Within the framework of the developed model, the spatial distribution of deformation, concentration of defects, conduction electrons and electric field intensity is investigated, depending on the value of the mismatch, the type of defects, the average concentrations of point defects and conduction electrons. It is established that the coordinate dependence of deformation and the concentration profile of defects of the type of stretching (compression) centers, along the axis of growth of the strained film, have a non-monotonic character with minima (maxima), the positions of which are determined by the average concentration of point defects. It is shown that due to the electron–deformation interaction in film with a lattice parameter mismatch, the spatial redistribution of conduction electrons is observed and n-n+ transitions can occur. Information about the self-consistent spatial redistribution of point defects, electrons and deformation of the crystal lattice in semiconductor materials is necessary for understanding the problems of their stability and degradation of nano-optoelectronic devices operating under conditions of intense irradiation.

Применение метода Бриджмена для получения термоэлектрического кремния, легированного германием и фосфором

Heavily donor or acceptor doped metallurgical silicon is a promising candidate as a high-temperature thermoelectric energy converter due to the extremely low cost of its fabrication. The problem of silicon-based thermoelectric materials is the high value of the thermal conductivity; however, modern technologies offer several options for solving this problem at once. In the present work, silicon ingots heavily doped using Si:P compound were grown by the Bridgman directional crystallization method with a small (up to 5 at. %) germanium impurity fraction. The main thermoelectric parameters of the material were measured in a wide temperature range (50 – 800 °C). These are Seebeck coefficient, electrical conductivity and thermal conductivity. Based on the measurement results, the value of the thermoelectric figure of merit was calculated. The latter determines the value of the thermoelectric conversion efficiency. The study of electrical properties shows that phosphorus from the SiP compound is introduced into the lattice as a dopant and creates a high concentration of conduction electrons. The chemical analysis of the ingots showed the presence of additional background impurities, the concentration and composition of these impurities vary over the bulk of the sample. Despite the presence of impurities, the material demonstrates relatively high thermoelectric characteristics, and the efficiency is at the level of the best world results. A further potential for optimizing thermoelectric characteristics due to the possibility of a fine-grained polycrystalline structure formation is discussed.

Phase Conjugation of Sound Beams in Piezoelectric Semiconductors in an Alternating Magnetic Field

The propagation of a linearly polarized transverse sound wave in a semiconductor layer with a certain crystallographic structure with a sufficiently high concentration of conduction electrons is considered. The semiconductor layer is located in a spatially uniform magnetic field directed perpendicular to the plane of incidence of the sound wave and modulated with the sound frequency. The relationship between the plasma subsystem and acoustic oscillations in the layer is achieved due to the internal piezoelectric field. It is shown that propagation of a sound beam under these conditions is accompanied by a counterpropagating sound wave with a conjugated wave front. The conversion coefficient of direct and phase-conjugated waves is found.

Correct determination of electron concentration in n-GaSb by electrical measurements

The concentrations of conduction electrons in n-GaSb at 295 and 77 K have been calculated taking into account the non-parabolic deviation of the conduction band shape. We show that at T = 295 K the concentration of heavy electrons in the L-valley of the conduction band is higher than the concentration of light electrons in the Г-valley. On the contrary, at T = 77 K the conduction electrons are mostly concentrated in the Г-valley. Hall data for tellurium doped CZ n-GaSb specimens have been reported. Analysis of experimental data for T = 295 K requires the existence of two types of electrons be taken into account (the light and the heavy ones), the concentrations of which cannot be determined. The apparent increase in the electron concentration with a decrease in the temperature from 295 to 77 K is not true. The concentration of conduction electrons at T = 77 K can be measured correctly with the Hall method.

Occupation preferences and impacts of interstitial H, C, N, and O on magnetism and phase stability of Ni2MnGa magnetic shape memory alloys by first-principles calculations

Microalloying by introducing small atoms into the interstitial sites of crystal represents an important strategy in composition design, usually enabling a leap in material performance under a tiny doping concentration. However, for the Ni–Mn-based magnetic shape memory alloys, plenty of critical scientific issues related to interstitial alloying remains ambiguous. In this work, by first-principles calculations, the occupation preferences, and the impacts and the underlying mechanisms of H, C, N, and O on magnetism, phase stability, and electronic structures of Ni2MnGa, were systemically investigated. By using a two-stage relaxation strategy, it is confirmed that all the studied interstitial atoms prefer to occupy the octahedral interstice, although the undistorted octahedral interstice possesses a smaller size than that of the tetragonal interstice. The magnetic moments of Ni and Mn around the interstitial element are highly modified, which is attributed to the decreased concentration of conduction electrons, resultant from the formation of covalent bonds between Ni and the interstitial atoms, and the revised distances between Mn–Ni(Mn) caused by the local lattice distortion. Interstitial alloying can highly tailor the phase stability and the c/a ratio of martensite. The doping of C has a great potential to destabilize the austenite owing to the reduced ferromagnetism, which is opposite to the case in steel. The atom radius of an interstitial atom may be the critical factor dominating the elastic stability of the alloyed systems. This work is expected to provide fundamental information for interstitial alloying to promote the design of advanced magnetic shape memory alloys.

Surface Plasmons Excited by X-rays in the Surface Layers of Solids

The phenomenon of total external reflection of X-rays at a sliding angle of incidence of a beam of incident X-rays is investigated. For metals, a quantitative law of direct dependence of the refractive index decrement on the interplane distance is obtained. The excitation of surface plasmons by X-rays that have experienced complete external reflection is detected. For surface plasmons, a dimensional effect was observed, expressed in an increase in the energy of plasmons and the concentration of conduction electrons with an increase in the depth of the output of surface plasmons. By the method of dispersion of surface plasmons, internal mechanical micro-stresses and spontaneous polarization of the surface layers of glassy dielectrics and in thin layers of vanadium dioxide were determined. The absence of micro-stresses in the lithium fluoride ionic single crystal was found out, and the polarization observed in it is due to the large dipole moment of the molecules of this crystal. In thin films of vanadium dioxide, the dependence of micro-stresses on the stresses in the substrates was found.

Influence of back barrier layer thickness on device performance of AlGaN/GaN MOS-HEMT

In this work, we have performed the influence of back barrier layer thickness variation on AlGaN/GaN Metal Oxide Semiconductor High Electron Mobility Transistor (MOS-HEMT) device with 0.5 µm Schottky gate length. The AlGaN back barrier layer presented increases the conduction band with respect to GaN channel layer so that more no of electron confinement into the GaN channel layer and improve the high-frequency performance. The effect of the back-barrier layer thickness is performed by using 2-D TCAD Atlas Silvaco numerical simulation tool by taking Hydrodynamic mobility model. Due to a large amount of two-dimensional electron gas (2-DEG) density at the AlGaN/GaN heterointerface of the MOS-HEMT device higher drain current density is obtained. The 2-D simulation is carried out with a variation of back barrier layer thickness for various device parameter such as transfer characteristics (Id-Vg), drain current with a drain voltage (Id-Vd), transconductance (gm), drain induced barrier lowering (DIBL), conduction band energy and electron concentration into the channel. In this simulation, we have also performed the RF performance like a gate to source capacitance (Cgs) and current gain cut-off frequency of AlGaN/GaN MOS-HEMT device. The results obtained by variation of AlGaN back barrier layer thickness can be a better solution in future analog and RF device application. Copyright © 2018 VBRI Press.

Pseudogap metal induced by long-range Coulomb interactions

In correlated electron systems the metallic character of a material can be strongly suppressed near an integer concentration of conduction electrons as Coulomb interactions forbid the double occupancy of local atomic orbitals. While the Mott-Hubbard physics arising from such on-site interactions has been largely studied, several unexplained phenomena observed in correlated materials challenge this description and call for the development of new ideas. Here we explore a general route for obtaining correlated behavior that is decidedly different from the spin-related Mott-Hubbard mechanism and instead relies on the presence of unscreened, long-range Coulomb interactions. We find a pseudogap metal phase characterized by a divergent quasiparticle mass and the opening of a Coulomb pseudogap in the electronic spectrum. The destruction of the Fermi-liquid state occurs because the electrons move in a nearly frozen, disordered charge background, as collective charge rearrangements are drastically slowed down by the frustrating nature of long-range potentials on discrete lattices. The present pseudogap metal realizes an early conjecture by Efros, that a soft Coulomb gap should appear for quantum lattice electrons with strong unscreened interactions due to self-generated randomness.

External light control of three-dimensional ultrashort far-infrared pulses in an inhomogeneous array of carbon nanotubes

We present a study of the propagation of three-dimensional (3D) bipolar electromagnetic ultrashort pulses in an inhomogeneous array of semiconductor carbon nanotubes (CNTs) in the presence of a control high-frequency (HF) electric field. The inhomogeneity is present in the form of a layer with an increased concentration of conduction electrons, which acts as a barrier for the propagation of ultrashort electromagnetic pulses through the CNT array. The dynamics of the pulse is described by a nonlinear equation for the vector potential of the electromagnetic field (it takes the form of a 3D generalization of the sine-Gordon equation), derived from the Maxwell's equations and averaged over the period of the HF control field. By means of systematic simulations, we demonstrate that, depending on the amplitude and frequency of the HF control, the ultrashort pulse approaching the barrier layer either passes it or bounces back. The layer's transmissivity for the incident pulse is significantly affected by the amplitude and frequency of the HF control field, with the reflection coefficient nearly vanishing in intervals that make up a discrete set of transparency windows, which resembles the effect of the electromagnetically-induced transparency. Having passed the barrier, the ultrashort pulse continues to propagate, keeping its spatiotemporal integrity. The results may be used for the design of soliton valves, with the transmissivity of the soliton stream accurately controlled by the HF field.

Electron distribution and electrical resistance in nanostructured mixed oxides CeO2-In2O3

The electronic structure of nanoparticles in binary mixtures of oxides are considered by the example of CeO2-In2O3. The effect of catalytically active CeO2 nanoclusters on charge distribution in nanoparticles with a high concentration of conduction electrons (In2O3) is studied. The spillover of oxygen atoms from CeO2 nanoclusters to In2O3 nanoparticles, which enhances the inhomogeneous distribution of conduction electrons in the In2O3 nanoparticle, is taken into account, and it is shown that exactly spillover leads to depletion of the near-surface layer. The film resistance rises with an increase in the CeO2 concentration, due to a decrease in the near-surface electron density in In2O3 nanoparticles and the overlap of the current flow paths by CeO2 nanoclusters. An expression for the temperature dependence of a nanostructured film resistance is found, taking into account the inhomogeneous distribution of electrons in nanoparticles. The temperature dependence of the CeO2-In2O3 film was measured and the results were compared with theory. A good agreement between theory and experiment was obtained.

Study on Basic Characteristics of CuAlBe Shape Memory Alloy

In this work, the CuAlBe shape memory alloy (SMA) with a new composition of 77.05Cu-22.02Al-0.93Be (at.%) detected by the EDS test was fabricated in a vacuum arc melter under a pure argon atmosphere. At the beginning, the high-purity (%99.9) elements of Cu, Al, and Be powders were mixed and the mixture (~ 10 g) was formed as pellets by pressure. By melting the pellets in arc melter, the as-cast ingot alloy was obtained. Then, the ingot alloy was cut into small pieces (~ 30–50 mg) and then all of these samples were all homogenized in the β-phase region (at 900 °C for 1 h) and immediately submerged in traditional iced-brine water to form β1’ martensite phase in the alloy which results from such fast cooling by suppressing the formation of hypoeutectoid precipitations (α and γ2). Then, to uncover and evaluate the existence of martensite structure and the characteristics of shape memory alloy properties of the alloy, the specimens were tested by calorimetric and structural measurements. The results of thermal heating/cooling cycles of the alloy were obtained from differential scanning calorimetry (DSC) measurements that were taken twice at 5 °C/min of heating/cooling rate under constant argon gas flow (100 ml/min) and by using liquid nitrogen cooling support to reach lower temperatures than room temperature. The DSC thermograms of the alloy revealed the characteristic martensitic transformation peaks that occurred endothermic by heating and exothermic by cooling at moderate temperatures ranging between 19 and 66 °C, regarded as evidence for the presence of shape memory effect property in the alloy. Important thermodynamical parameters such as transformation temperatures, hysteresis gap, and entropy and enthalpy change amounts for these back and forward martensitic phase transition peaks were determined directly by using data of DSC peak analyses and by calculation. Differential thermal analysis (DTA) measurement that was taken from room temperature to 900 °C at a heating/cooling rate of 25 °C/min displayed a high-temperature behavior of the alloy as compatible with the common behavior of Cu-Al SMAs. The X-ray test of the alloy conducted at room conditions showed sharp diffraction peaks and their matching crystal planes which indicate the existence of β1′ martensite phase in the alloy and its high single-like crystallinity i.e. its large Debye-Scherrer sub-micrometer crystallite size. Besides, theoretical forecasting about the existence and volumetric dominance of martensite phases was deduced from the calculated value of e/a (average conduction electron concentration per atom) parameter of the alloy. Furthermore, the mechanical Vickers microhardness tests that were performed at room temperature and under 100-gf load applied for 10 s revealed the high ductility and softness features of the alloy. Considering all the results, the highly ductile CuAlBe alloy with new composition owing shape memory alloy properties and intermediate working temperatures can be useful in various kinds of research and applications in which Cu-based SMAs are exploited.

Investigation of Electrophysical Properties of ITO Films

The results of a study of the electrophysical characteristics of ITO films obtained by magnetron sputtering are presented. It is shown that a significant increase in the electrical conductivity of ITO films is facilitated by high-temperature annealing due to two processes. First, the high-temperature treatment of ITO films after their synthesis promotes the formation of a crystal structure, which leads to an increase in the mobility of charge carriers. Second, as a result of high-temperature annealing, the impurity in ITO films becomes completely electrically active, which leads to an increase in the concentration of conduction electrons and a change of the semiconductor mechanism of electrical conductivity to the metallic one.

Spin filtering and spin separation in 2D materials by topological spin Hall effect

The needs of high speed performance electronic devices for various applications require novel materials and new physical phenomena. For these purposes we propose to study new physical effects based on electron scattering on magnetic skyrmions and vortices distributed in a 2D ferromagnetic material. We show that the topological spin Hall effect can be efficiently employed for the filtering, switching, and separation of spin currents. For some values of the parameters (conduction electron concentrations, and skyrmion/vortex sizes) it is possible to separate Hall currents for different electron spin projections as it is like for different carrier charges (electrons and holes) in the normal Hall effect. The calculations are performed using the Boltzmann kinetic equation for the nonequilibrium distribution function and the Lippmann–Schwinger equation for the transition matrix in the whole range of the adiabaticity parameter. The spin filtering due to the skyrmion/vortex scattering can be several orders of magnitude more efficient in the narrow range of the electron concentrations than that of the ordinary ferromagnetic spin polarization in spintronics.

Spectral-Kinetic Analysis of State of Silver Sol Particles During Synthesis in Various Media

Silver nanoparticles (Ag NPs) have been synthesized in various media using chemical reduction of silver ions with tannin and absorption spectra have been recorded during synthesis of organosol, organohydrosol, and hydrosol. Analysis of the spectra has provided the concentration of Ne conduction electrons, volume fraction of particles NV, and damping coefficient of plasma oscillations γ, which characterizes the imperfection of particle surface. It has been determined that there is an increase in NV and a decrease in γ during formation of Ag NPs of hydrosol and organohydrosol. Parameters NV and γ, which vary upon synthesis of Ag sols, possess a period with self-excited oscillations. Explanations of the nonstationary character of Ag NP formation in sols and a formal second kinetic order of the process in nonaqueous medium have been suggested.

The piezotronic effect in InGaN/GaN quantum-well based microwire for ultrasensitive strain sensor

By introducing external mechanical strain, the carriers transport of certain semiconductors with non-centrosymmetric structure would be modulated, which is referred to the piezotronic effect. Herein, a piezotronic strain sensor with ultrahigh sensitivity based on InGaN/GaN multiple quantum-well heterojunction microwire (MQWH-MW) was designed. The InGaN/GaN MQWH-MW orientated along a-axis was directly obtained by conductivity-selective electronchemical (EC) etching of epitaxial film on sapphire. The strain sensor was fabricated into a back-to-back Schottky junction. The gauge factor of the fabricated strain sensor reaches over 2000 when applying 0.55% tensile strain under +0.4 V external bias. A theoretical analysis of energy band in the MQWH-MW based strain sensor is proposed to explain the observed result. Theoretical simulations about conduction band energy profiles and electron concentration of electron accumulation region (EAR) are performed and presented to systematically illustrate and confirm the proposed working mechanisms. This work demonstrates a promising approach to design and fabricate strain nanosensors with high sensitivety.

Dependence of the Kinetics of Radiation-Induced Defect Formation on the Energy Absorbed by Si and SiC when Exposed to Fast Charged Particles

The kinetics of the formation of radiation-induced defects in silicon and silicon carbide as a function of the absorbed energy is analyzed. The dependence of the concentration of conduction electrons n-Si and n-SiC on the irradiation dose is studied experimentally under conditions of irradiation with electrons with an energy of 0.9 MeV and protons with an energy of 15 MeV. The advantages and disadvantages of using the integral flux (fluence) and absorbed energy as kinetic parameters are discussed. It is established in the performed studies that the visual representation of the kinetics of radiation-induced defect formation as a function of the fluence of irradiating particles is clearer for tabulating the requirements imposed to equipment stability under suitable conditions. To study the physical processes underlying the formation of radiation-induced defects in semiconductors, it is more convenient to use the dependences of the effects observed under radiation exposure as functions of the absorbed energy.

Surface electromagnetic waves at the interface between dissipative porous nanocomposite and hypercrystal under different temperatures

In the current paper the properties of surface plasmon polaritons as well as hybrid polarized surface electromagnetic waves at the plane interface of a porous semiconductor nanomaterial and a hypercrystal are studied theoretically and numerically. The nanomaterial, as well as the hypercrystal, is made of n-type doped indium antimonide (n-InSb) semiconductor layers. The losses which inherent to the semiconductor materials are taken into account. To describe the properties of the constituent semiconductor layers the approximate analytical temperature models for concentration and mobility of conduction electrons are applied. This allows to consider the effect of electron scattering on the properties of supported electromagnetic modes in the wide frequency range. The contacting materials are considered in the subwavelength approximation when it is possible to introduce the effective permittivity components. It is shown that taking into account dissipation processes in the semiconductor constituent layers result in the appearance of additional surface modes branches, changes in the surface waves penetration depth, frequency and angular existence domains.

Determination of Plasma Oscillations in Metals Using Total External Reflection of X-Rays

A method to determine plasma oscillations in metals with the aid of total external reflection (TER) of X-rays is proposed. It is shown that the plasmon energy, concentration of conduction electrons, and Fermi energies can be calculated using experimentally measured grazing angles of X-rays after TER. Such characteristics depend on crystallographic directions and may depend on the TER depth.