Two-dimensional Charge Research Articles

Modeling and Analysis of IGBT Based on Two-Dimensional Charge Distribution

Modeling and Analysis of IGBT Based on Two-Dimensional Charge Distribution

Two-Dimensional Space Charge Measurement in Cable Joint Model Using the Pulsed Electroacoustic Method

Two-Dimensional Space Charge Measurement in Cable Joint Model Using the Pulsed Electroacoustic Method

Charge distributions of pseudoscalar and vector mesons from Dyson-Schwinger equations

We combine the Dyson-Schwinger/Bethe-Salpeter equations framework with modern numerical reconstruction methods to derive the three-dimensional and transverse two-dimensional charge distribution of an array of ground-state pseudoscalar and vector mesons from their elastic electromagnetic form factor in the low-momentum region. The charge radii obtained by averaging over the reconstructed charge distributions have been checked to be consistent with those calculated from the slope of the elastic electromagnetic form factor at zero transferred momentum. The capability of the reconstruction procedure for capturing a reliable low-distance charge distribution is discussed and argued to work down to distances of around 0.1 fm, such that it might be potentially applied to extract, e.g., mass densities from gravitational form factors. Published by the American Physical Society 2024

Molecular semiconductors with Carbazole versus Triazatruxene endcaps: A matter of self-assembly

In this work, we describe the design and synthesis of a novel soluble conjugated molecule, CAR-TzDPP, consisting of a central diketopyrrolopyrrole core (DPP) connected to carbazole endcaps through thiazole rings. To evaluate its properties, a triazatruxene-based molecule (TAT-TzDPP) was used as a model for comparison. The electrochemical and the in-solution optical properties are in good agreement with those obtained from DFT calculations. However, the thin-film self-assemblies of CAR-TzDPP exhibited distinct differences, forming a more standard crystalline structure including insulating side-chains layers that disallowed side interactions between aromatic rings. This resulted in a reduced charge transport ability to a two-dimensional (2D) ambipolar charge transport in CAR-TzDPP, compared to the nearly isotropic unipolar charge transport in TAT-TzDPP. Despite this, CAR-TzDPP can be considered a viable alternative for device configurations requiring semiconducting pathways aligned in the substrate plane due to its simplified and high yield synthesis route, ease of processing, and reliable charge mobility in OFETs. This research highlights the significance of molecular units in small semiconducting organic molecules, not only in terms of their individual optoelectronic properties but also in terms of their self-assembly capabilities, which ultimately influence their charge transport properties.

Two-dimensional space charge limited current in regime between accelerating diode and drift space for sheet and circular beam

We develop a general electrostatic steady-state model to calculate the two-dimensional (2D) space-charge-limited current (SCLC) with an initial velocity v0 (i.e., kinetic energy eKV=mv02/2) injected from a cathode into a vacuum diode with a spacing D and a potential difference of Vg. We consider two types of beam cross section: (a) a sheet beam with a width W and (b) a circular beam with a radius R, where both W and R are larger than D in the calculation. By introducing a parameter ε≡KV/Vg≥0, we can tune the operating regime to be in between the accelerating diode (ε≪1) and drift space (ε≫1). Hence, we verify the 2D SCLC model by converging to some prior analytical findings such as the 2D Child–Langmuir law at ε = 0, 2D SCLC at drift space at ε≫1, and their 1D limits at W/D≫1 or R/D≫1. Our 2D SCLC model indicates that there is a maximal value at ε∼1/60, which is independent of W/D or R/D. Furthermore, the hysteresis behavior is also studied when the injected current density is beyond the SCLC threshold and its dynamics are mainly influenced by ε instead of geometrical effect. This model is useful in high current electron beam emission with a finite emission area and with a non-negligible arbitrary injection kinetic energy.

Ethylenedioxythiophene-Based Small Molecular Donor with Multiple Conformation Locks for Organic Solar Cells with Efficiency of 19.3 .

Ternary organic solar cells (T-OSCs) represent an efficient strategy for enhancing the performance of OSCs. Presently, the majority of high-performance T-OSCs incorporates well-established Y-acceptors or donor polymers as the third component. In this study, a novel class of conjugated small molecules has been introduced as the third component, demonstrating exceptional photovoltaic performance in T-OSCs. This innovative molecule comprises ethylenedioxythiophene (EDOT) bridge and 3-ethylrhodanine as the end group, with the EDOT unit facilitating the creation of multiple conformation locks. Consequently, the EDOT-based molecule exhibits two-dimensional charge transport, distinguishing it from the thiophene-bridged small molecule, which displays fewer conformation locks and provides one-dimensional charge transport. Furthermore, the robust electron-donating nature of EDOT imparts the small molecule with cascade energy levels relative to the electron donor and acceptor. As a result, OSCs incorporating the EDOT-based small molecule as the third component demonstrate enhanced mobilities, yielding a remarkable efficiency of 19.3 %, surpassing the efficiency of 18.7 % observed for OSCs incorporating thiophene-based small molecule as the third component. The investigations in this study underscore the excellence of EDOT as a building block for constructing conjugated materials with multiple conformation locks and high charge carrier mobilities, thereby contributing to elevated photovoltaic performance in OSCs.

Charge interaction behaviors at interfacial domains in DC GIL insulators

Long-term operation of high voltage direct current at elevated temperatures can result in the accumulation of surface charges in DC gas-insulated transmission line (GIL) insulators. Such a phenomenon leads to localized electric field distortion, increasing the risk of surface discharge. The analysis of interaction behaviors between surface charge and space charge at interfacial domains of GIL insulators is a complex task, which requires a comprehensive understanding of physical mechanisms of the gas–solid interface charging. In this work, a two-dimensional bipolar charge transport and interaction (2D BCTI) model is established, with the consideration of both surface and space charge dynamics. Pulsed electroacoustic tests and surface potential measurements are conducted on DC GIL insulator materials under different electrical-thermal coupling conditions. Experimental results exhibit great consistency with the predictions from the 2D BCTI model. The local accumulation of space charge near interfaces has certain effects on surface potential distribution, which in turn influences charge injection behavior from electrodes. In comparison to traditional surface charge simulation models, the consideration of space charge–surface charge interaction behaviors proves to be essential for estimating the polarity and amplitude of surface potential distribution. This model holds promise for assessing charge characteristics in electrical equipment where direct measurement is challenging.

Endotaxial stabilization of 2D charge density waves with long-range order

Charge density waves are emergent quantum states that spontaneously reduce crystal symmetry, drive metal-insulator transitions, and precede superconductivity. In low-dimensions, distinct quantum states arise, however, thermal fluctuations and external disorder destroy long-range order. Here we stabilize ordered two-dimensional (2D) charge density waves through endotaxial synthesis of confined monolayers of 1T-TaS2. Specifically, an ordered incommensurate charge density wave (oIC-CDW) is realized in 2D with dramatically enhanced amplitude and resistivity. By enhancing CDW order, the hexatic nature of charge density waves becomes observable. Upon heating via in-situ TEM, the CDW continuously melts in a reversible hexatic process wherein topological defects form in the charge density wave. From these results, new regimes of the CDW phase diagram for 1T-TaS2 are derived and consistent with the predicted emergence of vestigial quantum order.

Ultrafast formation of topological defects in a two-dimensional charge density wave

Ultrafast formation of topological defects in a two-dimensional charge density wave

Physical mechanism of Ge doping enhanced Ruddlesden-Popper structure quasi-2D Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic hybrid improper ferroelectricity

Hybrid improper ferroelectricity with quasi-two-dimensional (quasi-2D) structure has attracted much attention recently due to its great potential in realizing strong magnetoelectric coupling and room-temperature multiferroicity in a single phase. However, recent studies show that there appears high coercive field and low remnant polarization in ceramics, which severely hinders the applications of this material. In this work, high-quality Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramics with a Ruddlesden-Popper (R-P) structure are successfully prepared, and their crystal structures and electrical properties are investigated in detail. It is found that the Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic exhibits a lower coercive field that is close to that of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> single crystal. Moreover, via a small amount of Ge doping, the polarization reaches 0.34 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and 0.61 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub>. Combining crystal lattice dynamic studies, we analyze the Raman and infrared responses of the samples, showing the information about the tilting and rotation of the oxygen octahedra in the samples. The improved ferroelectricity after doping may be attributed to the increased amplitude of the tilt mode and the reduced amplitude of rotation mode. Besides, the enhanced ferroelectric properties through Ge doping and its mechanism are further investigated by the Berry phase approach and the Born effective charge method. Furthermore, via the UV-visible spectra, the optical bandgap is determined to be 3.91 eV for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic and 3.95 eV for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramic. Using the Becke-Johnson potential combined with the local density approximation correlation, the bandgap is calculated and is found to be in close agreement with the experimental result. And the electronic excitations can be assigned to the charge transfer excitation from O 2p to Sn 5s (Ge 4s). The effects of Ge doping on the ability of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> to gain and lose electrons and the bonding strength of Sn-O bond are analyzed via two-dimensional charge density difference. In conclusion, this study provides insights into the synthesis method and modulation of ferroelectric properties of hybrid improper ferroelectrics Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub>, potentially facilitating their widespread applications in various capacitors and non-volatile memory devices.

Polar NaTaO3/LaAlO3 (001) superlattices: A comparison of PBEsol and PBEsol+[formula omitted] first-principles calculations

The increasing experimental and theoretical interest in two-dimensional charge carrier gases formed at the interfaces of heterostructured perovskite oxide systems has led to a demand for a deeper understanding regarding the influence of exchange–correlation functionals and associated corrections on the systems’ properties. Herein, we investigate the impact of implementing additive Hubbard corrections compared to standard DFT routines using the PBEsol functional in polar NaTaO3/LaAlO3 (001) superlattices. Structural analysis reveals that the qualitative assessment remains consistent regardless of the approach, with slight variations observed for the NaTaO3 phase due to more pronounced polar distortions. Electronically, standard PBEsol fails to accurately describe the electronic structure of the superlattice below the system’s critical thickness, whereas the inclusion of on-site Coulomb interactions through PBEsol+U routines provides the correct semiconductor character of the superlattice, emphasizing the role of strong correlation effects on d and f electrons at the material’s interface. Additionally, our investigation reveals the spatial charge separation in the semiconductor heterostructure, with potential implications for photocatalytic devices. This work not only contributes to demonstrate the importance of the chosen methodology when studying such systems but also sheds light on important electronic and structural properties of NaTaO3/LaAlO3 (001) superlattices, enabling new avenues for future applications.

Ab initio investigations of two-dimensional carrier gas at interfaces in GaN/AlN and GaN/AlN/Al2O3 heterostructures

The formation of a two-dimensional electron gas (2DEG) at the GaN (0001)/AlN interface holds significant implications for GaN-based high-voltage and high-frequency (RF) devices. Due to the promising results provided by the addition of a thin layer of AlN in metal–oxide-semiconductor channel high-electron-mobility transistor devices, this interface can be found in both the access region and near the dielectric gate. Recent ab initio simulations shed light on the crucial role played by spontaneous and piezoelectric polarizations within polar GaN and AlN crystals in driving the formation of the 2DEG. This study explores the underlying mechanisms behind the 2DEG formation and investigates the impact of fixed charges and additional layers, like Al2O3, on the carrier concentration. Consistent with the literature, our findings highlight the predominant role of polarizations within III–V materials in the formation of the 2DEG. Moreover, we examine the influence of fixed charges on the AlN surface, revealing their ability to accumulate or deplete the 2DEG, while maintaining charge conservation through the emergence of a new two-dimensional charge gas on the AlN surface. Additionally, we explore the effects of incorporating a β-Al2O3 crystal layer on the GaN/AlN structure, finding that the 2DEG’s carrier density is reduced, yet not entirely eliminated, while a significant positive charge concentration at the AlN/Al2O3 interface pins the Fermi level. This comprehensive investigation contributes to our understanding of microscopic phenomena in III–V heterostructures, paving the way for future advancements and applications in power electronics.

Importance of surface morphology on secondary electron emission: a case study of Cu covered with carbon, carbon pairs, or graphitic-like layers

Understanding the relationship between surface adsorbates and secondary electronic emission is critical for a variety of technologies, since the secondary electrons can have deleterious effects on the operation of devices. The mitigation of such phenomena is desirable. Here, using the collective efforts of first-principles, molecular dynamics, and Monte Carlo simulations, we studied the effects of a variety of carbon adsorbates on the secondary electron emission of Cu (110). It was demonstrated that the adsorption of atomic C and C_2 pair layers can both reduce and increase the number of secondary electrons depending on the adsorbate coverage. It was shown that under electron irradiation, the C–Cu bonds can be dissociated and reformed into C_2 pairs and graphitic-like layers, in agreement with experimental observation. It was verified that the lowest secondary electron emission was due to the formation of the graphitic-like layer. To understand the physical reason for changes in number of secondary electrons for different systems from an electronic structure perspective, two-dimensional potential energy surfaces and charge density contour plots were calculated and analyzed. It was shown that the changes are strongly influenced by the Cu surface morphology and depends highly on the nature of the interactions between the surface Cu and C atoms.

A New Analytical Large-Signal Model for Quasi-Ballistic Transport in InGaAs HEMTs Accommodating Dislocation Scattering

A surface-potential-based analytical large-signal model, which is applicable to both ballistic and quasi-ballistic transport in InGaAs high electron mobility transistors, is developed. Based on the one-flux method and a new transmission coefficient, a new two-dimensional electron gas charge density is derived, while the dislocation scattering is novelly taken into account. Then, a unified expression for Ef valid in all the regions of gate voltages is determined, which is utilized to directly calculate the surface potential. The flux is used to derive the drain current model incorporating important physical effects. Moreover, the gate-source capacitance Cgs and gate-drain capacitance Cgd are obtained analytically. The model is extensively validated with the numerical simulations and measured data of the InGaAs HEMT device with the gate length of 100 nm. The model is in excellent agreement with the measurements under I-V, C-V, small-signal conditions, and large-signal conditions.

A study on the luminescence properties of high-performance benzothieno[3,2-b][1]benzothiophene based organic semiconductors

In terms of molecular structures, benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives qualify as the best-performing organic semiconductors for hole transport, but it exhibits poor luminescence properties. For instance, a classical molecule of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) only shows photoluminescence quantum yields (PLQYs) of 5.0% in chlorobenzene. Herein, we present three kinds of organic semiconductors based on phenyl-BTBT molecule core, namely, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), 2,7-bis(4-ethylphenyl) benzothieno[3,2-b][1] benzothiophene (DBEP-BTBT), and 2,7-bis(4-methoxyphenyl)benzothieno[3,2-b][1]benzothiophene (DBOP-BTBT). Notably, blue fluorescence with relatively high PLQYs of 27%, 36%, and 48% in chlorobenzene was shown. Due to the surface defects and grain boundaries in the thin films, the PLQYs decrease to 3.4% for DPh-BTBT and 7.4% for DBOP-BTBT, respectively. Nevertheless, the DBEP-BTBT film still exhibits an absolute PLQY as high as 12%, which is a rare and precious characteristic in BTBT derivatives. Meanwhile, the differences in photophysical characteristics, crystal structures, and charge transport properties of the three BTBT derivatives were systematically studied. Similar to DPh-BTBT and DBOP-BTBT, the DBEP-BTBT crystals display typical layered herringbone packing structure with multi- and strong intermolecular interactions, which are conductive to form a two-dimensional charge transport network in layered thin films for high mobility. The mobility of DBEP-BTBT in polycrystalline thin film transistors is up to 3.22 cm2V−1s−1 in ambient air, which is close to DPh-BTBT (3.64 cm2V−1s−1) and much higher than that of DBOP-BTBT (0.5 cm2V−1s−1). Moreover, the calculations show that the intermolecular transfer integrals of HOMOs are almost isotropic for six nearest neighbour contacts in DBEP-BTBT and DPh-BTBT crystals, implying their possibility of isotropic hole transport property in single crystals.

Characterization of the bidimensional size and charge distribution of sub- and supermicrometer particles in an electrostatic precipitator

We develop and implement an approach to directly invert two-dimensional size and charge distributions for particles exiting a wire-plate electrostatic precipitator (ESP). Combined instruments used in measurement and data inversion include both a tandem differential mobility analyzer (TDMA) system and a DMA combined with an aerodynamic particle spectrometer. The two-dimensional size and charge distribution hence provides information about the charge levels attained by particles in the ESP, which can reach 300 elementary charges for supermicrometer particles. The main mode of the charge distribution is well-fit by a Gaussian profile over the entire range of measured particle sizes. We find the average charge scales in a power-law manner with exponents between 1.29 to 1.39. We compare measured size and charge distributions, along with particle size-dependent penetrations to a 1D model accounting for unipolar ionization and electrophoretic deposition.

Spectroscopic visualization and phase manipulation of chiral charge density waves in 1T-TaS2

The chiral charge density wave is a many-body collective phenomenon in condensed matter that may play a role in unconventional superconductivity and topological physics. Two-dimensional chiral charge density waves provide the building blocks for the fabrication of various stacking structures and chiral homostructures, in which physical properties such as chiral currents and the anomalous Hall effect may emerge. Here, we demonstrate the phase manipulation of two-dimensional chiral charge density waves and the design of in-plane chiral homostructures in 1T-TaS2. We use chiral Raman spectroscopy to directly monitor the chirality switching of the charge density wave—revealing a temperature-mediated reversible chirality switching. We find that interlayer stacking favours homochirality configurations, which is confirmed by first-principles calculations. By exploiting the interlayer chirality-locking effect, we realise in-plane chiral homostructures in 1T-TaS2. Our results provide a versatile way to manipulate chiral collective phases by interlayer coupling in layered van der Waals semiconductors.

Modeling and simulation assessment of dual material gate Delta(δ) doped fully depleted SOI-FET with effect of interface trap charges

A two–dimensional (2D) analytical model with surface potential changes in the delta doped dual material gate with fully depleted silicon on insulator-MOSFET (DMG-DD-SOIFET) is designed to evaluate device characteristics in consideration of interface trap charges (InTCs). The present paper introduced a novel δ-doped in SOIFET to improve device performance. The potential is modeled via two-dimensional Poisson's equations and interface trap charges (InTCs) impact which is verified with numerical simulations using TCAD tool of Silvaco ATLAS. Further, a parametric investigation is carried out to adjust the dimensions of device for a nano-scaled MOS architecture. The performance investigation model and simulation work also involve the calculation of surface potential, drain current and threshold voltage comprising effects of InTCs. It is found that the proposed DMG-DD-SOIFET suggests excellent immunity for short-channel-effects (SCEs) in addition to better device performance for different InTCs conditions.

Two-dimensional transverse charge distributions of the Δ baryon: Interpolation between the nonrelativistic and ultrarelativistic limits

We investigate how the charge distributions of both the unpolarized and transversely polarized $\Delta$ baryon change as the longitudinal momentum~($P_{z}$) of the $\Delta$ baryon increases from $P_{z}=0$ to $P_{z}=\infty$ in a Wigner phase-space perspective. When the $\Delta$ baryon is longitudinally polarized, its two-dimensional charge distribution is kept to be spherically symmetric with $P_{z}$ varied, whereas when the $\Delta$ baryon is transversely polarized along the $x$-axis, the quadrupole contribution emerges at the rest frame ($P_{z}=0$). When $P_{z}$ grows, the electric dipole and octupole moments are induced. The induced dipole moment dominates over other higher multipole contributions and governs the deformation of the charge distribution of the $\Delta$ baryon.

Correlation of Crystal Defects with Device Performance of AlGaN/GaN High-Electron-Mobility Transistors Fabricated on Silicon and Sapphire Substrates

Herein, the performance of AlGaN/GaN high-electron-mobility transistor (HEMT) devices fabricated on Si and sapphire substrates is investigated. The drain current of the AlGaN/GaN HEMT fabricated on sapphire and Si substrates improved from 155 and 150 mA/mm to 290 and 232 mA/mm, respectively, at VGS = 0 V after SiO2 passivation. This could be owing to the improvement in the two-dimensional electron gas charge and reduction in electron injection into the surface traps. The SiO2 passivation resulted in the augmentation of breakdown voltage from 245 and 415 V to 400 and 425 V for the AlGaN/GaN HEMTs fabricated on Si and sapphire substrates, respectively, implying the effectiveness of SiO2 passivation. The lower transconductance of the AlGaN/GaN HEMT fabricated on the Si substrate can be ascribed to the higher self-heating effect in Si. The X-ray rocking curve measurements demonstrated that the AlGaN/GaN heterostructures grown on sapphire exhibited a full-width half maximum of 368 arcsec against 703 arcsec for the one grown on Si substrate, implying a better crystalline quality of the AlGaN/GaN heterostructure grown on sapphire. The AlGaN/GaN HEMT fabricated on the sapphire substrate exhibited better performance characteristics than that on the Si substrate, owing to the high crystalline quality and improved surface.