Increasing Electron Concentration Research Articles

How Cr doping influences on the properties of ZnO nanoparticles obtained by aqueous synthesis: Exploring at low Cr concentrations

A detailed physico-chemical study of Cr3+-doped ZnO nanoparticles (NPs) obtained via aqueous synthesis was carried out. The NPs acquire greenish tonality as the Cr concentration increased. High efficiency of Cr incorporation into the ZnO lattice was found up to approximately 1 at% Cr. The ZnO:Cr NPs show wurtzite hexagonal phase with a lattice compressed for low Cr concentration and tensed for higher concentrations. Structural, chemical and optical analyses indicate that for low Cr concentration Cr occupies the Zn vacancy sites, owing to the reduction in acceptor compensation by passivating the Zn vacancies and the contribution of the unpaired electron of Cr3+, which induces an increase of electron concentration, changes in the conductivity, transport of carriers, optical properties and ferromagnetic properties, among others. At higher Cr concentrations, the passivation of Zn vacancies by Cr atoms reduces the acceptor contribution, and substantial substitution of Zn atoms by Cr atoms in the ZnO lattice strengthens the retention of O atoms in the Cr-O bonds. This results in a reduction in O vacancy concentration, donor contribution, strain, electron concentration, and luminescence intensity.

Electron mobility enhancement in n-GaN under Ohmic-metal

We investigated the electron transport properties of n-GaN under an Ohmic-metal. Hall measurement results were compared for n-GaN (A) before Ti-based metal deposition, (B) after Ti-based metal deposition but before annealing, (C) after Ohmic annealing, and (D) after Ohmic-metal removal, where multi-probe-Hall device measurements are required for (C), while the others, (A), (B), and (D), can be characterized by conventional Hall device measurements. The multi-probe-Hall device measurements for (C) elucidated that, under the Ohmic-metal, the electron concentration is increased and the electron mobility is enhanced in comparison with those for the other cases, (A), (B), and (D). The increased electron concentration indicates that high-density doping takes place in the n-GaN by the Ohmic annealing. However, the high-density doping is not observed after the Ohmic-metal removal. Moreover, the electron mobility enhancement under the Ohmic-metal cannot be explained by donor doping with ionized impurity scattering. These suggest that, under the Ohmic-metal, high-density donors are not formed, and high-density polarization doping owing to strain from the Ohmic-metal takes place. From theoretical calculations, we clarified that the increase in the electron density by polarization doping without donors leads to the suppression of ionized impurity scattering and consequently the electron mobility enhancement.

Alloying Effect on Elastic and Mechanical Properties of Refractory Medium-Entropy Alloys from First-Principles Calculations

There is still a challenge to rapidly and accurately acquire the composition dependence of mechanical properties of multi-principal element alloys due to their huge composition space and complex electronic structures. Aiming to straightforwardly tune the mechanical properties through alloying, we systematically calculate the elastic and mechanical properties for four selected body-centered-cubic (bcc) Ti–Nb–M (M = Zr, Mo, Sn, Ta) refractory medium-entropy alloys (RMEAs) using a first-principles method. It is shown that the elastic stability of the bcc RMEAs can be significantly (weakly) improved with the increase in Nb, Mo, and Ta (Sn) content, but it is insensitive to Zr. The elastic stability, elastic modulus, hardness, and strength generally enhance, whereas Pugh’s ratio and Zener anisotropy (AZ) decrease with the increase in valence electron concentration. Additionally, there is a correlation between the magnitude of the AZ and the shape of the single-crystal Young’s modulus (E[hkl]). The largest (smallest) E[hkl] appears in the <111> (<100>) direction. Particularly, the most isotropic Ti60Nb20Mo20 has the highest hardness and strength among all the Ti–Nb–M alloys. Moreover, the change of elastic properties induced by alloying is related to the change of density of states. This work sheds deep light on the design of high-performance Ti-based multi-principal element alloys.

Photo-thermal synergistic excitation: Feasible strategy to detect ethanol for wide bandgap ZIF-8 at low work temperature

Zeolitic Imidazolate Framework-8 (ZIF-8) material was prepared by chemical precipitation method. The microstructure and physical properties of the as-prepared samples were characterized by XRD, BET, FESEM and UV spectrophotometer. The self-made four-channel measurement device was used to test the gas sensitivity of ZIF-8 material toward ethanol gas under photo-thermal synergistic excitation. The results showed that the sample was typical ZIF-8 (Eg = 4.96 eV) with a regular dodecahedron shape and the specific surface is up to 1793 m2/g. The as-prepared ZIF-8 has a gas response value of 55.04 to 100 ppm ethanol at 75°C and it shows good gas sensing selectivity and repeated stability. The excellent gas sensitivity can be attributed to the increase of free electron concentration in the ZIF-8 conduction band by photo-thermal synergistic excitation, and the large specific surface area of ZIF-8 material provides more active sites for gas-solid surface reaction. The reaction mechanism of ZIF-8 material under multi-field excitation was also discussed.

Solar Flare Effects Observed over Mexico during 30–31 March 2022

Manifestations of two solar flares of March 2022 were studied over Mexico. The flare effects in the lower ionosphere had a ~3 min delay from the X1.3-flare onset and ~5 min from the M9.6-flare onset. The maximal impact on the HF signal amplitude was ~(14–15) min after the onset of both flares. The X1.3-flare provoked the shortwave fadeout during ~6 min. The effects in the lower ionosphere lasted longer than the flares and the effects at the F2 region and higher altitudes only during the flares. The interpretation of results showed the following. (1) Based on the absorption level estimated with minimum frequency and signal amplitude on ionograms, the major role of X-ray radiation in the electron concentration increase in the lower ionosphere was confirmed. At the same time, the EUV radiation impact on the lower ionosphere cannot be totally discarded. The lower ionosphere recovery began before and lasted after the X1.3-flare end, being more rapid at Eglin than in Mexico. During M9.6-flare, the responses at the two observation points were rather synchronized due to the more similar illumination conditions at the two meridians. (2) According to the dI variations characterizing the F2 region and higher, the M9.6-flare provoked medium-scale and the X1.3-flare provoked both medium- and small-scale ionospheric irregularities. The response duration corresponded to the dI series filtered with (10–20) min windows. The dI curve during the flares was characterized by the И-form and depended more on the active region position and the flare class than on the solar zenith angle. The available data do not allow us to unambiguously identify the reason for the negative dI: the applied filtering procedure or the physical effect. (3) During both flares, the major EUV impact on the lower ionosphere was by the flux at 133.5 nm and on the F2 region and higher altitudes at 25.6 nm. In addition, during the M9.6-flare, EUV 28.4, 30.4 and 121.6 nm spectral bands also played an important role in the F2 response. During the X1.3-flare, the EUV 25.6 nm flux and X-ray flux impacts on the F2 region were of the same level. The weakest impact was caused by the emission in the EUV 28.4 nm spectral band on the absorption in the lower ionosphere during both flares and on the electron density in the F2 region and higher during the X1.3-flare.

Investigations on Wide-Gap Al0.21Ga0.79N Channel MOS-HFETs With In0.12Al0.76Ga0.12N Barrier/Buffer and Drain Field-Plate

This work investigates, for the first time, wide-gap Al0.21Ga0.79N channel metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with In0.12Al0.76Ga0.12N barrier/buffer and drain field-plate (DFP) designs. High-k and wide-gap Al2O3 was grown as the gate oxide and surface passivation by using non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. A control device having the same epitaxial layers, except with In0.12Al0.88N barrier/buffer was studied in comparison. Enhanced spontaneous polarization effect, improved interfacial quality, and enhanced carrier confinement have been achieved by using the In0.12Al0.76Ga0.12N barrier/buffer design, which has successfully resulted in improved carrier transport, increased electron concentration, and high current densities. The present In0.12Al0.76Ga0.12N/AlN/Al0.21Ga0.79N MOS-HFET design with (without) DFP design has demonstrated superior maximum drain-source current density (IDS, max) of >1 (>1) A/mm at VDS = 20 V, high saturated drain-source current density at VGS = 0 V (IDSS0) of 791.1 (755) mA/mm, and low specific on-resistance (Ron, sp) of 2.83 (2.81) mΩ/cm2. High device figure-of-merit (FOM) on BVDS2/Ron, sp of 93.7 (75.4) MW/cm2 was also obtained with the three-terminal on-state drain-source breakdown voltage (BVDS) of 515 (460) V. The present design is promisingly advantageous to high-current and high-voltage power-switching circuit applications.

Theoretical insights into the amplified optical gain of hexagonal germanium by strain engineering.

Strain engineering is a versatile technique used to tune the electronic and optical attributes of a semiconductor. A proper degree of strain can induce the optimum amount of gain necessary for light-emitting applications. Particularly, photonic integrated chips require an efficient light-emitting material that can be easily assimilated into complementary metal-oxide semiconductor (CMOS) technology. Germanium falls in the same group of the periodic table as silicon, and thus, it completely complies with Si technology. Hence, we investigated extensively the electronic and optical properties of hexagonal germanium for both compressive and tensile strains using density functional theory. The electronic bandstructure, dielectric function, absorption, and reflectivity were calculated by employing a modified Becke-Johnson (mBJ) potential including spin-orbit coupling for uniaxial strains ±0.5-5%. We calculated the effective masses at various symmetry points and determined other band parameters, including the crystal field splitting and spin-orbit splitting energies. The partial, projected, and total density of states were discussed in great depth to unveil the characteristics of the energy states that take part in optical transitions. Finally, the optical gain for the semiconductor was calculated as a function of strain. After the band inversion phenomenon, hex-Ge generates a huge increase in the amplification and bandwidth of optical gain. This results from the increased electron concentration in Γ- 7c state and enhanced momentum matrix between the p-character valence states and sp-hybridized states of the conduction band. Conduction band to light hole recombination is observed to improve the light emission to a great extent.

Modification of the Ni-NiO-oxidant system by thermo-chemical oxidation with accelerated growth of the p-type dielectric oxide layer, by the action of substituted V2O5 on the system

Nickel is covered with very thin oxide film and is stable against corrosion under normal conditions. Nickel, as Ni-NiO-oxidant system, can make much thicker (even a thousand times) and more corrosion unstable oxides. With certain new oxidation process parameters, nickel can undergo so-called accelerated oxidation. The subject and goal of this work is to find the most favorable conditions for performing of accelerated - catastrophic Ni oxidation process, when substitution of V2O5 is carried out into NiO (2V (Ni)), with significant electron concentration increase and oxygen surplus. Unlimited main and secondary charge carriers transfer is significantly facilitated if oxide film with non-stoichiometric composition and defects is formed. The aim is to avoid compact oxide film formation by chemical-thermal treatments and to obtain porous and defective NiO. By using oxidized system Ni - NiO - oxidant in glass, ceramics and evaporated metal technology, extraordinary usable value of this work is ensured.

Magnetic-field-induced simultaneous charge separation and oxygen transfer in photocatalytic oxygen activation for algae inactivation

Photocatalytic oxygen activation is an excellent strategy for algae control in water. However, the fast recombination of photogenerated charge and slow rate of oxygen transfer limit the reactive oxygen species generation efficiency for algae inactivation. Herein, to solve above issues, magnetic field was introduced to the BiO2−x/Bi3NbO7 system to effectively covert oxygen into reactive radicals. The electrochemical experiment and DFT calculation results indicated the charge separation could be accelerated by the Lorentz force generated by the magnetic field, resulting in increase of electron concentration. Meanwhile, the value of volumetric gas-liquid mass transfer coefficient was increased by 59.79 % with magnetic field, thus more oxygen could be reduced to superoxide radical. Photocatalytic algae inactivation rate by BiO2−x/Bi3NbO7 with magnetic field could be increased by 2.07 times than that without magnet filed. This work further extends the strategy of using magnetic field to simultaneously facilitate the charge separation and oxygen transfer rate.

Quantitative measurement of the charge carrier concentration using dielectric force microscopy

The charge carrier concentration profile is a critical factor that determines semiconducting material properties and device performance. Dielectric force microscopy (DFM) has been previously developed to map charge carrier concentrations with nanometer-scale spatial resolution. However, it is challenging to quantitatively obtain the charge carrier concentration, since the dielectric force is also affected by the mobility. Here, we quantitative measured the charge carrier concentration at the saturation mobility regime via the rectification effect-dependent gating ratio of DFM. By measuring a series of n-type GaAs and GaN thin films with mobility in the saturation regime, we confirmed the decreased DFM-measured gating ratio with increasing electron concentration. Combined with numerical simulation to calibrate the tip–sample geometry-induced systematic error, the quantitative correlation between the DFM-measured gating ratio and the electron concentration has been established, where the extracted electron concentration presents high accuracy in the range of 4 × 1016 – 1 × 1018 cm−3. We expect the quantitative DFM to find broad applications in characterizing the charge carrier transport properties of various semiconducting materials and devices.

Impact of electron-phonon interactions on phonon transport in diamond and c-BN

Diamond and cubic polymorph of boron nitride (c-BN) are two promising next-generation ultrawide bandgap semiconductor materials owning superior thermal properties. Although lattice vibration is the dominant mechanism of heat conduction in semiconductors, electron-phonon interactions exist and may affect phonon transport in doped semiconductors; yet such effects in wide bandgap materials have not received much attention. In this study, we explore the effects of electron-phonon interactions on the lattice thermal conductivity and phonon transport in electron-doped Si, diamond, and c-BN under various electron concentrations and in a wide temperature range from 300 K to 900 K based on the first-principles calculation. It is found that the electron-phonon interactions will bring down the thermal conductivity of doped materials, and the depletion impact increases as the electron concentration increases but decreases as the temperature increases. This depression effect in ultrawide bandgap diamond and c-BN is apparent, though it is not as severe as in Si. At room temperature with a high electron concentration of 10 21 cm −3 , the reduction of thermal conductivity reaches 36 % in Si, and 17.4 % and 16.1 % in diamond and c-BN, respectively. • Thermal effect of electron-phonon interactions in doped c-BN and diamond is revealed. • Temperature and electron concentration dependent lattice thermal conductivity is quantified. • Thermal depletions exist in doped c-BN and diamond, but not as severe as in doped Si. • Depletion lessens as temperature rises, but increases with increasing electron concentration. • Ultrawide bandgap materials have great potentials in high-power high-temperature electronics.

Phonon-limited transport of two-dimensional semiconductors: Quadrupole scattering and free carrier screening

Two-dimensional (2D) semiconductors have demonstrated great potential for next-generation electronics and optoelectronics. An important property for these applications is the phonon-limited charge carrier mobility. The common approach to calculate the mobility from first principles relies on the interpolation of the electron-phonon coupling (EPC) matrix. However, it neglects the scattering by the dynamical quadrupoles generated by phonons, limiting its accuracy. Here we present a first-principles method to incorporate the quadrupole scattering, which results in a much better interpolation quality and thus a more accurate mobility as exemplified by monolayer MoS2 and InSe. This method also allows for a natural incorporation of the effects of the free carriers, enabling us to efficiently compute the screened EPC and thus the mobility for doped semiconductors. Particularly, we find that the electron mobility of InSe is more sensitive to the carrier concentration than that of MoS2 due to the stronger long-range scattering in intrinsic InSe. With increasing electron concentration, the InSe mobility can reach ~4 times of the intrinsic value, then decrease owing to the involvement of heavier electronic states. Our work provides accurate and efficient methods to calculate the phonon-limited mobility in the intrinsic and doped 2D materials, and improves the fundamental understanding of their transport mechanism.

Enhancing the injected electron concentration and thermionic emission property in C12A7:e− ceramic via Al site Zr doping

Abstract[Ca24Al28O64]4+(4e−) (C12A7:e−) has excellent potential in electric propulsion owing to various advantages, including low work function, low working temperature, and stable chemical properties. However, increasing electron concentration enhances the thermionic emission property of C12A7:e−. This study used a chemical method—spark plasma sintering to synthesize Zr‐doped C12A7:e− (C12(A1 − xZx)7:e−, where x = 0 to 0.14). The study results show that C12(Al0.86Zr0.14)7:e− maximum injected electron concentration (n), conductivity (σ), electron mobility (μn), and zero‐field emission current density (Jo) improved to 2.4 × 1021 /cm3, 1507 S/cm, 5.2 cm2/V·s, and 2.4 A/cm2, respectively. The emission current density of the C12(Al0.86Zr0.14)7:e− could be maintained at about 2.9 A/cm2 under 1100°C, 4000 V, and 600 min. Theoretical calculation results prove that Zr doping broadens the cage conduction band (CCB) of C12A7:e− and reduces the bandgap between the CCB and the frame conduction band. In addition, Zr doping increases the electronic density of states near the Fermi level, and the density of states presents a continuous state. This helps to increase the injected electron concentration and enhance the electrical transport and thermionic emission characteristics of the sample. The calculation results provide theoretical support for the experiment.

Carrier-induced metal-insulator transition in trirutile MgTa2O6

Materials exhibiting metal-insulator transitions (MITs) are proposed platforms for next-generation low-power electronics. Many of these materials exhibit strong coupling between the electronic and lattice degrees of freedom, which makes them ideal systems to examine the interplay between lattice dynamics, electronic structure, and magnetic order. The rutile structure, featuring edge-connected octahedral chains along its $c$ axis, permits metal-metal interactions that can induce metal-insulator transitions. Although the MITs in rutile-structured compounds have been thoroughly studied, the derivative trirutile phase, which accommodates similar metal-metal interactions, has yet to be examined in the context of MITs. Here we use density functional theory calculations to investigate a carrier-driven MIT in trirutile ${\mathrm{MgTa}}_{2}{\mathrm{O}}_{6}$, a ${d}^{0}$ insulator with a suitable axial ratio. Our calculations suggest the existence of four distinct phases in ${\mathrm{MgTa}}_{2}{\mathrm{O}}_{6}$ with increasing electron concentration: nonmagnetic insulator, ferromagnetic (FM) metal, FM half-metal, and nonmagnetic insulator. We explain how the resulting electronic phases arise from changes in atomic structure with increasing carrier density. Our results indicate that trirutile oxides may be a promising materials class for which to access and functionalize MITs.

Ni substitution improves the high-temperature thermoelectric performance of electronegative element Se-filled skutterudite Se0.05Ni Co4-Sb12

The electronegative element-filled skutterudite (EN-SKD) has become an interesting thermoelectric material in the mid-temperature region in recent years. However, the strong intrinsic excitation above ~650 K affects its thermoelectric performance, limiting the practical applications in thermoelectric generators (TEG). To alleviate this situation, a series of EN-SKD compounds with different Ni substitutions, Se0.05NixCo4-xSb12 (x = 0.1, 0.2, 0.3, 0.4) are fabricated by solid-state reaction. It is found that the substitution of Ni for Co significantly suppresses the bipolar effect as the temperature above ~700 K due to the increase of electron concentration, reduces the lattice thermal conductivity (κL) by introducing phonon scattering centers, and consequently performs a significant enhancement of zT. Se0.05Ni0.4Co3.6Sb12 realizes a zT value two times higher than Se0.05Ni0.1Co3.9Sb12 at 823 K. This work demonstrates an approach to optimize thermoelectric performance by Ni substitution at Co sites in EN-SKD through the suppression of the bipolar effect on S and κ.

Improved thermoelectric properties of C-doped Bi2Te3 films following short-range atomic diffusion induced by pulsed electric field treatment

Bi2Te3 is a successful and commercial thermoelectric material. However, Bi2Te3 is easily generates point defects, which limits its thermoelectric (TE) properties. This study presents a novel method to tune point defects by using short-range atomic diffusion via pulsed electric field (PEF) treatment in C-doped Bi2Te3 films. The C atoms migrate from the interstitial position in the Bi layer (Ci) and the Van der Waals gap (Cvdw) to the Bi vacancy after the PEF treatment. For this case, it increases the content of the Te–C bonds for the PEF films, resulting in increased electron concentration. And the PEF can eliminate amorphous regions of the C-doped Bi2Te3 films. In addition, the shorter bond length of the Te–C improves the compressive stress of the film and increases the effective mass of carriers. Thus, the conductivity and the Seebeck coefficient are improved simultaneously. In this study, the power factor was 2450.32 μW m−1·K−2 at 513 K after the PEF treatment. Therefore, the PEF is an effective method to improve TE properties by short-range atomic diffusion.

High room temperature mobility in Al2O3/SrTiO3 heterostructures

Al2O3/SrTiO3 heterostructures have attracted wide attention because of high electron mobility at cryogenic temperatures. But very low room temperature mobility (typically less than 12 cm2V−1s−1) limits its potential application. Here, the Al2O3/SrTiO3 heterostructures are successfully prepared by the low-cost magnetron sputtering. We observed that room temperature mobility is gradually enhanced on increasing electron concentration, and a maximum of 41.1 cm2V−1s−1 is obtained. This can be attributed to the high electron concentration due to the longitudinal optical phonon scattering. Our findings not only offer a route to improve room temperature mobility of the SrTiO3-based heterostructures but also provide a new experimental method for fabricating Al2O3/SrTiO3 heterostructures.

Effects of Co-doping and Microstructure on Charge Carrier Energy Filtering in Thermoelectric Titanium-Doped Zinc Aluminum Oxide

ZnO is a promising thermoelectric (TE) material for high-temperature applications; however, its TE performance is limited by strong coupling between electrical and thermal transport. In this study, we synthesized Al and Ti co-doped ZnO by a solid-state reaction and air sintering at 1500 °C and analyzed the microstructure to establish its correlation with TE properties. The TE transport properties were measured between room temperature and 800 °C, and electronic properties were calculated from first principles calculations. Herein, we introduced second and third phases into a ZnO-based matrix to enhance its power factor (PF) by charge carrier energy filtering by applying co-doping with Al and Ti. Although multiphase materials usually do not exhibit high PF, in this study, it is observed that three-phase ZnO-based materials exhibit higher PF values compared to the two-phase materials. We observed unusual behavior, in which the Seebeck coefficient (S ) and electrical conductivity (σ) values increased simultaneously with temperature for Zn1-x-yAlxTiyO, originating from energy filtering of charge carriers due to both co-doping and the peculiar multiphase structure. High σ values were associated with the increase of electron concentration, while high S values were due to Fermi energy tuning and heavier effective masses initiated by Al and Ti co-doping. Besides increasing the PF, the multiphase structure played an essential role in reducing lattice thermal conductivity due to phonon scattering by the Umklapp, point defect, and second-phase mechanisms. Our approach yielded an increase of the TE figure of merit upon formation of a three-phase 2 wt % Ti-doped Zn0.98Al0.02O compound of ca. 10 times compared to the corresponding value attained for its two-phase ZnAl0.02O counterpart.

Understanding the influence of Bi/Sb substitution on carrier concentration in Mg3Sb2-based materials: decreasing bandgap enhances the degree of impurity ionization.

Experimental results show an intriguing phenomenon that although Bi and Sb have the same number of valence electrons, Bi/Sb substitution increases the electron concentration of n-type Mg3Sb2-based materials. Using a combination of theoretical calculations and experimental synthesis, this work reveals the physical mechanism of the effect of Bi doping on carrier concentration. The increase in electron concentration mainly originates from the enhanced degree of ionization of donor impurity because of the decrease of conductivity effective mass and increase of dielectric constant caused by the narrowing of bandgap with Bi doping. Based on the collaborative optimization of the electrical and thermal transports, n-type Mg3.175Mn0.025Sb1.48Bi0.48Te0.04 exhibits the best thermoelectric performance with a peak zT of 1.85 at 725 K and an average zT of 1.21. This work demonstrates an effective strategy of bandgap engineering for the optimization of carrier concentration and provides insightful guidance for designing other thermoelectric materials.

The Magneto Electron Statistics in Heavily Doped N Type-Intrinsic-P Type-Intrinsic Structures.

In this paper we study the Electron Statistics in Heavily Doped N Type-Intrinsic-P Type-Intrinsic structures of non-linear optical, tetragonal and opto-electronic materials in the presence of magnetic quantization. It is found taking such heavily doped structures of Cd₃As₂, CdGeAs₂, InAs, InSb, Hg1-xCdxTe, In1-xGaxAsyP1-y as examples that the Fermi energy (EF) oscillates with inverse quantizing magnetic field (1/B) and increases with increasing electron concentration with different numerical magnitudes which is the signature of respective band structure. The numerical value of the Fermi energy is different in different cases due to the different values of the energy band constants.