Semiclassical Transport Research Articles

Signature of pressure-induced topological phase transition in ZrTe5

The layered van der Waals material ZrTe5 is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe5 nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe5 is a weak TI in ambient conditions.

Band gap engineering of novel double perovskites Na2LiYX6 (X=Cl, Br) through halide ion replacement for solar cell and energy harvesting applications

Using the Perdew − Burke − Ernzerhof-generalized-gradient approximation (PBE-GGA) and PBE+SOC exchange − correlation functional, this work investigates the physical properties and energy storage capacity of novel double perovskite Halides Na2LiYX6 (X=Cl and Br) through Density Functional Theory (DFT) and semiclassical transport theory under ideal conditions. Thermal stability has been verified through the computation of thermodynamic parameters and formation energies. Brittle and anisotropic nature has been confirmed by the calculation of elastic and mechanical parameters. With PBE+SOC, the band gaps (Eg) for Na2LiYCl6 and Na2LiYBr6 are 2.90 and 3.76 eV, respectively, whereas with PBE-GGA, they are 3.061 and 3.824 eV, respectively. The analysis of the computed optical characteristics determines whether double perovskite halides are suitable for use in photovoltaic applications. The calculations for the transport properties were carried out using the BoltzTraP code. The effectiveness and transport mechanism for thermal energy conversion have been studied by determining the thermoelectric properties. To assess the stability at high temperatures and suitability for industrial applications, thermal properties such as heat capacity, entropy, Debye temperature, and thermal expansion are investigated. Overall, evaluations of the compounds’ thermoelectric characteristics show that they have a high-power factor and remarkably low thermal conductivity, which makes them ideal for energy harvesting devices.

Lorentz-violating extension of Wigner function formalism and chiral kinetic theory

The quantum kinetic equation for the gauge-invariant Wigner function, constructed from spinor fields that obey the Dirac equation modified by CPT and Lorentz symmetry-violating terms, is presented. The equations for the components of the Wigner function in the Clifford algebra basis are accomplished. Focusing on the massless case, an extended semiclassical chiral kinetic theory in the presence of external electromagnetic fields is developed. We calculate the chiral currents and establish the anomalous magnetic and separation effects in a Lorentz-violating background. The chiral anomaly within the context of extended quantum electrodynamics is elucidated. Finally, we derive the semiclassical Lorentz-violating extended chiral transport equation. Published by the American Physical Society 2024

Analytical modeling of drain current for DG-Graphene Nanoribbon Vertical Tunnel FET

The semi-classical current transport model is used in this study to examine the drain current model for Double-Gate (DG) Dual-Material-Gate (DMG) Graphene-Nanoribbon (GNR) Vertical TFETs. It takes into account the contact potential (VGS, VDG), the impact of the oxide thickness (tOX), and the carrier mobility (μ). The device’s channel length is 50 nm, and the energy bandgap is 0.26 eV when the GNR ribbon width is 10 nm. The bandgap varies with the GNR ribbon width. The data from the simulation of TCAD tools are compared to an analytical result of the drain current. The current transport model shows good agreement. It is clear that a GNR with a 5 nm width can switch ON and OFF at 0.5 V drain voltage, which shows low power bias operation. The suggested GNR-TFET is capable of producing an average subthreshold current of 16 mV/Dec, 1.74 × 10−4 A/μm on-state current, and 1.66 × 10−18 A/μm off-state current. The exceptional performance is made possible by the narrow bandgap, high mobility, and two-dimensional (2-D) nature of GNR.

Effect of doping on domain suppression in the presence of hot electrons in semiconducting achiral carbon nanotubes

Effect of doping semiconducting achiral carbon nanotubes on domain suppression prerequisite for potential generation of terahertz radiations in the presence of hot electrons has been theoretically considered. This was done by solving the semiclassical Boltzmann's transport equation in the presence of hot electrons source to derive the current density as a function of constant electric field and chemical potential of semiconducting achiral carbon nanotubes (CNTs). Plots of normalized current density versus electric field strength for semiconducting achiral CNTs as the chemical potential (μ) representing the degree of doping increases from 1.4 eV to 2.0 eV (i.e. heavily doped CNT) revealed an increase in positive differential conductivity (PDC) that results in enhancement of domain suppression prerequisite for generation of terahertz radiations. Also, it has been observed that PDC which is associated with domain suppression increases as the relative thickness and carrier temperature of semiconducting CNTs increase. Therefore, the study predicts heavily doped larger radius semiconducting achiral CNT at a higher carrier temperature as a better candidate for generation of terahertz radiations whose applications are relevant in current-day technology, industry, and research.

A Simulator for Investigation of Breakdown Characteristics of SiC MOSFETs

Breakdown characteristics play an important role in silicon carbide (SiC) power devices; however, the wide bandgap of SiC poses a challenge for numerical simulation of breakdown characteristics. In this work, a self-developed simulator employing a novel numerical processing method to prevent convergence issues, based on semi-classical transport models and including several kinds of mobility, generation and recombination models, is used to investigate the performance and breakdown characteristics of 4H-SiC MOSFETs in high-power applications. Good agreement between our simulator and an experiment and commercial TCAD was achieved. The simulator has good stability and convergence and can be used as a powerful tool to design and optimize semiconductor devices. Further, the breakdown characteristics are evaluated with different factors, including lattice temperature, device structure and doping profiles. Our results show that the doping profile plays the most important role in the breakdown voltage, followed by the device structure, while the impact of lattice temperature is found to be minimal.

(Invited) Simulations of Ultra-Scaled Electronic Devices with a Novel Flexible Nano-TCAD Nano-Electronic Simulation Software (NESS) Environment

Simulation of conventional and emerging electronic devices using Technology Computer Aided Design (TCAD) tools has been an essential part of the semiconductor industry as well as academic research. Computational efficiency and accuracy of the numerical modeling are the key criteria on which quality and utility of a TCAD tool is ascertained. Further, the ability of the tool to incorporate different modeling paradigms and to be applicable to a wide range of device architectures and operating conditions is essential.In this talk, I will provide an overview of the new device simulator NESS (Nano-Electronic Software Simulator) developed at Device Modelling Group at University of Glasgow. NESS is a fast, modular software with its own structure and mesh generators, and contains different modules for classical, semi-classical, and quantum transport solvers, mobility calculation, kinetic Monte-Carlo etc. NESS can simulate numerous device architectures such as nanowire and bulk transistors, tunnelling field effect transistors (TFET) and resonant tunnelling diodes. Moreover, NESS accounts for various sources of statistical variability in nano-size electronic devices and it can perform statistical simulations for thousands microscopically different devices.

Epitaxial Kagome Thin Films as a Platform for Topological Flat Bands.

Systems with flat bands are ideal for studying strongly correlated electronic states and related phenomena. Among them, kagome-structured metals such as CoSn have been recognized as promising candidates due to the proximity between the flat bands and the Fermi level. A key next step will be to realize epitaxial kagome thin films with flat bands to enable tuning of the flat bands across the Fermi level via electrostatic gating or strain. Here, we report the band structures of epitaxial CoSn thin films grown directly on the insulating substrates. Flat bands are observed by using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). The band structure is consistent with density functional theory (DFT) calculations, and the transport properties are quantitatively explained by the band structure and semiclassical transport theory. Our work paves the way to realize flat band-induced phenomena through fine-tuning of flat bands in kagome materials.

Semiclassical electron and phonon transport from first principles: application to layered thermoelectrics

Semiclassical electron and phonon transport from first principles: application to layered thermoelectrics

Enhanced thermoelectric properties of SWCNT by new nitrogen chains doping

Single walled carbon nanotubes (SWCNT) are attractive materials for thermoelectric (TE) devices because of their unique one-dimensional structure and provide us for developing a flexible TE devices due to their mechanical properties. In this paper, the TE properties of pristine small SWCNT (3,3) and three new nitrogen (N) doping models namely CN, C2N2 (doping by two N-chains) and C4N2 (doping by three N-chains) are calculated using the density functional theory (DFT) combined with Boltzmann's semi-classical transport theory. Dynamical and energetic stability of the compounds are confirmed from the phonon density of states curve and formation energy calculations. The values of Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), power factor (PF) and figure of merit (ZT) are reported at different temperature and discussed in detail. The results show that N-doping significantly improves the TE properties of the pristine nanotube and show a reasonable agreement with previous experiments and ab-initio calculations. It was found that the C2N2 configuration presents the largest Seebeck coefficient of 70.53 μV/K and the figure of merit ZT of 0.25 compared to other doping models. N-chains doping is therefore a new suitable way to improve the TE properties of small SWCNT in order to making them a good candidate for flexible TE devices.

Using ensemble Monte Carlo methods to evaluate non-equilibrium Green’s functions, II. Polar-optical phonons

In semi-classical transport, it has become common practice over the past few decades to use ensemble Monte Carlo methods for the simulation of transport in semiconductor devices. This method utilizes particles while still addressing the full physics within the device, leaving the computational difficulties to the computer. More recently, the study of quantum mechanical effects within the devices, have become important, and have been addressed in semiconductor devices using non-equilibrium Green’s functions (NEGF). In using NEGF, one faces considerable computational difficulties. Recently, a particle approach to NEGF has been suggested and preliminary results presented for non-polar optical phonons in Si, which are very localized scattering centers. Here, the problems with long-range polar-optical phonons are discussed and results of the particle-based simulation are used to examine quantum transport in InN at 300 K.

Tensile strain as an efficient way to tune transport properties of Graphdiyne/Borophene hetero-bilayers; a first principle investigation

Thermoelectric materials have attracted much attention due to their crucial role in heat conversion and cooling applications. Transport properties effectively conduct thermoelectricity efficiency, and therefore, many studies have particularly focused on enhancing these features. In the present study, through periodic density functional theory combined with semi-classical transport formulations, the influence of strain on thermoelectric characteristics of Graphdiyne/Borophene hetero-bilayers has been investigated. Based on mechanical properties data, we have shown that bilayer construction causes an increase in the ultimate stress of constituent monolayers. The maximum strain that each hetero-bilayer can withstand is determined by using stress-strain behavior. Among the studied hetero-bilayers, GBS0 has the most value of Young’s modulus [1,2] under equibiaxial strain. Temperature-dependent transport properties of strained hetero-bilayers suggest strain as an efficient way to tune the thermoelectric behavior of hetero-bilayers. The introduction of uni- and biaxial strains affects the electrical and thermal conductivity of GBS0 and GBS1. Furthermore, the power factor and figure of merit, as the key elements of thermoelectric devices, can be significantly engineered under external tensions. The impact of tensile strain on the Seebeck effect is disclosed. Surprisingly, the studied hetero-bilayers can possess either positive or negative Seebeck coefficients under different modes of external strains. The present research extends the strain field, particularly in the rational design of thermoelectric devices.

Revealing the band structure of ZrTe5 using multicarrier transport

The layered material ZrTe$_5$ appears to exhibit several exotic behaviors which resulted in significant interest recently, although the exact properties are still highly debated. Among these we find a Dirac/Weyl semimetallic behavior, nontrivial spin textures revealed by low temperature transport, and a potential weak or strong topological phase. The anomalous behavior of resistivity has been recently elucidated as originating from band shifting in the electronic structure. Our work examines magnetotransport behavior in ZrTe$_5$ samples in the context of multicarrier transport. The results, in conjunction with ab-initio band structure calculations, indicate that many of the transport features of ZrTe$_5$ across the majority of the temperature range can be adequately explained by the semiclassical multicarrier transport model originating from a complex Fermi surface.

Modeling and analyzing near-junction thermal transport in high-heat-flux GaN devices heterogeneously integrated with diamond

Gallium nitride (GaN) electronic devices possess excellent electrical characteristics that make them promising for advanced power and radiofrequency device applications. However, elevated channel temperatures associated with localized Joule heating degrade the device reliability and limit the device performance. To address this, recent intensive efforts have heterogeneously integrated diamond, the best thermal conductor among three-dimensional materials, near the device junction and have fabricated GaN-on-diamond devices. Numerous efforts have also been made to investigate their thermal characteristics via finite element simulations. The majority of these efforts have adopted several simplifying assumptions to facilitate thermal analysis and reduce computational loads. However, few have examined the impact of these assumptions on the device junction temperature. Here, we evaluate this impact by constructing a three-dimensional near-junction thermal transport model for GaN-on-diamond devices and calculating the temperature- and thickness-dependent GaN thermal conductivity utilizing semiclassical phonon transport theory. We further explore the influence of several near-junction thermal design parameters, including GaN layer and diamond substrate thicknesses, GaN/diamond thermal boundary resistance, and gate pitch and width, on the device junction temperature. Overall, this work may provide detailed guidelines and best practices for modeling and analyzing near-junction thermal transport in high-heat-flux GaN devices heterogeneously integrated with diamond.

Thermoelectric properties of C2P4 monolayer: A first principle study

We have theoretically explored the electronic and thermoelectric properties of the C2P4 monolayer with the interface of density functional theory and semi-classical transport theory. Our calculation shows a high Seebeck coefficient and low electronic thermal conductivity in the vicinity of zero chemical potential (μ = 0), resulting in a good power factor (PF) and a high figure of merit (ZT). More particularly, the electronic figure of merit (ZTe) exhibits two high peak values around μ = 0 due to the significant contribution of thermoelectric parameters. Furthermore, ZTe decreases by increasing the temperature, giving a peak value of 0.98 in the negative chemical potential (μ), whereas, for μ > 0, the peak value increases slightly with temperature. Additionally, the ZTe peak value is robust against ±10% of uni- and biaxial strains at room temperature. To make our calculation more realistic, we add phonon contributions to the thermal conductivity in pristine C2P4 and calculate the total ZT. We have found that phonon contribution dominates at low temperatures, and the ZT peak is reduced to 0.78. These optimal thermoelectric parameters of the C2P4 monolayer may be suitable for demonstrating the feasibility of a good thermoelectric material.

Unexpected nontriviality of inter-band contribution and huge anisotropy of conductivity in tilted Weyl semimetals

For tilted type-I Weyl semimetals, our theoretical study indicates that the Kubo conductivity parallel to cone tilt direction increases rapidly with the Fermi energy. Hence it deviates from its Boltzmann counterpart sizably even though the electronic wavelength is so short that the quantum interference is seemingly trivial. As a result, such a system shows huge anisotropy of the conductivity. By means of the diagrammatic expansion technique, we find that the absence of backscattering and the breakdown of the time reversal symmetry in tilted Weyl semimetals are two crucial factors which bring about the nontrivial inter-band contribution to the Kubo conductivity. Our findings indicate that Boltzmann theory fails to describe reasonably the electronic transport of a tilted Weyl cone, even in the case of weak scattering and short Fermi wavelength. Though in this regime such a semiclassical transport theory is believed to be valid according to conventional arguments. This situation should be paid attention. • Via SCBA, the high order quantum corrections to conductivity are considered and it makes huge enhancement to diagonal conductivity σ z z , which is missed by the Boltzmann transport equation with the lowest order Born approximation. • The huge anisotropy of diagonal conductivity in tilted WSMs origin from the nature of the absence of backscattering and breakdown of TRS. • The interaction range of impurity potential affects the extent of anisotropy.

Hydrodynamic charge transport in an GaAs/AlGaAs ultrahigh-mobility two-dimensional electron gas

Viscous fluid in an ultrahigh-mobility two-dimensional electron gas (2DEG) in GaAs/AlGaAs quantum wells is systematically studied through measurements of negative magnetoresistance (NMR) and photoresistance under microwave radiation, and the data are analyzed according to recent theoretical work [P. S. Alekseev, Phys. Rev. Lett. 117, 166601 (2016)]. Size-dependent and temperature dependent NMR are found to conform to the theoretical predictions. The size dependence of microwave induced resistance oscillations and that of the second harmonic peak indicate that 2DEG in a moderate magnetic field should be regarded as viscous fluid as well. Size-dependent radiation heating effect is found by using NMR as electron thermometry. Our results suggest that the hydrodynamic effects must be considered in order to understand semiclassical electronic transport in a clean 2DEG.

Semiclassical spin transport in LaO/STO system in the presence of multiple Rashba spin-orbit couplings

The interaction between linear and cubic spin–orbit couplings with magnetic moments and mobile spin-polarized carriers in the LaAlO3/SrTiO3 (LaO/STO) system provides new avenues for spin transport applications. We study the interplay between linear and cubic Rashba spin orbit coupling (RSOC) on in-plane magnetic moments in the LaO/STO system using the Boltzmann transport theory based on the relaxation time approximation (RTA) and the more refined Schliemann-Loss (SL) delta-potential scattering model. In general, both methods yield a linear (quadratic) relationship in the spin accumulation (spin current) when one of the three RSOC strengths is varied and the other two fixed. The simultaneous presence of multiple types of RSOC with distinct angular dependencies facilitates the breaking of the k-space symmetry of the Fermi surface, thus ensuring a finite spin accumulation upon integration over the entire Fermi surface. While the oft-used RTA method is sufficiently accurate for spin accumulation calculations, the more refined SL model is required for spin current calculations because the RTA method neglects the anisotropy of the Fermi contour arising from the cubic RSOC terms. Based on the refined SL model and under optimal tuning of the RSOC parameters, the spin charge conversion in LaO/STO is predicted to reach a remarkable efficiency of 30%.

Hot and cold phonons in electrically biased graphene

Based on first-principles calculations and semiclassical transport theory, we study the nonequilibrium phonon distribution in graphene in the presence of electrical current. In addition to the hot-phonon effect, we observe a cooling of phonon modes at the same time. Interestingly, the presence of electric current along the direction connecting $K$ and ${K}^{\ensuremath{'}}$ valleys induces an opposite dipolelike temperature distribution in the two valleys and at the $\mathrm{\ensuremath{\Gamma}}$ point. This leads to a ``high-order'' valley polarization of phonon distribution between $K$ and ${K}^{\ensuremath{'}}$. Based on the nonequilibrium phonon distribution, we furthermore study their effect on the lattice parameters and find a negative current-induced expansion coefficient. Similar to graphene's negative thermal expansion, this is rooted in the dominant contribution of out-of-plane acoustic phonons, which do not couple directly to electrons, but gain energy from in-plane phonons through anharmonic scattering.

Nonlinear anomalous Hall effects probe topological phase-transitions in twisted double bilayer graphene

Nonlinear anomalous (NLA) Hall effect is the Berry curvature dipole induced second-order Hall voltage or temperature difference induced by a longitudinal electric field or temperature gradient. These are the prominent Hall responses in time-reversal symmetric systems. These band-geometry induced responses in recently realized twistronic platforms can probe their novel electronic band structure and topology. Here, we investigate the family (electrical, thermoelectric, and thermal) of second-order NLA Hall effects in the moiré system of twisted double bilayer graphene (TDBG). We combine the semiclassical transport framework with the continuum model of TDBG to demonstrate that the NLA Hall signals can probe topological phase transitions in moiré systems. We show that the whole family of NLA Hall responses undergo a sign reversal across a topological phase transition. Our study establishes a deeper connection between valley topology and nonlinear Hall effects in time-reversal symmetric systems.