Simulation Of Carrier Transport Research Articles

Carrier transport simulation methods for electronic devices with coexistence of quantum transport and diffusive transport

In this manuscript, carrier transport simulation methods are proposed for devices with the coexistence of quantum transport and diffusive transport by combining the nonequilibrium Green's function method with the drift-diffusion transport simulation method. Current continuity between quantum transport and drift-diffusion transport is ensured by setting quantum transport current as the connection boundary condition of drift-diffusion simulation or by introducing quantum transport-induced carrier generation rates to drift-diffusion simulation. A comprehensive study of our method and the method combining the Wentzel–Kramers–Brillouin (WKB) method with the drift-diffusion transport simulation method is performed for n-type tunnel oxide passivating contact solar cell to investigate their applicable conditions and balance the accuracy and computational cost. As the oxide barrier width, barrier height, and electron effective mass increase, or the doping concentration in the electron transport layer decreases to the extent that the blocking effect of the oxide barrier on light-generated electrons becomes significant, method I is more accurate since the transmission coefficient near the conduction band edge calculated by WKB is overestimated; otherwise, method II is more suitable due to its low computational cost without the loss of accuracy. In addition, the differences between current densities, carrier densities, and Shockley–Read–Hall recombination rates simulated under the two current continuity conditions for the solar cell with different carrier mobilities are also further explored and analyzed.

Carrier transport simulations in twisted bilayer and turbostratic multilayer graphene systems

The effects of inserting a twisted bilayer graphene (tBLG) between turbostratic graphene layers and a SiO2 substrate on the transport properties are investigated, to explore the possibility that using a tBLG could be an effective way to screen the potential fluctuations due to the impurities on the substrate. The Fermi velocity in a tBLG changes from that of the pristine graphene depending on the twist angle. In the present study, a parameter α is introduced, which is defined as the ratio of the Fermi velocity in the tBLG to that of pristine graphene, and the transport properties are calculated as a function of α. The self-consistent calculation of the Poisson equation with the Thomas–Fermi approximation is performed for multilayer graphene systems consisting of a tBLG and turbostratic graphene layers, and the potential profiles are incorporated into a Monte Carlo simulator to calculate the drift velocities and mobilities. It is shown that the transport properties of the whole system strongly depend on the parameter α and the number of layers in the system.

Time-resolved analysis of dual-gate FETs with non-parabolic energy dispersion for THz applications

The investigation of charge carrier transport in state-of-the-art nanoelectronic devices based on III/V semiconductors proves to be challenging, even more so when the highly non-parabolic energy dispersion exhibited by these materials is taken into account. Unlike the common approach of neglecting this behavior by the use of the parabolic band approximation, a novel combination of a tight-binding approach with a quantum Liouville-type equation is introduced here, where any arbitrary energy dispersion can effectively be included. This leads to a discretization based on the atomic structure without the need for finite difference approximations of the Hamiltonian. Because this allows for the stationary as well as the transient simulation of quantum charge carrier transport, it is well suited for the analysis of ultrathin FETs such as dual-gate FETs when it is combined with a mode-space approach. We demonstrate that the parabolic approximation not only vastly underestimates the current densities when compared to the non-parabolic case but also fails to capture transient effects such as gain compression when amplifier operation is considered.

Self-consistent simulations of intracavity terahertz comb difference frequency generation by mid-infrared quantum cascade lasers

Portable terahertz (THz) frequency comb sources are highly desired for applications in rotational molecular spectroscopy and sensing. To date, direct THz quantum cascade laser (QCL) frequency comb generation is not achievable at room temperature. However, THz comb generation based on intracavity difference frequency generation (DFG) in mid-infrared (mid-IR) QCLs is a promising alternative. Here, we present a numerical study of THz DFG-QCL comb formation in mid-IR QCLs based on a self-consistent multi-domain simulation approach. The dynamical simulations are performed using our open-source software tool mbsolve, which provides a flexible and efficient codebase for solving the generalized full-wave Maxwell–Bloch equations. Here, DFG in the active region of a dual-wavelength mid-IR QCL is considered for the generation of THz radiation. The mixing process and, thus, THz generation require a high second-order intersubband nonlinear susceptibility in the QCL active region and can be obtained by targeted quantum engineering. The associated nonlinear effects are included in the Hamiltonian of our Maxwell–Bloch simulation approach. All necessary input parameters for the description of the quantum system are determined self-consistently using our in-house ensemble Monte Carlo software tool for stationary carrier transport simulations. Notably, such simulations require a full-wave Maxwell–Bloch solver that does not employ the common rotating wave approximation, as a broadband optical field extending from the THz to the mid-IR region is investigated. Our modeling approach and the obtained simulation results for two THz DFG-QCL comb setups are validated against experimental data, showing reasonable agreement. Furthermore, we obtain a locked frequency modulated comb state for mid-IR and THz regimes.

Definition of electron polar scattering angle on ionized impurities for Monte Carlo simulation of charge carrier transport in semiconductors

Procedures of polar scattering angle simulation for electron scattering on ionized impurities are examined for Brooks--Herring, Conwell--Weisskopf and Ridley models as the most frequently used in Monte Carlo simulation of charge carrier transport in semiconductors. A more correct procedure for polar scattering angle simulation is proposed for Ridley model. Peculiarities of scattering angle distribution densities calculated in the framework of regarded models are analyzed taking silicon as an example. Comparison of electron mobility calculated by ensemble Monte Carlo method using considered models has been done for doped silicon at 300 K and for constant electric field strength F = 7· 104 V/m. Keywords: ionized impurity, electron scattering, Monte Carlo method.

Разыгрывание полярного угла рассеяния электронов на ионах примеси при моделировании процессов переноса заряда в полупроводниках методом Монте-Карло

Procedures of polar scattering angle simulation for electron scattering on ionized impurities are examined for Brooks-Herring, Conwell-Weisskopf and Ridley models as the most frequently used in Monte Carlo simulation of charge carrier transport in semiconductors. A more correct procedure for polar scattering angle simulation is proposed for Ridley model. Peculiarities of scattering angle distribution densities calculated in the framework of regarded models are analyzed taking silicon as an example. Comparison of electron mobility calculated by ensemble Monte Carlo method using considered models has been done for doped silicon at 300 K and for constant electric field strength F = 7∙104 V/m

Inverse Design of Silicon Photonic Modulators

A novel shape optimization procedure is formulated for obtaining a higher phase change while maintaining high transmission in phase modulators. The formulation is implemented as a shape optimization tool. Subwavelength grating-based Si carrier depletion and carrier injection modulators are considered for testing the optimizer. Carrier transport simulations were performed to extract the carrier concentration profiles and were used in the optimizer. The optimized structures are then validated using simulation for transmission and phase change. The results show that the optimization tool developed can optimize both carrier injection and depletion type modulators, in terms of both transmission and phase modulation. The optimized structures can be placed in a ring or Mach-Zehnder modulator to make compact modulators with low drive voltages.

Advancements in modeling conformally doped X-MSND radiation imagers

A novel bimodal radiation imager (X-ray and thermal neutron) was designed using a fin–trench microstructured silicon diode backfilled with 6LiF, then bonded to a Timepix pixelated imaging ASIC (X-MSNDs). A simulation scheme had been devised which combined finite element analysis and discrete charge carrier transport to simulate the imaging characteristics of such arbitrarily doped 3D silicon Timepix sensors. However, the precise dopant profile of the deeply etched silicon surface that forms the X-MSND diode junction is difficult to measure with conventional optical or chemical staining means. A set of boundary conditions was instead derived from simple particle transport simulations and the constant-surface-concentration silicon diffusion model. Finite element analysis of the microstructured devices using COMSOL Multiphysics and this derived boundary condition revealed that the fin region of the sensors was generally un-depleted under the maximum 300V reverse bias, and the calculated electric field along the thickness of the sensor in the microstructured region is negligible. Charge carrier transport simulations using Allpix2 predicted charge carrier motion in the fin region is dominated by slow diffusion along the fin dimension, resulting in mean charge drift times of 1000ns to collect 95% of charge carriers. Neutron transport simulations indicated that single neutron detection events resulted in highly elongated pixel clusters, typically 1-pixel wide by 6-pixels along the fin dimension. Measured data collected from KSU fabricated X-MSND imagers matched these simulation results.

Charge Transport in Organic Semiconductors: The Perspective from Nonadiabatic Molecular Dynamics.

ConspectusOrganic semiconductors (OSs) are an exciting class of materials that have enabled disruptive technologies in this century including large-area electronics, flexible displays, and inexpensive solar cells. All of these technologies rely on the motion of electrical charges within the material and the diffusivity of these charges critically determines their performance. In this respect, it is remarkable that the nature of the charge transport in these materials has puzzled the community for so many years, even for apparently simple systems such as molecular single crystals: some experiments would better fit an interpretation in terms of a localized particle picture, akin to molecular or biological electron transfer, while others are in better agreement with a wave-like interpretation, more akin to band transport in metals.Exciting recent progress in the theory and simulation of charge carrier transport in OSs has now led to a unified understanding of these disparate findings, and this Account will review one of these tools developed in our laboratory in some detail: direct charge carrier propagation by quantum-classical nonadiabatic molecular dynamics. One finds that even in defect-free crystals the charge carrier can either localize on a single molecule or substantially delocalize over a large number of molecules depending on the relative strength of electronic couplings between the molecules, reorganization, or charge trapping energy of the molecule and thermal fluctuations of electronic couplings and site energies, also known as electron–phonon couplings.Our simulations predict that in molecular OSs exhibiting some of the highest measured charge mobilities to date, the charge carrier forms “flickering” polarons, objects that are delocalized over 10–20 molecules on average and that constantly change their shape and extension under the influence of thermal disorder. The flickering polarons propagate through the OS by short (≈10 fs long) bursts of the wave function that lead to an expansion of the polaron to about twice its size, resulting in spatial displacement, carrier diffusion, charge mobility, and electrical conductivity. Arguably best termed “transient delocalization”, this mechanistic scenario is very similar to the one assumed in transient localization theory and supports its assertions. We also review recent applications of our methodology to charge transport in disordered and nanocrystalline samples, which allows us to understand the influence of defects and grain boundaries on the charge propagation.Unfortunately, the energetically favorable packing structures of typical OSs, whether molecular or polymeric, places fundamental constraints on charge mobilities/electronic conductivity compared to inorganic semiconductors, which limits their range of applications. In this Account, we review the design rules that could pave the way for new very high-mobility OS materials and we argue that 2D covalent organic frameworks are one of the most promising candidates to satisfy them.We conclude that our nonadiabatic dynamics method is a powerful approach for predicting charge carrier transport in crystalline and disordered materials. We close with a brief outlook on extensions of the method to exciton transport, dissociation, and recombination. This will bring us a step closer to an understanding of the birth, survival, and annihiliation of charges at interfaces of optoelectronic devices.

Modelling and simulation of carrier transport in quantum dot memory device for longer data retention and minimized power consumption

The performance of a group III–V material quantum dot (QD) nanostructure memory is investigated using a self-consistent Schrodinger solver, eight-band k·p model, and carrier dynamics modelling. This model is used to explore the information loss due to the carrier emission rate in the QDs as a function of temperature, size and confinement potential. The results reveal the dominant emission mechanisms that should occur at different operating temperatures. To minimize the loss and improve the performance at room temperature, our findings reveal an increase in the carrier storage time and a reduction in the power dissipation with increasing dot size. It is further illustrated that electrons are advantageous as information carriers over holes and that the inclusion of high-bandgap barrier layers favours longer-duration data retention. The model is extended to include trap states in realistic QDs, whose effect is found to become more prominent with performance optimization. The computed results are in close agreement with other experimental data for different QDs along with barrier layer. This validates the efficacy of the model, which can be utilized as a design tool for fabricating nanoscale memories with better data retention capability.

Analysis of the triplet exciton transfer mechanism at the heterojunctions of organic light-emitting diodes

In this study, a model was proposed to analyze the triplet exciton transfer at the heterojunction interface and emission efficiency. The experimental results obtained for four devices with and without exciton blocking layers (EBLs) were analyzed. Although the incorporation of an EBL improved the peak external quantum efficiency (EQE) due to exciton blocking, the efficiency droop behavior became stronger, and the EQE was worse than that of the reference device without an EBL at current densities over 5 mA cm−2. To understand the physical mechanism underlying this observation, a new carrier transport simulation program was developed. This program considered a multi-Gaussian shape density of states, field-dependent mobility, exciton diffusion, triplet–triplet annihilation, triplet–polaron quenching and triplet exciton transfer at the interface. The simulation results showed that although the excitons were blocked at low current density when an EBL was used, the large exciton density generated at the interface and strong annihilation behavior resulted in a strong droop behavior. The other key factor was that the EBL limits hole injection; this resulted in stronger overflow at high current density, which reduced the efficiency and exhibited such switching behavior.

Studies on Charge Carrier Transport in an Injection Laser with Frequency Modulation of the Optical Radiation

One of the promising approaches to solve the problem of increasing the efficiency for interelement connections consists in the use of integrated optical switching systems, whose main elements represent injection lasers with functionally integrated optical radiation modulators. Injection lasers make it possible to modulate the laser radiation by subpicosecond controlling pulses at a constant pumping current, as well as making it possible to implement the sources and modulators of optical radiation in a united AIIIBV nanoheterostructure with second-type heterojunctions. This work is devoted to the studies concerning the transport of charge carriers within a functionally integrated modulator laser with internal frequency modulation of the generated optical radiation using a proposed two-dimensional diffusion-drift model and a numerical simulation technique. The results of the numerical simulation of charge carrier transport in a modulator laser when the pumping current is switched on, as well as under pulsed variation of the controlling voltage, take the structural features, the transport effects, the mechanisms of stimulated and spontaneous radiative recombination, and the photon lifetime into account. It is shown that the maximum modulation frequency of laser radiation is determined by the subpicosecond time of the controlled spatial relocation of the charge-carrier density maxima in the quantum zones of the modulator laser, as well as by the photon lifetime in the laser resonator, and corresponds to the terahertz range. To increase the maximum modulation frequency, it is necessary to reduce the photon lifetime in the active zone of the modulator laser to values lower than 3 ps by changing the resonator’s parameters in a corresponding manner. The proposed model and the method of numerical simulation make it possible to optimize the parameters of a functionally integrated modulator laser, and to provide the required relationships between the maximum modulation frequency of the optical radiation, the modulation coefficient, and the density of the threshold pumping current.

Drift-Diffusion Simulation of High-Speed Optoelectronic Devices

In this paper, we address the problem of research and development of the advanced optoelectronic devices designed for on-chip optical interconnections in integrated circuits. The development of the models, techniques, and applied software for the numerical simulation of carrier transport and accumulation in high-speed AIIIBV (A and B refer to group III and V semiconductors, respectively) optoelectronic devices is the purpose of the paper. We propose the model based on the standard drift-diffusion equations, rate equation for photons in an injection laser, and complex analytical models of carrier mobility, generation, and recombination. To solve the basic equations of the model, we developed the explicit and implicit techniques of drift-diffusion numerical simulation and applied software. These aids are suitable for the stationary and time-domain simulation of injection lasers and photodetectors with various electrophysical, constructive, and technological parameters at different control actions. We applied the model for the simulation of the lasers with functionally integrated amplitude and frequency modulators and uni-travelling-carrier photodetectors. According to the results of non-stationary simulation, it is reasonable to optimize the parameters of the lasers-modulators and develop new construction methods aimed at the improvement of photodetectors’ response time.

Development of a Physical-Topological Model for the Response of a High-Power Vertical DMOS Transistor to the Effect of Pulsed Gamma-Radiation

A method for developing models of semiconductor devices with two-dimensional nonuniform concentration profiles of donors and acceptors in working regions of a semiconductor device structure is for the first time proposed based on a complex of physical-topological modeling of charge carrier transport and process simulation of the forming processes of the device structure. The application of process simulation is due to the need to correctly define the parameters of the semiconductor device structure, which are used as initial data to calculate the electron transport according to the physical-topological model. The parameters of production processes of ion implantation, diffusion, and lithography, which were refined during process simulation, are determined for a high-power metal—oxide—semiconductor (MOS) transistor forming by the double diffusion method in accordance with known electrical characteristics and measured geometrical sizes of the structure. This results in two-dimensional distribution profiles of donors and acceptors in p–n junctions necessary to calculate the transistor breakdown under the effect of pulsed γ radiation. Breakdown processes are modeled with the help of the physical-topological model based on the Poisson and continuity equations as well as expressions for the diffusion and drift current densities in the transistor. Accounting for carriers formed at the instant of γ irradiation is implemented by the introduction of the dependence of the generation coefficient of electron–hole pairs on the radiation-dose power. The results of calculations correlate well with the experimental data, which makes it possible to give a conclusion regarding the adequacy of the proposed complex model.

First-Principles Simulations of FETs Based on Two-Dimensional InSe

We explore the performance limits of monolayer InSe ${n}$ -type and ${p}$ -type FETs by means of first-principle simulations of carrier transport in nanoscale devices. We evaluate the impact on device performance of different device parameters, such as channel length, oxide thickness, and gate underlap. Finally, we assess the operation of a 32-bit CMOS ALU, based on InSe FETs with 10-nm channel length, for both high-performance and low-power applications, and find promising figures of merit with respect to CMOS and other beyond-CMOS proposals.

Introducing correlations into carrier transport simulations of disordered materials through seeded nucleation: impact on density of states, carrier mobility, and carrier statistics

Disorder in organic semiconductors has made it challenging to achieve performance gains; this is a result of the many competing and often nuanced mechanisms effecting charge transport. In this article, we attempt to illuminate one of these mechanisms in the hopes of aiding experimentalists in exceeding current performance thresholds. Using a heuristic exponential function, energetic correlation has been added to the Gaussian disorder model (GDM). The new model is grounded in the concept that energetic correlations can arise in materials without strong dipoles or dopants, but may be a result of an incomplete crystal formation process. The proposed correlation has been used to explain the exponential tail states often observed in these materials; it is also better able to capture the carrier mobility field dependence, commonly known as the Poole–Frenkel dependence, when compared to the GDM. Investigation of simulated current transients shows that the exponential tail states do not necessitate Montroll and Scher fits. Montroll and Scher fits occur in the form of two distinct power law curves that share a common constant in their exponent; they are clearly observed as linear lines when the current transient is plotted using a log–log scale. Typically, these fits have been found appropriate for describing amorphous silicon and other disordered materials which display exponential tail states. Furthermore, we observe the proposed correlation function leads to domains of energetically similar sites separated by boundaries where the site energies exhibit stochastic deviation. These boundary sites are found to be the source of the extended exponential tail states, and are responsible for high charge visitation frequency, which may be associated with the molecular turnover number and ultimately the material stability.

Разработка физико-топологической модели реакции мощного вертикального ДМОП транзистора на воздействие импульсного гамма-излучения

AbstractA method for developing models of semiconductor devices with two-dimensional nonuniform concentration profiles of donors and acceptors in working regions of a semiconductor device structure is for the first time proposed based on a complex of physical-topological modeling of charge carrier transport and process simulation of the forming processes of the device structure. The application of process simulation is due to the need to correctly define the parameters of the semiconductor device structure, which are used as initial data to calculate the electron transport according to the physical-topological model. The parameters of production processes of ion implantation, diffusion, and lithography, which were refined during process simulation, are determined for a high-power metal—oxide—semiconductor (MOS) transistor forming by the double diffusion method in accordance with known electrical characteristics and measured geometrical sizes of the structure. This results in two-dimensional distribution profiles of donors and acceptors in p – n junctions necessary to calculate the transistor breakdown under the effect of pulsed γ radiation. Breakdown processes are modeled with the help of the physical-topological model based on the Poisson and continuity equations as well as expressions for the diffusion and drift current densities in the transistor. Accounting for carriers formed at the instant of γ irradiation is implemented by the introduction of the dependence of the generation coefficient of electron–hole pairs on the radiation-dose power. The results of calculations correlate well with the experimental data, which makes it possible to give a conclusion regarding the adequacy of the proposed complex model.

Self-consistent simulations of quantum cascade laser structures for frequency comb generation

Maxwell–Bloch equations are widely used to model the dynamics due to coherent light-matter interaction in quantum cascade laser (QCL) structures, which plays an essential role especially for the generation of frequency combs and mode-locked pulses. While the modest numerical complexity of the Maxwell–Bloch system allows for a full spatiotemporal treatment, its main disadvantage is the inclusion of dissipation by empirical dephasing rates and electron lifetimes. We present a self-consistent multi-domain approach which couples the Maxwell–Bloch equations to advanced carrier transport simulations based on a density matrix Monte Carlo technique, yielding the scattering and dephasing rates. In this way, the compact spatiotemporal modeling of the carrier-light dynamics by the Maxwell–Bloch system can be combined with the versatility and reliability of self-consistent carrier transport approaches. Simulation results are shown for a QCL-based terahertz frequency comb source, and good agreement with experiment is obtained.

Comparative Monte-Carlo simulations of charge carrier transport in amorphous molecular solids as given by three most common models of disorder: The dipolar glass, the Gaussian disorder, and their mix.

We have performed Monte-Carlo simulations of the charge carrier transport in a model molecularly doped polymer using three most popular hopping theories (the dipolar glass model, the Gaussian disorder model, and an intermediate between them) in a wide range of applied electric fields and temperatures. Time of flight transients have been computed and analyzed in logarithmic coordinates to study the Poole-Frenkel field dependence, the non-Arrhenius mobility temperature dependence, and the nondispersive versus dispersive current shapes. We also have made an attempt to estimate the total disorder energy directly from simulation data at the lowest electric field thus checking the consistency of the model fitting. Computational results have been compared with the analytical and experimental information available in the literature.

Electro-plasmonic Modal Power Shifting in Metal/Insulator/Semiconductor Structure Tailored as a CMOS-compatible Plasmonic Waveguide

In this paper, we have proposed a new npn-type design of a CMOS-compatible metal/semiconductor/insulator/metal (MSIM) plasmonic structure, to be used as a different geometry to manipulate, guide, and route surface plasmon polaritons (SPPs). Relying on the sub-wavelength diffraction-free plasmonic technology, the proposed ultra-compact structure has only a 20-nm-thick dynamic region accompanied by 1 to 2-nm thin-film HfO2 gate insulator as a distinct carrier barrier to serve both electronic and optic characteristics. The device is tailored as a mixture of MOSFET and BJT transistors in which to attain electro-plasmonic tuning goals; the npn-type structure uses rather large electron concentration densities accumulated near the oxide/semiconductor interface under 7.6-v switching voltage. This fact leads to adequate modal index variation of a doped Si, dynamic region by which routing of plasmonic traveling waves at a fiber communications wavelength of λ = 1550 nm will be possible. To investigate the optical and electronic behaviors of design, we have launched electromagnetic simulations solved on the rigorous finite element method (FEM) and also quantum mechanical (QM) included carrier transport simulations, respectively. To be reported, the estimated plasmonic modal power movement from the left-side-half-Si-core to the right-side-half-Si-core can reach to percentages of even 23.7%. This MSIM electro-plasmonic-addressed structure can be dramatically used to design specific-application routing/switching devices.