Total Electron Concentration Research Articles

High-temperature transport properties of a two-dimensional weakly doped parabolic semiconductor

A version of the Mexican-hat Hamiltonian is used to study high-temperature transport properties of a two-dimensional weakly doped semiconductor with electron-hole symmetric bands. For a finite doping level and a temperature-dependent band gap, we find a closed analytical form of the temperature-dependent chemical potential. The effective concentrations of charge carriers participating in transport coefficients are analyzed in the space spanned by the total electron concentration and temperature. It is shown that these concentrations are the sum of a residual contribution and two thermally activated contributions, with a complicated dependence on temperature. The analytical expression for the Hall coefficient RHis also found. It is argued that it is a non-monotonic function of the doping level with the maximum at the doping nmax that is a linear function of temperature at high enough temperatures. The analysis of the real part of the interband conductivity shows that it is inversely proportional to incoming photon energy at low temperatures and that it is nearly constant over a wide energy range at high temperatures. This results are expected to be of significant importance in understanding transport and optical properties of weakly doped two-dimensional semiconductors with nearly symmetric parabolic bands.&#xD.

Analysis of multilayer OLED for improvement in drive current and luminescent power

This research paper discusses the performance enhancement techniques of multi-layered OLED structures. OLED’s electrical behaviour is evaluated using an analytical model using benchmarked industry-standard Atlas 2-D numerical device simulator. It highlights the extraction of emission and charge transport phenomena and charges injection role. Multilayer OLED device has been investigated by extracting the key performance parameters, including the current density and luminescent power as a function of anode voltage. There is an improvement of 15% in the current density. Furthermore, this work has proceeded with internal analysis and using a mathematical model, where the electric field, total current density, electron and hole concentration, and internal device parameters have been extracted to understand the performance of OLED devices better. Additionally, the internal physics and behaviour of the device have been studied in terms of Langevin recombination. Internal analysis is performed by making cutlines in a vertical fashion so that device physics and the process occurring in the internal part can be clearly understood. The OLED device behaviour analysis is performed using the industry standard state of art device simulation tool.

Ionospheric Inhomogeneities and Their Influences on the Earth’s Remote Sensing from Space

An important problem that arises when planning experiments on remote sensing from space in the P-band is taking into account the influence of the Earth’s ionosphere. We investigated the influence of ionospheric inhomogeneities on the results of remote sensing of the Earth from space, taking into account the curvature of the propagation medium. One- and two-layer models of the ionosphere, both with and without large-scale inhomogeneities of the cold ionospheric plasma, were considered. To obtain numerical results, a bicharacteristic system was used, which makes it possible to adequately take into account the complex structures of ionospheric plasma layers. The dependence of the rate of phase change on the height and the dependence of the total electron concentration on the horizontal distance and group time were investigated. The case was compared when the vector of the strength of the Earth’s magnetic field is perpendicular to the plane of propagation, and the case when this vector lies in the plane of propagation. The dependence of the difference between the refractive indices on the height along the rays was studied. Estimates of the Faraday rotation angle and phase deviation were obtained for various models. The magnitude of the angle of Faraday rotation depends significantly on the orientation of the trajectory relative to the Earth’s magnetic field. Polarization coefficients are investigated. It is shown that the o- and x-waves are separately circularly polarized, and the contribution of the longitudinal component of the electric field in the electromagnetic wave is insignificant.

Variations of the total electronic concentration in the ionosphere in seismically active region

In this work we studied the variations in the total electron concentration (TEC) obtained from measurements of the global navigation system GPS in the preparation zone for the 2010 catastrophic Chilean earthquake (Mw = 8.8) under calm background conditions at a minimum of 24 solar activity (SA) cycles. The analysis of the geodynamic activity and ionospheric TEC disturbances in the seismically active region of this catastrophic earthquake is carried out. A computational technique has been developed that can be used to study TEC variations over seismically active regions.

Reversible room temperature hydrogen storage in high-entropy alloy TiZrCrMnFeNi

Despite potential of hydride-forming alloys for hydrogen storage, there have been few alloys which can reversibly store hydrogen without heating or activation treatment. In this study, a high-entropy alloy is designed for room temperature hydrogen storage based on three criteria: total valence electron concentration (VEC) of 6.4, single-phase thermodynamic stability (examined by CALPHAD calculations) and AB2H3 hydride formation (A: hydride-forming elements, B: elements without affinity to hydrogen, H: hydrogen). The designated alloy, TiZrCrMnFeNi containing 95 wt% C14 Laves phase, absorbs and desorbs 1.7 wt% of hydrogen (hydrogen-to-metal ratio: 1) at room temperature with a fast kinetics and without activation treatment.

Potential Predictability and Forecasting of the State of the Field of Anomalies of the Total Electron Concentration from Observational Data

An attempt is made in this study to evaluate the potential predictability of the TEC field (its deviation from the weekly average) at hourly time scales based on observational data. The method is based on the calculation of the distance between the group of observations and its subsample (group of analogues) contaning pairs of points separated in time but at minimum distance from each other. The time of potential predictability is defined as the time at which the distance between the two groups becomes close to the root-mean-square distance over the archive of observations. Calculations performed for the TEC SDDIS NASA data archive give the predictability time for anomalies within 6–12 h. Strong anomalies of the TEC field are characterized by the highest predictability; the predictability is maximal for the winter hemispheres and in years of high solar activity (1999–2004 and 2011–2015). These conclusions are confirmed by the forecasts based on the simplest empirical linear dynamic stochastic model. The forecasting calculations also show that there are statistical relationships between the predictability time and the minima/maxima of daily values of the F10.7 and ap indices.

Measurement independent magnetocaloric effect in Mn-rich Mn-Fe-Ni-Sn(Sb/In) Heusler alloys

We report a systematic study on the magneto-structural transition in Mn-rich Fe-doped Mn-Fe-Ni-Sn(Sb/In) Heusler alloys by keeping the total valence electron concentration (e/a ratio) fixed. The martensitic transition (MT) temperature is found to shift by following a proportional relationship with the e/a ratio of the magnetic elements alone. The magnetic entropy change (ΔSM) across MT for a selected sample (Mn49FeNi40Sn9In) has been estimated from three different measurement methods (isofield magnetization (M) vs temperature (T), isothermal M vs field (H) and heat capacity (HC) vs T). We observed that though the peak value of ΔSM changes with the measurement methods, the broadened shape of the ΔSM–T curves and the corresponding cooling power (∼140 Jkg−1) remains invariant. The equivalent adiabatic temperature change (ΔT) ∼−2.6 K has been obtained from indirect measurements of ΔT. Moreover, an exchange bias field ∼783 Oe at 5 K and a magnetoresistance of −30% are also obtained in one of these materials.

Mitigation of magnetic noise originating from ionosphere using TEC data retrieved by GNSS for detection of magnetic anomaly signals

Magnetic noise originating from the ionosphere may be estimated by using the total electron concentration data retrieved by the global navigation satellite systems. This provides a useful scheme for mitigation of magnetic noise coming from the fluctuating ionosphere, which is a major interference source in the magnetic anomaly detection applications.

Rashba induced spin multistability of the intersubband optical absorption in asymmetric coupled quantum wells

We study the multistable behavior of the intersubband optical absorption for InSb-based tunnel-coupled quantum wells. We consider four sublevels coming from the splitting of the two deepest levels due to the inversion asymmetry of the structure (Rashba effect), and a weak external in-plane magnetic field (Zeeman effect). Photoexcitation with an intense terahertz pump produces the redistribution of nonequilibrium electrons among the four spin sublevels. The redistribution produces a photoinduced self-consistent potential, giving rise to the renormalization of energy distance between sublevels. Depending on total electron concentration, magnetic field intensity, and pumping efficiency, we find different multistable behaviors in the intersubband optical absorption spectrum. Based on the matrix density, we describe the electron redistribution by means of a system of balance equations for electron concentrations.

On the Thermodynamics of a Two-Dimensional Electron Gas with Non-Parabolic Dispersion

A thermodynamic density of states, electron density in the subband and the entropy of the gas as function of the temperature and the total two-dimensional electron density are studied. Semiconductor conduction band dispersion is described by the simplified Kane model. Numerical simulation shows that with an increase in the total electron concentration, thermodynamic density of states at low temperatures changes abruptly and smoothes jumps at high temperatures. This change manifests itself in the peculiar thermodynamic characteristics. The results are used to interpret existing experimental data.

Influence of electron distribution on efficiency droop for GaN-based light emitting diodes

Abstract By modulating the indium composition in the quantum barriers of InGaN-based LEDs, the influence of electron distribution, electron overflow and Auger recombination on the external quantum efficiency (EQE) and droop effect have been investigated. Experimental results as well as numerical simulations reveal that the electron distribution is the key factor to influence both the peak efficiency and droop effect. The results show that the high electron concentration in the individual quantum well can stimulate the Auger recombination and lead to the droop effect instead of the total effective electron concentration, which is more related to the external quantum efficiency. If we modulate the indium composition in the quantum barriers of the InGaN-based LEDs, a uniform electron distribution can be achieved, which can not only enhance the EQE but also avoid the Auger recombination and improve the droop effect.

Criticalities in the itinerant ferromagnetUGe2

We provide a microscopic description of the magnetic properties of UGe$_2$ and in particular, of its both classical and quantum critical behavior. Namely, we account for all the critical points: the critical ending point (CEP) at the metamagnetic phase transition, the tricritical point, and the quantum critical end point at the ferromagnetic to paramagnetic phase transition. Their position agrees quantitatively with experiment. Additionally, we predict that the metamagnetic CEP can be traced down to zero temperature and becomes quantum critical point by a small decrease of both the total electron concentration and the external pressure. The system properties are then determined by the quantum critical fluctuations appearing near the instability point of the Fermi surface topology.

COMBINED METHOD FOR SIMULATING ELECTRON SPECTRUM OF δ-DOPED QUANTUM WELLS IN N-SI WITH MANY-BODY CORRECTIONS

The combined method to investigate the electron spectrum of single n-type --doped quantum wells in silicon is proposed. It is based on computing the electron potential energy by means of the Thomas-Fermi method at flnite temperatures; then the obtained electron potential energy is applied to the iteration procedure with solving the Schrodinger equations for the electron spectrum and the Poisson one for the potential energy. The many-body corrections to the electron spectrum in the quantum well also have been investigated. The combined method demonstrates a rapid convergence. It is shown that that the simple Thomas-Fermi method gives a good approximation for the electron potential energy and for the total electron concentration within the well.

Effect of point defects on the electronic density of states of ScN studied by first-principles calculations and implications for thermoelectric properties

We have investigated the effect of defects and impurities on the electronic density of states of scandium nitride using first-principles calculations with the generalized gradient approximation and hybrid functionals for the exchange correlation energy. Our results show that Sc and N vacancies can introduce asymmetric peaks in the density of states close to the Fermi level. We also find that the N vacancy states are sensitive to total electron concentration of the system due to their possibility for spin polarization. Substitutional point defects shift the Fermi level in the electronic band according to their valence but do not introduce sharp features. The energetics and electronic structure of defect pairs are also studied. By using hybrid functional calculations, a correct description of the band gap of scandium nitride is obtained. Our results envisage ways for improving the thermoelectric figure of merit of ScN by electronic structure engineering through stoichiometry tuning and doping.

Half-metallic properties for the Ti2YZ (Y = Fe, Co, Ni, Z = Al, Ga, In) Heusler alloys: A first-principles study

Using the full-potential local orbital minimum-basis method, the Ti2-based full-Heusler alloys are studied. The results show that these compounds exhibit a half-metallic behavior, however, in contrast to the conventional full-Heusler alloys, the full-Heusler alloys show a Slater–Pauling rule Mt = Zt − 18 between the total spin magnetic moment (Mt) and valence electron concentration (Zt) per unit cell. Low formation enthalpy implies these Heusler alloys can be fabricated experimentally. The origin of the gap in these half-metallic alloys are well understood. It is found that the half-metallic properties of Ti2-based compounds are insensitive to the lattice distortion and a fully spin polarization can be obtained within a wide range of lattice parameters. This is favorable in practical application.

Parameterisation of injection-dependent lifetime measurements in semiconductors in terms of Shockley-Read-Hall statistics: An application to oxide precipitates in silicon

Injection-dependent minority carrier lifetime measurements are a valuable characterisation method for semiconductor materials, particularly those for photovoltaic applications. For a sample containing defects which obey Shockley-Read-Hall statistics, it is possible to use such measurements to determine (i) the location of energy levels within the band-gap and (ii) the ratios of the capture coefficients for electrons and holes. In this paper, we discuss a convenient methodology for determining these parameters from lifetime data. Minority carrier lifetime is expressed as a linear function of the ratio of the total electron concentration to the total hole concentration for p-type (or vice versa for n-type) material. When this is plotted on linear scales, a single-level Shockley-Read-Hall centre manifests itself as a straight line. The gradient and intercepts of such a plot can be used to determine recombination parameters. The formulation is particularly instructive when multiple states are recombination-active in a sample. To illustrate this, we consider oxide precipitates in silicon as a case study and analyse lifetime data for a wide variety of p-type and n-type samples as a function of temperature. We fit the data using both a single two-level defect and two independent single-level defects and find the latter can fit the lifetime curves in all cases studied. The first defect is at EV + 0.22 eV and has a capture coefficient for electrons ∼157 times greater than that for holes at room temperature. The second defect is at EC − 0.08 eV and has a capture coefficient for holes ∼1200 times greater than that for electrons at room temperature. We find that the presence of dislocations and stacking faults around the precipitates acts to increase the density of both states without introducing new levels. Using the analysis method described, we present a parameterisation of the minority carrier lifetime in silicon containing oxide precipitates.

On relation between mid-latitude ionospheric ionization and quasi-trapped energetic electrons during 15 December 2006 magnetic storm

We report simultaneous observations of intense fluxes of quasi-trapped energetic electrons and substantial enhancements of ionospheric electron concentration (EC) at low and middle latitudes over the Pacific region during the geomagnetic storm on 15 December 2006. Electrons with energy of tens of keV were measured at altitude of ∼800–900km by POES and DMSP satellites. Experimental data from COSMIC/FS3 satellites and global network of ground-based GPS receivers were used to determine height profiles of EC and vertical total EC, respectively. A good spatial and temporal correlation between the electron fluxes and EC enhancements was found. This fact allows us to suggest that the quasi-trapped energetic electrons can be an important source of ionospheric ionization at middle latitudes during magnetic storms.

Magnetic properties of vanadium oxide nanotubes and nanolayers

We report results of magnetic measurements of VO${}_{x}$ nanolayers (stacks of four to eight distorted layers of vanadium oxide separated by an organic template) obtained at the early stages of a hydrothermal synthesis of VO${}_{x}$ nanotubes (VO${}_{x}$ NTs), and VO${}_{x}$ NTs themselves, which are the product of scrolling of VO${}_{x}$ nanolayers (VO${}_{x}$ NLs). Static and dynamic magnetic properties of VO${}_{x}$ NLs and VO${}_{x}$ NTs have been studied by means of a high-frequency (60 GHz) ESR and SQUID magnetometry in a temperature range 1.8--300 K. The temperature dependence of the magnetic susceptibility $\ensuremath{\chi}$ of the V${}^{4+}$ quasifree spins in VO${}_{x}$ NTs and VO${}_{x}$ NLs shows a transition from the Curie-Weiss laws with close paramagnetic temperatures ${\ensuremath{\Theta}}_{\mathrm{AFM}}=\ensuremath{-}25$ K (VO${}_{x}$ NTs) and ${\ensuremath{\Theta}}_{\mathrm{AFM}}=\ensuremath{-}19$ K (VO${}_{x}$ NLs) to the low-temperature power law $\ensuremath{\chi}\ensuremath{\propto}1/{T}^{\ensuremath{\xi}}$ with $\ensuremath{\xi}=0.75$ (VO${}_{x}$ NTs) and $\ensuremath{\xi}=0.85$ (VO${}_{x}$ NTs). The observed $\ensuremath{\chi}(T)$ dependence for quasifree spins was adequately described within an analytical model of an antiferromagnetic system with a disorder-driven quantum critical behavior. Comparison of static and dynamic susceptibilities reveals that the total magnetic susceptibility in VO${}_{x}$ NLs and VO${}_{x}$-NTs consists of several contributions. Apart from the oscillating susceptibility, there is a nonoscillating background, likely, consisting of the Van Vleck-type and Pauli-type terms. In the case of VO${}_{x}$ NTs, a correlated change of the Curie constant for the nondimerized V${}^{4+}$ quasifree spins and a variation of the background are observed: the decrease in temperature in the interval $70\phantom{\rule{4.pt}{0ex}}\text{K}<T<120$ K induces 1.8-fold growth of the Curie constant followed by a nonoscillating background decrease along with a change of its sign. The estimated concentration of magnetic sites indicates that a characteristic feature of VO${}_{x}$ NLs and VO${}_{x}$ NTs is a discrepancy between the maximal possible concentration of V${}^{4+}$ magnetic ions that is allowed by a chemical composition, and a concentration of the localized magnetic moments. The number of electrons in the V${}^{4+}$ state is always less than the total electron concentration, and the concentration of the ``excessive'' electrons may vary in the range 0.11--0.46 per vanadium site. At the same time, the concentration of V${}^{4+}$-V${}^{4+}$ antiferromagnetic dimers in VO${}_{x}$ NTs is rather high and reaches 49% of all vanadium sites. The effects of scrolling and Mott-Hubbard-type models of unusual magnetism in the VO${}_{x}$ nanomaterials are discussed.

Quantifying Regeneration in Dye-Sensitized Solar Cells

We propose and apply a new experimental pro-tocol for determining the kinetics of the oxidation of iodide (aka dye regeneration) in dye sensitized solar cells (DSSCs) using measurements on full cells under operating conditions. Transient absorption (TA) decays of the oxidized dye after a laser pulse are correlated with the short circuit current (Jsc) and electron concentration in the TiO2, measured during the TA experiment. Results for a series of cells with N719 [(Bu4N)2][Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl), a standard electrolyte, and varying iodide concentration allow us to fit for the fundamental regeneration rate constant (krg = 7.8 × 105 M−1 s−1) and to determine that the order in iodide, in these cells, is near 1 (0.98 ± 0.16) and clearly not 2. The rate and order allow us to rank various reaction mechanisms and to discuss possible rate-limiting steps that could be catalyzed to improve regeneration. The method can also give estimates of the fundamental rate constant for electron dye recombination (EDR) and the apparent reaction order in total electron concentration.

Influence of Nonlocality on Amplification of Space Charge Waves in n-GaN Films

It is investigated theoretically the amplification of space charge waves (SCWs) due to the negative differential conduc-tivity (NDC) in n-GaN films of submicron thicknesses placed onto a semi-infinite substrate. The influence of the nonlo-cal dependence of the average electron velocity on the electron energy is considered. The simplest nonlocal model is used where the total electron concentration is taken into account. The relaxation momentum and energy frequencies have been calculated. The influence of the nonlocality on NDC results in the decrease of the absolute value of its real part and appearance of the imaginary part. The calculation of the diffusion coefficient leads to essential errors. The simulations of spatial increments of the amplification of SCWs demonstrate that the nonlocality is essential at the fre-quencies f ? 150 GHz, and the amplification is possible up till the frequencies f ? 400 ??? 500 GHz.