High Drift Velocity Research Articles

Plasmon scattering at a junction in double-layer two-dimensional electron gas plasmonic waveguide

Abstract The scattering of plasmons at a junction within a double-layer two-dimensional electron gas plasmonic waveguide is studied via a full electromagnetic method. The dispersion relation is derived by utilizing the transfer matrix method and can be extended to the situation of an arbitrary number of layers. By numerically solving the dispersion equations, both the acoustic and optical plasmon modes are identified in this double layer system, and the unstable plasmon modes arising from plasmon coupling in different layers are discussed elaborately. Subsequently, the total fields are expanded with eigenmodes and matched at the interface to analyze the scattering characteristics at the junction. The results indicate that the total power of the plasmon mode is amplified when the electron fluid flows from a high concentration region to a low concentration region, and the amplification is more evident at a higher drift velocity. Additionally, we address the scattering of unstable plasmons caused by the two-stream instability and find that the transmitted plasmons are excited intensively at the incidence of the growing plasmon, leading to the plasmon amplification. The detailed examination of plasmon scattering at junction is the prerequisite for studying more complex structures of terahertz plasmonic devices and comprehending the corresponding amplification mechanism.

Transient dispersion of settling gyrotactic microorganisms in an open channel flow

Dispersion of microorganisms is a key issue in bio-physics and has many applications in the fields of algae cultivation, biomass energy, and wetland ecology. However, there has been limited exploration of the effects of settling behavior and initial release conditions on the transient dispersion of gyrotactic microorganisms. This paper explores the transient dispersion of settling gyrotactic microorganisms in an open channel flow. The moment equations derived from the Smoluchowski equation are solved by the biorthogonal expansion method, and the results are compared with random walk simulations, showing good agreement. The time variations of concentration distribution, drift velocity, and dispersivity of settling gyrotactic microorganism suspension are explored in detail under typical initial release conditions. As illustrated and characterized, settlement weakens the gravitactic focusing of microorganisms near the free surface, leads to accumulation at the bottom, and increases the dispersivity; from a line source release, the relaxation time is shortest, and the microorganisms scatter fastest in the longitudinal direction, while the point source at the water surface leads to the most concentrated longitudinal distribution and the highest drift velocity; furthermore, the initial release condition assumes an important role in shaping the concentration distribution and drift velocity.

Transient behaviour analysis in silicon carbide alpha particle detector using TCAD and SRIM simulation

Time response characteristics of α particle detector are crucial for monitoring radiation fields varied with time and its characterization of pulse radiation field. Here, SRIM-informed TCAD simulation is utilized to visually investigate the transient behaviors of carriers and alpha particles in 4H-SiC Schottky barrier detectors for the time response characteristics. We identified external bias voltage and incident particle energy as key factors to influence transient current pulse broadening. Low-energy alpha particles result in low initial kinetic energy of the ionization-generated carriers, leading to transient current broadening and reduced time resolution characteristics. Conversely, high-energy alpha particles ionize carrier with high drift velocity, preventing the broadening effect. Our simulation provides a planform and valuable guidance for optimizing alpha particle detector, selecting appropriate bias voltages, and enhancing time resolution capabilities.

Intermittency, bursty turbulence, and ion and electron phase-space holes formation in collisionless current-carrying plasmas

In the previous studies of nonlinear saturation of the Buneman instability caused by high electron drift velocity relative to ions, the phase-space holes and the plateau on the electron velocity distribution function were identified as features of the saturation stage of instability [notably in the paper by Omura et al., J. Geophys. Res. 108, 1197 (2003)]. We have performed a much longer simulation of the Buneman instability and observed a secondary instability. This secondary instability generates fast electron-acoustic waves. By analyzing the phase-space plot of ions and electrons, we show that the fast electron heating and the formation of the plateau of electron velocity distribution function are not due to the quasi-linear diffusion but due to the nonlinear interaction of ion- and electron-acoustic solitary waves (phase-space holes) by exchange of trapped electrons in each wave. We also report the details on the intermittent and bursty nature of turbulence driven by this instability.

UnDrift: A versatile software for fast offline SPM image drift correction.

Scanning probe microscopy (SPM) techniques are widely used to study the structure and properties of surfaces and interfaces across a variety of disciplines in chemistry and physics. One of the major artifacts in SPM is (thermal) drift, an unintended movement between sample and probe, which causes a distortion of the recorded SPM data. Literature holds a multitude of strategies to compensate for drift during the measurement (online drift correction) or afterwards (offline drift correction). With the currently available software tools, however, offline drift correction of SPM data is often a tedious and time-consuming task. This is particularly disadvantageous when analyzing long image series. Here, we present unDrift, an easy-to-use scientific software for fast and reliable drift correction of SPM images. unDrift provides three different algorithms to determine the drift velocity based on two consecutive SPM images. All algorithms can drift-correct the input data without any additional reference. The first semi-automatic drift correction algorithm analyzes the different distortion of periodic structures in two consecutive up and down (down and up) images, which enables unDrift to correct SPM images without stationary features or overlapping scan areas. The other two algorithms determine the drift velocity from the apparent movement of stationary features either by automatic evaluation of the cross-correlation image or based on positions identified manually by the user. We demonstrate the performance and reliability of unDrift using three challenging examples, namely images distorted by a very high drift velocity, only partly usable images, and images exhibiting an overall weak contrast. Moreover, we show that the semi-automatic analysis of periodic images can be applied to a long series containing hundreds of images measured at the calcite-water interface.

Three-component modelling of O-rich AGB star winds

Stellar winds of cool and pulsating asymptotic giant branch (AGB) stars enrich the interstellar medium with large amounts of processed elements and various types of dust. We present the first study on the influence of gas-to-dust drift on ab initio simulations of stellar winds of M-type stars driven by radiation pressure on forsterite particles. Our study is based on our radiation hydrodynamic model code T-800 that includes frequency-dependent radiative transfer, dust extinction based on Mie scattering, grain growth and ablation, gas-to-dust drift using one mean grain size, a piston that simulates stellar pulsations, and an accurate high spatial resolution numerical scheme. To enable this study, we calculated new gas opacities based on the EXOMOL database, and we extended the model code to handle the formation of minerals that may form in M-type stars. We determine the effects of drift by comparing drift models to our new and extant non-drift models. Three out of four new drift models show high drift velocities, 87–310 km s−1. Our new drift model mass-loss rates are 1.7–13 per cent of the corresponding values of our non-drift models, but compared to the results of two extant non-drift models that use the same stellar parameters, these same values are 0.33–1.5 per cent. Meanwhile, a comparison of other properties such as the expansion velocity and grain size show similar values. Our results, which are based on single-component forsterite particles, show that the inclusion of gas-to-drift is of fundamental importance in stellar wind models driven by such transparent grains. Assuming that the drift velocity is insignificant, properties such as the mass-loss rate may be off from more realistic values by a factor of 50 or more.

New optimized HIT solar cell using wide-bandgap nc-3C-SiC:H as a window layer

This study focuses on the investigation of HIT solar cell using nanocrystalline hydrogenated cubic silicon carbide (nc-3C-SiC:H) as a window layer. The selection of nc-3C-SiC:H is motivated by its wide bandgap, high electron drift velocity, and exceptional optical, electrical, and structural properties. Employing SCAPS-1D simulator, a comprehensive simulation analysis of thin nc-3C-SiC:H-based solar cell was conducted. By analyzing the current–voltage and quantum efficiency curves, the impact of doping concentration in the window layer was examined, covering a range from [Formula: see text] to [Formula: see text]. A rapid degradation of [Formula: see text] was observed for high doping concentrations. Additionally, the temperature dependence of external DC parameters for the temperatures 273, 300, and 318[Formula: see text]K was explored under AM1.5G and AM1.5D sunlight spectra. The application of two distinct light spectra resulted in the DC parameters exhibiting minimal variation or similarity. The study also took into account the influence of window thickness variation, temperature, and doping concentration. The findings demonstrate a remarkable influence of temperature and doping concentration on the DC parameters. [Formula: see text] value of 30.29[Formula: see text]mA/cm2 was achieved with a 3-nm thickness and an optimal efficiency of 19.48% was registered with a doping concentration of up to [Formula: see text], showcasing the promising potential of nc-3C-SiC:H.

Effect of band-gap tuning on absorption of phonons and acoustoelectric current in graphene nanoribbon

We report the use of Boltzmann Transport Equation (BTE) in the hypersound regime for investigating the generation of acoustoelectric (AE) current in an armchair graphene nanoribbon (AGNR). The AE current obtained was deduced from the absorption due to the Landau damping of quantized sound waves (LD-QSW) energy. By stimulating the AGNR with a non-quantized electric field, the absorption and the AE current were analysed against the frequency, the wavenumber, the width of the bandgap and the drift velocity. The absorption was observed to switch to amplification as the drift velocity was varied. At high drift velocities, the amplitude of the AE current decreases as the band-gap (that depends on the applied electric field) was increased, suggesting a potential use of AGNR in the design of tunable acoustoelectric current devices. • Graphene Nanoribbon. • Landau Damping. • Absorption. • Acoustoelectric current.

Nondestructive microstructural investigation of defects in 4H-SiC epilayers using a multiscale luminescence analysis approach

The development of metal oxide semiconductor field effect transistors (MOSFETs) utilizing epitaxially grown 4H-SiC has accelerated in recent years due to their favorable properties, including a high breakdown field, high saturated electron drift velocity, and good thermal conductivity. However, extended defects in epitaxial 4H-SiC can affect both device yields and operational lifetime. In this work, we demonstrate the importance of a multiscale luminescence characterization approach to studying nondestructively extended defects in epitaxial 4H-SiC semiconducting materials. Multiscale luminescence analysis reveals different aspects of excess charge carrier recombination behavior based on the scale of a particular measurement. Combining measurements of the same extended defect area at different scales tells us more about the essential nature of that defect and its microstructure. Here, we use photoluminescence imaging and cathodoluminescence spectrum imaging to investigate the recombination behavior of several different types of extended defects, including stacking faults, inclusions, and basal plane dislocations. A detailed understanding of the optoelectronic properties of extended defects in epitaxial SiC helps elucidate the microstructure of extended defects and can provide pathways to mitigate detrimental changes during device operation related to their evolution, such as the recombination enhanced dislocation glide effect that affects SiC-based MOSFETs.

GaN Schottky diode on sapphire substrate for THz frequency multiplier applications

Gallium Nitride, because of its high breakdown field, and peak and saturated electron drift velocity, high chemical stability and high thermal conductivity, is a very promising material as candidate for Schottky varactor diodes for next-generation high-power frequency multipliers. Similarly to the huge interest in the field of HEMT for high frequency and power application, GaN Schottky diodes is of a high interest for the next technological step for frequency multipliers for THz generations, replacing GaAs Schottky diodes. Due to a lack of research focused on GaN Schottky diodes for frequency multiplication, the design and fabrication of GaN frequency multipliers remains a technological challenge. In this paper, we present GaN Schottky diodes fabricated on a heteroepitaxial GaN on sapphire. The barrier height obtained is around 0.53 ​eV, and it increases up to 0.73 ​eV after Schottky contact annealing; and the ideality factor is equal to 1.1 before annealing and 1.2 after annealing. The resulting series resistance is very low, 7Ω and 1Ω, before and after annealing, respectively.

Simulation study of ternary gas mixture transport properties and their gain in GEM detectors used for muon tracking

Gas electron multiplier (GEM) has been installed in the high η region of the Compact Muon Solenoid experiment (CMS) because as example of its high-rate capability. The operational gas of the GEM plays an important role in providing an efficient detection of muon. This work aims to search for a safe and a high-performance gas mixture through a numerical study of the transport properties of ternary gas mixtures. The gas mixtures consists of a noble gas (Ar-Ne-Xe-He) mixed with CO2 and a third gas (N2-C5H12 -CH4 - iC4H10) with different proportions. The gain of single and triple GEM for the mixtures, which show optimal properties, is calculated to estimate their performance in the detector. It is found that the N2 included mixture is giving a high drift velocity while the C5H12 based mixture has the highest Townsend coefficient, which results in higher gain in both single and triple GEM.

Experimental Validation of Fuel Cell Powered Energy Efficient Gallium Nitride Multilevel Inverter for Industrial Applications

This paper presents the air breathing fuel cell (ABFC) power system and its investigation for industrial applications. The ABFC actual behaviour is realized through the development of an empirical model by considering activation polarisation, ohmic polarisation and mass transport effects. The developed stack model feeds the power conditioning unit of the proposed power system. This paper will throw light on the usage of GaN wide bandgap (WBG) material based power semiconductor devices (PSDs) that are used to build 5-level cascaded H- bridge multilevel inverter and a boost converter. This in turn enhances the energy efficiency of the entire FC power system. These WBG PSDs present the phenomenal properties such as high saturated drift velocity, wide bandgap and high critical breakdown field. This multilevel inverter is implemented for facilitating a quality AC output besides offering the voltage control. The investigation is carried out with the development of power conditioning unit hardware setup besides the fabricated FC module for the validation the developed ABFC model.

The dependence of the Irregularity Parameter Characterizing the Equatorial Ionospheric Irregularities on the background Ionospheric Density

The low-latitude ionosphere is characterized by large-scale instabilities in the post-sunset hours due to the distinct geometry of the earth’s magnetic field lines at the equator. The magnetic field lines are horizontal at the equator contributing to the high vertical drift velocity of the plasma bubbles growing from the bottomside of the ionospheric F-region. The phenomenon, commonly known as equatorial spread F, is an important problem in aeronomy as it can cause radio wave scintillation effects representing the most critical impacts of space weather on man-made technologies, such as satellite communications and global navigation satellite systems (GNSS). Here, we report results on the dependence of the peak heights of the irregularities at the magnetic equator, also called as apex-altitude, on solar flux by analyzing in-situ observations made on-board the Communications/Navigations Outage Forecasting System (C/NOFS) satellite mission. Our analysis indicates the median of the peak-height distributions of the irregularities increases linearly from about 491 km at solar minimum to 737 km during solar maximum. The Physics-Based Model (PBMOD) has been used to confirm the space-based observational results and we find the field-line integrated conductivity is the key parameter which controls the peak-heights of the irregularities. In this investigation, we also seek to understand the possible dependence of the irregularity parameter characterizing the equatorial ionospheric irregularities on the background ionospheric density.

Role of strain and composition on the piezoelectric and dielectric response of AlxGa1−xN: Implications for power electronics device reliability

Gallium nitride (GaN) and AlxGa1−xN, its solid solution with Al, play a vital role in a variety of high-power applications owing to their high breakdown voltage, drift velocity, and sheet charge density. Their piezoelectric nature is critical for both the operation and reliability of GaN-based devices, and this is compounded by the lack of lattice-matched substrates and the lattice mismatch between GaN and AlxGa1−xN, which invariably results in strained films. Thus, accurate models of performance and reliability require knowledge of how strain affects dielectric and piezoelectric response. We used density functional theory to calculate the piezoelectric and dielectric constants for different compositions of AlxGa1−xN as a function of biaxial strain and use Gaussian process regression to develop models, including uncertainties, from the ab initio results. We find that the dielectric constants decrease with compressive biaxial strain and increasing Al content due to an increase in phonon frequencies. Meanwhile, the piezoelectric constants increase with compressive biaxial strain and with Al doping. Our results show that the presence of strain can explain discrepancies in experimental measurements of dielectric constants but not piezoelectric ones. Interestingly, the piezoelectric constants e33 and e31 (which control the elastic energy induced by the application of gate voltage in GaN high electron mobility transistors, which have been related to their degradation) vary by almost 100% within a biaxial strain range of ∼3%. These results indicate that incorporating strain-dependent and composition-dependent piezoelectric response into current degradation models based on inverse piezoelectricity is crucial for accurate reliability predictions in GaN-based transistors.

Current Sheets with Multicomponent Plasma in Magnetospheres of Planets of the Solar System

A self-consistent hybrid model of a thin current sheet (TCS) with a thickness of the order of several ion gyroradii is proposed that takes into account the multicomponent nature of collisionless space plasma. Several plasma components can be present in the tails of the magnetospheres of terrestrial planets (for example, the Earth, Mercury, Mars, and Venus). Variations in the current sheet (CS) structure in magnetospheric plasma in the presence of heavy oxygen ions with different characteristics are analyzed. It is shown that high relative concentrations of oxygen ions, as well as their relatively high temperatures and drift velocities, lead to significant thickening of CS and the formation of an additional embedded scale. In this case, on the profiles of the main characteristics—current density and magnetic field—symmetric breaks appear, which correspond to a sharp change in the gradients of variation in values. A comparison is performed and a qualitative agreement is shown between the simulation results and observational data in the tail of the Martian magnetosphere.

Implantation-free edge termination structures in vertical GaN power diodes

Gallium nitride (GaN) is a promising candidate for power applications, due to its superior electrical properties such as high critical breakdown field and saturation drift velocity. The recent emergence of GaN substrates with low defect densities has made it feasible to develop efficient vertical GaN power diodes. However, the breakdown voltage of these diodes is far from the theoretical value, due to the lack of suitable edge termination techniques in GaN such as junction termination extension (JTE). This is attributed to the processing challenges in fabricating GaN power diodes where it is difficult to form p-type regions through implantation. This work addresses this challenge and proposes the design, optimization, and a comprehensive design methodology for implantation-free, edge termination techniques such as field plate (FP), single and multi-etch JTE, and a hybrid FP-JTE structure, suitable for vertical GaN diodes. The proposed approaches demonstrate that close to theoretical breakdown voltages (>80%) are achievable, with greater area efficiency, simple processing, and high tolerance in process variation.

Recent progress of SiC UV single photon counting avalanche photodiodes

4H-SiC single photon counting avalanche photodiodes (SPADs) are prior devices for weak ultraviolet (UV) signal detection with the advantages of small size, low leakage current, high avalanche multiplication gain, and high quantum efficiency, which benefit from the large bandgap energy, high carrier drift velocity and excellent physical stability of 4H-SiC semiconductor material. UV detectors are widely used in many key applications, such as missile plume detection, corona discharge, UV astronomy, and biological and chemical agent detection. In this paper, we will describe basic concepts and review recent results on device design, process development, and basic characterizations of 4H-SiC avalanche photodiodes. Several promising device structures and uniformity of avalanche multiplication are discussed, which are important for achieving high performance of 4H-SiC UV SPADs.

Growth evolution of N-polar indium-rich InAlN layer on c-sapphire via strain relaxation by ultrathin AlON interlayer

InAlN as a functional inorganic material is a promising alternative to the commonly used InGaN in tunnel diodes and optoelectronic devices, due to its tunable wider range of energy bandgap (0.65–6.2 eV), thus empowering utilization of the whole solar spectrum. Moreover, high electron drift velocity and carrier concentration are considered as the most desirable prerequisite of indium-rich InAlN. N-polar indium-rich InAlN could be more beneficial due to the reverse direction of the polarization compared to Ga-polar. However, unanswered questions persist concerning growth evolution of N-polar indium-rich InAlN grown by organometallic chemical vapor deposition (OMCVD). In this study, energy dispersive X-ray spectroscopy (EDX) and high-angle annular dark-field (HAADF) imaging are used to characterize N-polar In0.60Al0.40N layer at nanometer scale in order to determine the evolution of the layer on (0001) sapphire substrate. Long nitridation of sapphire substrate leading to the formation of ~2 nm AlON ultrathin interlayer, which relaxes strain at the InAlN/sapphire interface with assistance of a low-temperature AlN interlayer is observed. EDX analysis confirms that after strain relaxation of InAlN layer, the indium-incorporation has only a weak dependence on the polarity of the layer. The incorporation of indium at preferential sites is also discussed at length.

Thermionic emission current in graphene-based electronic devices

A new current equation for graphene/semiconductor or graphene/metal junctions in graphene-based electronic devices is proposed based on the thermionic emission theory. Temperature-dependent current density predicted by the proposed model agrees well with those experimental data reported in the literature. It can also explain the electric field and temperature-dependent effective Schottky barrier height observed in experiments. This is because a high drift velocity in graphene and its dependence on temperature can lead to a change in the effective Schottky barrier height. Due to the nonlinearity between current and temperature, the Richardson’s law will be broken down. The proposed model will benefit to better understand the current transport mechanism in graphene-like materials and graphene-based electronic devices.

Electron-phonon instability in graphene revealed by global and local noise probes.

Understanding and controlling nonequilibrium electronic phenomena is an outstanding challenge in science and engineering. By electrically driving ultraclean graphene devices out of equilibrium, we observe an instability that is manifested as substantially enhanced current fluctuations and suppressed conductivity at microwave frequencies. Spatial mapping of the nonequilibrium current fluctuations using nanoscale magnetic field sensors reveals that the fluctuations grow exponentially along the direction of carrier flow. Our observations, including the dependence on density and temperature, are consistently explained by the emergence of an electron-phonon Cerenkov instability at supersonic drift velocities. These results offer the opportunity for tunable terahertz generation and active phononic devices based on two-dimensional materials.