Carrier Distribution Research Articles

Analytic theory for current-voltage characteristic of a nanowire radial p-i-n diode

In this article, process of current flow in a nanowire radial p-i-n diode has been considered in detail. It has been shown that cylindrical geometry of the structure gives rise to specific asymmetry of the concentration distribution for current carriers injected to the i-layer, which is opposite to asymmetry that is due to inequality of carriers’ mobilities. This specific asymmetry rises with bringing the i-layer nearer to the nanowire center. Together with that, decrease of the current density in a “long” p-i-n diode and its increase in a “short” p-i-n diode take place (at given forward voltage). And variation of radial thickness of the i-layer demonstrates maximums in the current density at the thickness close to the bipolar diffusion length.

Effect of Step Gate Work Function on InGaAs p-TFET for Low Power Switching Applications.

In this study, we theoretically investigated the effect of step gate work function on the InGaAs p-TFET device, which is formed by dual material gate (DMG). We analyzed the performance parameters of the device for low power digital and analog applications based on the gate work function difference (∆ϕS-D) of the source (ϕS) and drain (ϕD) side gate electrodes. In particular, the work function of the drain (ϕD) side gate electrodes was varied with respect to the high work function of the source side gate electrode (Pt, ϕS = 5.65 eV) to produce the step gate work function. It was found that the device performance varies with the variation of gate work function difference (∆ϕS-D) due to a change in the electric field distribution, which also changes the carrier (hole) distribution of the device. We achieved low subthreshold slope (SS) and off-state current (Ioff) of 30.89 mV/dec and 0.39 pA/µm, respectively, as well as low power dissipation, when the gate work function difference (∆ϕS-D = 1.02 eV) was high. Therefore, the device can be a potential candidate for the future low power digital applications. On the other hand, high transconductance (gm), high cut-off frequency (fT), and low output conductance (gd) of the device at low gate work function difference (∆ϕS-D = 0.61 eV) make it a viable candidate for the future low power analog applications.

Structural and Optical Properties of Silicon Carbide Powders Synthesized from Organosilane Using High-Temperature High-Pressure Method.

The development of new strategies for the mass synthesis of SiC nanocrystals with high structure perfection and narrow particle size distribution remains in demand for high-tech applications. In this work, the size-controllable synthesis of the SiC 3C polytype, free of sp2 carbon, with high structure quality nanocrystals, was realized for the first time by the pyrolysis of organosilane C12H36Si6 at 8 GPa and temperatures up to 2000 °C. It is shown that the average particle size can be monotonically changed from ~2 nm to ~500 nm by increasing the synthesis temperature from 800 °C to 1400 °C. At higher temperatures, further enlargement of the crystals is impeded, which is consistent with the recrystallization mechanism driven by a decrease in the surface energy of the particles. The optical properties investigated by IR transmission spectroscopy, Raman scattering, and low-temperature photoluminescence provided information about the concentration and distribution of carriers in nanoparticles, as well as the dominant type of internal point defects. It is shown that changing the growth modes in combination with heat treatment enables control over not only the average crystal size, but also the LO phonon—plasmon coupled modes in the crystals, which is of interest for applications related to IR photonics.

Charge transport in semiconducting carbon nanotube networks

Efficient and controlled charge transport in networks of semiconducting single-walled carbon nanotubes is the basis for their application in electronic devices, especially in field-effect transistors and thermoelectrics. The recent advances in selective growth, purification, and sorting of semiconducting and even monochiral carbon nanotubes have enabled field-effect transistors with high carrier mobilities and on/off current ratios that were impossible a few years ago. They have also allowed researchers to examine the microscopic interplay of parameters such as nanotube length, density, diameter distribution, carrier density, intentional and unintentional defects, dielectric environment, etc., and their impact on the macroscopic charge transport properties in a rational and reproducible manner. This review discusses various models that are considered for charge transport in nanotube networks and the experimental methods to characterize and investigate transport beyond simple conductivity or transistor measurements. Static and dynamic absorption, photoluminescence and electroluminescence spectroscopy, as well as scanning probe techniques (e.g., conductive atomic force microscopy, Kelvin probe force microscopy), and their unique insights in the distribution of charge carriers in a given nanotube network and the resulting current pathways will be introduced. Finally, recommendations for further optimization of nanotube network devices and a list of remaining challenges are provided.

Patterns of exposure to SARS-CoV-2 carriers manifest multiscale association between urban landscape morphology and human activity

The outbreak of the Coronavirus disease 2019 (COVID-19), and the drastic measures taken to mitigate its spread through imposed social distancing, have brought forward the need to better understand the underlying factors controlling spatial distribution of human activities promoting disease transmission. Focusing on results from 17,250 epidemiological investigations performed during early stages of the pandemic outbreak in Israel, we show that the distribution of carriers of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), which causes COVID-19, is spatially correlated with two satellite-derived surface metrics: night light intensity and landscape patchiness, the latter being a measure to the urban landscape’s scale-dependent spatial heterogeneity. We find that exposure to SARS-CoV-2 carriers was significantly more likely to occur in “patchy” parts of the city, where the urban landscape is characterized by high levels of spatial heterogeneity at relatively small, tens of meters scales. We suggest that this spatial association reflects a scale-dependent constraint imposed by the city’s morphology on the cumulative behavior of the people inhabiting it. The presented results shed light on the complex interrelationships between humans and the urban landscape in which they live and interact, and open new avenues for implementation of multi-satellite data in large scale modeling of phenomena centered in urban environments.

Generation of a Charge Carrier Gradient in a 3C-SiC/Si Heterojunction with Asymmetric Configuration.

It is critical to investigate the charge carrier gradient generation in semiconductor junctions with an asymmetric configuration, which can open a new platform for developing lateral photovoltaic and self-powered devices. This paper reports the generation of a charge carrier gradient in a 3C-SiC/Si heterojunction with an asymmetric electrode configuration. 3C-SiC/Si heterojunction devices with different electrode widths were illuminated by laser beams (wavelengths of 405, 521, and 637 nm) and a halogen bulb. The charge carrier distribution along the heterojunction was investigated by measuring the lateral photovoltage generated when the laser spot scans across the 3C-SiC surface between the two electrodes. The highest lateral photovoltage generated is 130.58 mV, measured in the device with an electrode width ratio of 5 and under 637 nm wavelength and 1000 μW illumination. Interestingly, the lateral photovoltage was generated even under uniform illumination at zero bias, which is unusual for the lateral photovoltage, as it can only be generated when unevenly distributed photogenerated charge carriers exist. In addition, the working mechanism and uncovered behavior of the lateral photovoltaic effect are explained based on the generation and separation of electron-hole pairs under light illumination and charge carrier diffusion theory. The finding further elaborates the underlying physics of the lateral photovoltaic effect in nano-heterojunctions and explores its potential in developing optoelectronic sensors.

Minority carrier decay length extraction from scanning photocurrent profiles in two-dimensional carrier transport structures

Carrier transport was studied both numerically and experimentally using scanning photocurrent microscopy (SPCM) in two-dimensional (2D) transport structures, where the structure size in the third dimension is much smaller than the diffusion length and electrodes cover the whole terminal on both sides. Originally, one would expect that with increasing width in 2D transport structures, scanning photocurrent profiles will gradually deviate from those of the ideal one-dimensional (1D) transport structure. However, the scanning photocurrent simulation results surprisingly showed almost identical profiles from structures with different widths. In order to clarify this phenomenon, we observed the spatial distribution of carriers. The simulation results indicate that the integrated carrier distribution in the 2D transport structures with finite width can be well described by a simple-exponential-decay function with the carrier decay length as the fitting parameter, just like in the 1D transport structures. For ohmic-contact 2D transport structures, the feasibility of the fitting formula from our previous 1D analytical model was confirmed. On the other hand, the application of a simple-exponential-decay function in scanning photocurrent profiles for the diffusion length extraction in Schottky-contact 2D transport structures was also justified. Furthermore, our simulation results demonstrate that the scanning photocurrent profiles in the ohmic- or Schottky-contact three-dimensional (3D) transport structures with electrodes covering the whole terminal on both sides will reduce to those described by the corresponding 1D fitting formulae. Finally, experimental SPCM on a p-type InGaAs air-bridge two-terminal thin-film device was carried out. The measured photocurrent profiles can be well fitted by the specific fitting formula derived from our previous 1D analytical model and the extracted electron mobility-lifetime product of this thin-film device is 6.6 × 10–7 cm2·V−1. This study allows us to extract the minority carrier decay length and to obtain the mobility-lifetime product which can be used to evaluate the performance of 2D carrier transport devices.

Conditions for the identical distribution of free carriers in thin films

The condition of the same distribution of free carriers in thin films is necessary for comparing the mobility and analyzing the scattering mechanisms of carriers near semiconductor film/insulator interfaces. In thin film/insulator systems with different design parameters, it is difficult to ensure the same distribution of free carriers due to physical phenomenon such as the coupling effect. In this study, TCAD simulations of thin-film transistors, which have been used to monitor Si film properties, were applied to find parameters that allow tuning the potential distribution and, accordingly, the distribution of free carriers in films. It was found that such parameters are the film regime, the density of induced carriers, the gate voltage or threshold voltage of transistors. The conditions for the selection of parameters were found that ensure the same distribution of free carriers in thin-film structures for the cases of different thicknesses of films and the surrounding dielectrics. It was shown that the proposed approach can be used for a comparative analysis of the mobility in thin films and makes it possible to eliminate errors associated with different distributions of carriers in the films due to the coupling effect.

Recombination Dynamics in PbS Nanocrystal Quantum Dot Solar Cells Studied through Drift–Diffusion Simulations

The significant performance increase in nanocrystal (NC)-based solar cells over the last decade is very encouraging. However, many of these gains have been achieved by trial-and-error optimization, and a systematic understanding of what limits the device performance is lacking. In parallel, experimental and computational techniques provide increasing insights into the electronic properties of individual NCs and their assemblies in thin films. Here, we utilize these insights to parameterize drift–diffusion simulations of PbS NC solar cells, which enable us to track the distribution of charge carriers in the device and quantify recombination dynamics, which limit the device performance. We simulate both Schottky- and heterojunction-type devices and, through temperature-dependent measurements in the light and dark, experimentally validate the appropriateness of the parameterization. The results reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact, while heterojunction devices are currently limited by NC dopants and electronic defects in the PbS layer. The simulations highlight a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. For example, reduction in the percentage of p-type NCs from the current 1–0.01% in the heterojunction device can lead to a 25% percent increase in the power conversion efficiency.

Bifurcated Current Sheets in Mercury's Magnetotail: Observations and Implications

AbstractCross‐tail current sheets play an important role in magnetospheric dynamics and plasma transport. We present a comprehensive study of the structure and behavior of Mercury's cross‐tail current sheet, as well as constraints on the plasma environment imposed by this structure, using magnetic field measurements from the MESSENGER spacecraft. We find that the current sheet dominantly exhibits a bifurcated, or double‐peaked, current density structure throughout the tail region, but that a single‐peaked sheet is sometimes observed. The time‐averaged current distribution in the nightside magnetosphere is as follows: close to the planet on the midnight plane at an altitude of 0.15 RM (where RM is Mercury radius = 2,440 km), current density peaks are 0.2 RM north and south of the magnetic equator. The peak‐to‐peak separation decreases with down‐tail distance, until the current density profile resembles that of a single‐peaked sheet. We find no correlation between the sheet geometry and long‐period variations in solar wind and magnetospheric conditions. We attribute the observed bifurcated structure in the current sheet to non‐adiabatic behavior (Speiser orbits) of the major plasma constituents in Mercury's magnetosphere (H+, Na+ and, to a lesser extent e−). Finally, we infer the spatial distribution of the current carriers by comparing a simple model of their resultant current sheets to observations. We find that, close to the planet, a combination of planetary sodium ions and solar wind protons with energies of a few keV are the main current carriers, but in the far‐tail region, the current is dominantly carried by solar wind protons.

Erratum: “Improved carrier confinement and distribution in InGaN light-emitting diodes with three-layer staggered QWs” [AIP Adv. 11, 075027 (2021)

Erratum: “Improved carrier confinement and distribution in InGaN light-emitting diodes with three-layer staggered QWs” [AIP Adv. 11, 075027 (2021)

The Quantum Calculation for Valence Band Structure of Strained Zinc-blende GaN Using Six-Band Based k·p Method

The variations of valence band energy with stress effects in zinc-blende GaN are proposed in this paper. The calculations are based on a six-band strain dependent k·p Hamiltonian, and can be self-consistently solved by Schrödinger-Poisson equation. Accurate physical pictures are given for the quantized valence subband structure under biaxial and uniaxial stress in (001) surface along the [110] direction accounting the quantum confinement effect. The warping of the energy profile results in carrier distribution change. This research will be beneficial for improving the hole mobility and the selective of optimum stress for group-III nitride semiconductor based devices.

Microscopic theory for the incoherent resonant and coherent off-resonant optical response of tellurium

An $\mathit{ab}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathit{initio}$-based fully microscopic approach is applied to study the nonlinear optical response of bulk tellurium. The structural and electronic properties are calculated from first principles using the shLDA-1/2 method within density functional theory. The resulting band structure and dipole matrix elements serve as input for the quantum mechanical evaluation of the anisotropic linear optical absorption spectra, yielding results in excellent agreement with published experimental data. Assuming quasiequilibrium carrier distributions in the conduction and valence bands, absorption/gain and spontaneous emission spectra are computed from the semiconductor Bloch and luminescence equations. For ultrafast intense off-resonant excitation, the generation of high harmonics is evaluated and the emission spectra are calculated for samples of different thicknesses.

P-AlInN electron blocking layer for AlGaN-based deep-ultraviolet light-emitting diode

Commonly, the Al-rich AlGaN layer acts as electron blocking layer (EBL) to block the overflow of electrons from the active region in conventional AlGaN deep-ultraviolet (DUV) light-emitting diode (LED). However, the Al-rich AlGaN layer leads to the disadvantages of severe electron overflow and hole blocking effect. Herein, we proposed a new AlInN-based EBL to improve the optoelectronic characteristics of AlGaN-based DUV LED. The calculated results show that conventional (fixed Al composition) AlInN EBL has superior hole injection, reduced electron overflow, and lower electric field as compared to conventional AlGaN EBL. By using AlInN EBL, internal quantum efficiency (IQE) drop is reduced by 20%, and light output power improved by 165.30%. It was noticed that conventional AlInN EBL has a 28% IQE drop which can further be minimized by employing AlInN/AlInN superlattice EBL structure. It is found that superlattice EBL improved carrier distribution and reduced electric field resulting in a higher electron-hole wave-function overlap of 55% within multiple quantum wells (MQW) which elevate radiative recombination rate. AlInN superlattice EBL LED has drop-free 93% IQE, twice light output power, and spontaneous emission rate compared to conventional AlInN EBL LED.

Correlation between photoluminescence and electroluminescence in GaN-related micro light emitting diodes: Effects of leakage current, applied bias, incident light absorption and carrier escape

Accurately predicting the peak wavelength and intensity of the electroluminescence spectrum through the photoluminescence method which can be measured without connecting electrodes is not only practically important for the development of micro-LEDs that cannot be fully inspected for device development, but there is also a fundamental importance in understanding the light emitting mechanism of the LEDs. In this study, the correlation and detailed mechanism between PL and EL are systematically analyzed by considering the effects of carrier escape, carrier accumulation, applied bias, leakage current, incident light wavelength and light absorption in order to provide a clear path for all-optical inspection of light emitting diodes and complete understanding of the luminescence mechanism in GaN-related quantum wells. • To correlate the photoluminescence to the electroluminescence in GaN-related micro LEDs, detailed mechanism of PL and EL are systematically analyzed. • Direct experimental results on the sequential depletion process and on the wavefunction overlap in an individual quantum well were obtained. • In order to correlate the PL with EL, matching the concentration of carriers participating in recombination in both measurements is important. • Effect of carrier leakage plays an important role in EL in micro-LEDs which operates just above the turn on voltages. • Simultaneous excitation of 325 nm and 405 nm lasers can provide most similar carrier distribution as in EL by injecting carriers both in QWs and p-GaN.

Near-field optical imaging and spectroscopy of 2D-TMDs

Abstract Two-dimensional transition metal dichalcogenides (2D-TMDs) are atomically thin semiconductors with a direct bandgap in monolayer thickness, providing ideal platforms for the development of exciton-based optoelectronic devices. Extensive studies on the spectral characteristics of exciton emission have been performed, but spatially resolved optical studies of 2D-TMDs are also critically important because of large variations in the spatial profiles of exciton emissions due to local defects and charge distributions that are intrinsically nonuniform. Because the spatial resolution of conventional optical microscopy and spectroscopy is fundamentally limited by diffraction, near-field optical imaging using apertured or metallic probes has been used to spectrally map the nanoscale profiles of exciton emissions and to study the effects of nanosize local defects and carrier distribution. While these unique approaches have been frequently used, revealing information on the exciton dynamics of 2D-TMDs that is not normally accessible by conventional far-field spectroscopy, a dedicated review of near-field imaging and spectroscopy studies on 2D-TMDs is not available. This review is intended to provide an overview of the current status of near-field optical research on 2D-TMDs and the future direction with regard to developing nanoscale optical imaging and spectroscopy to investigate the exciton characteristics of 2D-TMDs.

IMPACT OF THE GRAPHENE SYNTHESIS AND CONCENTRATION CONDITIONS ON ELECTRICAL PARAMETERS OF GRAPHENE — GRAPHITE SYSTEM

Electrochemical impedance spectroscopy was used to study the electrical parameters of graphite-graphene systems with different mass concentrations of graphene. Graphene was synthesized using two methods of plasma arc discharge from aqueous and non-aqueous medium (water and liquid nitrogen) to determine the impact of graphite concentration, water, and heat treatment of graphene on electrical parameters (conductivity and electrostatic capacity) of the graphite-graphene mixture. The average va­lues ​​of active resistance and electrostatic capacity of these systems are obtained. The optimal ratio of components with high capacitance and conductivity, which was 1: 1. The influence of heat treatment adsorbed on the graphene surface of the water and mass fraction of graphite on the change of electrical parameters of the system is shown. Comparison of the values ​​of capacity and active resistance of the samples showed that the presence of water in graphene reduces the average values ​​of capacity relative to graphene without water by 10 times and symbolically increases the active resistance at a mass ratio of graphene to graphite 1: 3, and at a ratio of 1: 1 values ​​are proportional. Comparison of resistance, capacitance, and charge distribution calculations in a graphite-graphene mixture in the frequency range 10–2 ÷ 103 Hz established the effect of heat treatment on increasing the values ​​of capacitance and active resistance. Heat treatment at 2500C of graphene, synthesized from an aqueous medium, leads to an increase in the values ​​of capacitance and conductivity, which occurs due to a different distribution of charges on the surface. Analysis of charge distribution maps shows that water adsorbed on the surface of graphene in the presence of a signi­ficant amount of graphite can be a factor in interfering with the distribution of charge carriers and significantly reduce the conductivity and electrostatic capacity of the system.

Electron–Hole Plasma‐Induced Dephasing in Transition Metal Dichalcogenides

Electron–hole plasma‐induced dephasing and its influence on the excitonic absorption and the degenerate four‐wave mixing spectra in monolayer transition metal dichalcogenides are investigated. A systematic microscopic theory is presented that combines density functional calculations for the linear material properties with a many‐body equation of motion approach for the optical response. Numerical results are obtained for the example of an hBN‐encapsulated layer of . It is shown that the influence of the excitation‐induced dephasing depends only weakly on the exact shape of the carrier distribution for small densities, whereas distribution details become more important with increasing density.

Effect of indium content and carrier distribution on the efficiency and reliability of InGaN/GaN-based multi quantum well light emitting diode

This paper investigates the mechanisms that limit the efficiency and the reliability of InGaN-based LEDs, by presenting an analysis carried out on devices having two quantum wells that emit at different wavelengths (405 nm and 495 nm). A color-coded test structure was designed in order to easily quantify the optical characteristics of both quantum wells, and to describe in detail the stress dynamics as a function of In-content and of the position of the quantum wells.The attention was focused on: (i) electrical degradation induced by constant current stress at high temperature, that was found to be related to a diffusion process; (ii) degradation of the optical parameters, which was dominantly ascribed to an increase in SRH recombination and to a decreased carrier injection efficiency; (iii) effect of indium content and carrier distribution on the efficiency and reliability of the devices. The results strongly suggest that the internal quantum efficiency is limited by a low injection efficiency and by the quantum confinement Stark effect. In addition, the results on optical degradation highlight the presence of different mechanisms that contribute to the degradation: the role of Shockley-Read-Hall recombination, carrier escape and Auger processes are discussed.

An analysis of flexoelectric coupling associated electroelastic fields in functionally graded semiconductor nanobeams

Strain gradient with strong size dependency and structural association (geometry or microstructure) can efficiently tune the performances of semiconductors by the flexoelectric coupling effect. In this work, we studied a novel asymmetric beam-like semiconductor made by functionally graded (FG) flexoelectric materials. When being applied with pure bending loads at two ends, it can generate a relatively large inhomogeneous strain field to achieve obvious semiconducting behaviors. Unlike the analysis for piezoelectric semiconductor materials, we considered the effects of flexoelectricity and strain gradient elasticity in constitutive equations for flexoelectric semiconductor materials. Then, the complicated mutual coupling governing equations and associated boundary conditions are rederived strictly. By the Fourier series expansion and spatial integration methods, we obtained the solutions of the set of partial differential equations with non-constant coefficients. Results show that the semiconducting electromechanical coupling performances of the beam-like FG flexoelectric semiconductor depend heavily on the ratio and structural distributions of its constituent. Moreover, it is found that the inner carrier distributions and electromechanical characteristics can be significantly tuned by the strain gradient elasticity, the flexoelectricity, and the structural size. We believe this work provides a useful guideline for the practical design and manufacture of novel electromechanical semiconductor devices.