Variation Of Carrier Concentration Research Articles

Decompression-Driven Superconductivity Enhancement in In2 Se3.

An unexpected superconductivity enhancement is reported in decompressed In2 Se3 . The onset of superconductivity in In2 Se3 occurs at 41.3 GPa with a critical temperature (Tc ) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest Tc that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of Tc is driven by the pressure-induced R-3m to I-43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance.

Tuning of conductive type and magnetic properties of Ca3Co2O6 ceramics through Pb‐doping

AbstractThe effect of Pb doping on structural, electrical, magnetic, and thermal transport properties of Ca3−xPbxCo2O6 (x=0‐0.3) ceramics has been investigated systemically. It is found that the substituted Pb‐ions have a mixed valence state of +2 and +4, and a small amount of Co3+ ions will transfer into Co2+ due to the substitution of Pb4+ for Ca2+. The resistivity decreases monotonically with increasing Pb content, which is related to the variation in carrier concentration and the enhanced grain connectivity. The signs of both Hall coefficient and thermopower changed from positive to negative by a proper Pb doping, indicating the conductive type of Ca3Co2O6 can be effectively tuned from p to n through the doping. The low‐temperature magnetization, the magnetic exchange coupling constant J and Weiss temperature θ decrease monotonically with the increase in Pb‐doping content, indicating the strength of the ferromagnetic interaction between adjacent high spin Co3+ ions has been weakened due to the reduced magnetic correlation length in these Pb‐doped samples.

Electrically tunable diffraction efficiency from gratings in Al-doped ZnO

Transparent conducting aluminum-doped zinc oxide (AZO) can be used as part of an active plasmonic device due to its electrically tunable permittivity, which is accomplished by changing the carrier concentration with electrical biasing. In this letter, we report a continuous electrical tuning of diffraction efficiency from AZO gratings in the visible range (specifically 532 nm) when the AZO is under bias voltages between −1 V and −3.5 V. The carrier concentration in AZO under negative bias has been measured and simulated. The diffraction efficiency changes have been explained by the carrier concentration variation and induced complex refractive index change at the Al2O3 and AZO interface. The reported results can lead toward the application of post-fabrication tuning of optoelectronic devices using AZO.

Mesoporous niobium-doped titanium dioxide films from the assembly of crystalline nanoparticles: study on the relationship between the band structure, conductivity and charge storage mechanism

The charge storage mechanism of nanostructured intercalation materials such as anatase (TiO2) is still a matter of intense research, because it provides the basis for designing electrochemical energy storage and conversion materials. In this work, we addressed the relationship between conductivity and the charge storage mechanism exemplified with nanostructured Nb-doped TiO2. Nanoparticles of Nb-doped TiO2 were prepared by a novel two-step solvothermal process based on tert-amyl alcohol as the solvent. Several state-of-the-art techniques, including X-ray diffraction along with Rietveld refinement and Raman and X-ray photoelectron spectroscopy, were used to analyze the phase composition, the chemical oxidation state and the structure after doping with different niobium contents. Mesoporous Nb-doped TiO2 films with uniform pore sizes of 15 to 18 nm were produced by dip-coating employing these nanoparticles as building blocks. It is found that 5 at% Nb-doped TiO2 exhibits the highest conductivity. The analysis of the peak currents in the cyclic voltammetry of the mesoporous films indicates that the pseudocapacitive current contribution varies with the conductivity in the same trend. This correlation can be described qualitatively by the changes in the space charge layer at the interface with varying carrier concentration. Such a finding implies that doping not only affects the charge storage in the bulk due to changes of kinetic parameters including the chemical diffusion coefficient and electronic conductivity, but also influences the surface-related storage mechanism of a given nanostructured material. These results further provide a valuable insight in searching for and designing materials applied in pseudocapacitors.

Evolution of the charge density wave order on the two-dimensional hexagonal lattice

Experimental data on the monolayer niobium diselenide show that the single band crosses the Fermi energy. This is in striking contrast to the bulk systems in which at least three bands contribute to the Fermi surface of the material. Together with the fact that the charge density wave (CDW) transition temperature Tc of a bulk system and of a single layer are the same one concludes that it should allow description of the two dimensional system by a single band in the tight-binding approximation. We analyse the development of CDW order of triangular symmetry in two-dimensional lattice of hexagonal symmetry. We assume the existence of four sublattices and calculate the resulting density of states and the formation of CDW gap for systems with varying carrier concentration. The mechanism of CDW instability, most probably related to the strong electron-phonon coupling in the material, is modeled by the local attractive interaction U of the same strength on all sites.

Tuning of magnetic interaction and transport properties of Ca3Co2O6 through Sm-doping

The structure, magnetic and transport properties of quasi-one-dimensional spin chain compounds Ca3-xSmxCo2O6 (0 ≤ x ≤ 0.8) were investigated. It is found that the intra-chain magnetic exchange coupling constant J and Weiss temperature θ decrease considerably with increasing Sm-doping content x and the sign of J and θ changes from positive to negative as x reaches to 0.6. The result shows that the dominated intra-chain magnetic coupling can be changed from ferromagnetic (FM) to antiferromagnetic (AFM) by Sm-doping. Moreover, the magnetic ground state of Ca3Co2O6 is changed from the spin density wave (SDW) phase to AFM state in Sm-doped samples. The resistivity decreases sharply first with a small amount of Sm-doping and then increases slightly as x increases further, which is a result of the competing effect between carrier concentration variations and lattice distortion. The thermopower has a similar variation trend with that of resistivity. The increased carrier concentration and enhanced spin entropy may be the origin of the change of the thermopower.

Facilitated transport of Europium through supported liquid membrane using Cyanex272 as carrier and mass transfer modelling

AbstractIn this study, the extraction of europium from nitric acid solution by a flat sheet supported liquid membrane using Cyanex272 as a carrier was investigated. The influence of effective parameters on the extraction of Eu was studied and the optimum conditions were derived. Then a theoretical model was developed for unsteady‐state transport of Eu ions through the supported liquid membrane. In this model, the variation of carrier concentration in the membrane phase was taken into account. The partial and complete differential equations obtained for the feed, membrane, and stripping phases were numerically solved and the results were compared with the experimental data. The verified model was used to simulate the concentration distribution of the metal‐carrier complex and carrier as a function of position and time in the membrane phase. Using the results of numerical solution, the derived equations were also solved analytically.

Synthesis and characteristics of (Bi2Ba3O4−δ)b1/b2CoO2 thin films by chemical solution deposition

(Bi2Ba3O4−δ)b1/b2CoO2 (BBC) thin films are prepared on LaAlO3 (001) single crystal substrates by chemical solution deposition. The annealing temperature effects on the thin film microstructures as well as transport properties are investigated. The results of X-ray diffraction, energy dispersive spectrometry and transmission electron microscopy confirm the formation of stoichiometric BBC thin films with c-axis orientation. Annealing temperature plays a very important role in determination of resistivity and Seebeck coefficient due to the variations of grain size, carrier concentration and mobility. The resistivity behaviors for the different temperature-annealed thin films obey different electrical transport mechanisms at low, medium and high measured temperature ranges. The optimized BBC thin film is annealed at 600 °C, showing a metal-insulator transition at about 100 K and the resistivity and Seebeck coefficient at 300 K is of 8.7 mΩ cm, 74.4 μV K−1, respectively. The results will provide an effective route to fabricate BBC thin films as well as a guidance for investigation about its transport properties.

Verification of Fowler–Nordheim electron tunneling mechanism in Ni/SiO2/n-4H SiC and n+ poly-Si/SiO2/n-4H SiC MOS devices by different models

This article emphasizes verification of Fowler–Nordheim electron tunneling mechanism in the Ni/SiO2/n-4H SiC MOS devices by developing three different kinds of models. The standard semiconductor equations are categorically solved to obtain the change in Fermi energy level of semiconductor with effect of temperature and field that extend support to determine sustainable and accurate tunneling current through the oxide layer. The forward and reverse bias currents with variation of electric field are simulated with help of different models developed by us for MOS devices by applying adequate conditions. The latter is quite different from former in terms of tunneling mechanism in the MOS devices. The variation of barrier height with effect of quantum mechanical, temperature, and fields is considered as effective barrier height for the generation of current–field (J–F) curves under forward and reverse biases but quantum mechanical effect is void in the latter. In addition, the J–F curves are also simulated with variation of carrier concentration in the n-type 4H SiC semiconductor of MOS devices and the relation between them is established.

Synergistic effects of Lanthanum substitution on enhancing the thermoelectric properties of β-Zn4Sb3

A core challenge of thermoelectric research is decoupling the otherwise inversely interdependent properties electrical conductivity, thermopower, and lattice thermal conductivity offering synergistic effects. Herein, we present a systematic study in which we have substituted Lanthanum into the Zn-site in β-Zn4Sb3 compound. We observed that La-substitution not only simultaneously enhances electrical conductivity and thermopower over a wide temperature range but also substantially reduces the lattice thermal conductivity. These synergistic effects of La-substitution are discussed in terms of the substitution-induced variation in carrier concentration and effective mass as well as stronger phonon scattering by point defects. As a result, a ∼30% increase of the power factor and a ∼23% reduction in the lattice thermal conductivity led to a state-of-the-art ZT ∼1.3 at 648 K for the bulk sample of β-La0.01Zn3.99Sb3.

Electrical properties of Dy3+/Na+ Co-doped oxide thermoelectric [Ca1-x(Na1/2Dy1/2)x]MnO3 ceramics

A new Dy3+/Na+ co-doping system was attempted for improving the thermoelectric properties of CaMnO3 system in this paper. [Ca1-x(Na1/2Dy1/2)x]MnO3 samples were produced by the solid state reaction method, and the electrical properties and the microstructure were investigated. The X-ray diffraction data and tolerant factor were obtained, indicating the second phase and the geometric distortion in this system. Besides, compared with the resistivity and the Seebeck coefficient of single Dy-doped system, Dy3+/Na+ co-doping method could partially decrease the variation of carrier concentration as our expectation. More specifically, the holes introduced by doping Na+ could compensate electrons introduced by doping Dy3+, leading to a low variation of carrier concentration. As the result, a relatively high Seebeck coefficient (−251.6 μV/K) and a relatively low resistivity (8.7 mΩ cm) were gained in this system, and the highest power factor value of 298.0 μW/K2m was obtained at 323 K in [Ca0.95(Dy1/2Na1/2)0.05]MnO3.

Tuning Superconductivity in FeSe Thin Films via Magnesium Doping

In contrast to its bulk crystal, the FeSe thin film or layer exhibits better superconductivity performance, which recently attracted much interest in its fundamental research as well as in potential applications around the world. In the present work, tuning superconductivity in FeSe thin films was achieved by magnesium-doping technique. Tc is significantly enhanced from 10.7 K in pure FeSe films to 13.4 K in optimized Mg-doped ones, which is approximately 1.5 times higher than that of bulk crystals. This is the first time achieving the enhancement of superconducting transition temperature in FeSe thin films with practical thickness (120 nm) via a simple Mg-doping process. Moreover, these Mg-doped FeSe films are quite stable in atmosphere with Hc2 up to 32.7 T and Tc(zero) up to 12 K, respectively, implying their outstanding potential for practical applications in high magnetic fields. It was found that Mg enters the matrix of FeSe lattice, and does not react with FeSe forming any other secondary phase. Actually, Mg first occupies Fe-vacancies, and then substitutes for some Fe in the FeSe crystal lattices when Fe-vacancies are fully filled. Simultaneously, external Mg-doping introduces sufficient electron doping and induces the variation of electron carrier concentration according to Hall coefficient measurements. This is responsible for the evolution of superconducting performance in FeSe thin films. Our results provide a new strategy to improve the superconductivity of 11 type Fe-based superconductors and will help us to understand the intrinsic mechanism of this unconventional superconducting system.

Surface studies and photoluminescence of GaN epitaxial layers grown by MOCVD technique for applications of solar cells and light emitting devices

This article presents theoretical and experimental results of n-type GaN semiconductor. In the first part, the variation of surface potential of GaN with varying carrier concentration in the range from 1015 to 1019 cm−3 is determined as well as the change in the electric field with variation of surface potential is also estimated. In addition, the change in the electric field (F) with effect of irradiation of laser beam on the GaN sample, while measuring photoluminescence, is found by developing appropriate theoretical models using known activation energies of traps, which are obtained by the experimental photoluminescence (PL) measurements. In the second part, the experimental results of n-type GaN such as growth by MOCVD, electrical, structural and optical properties are presented. The un-doped GaN thin films have been successfully deposited onto c-sapphire substrates by MOCVD technique with different recipes. It is observed that the carrier concentration of the films varies with varying deposition parameters. The PL studies are systematically carried out on these samples at low temperature. The correlation between PL results and the electrical data such as carrier concentration (n) or mobility (μ) is analyzed for GaN thin films. Based on the activation energies of donor–acceptor pair (DAP) and the yellow band emissions obtained from the PL measurements, the possible transition energy levels in the energy band are hypothetically proposed. The highly resolved phonon replicas in PL spectra are observed for high carrier density samples. The n and μ are found to be in the range of 1017–1018 cm−3 and 65–120 cm2/V−s, respectively. The XRD analysis reveals that the samples show preferred orientation and hexagonal structure. On the other hand, the nature of the samples changes from epitaxy to polycrystalline with increasing carrier concentration in the samples.

Numerical Analysis of Terahertz Emissions From an Ungated HEMT Using Full-Wave Hydrodynamic Model

In this paper, we show how plasma-wave instability in an asymmetrically biased ungated InGaAs high-electron mobility transistor (HEMT) leads to terahertz emissions. Numerical calculations are provided using a new Maxwell-hydrodynamic solver. Using this solver, an accurate plasma-wave model is presented, accounting for nonuniform surroundings and finite dimensions of the 2D electron gas (2DEG) layer within the HEMT. We estimate that hundreds of nanowatts of power can be expected from such devices under ideal boundary conditions and sufficient channel mobility. Effects due to variations of carrier velocity, carrier concentration, and 2DEG confinement on the emitted power levels are also considered to provide design guidelines.

Sequence of Quantum Phase Transitions in Bi2Sr2CaCu2O(8+δ) Cuprates Revealed by In Situ Electrical Doping of One and the Same Sample.

Our recently discovered electrical doping technique allows a broad-range variation of carrier concentration without changing the chemical composition. We show that it is possible to induce superconductivity in a nondoped insulating sample and to tune it reversibly all the way to an overdoped metallic state. This way, we can investigate the whole doping diagram of one and the same sample. Our study reveals two distinct critical points. The one at the overdoped side is associated with the onset of the pseudogap and with the metal-to-insulator transition in the c-axis transport. The other at optimal doping is associated with the appearance of a "dressed" electron energy. Our study confirms the existence of multiple phase transitions under the superconducting dome in cuprates.

Effects of doping on the positional uniformity of the thermoelectric properties of n-type Bi2Te2.7Se0.3 polycrystalline bulks

The reproducibility problem is one of the central issues in n-type Bi2Te3-based thermoelectric materials. We herein investigate the effects of doping on the positional uniformity of the electronic transport properties of spark-plasma-sintered Bi2Te2.7Se0.3 polycrystalline bulks. Both the electrical conductivity and the Seebeck coefficient showed severe deviations with position in the pristine Bi2Te2.7Se0.3 bulk due to the variation in carrier concentration originating from the formation of Te- and Se-site vacancies. This non-uniformity problem could be significantly improved by the addition of excess elemental Cu and Te. Uniformity enhanced by Cu addition might be considered to be related with the suppression of Te/Se evaporation in the presence of intercalated Cu atoms. A maximum power factor value of ∼2.72 mW/mK2 at 300 K was obtained for Cu0.01Bi2Te2.7Se0.3.

Excitation dependent Raman studies of self-seeded grown InN nanoparticles with different carrier concentration.

High quality InN nanoparticles are grown using an atmospheric chemical vapour deposition technique via a self-seeded catalytic approach in the temperature range of 580-650 °C. In this temperature region, the nucleation barrier of InN is overcome by seeding low density In nanoparticles prior to introduction of reactive NH3. Samples with increasing carrier densities are grown, with the help of increasing growth temperature, to understand the role of carrier density in the optical phonon structure. Near-resonance Raman spectra show completely different phonon pictures compared to those for the off-resonance spectra. A Raman forbidden mode of B1(high), because of the possible breakdown of selection rules in the near-resonance conditions, is invoked for the first time. The intensity and frequency of this mode strongly depend on the carrier concentration in the sample. In off-resonance conditions, the A1(LO) mode for the sample with higher carrier concentration is dominated by Fano interference rather than plasmon-phonon coupling. Variation of the intensity of the B1(high) mode is correlated with a band filling effect, which is substantiated by the luminescence studies of the InN samples with different carrier concentrations.

Optical Measurements Revealing Nonuniform Hole Mobility in Organic Electrochemical Transistors

Theoretical predictions and experimental results show that the carrier mobility in conjugated polymers depends on carrier concentration. However, existing models for organic electrochemical transistors (OECTs) assume uniform carrier mobility along the transistor channel despite variations in carrier concentration. Here, a model incorporating disorder‐induced nonuniform mobility is developed to describe the steady‐state behavior of OECTs. This model is tested using in situ optical measurements of an OECT channel to decouple the mobility and carrier concentration contributions to channel conductivity. It is found that unlike existing models, the nonuniform mobility model agrees with these measurements. Furthermore, it is found that the model matches current–voltage data over a wide range of device geometries and two different device architectures. Finally, it is shown that a 120% improvement of transconductance can be obtained by operating a sensor according to device parameters given by the nonuniform mobility model rather than those extracted from an existing model that assumes a uniform mobility. Ultimately, the model presented allows more accurate measurement of material properties via transistor characterization. This will enable better‐informed material optimization, development of more accurate transient models for OECTs, and more effective use of OECTs made from existing materials.

Atomistic aspects of carrier concentration variation in post-annealed indium tin oxide films

Post-annealing environment-dependent optical and electrical properties of indium tin oxide films grown on glass were examined. X-ray diffraction measurements revealed that all of the films exhibited poly-crystallinity after annealing at 400 °C for 10 min O2, in-air and N2. The optical property measurements yielded >80% transmittances for all the films except for the as-grown and O2-annealed films, even though there were no significant optical band-gap energy differences. In the Hall measurements, all of the films exhibited n-type characteristics. However, the film annealed under the N2 environment showed the best electrical properties (highest carrier concentration and conductivity). The physical origin of electrical property variations due to annealing environment differences was explained by examining the core-level x-ray photoelectron spectra.

Highly Robust Flexible Oxide Thin-Film Transistors by Bulk Accumulation

We report the achievement of flexible oxide thin-film transistors (TFTs) that are highly robust under mechanical bending stress. Fabricated on solution-processed polyimide, the oxide TFTs employ the dual-gate structure with an amorphous-indium-gallium-zinc oxide (a-IGZO) semiconductor, silicon dioxide gate insulators, and molybdenum gate and source/drain electrodes. High mechanical stability is achieved by shorting the two gates together to induce bulk accumulation (BA)—a condition in which the channel accumulation layer of electrons extends the entire depth of the active layer. It is shown experimentally that the BA a-IGZO TFTs exhibit better stability under bending stress compared with single gate-driven TFTs. From TCAD simulations, the immunity to slight variations in carrier concentration under tensile strain is found to be a result of the high gate-drive intrinsic of the BA TFTs.