arXiv subjects

Michael Povolotskyi

Publications and source records attributed to Michael Povolotskyi.

At least 19 recordsLinked to original sources

arXiv2025

A quasi-bound band in the continuum in a photonic slab

Bound states in the continuum (BIC) are localized waves in electronic, photonic and acoustic systems, which remain decoupled from surrounding propagating waves and hence maintain their oscillation for extraordinary long time [Nat Rev Mater 1, 16048 (2016)]. In photonic crystals, symmetry-protected quasi-BICs (SP-qBIC) have been realized at high symmetry points of the Brillouin zone and utilized in photonic crystal and distributed feedback lasers. In the present work, we measure wavevector-resolved photoluminescence (PL) of monolayer WSe2 weakly coupled to a photonic slab, consisting of a square array of aluminum nanodisks. The results show that the slab supports a continuous band of symmetry-protected quasi-bound states along the Gamma-X direction, extending from the previously reported SP-qBIC at the Gamma point. The spectral width of this quasi-bound band in the continuum remains narrow through at least a half of the Brillouin zone, indicating its long lifetime.

physics.optics↗

arXiv2020

Doping profile engineered triple heterojunction TFETs with 12 nm body thickness

Triple heterojunction (THJ) TFETs have been proposed to resolve the low ON-current challenge of TFETs. However, the design space for THJ-TFETs is limited by fabrication challenges with respect to device dimensions and material interfaces. This work shows that the original THJ-TFET design with 12 nm body thickness has poor performance, because its sub-threshold swing is 50 mV/dec and the ON-current is only 6 $μA/μm$. To improve the performance, the doping profile of THJ-TFET is engineered to boost the resonant tunneling efficiency. The proposed THJ-TFET design shows a sub-threshold swing of 40 mV/dec over four orders of drain current and an ON-current of 325 uA/um with VGS = 0.3 V. Since THJ-TFETs have multiple quantum wells and material interfaces in the tunneling junction, quantum transport simulations in such devices are complicated. State-of-the-art mode-space quantum transport simulation, including the effect of thermalization and scattering, is employed in this work to optimize THJ-TFET design.

physics.app-ph↗

arXiv2020

Impact of body thickness and scattering on III-V triple heterojunction Fin-TFET modeled with atomistic mode space approximation

The triple heterojunction TFET has been originally proposed to resolve TFET's low ON-current challenge. The carrier transport in such devices is complicated due to the presence of quantum wells and strong scattering. Hence, the full band atomistic NEGF approach, including scattering, is required to model the carrier transport accurately. However, such simulations for devices with realistic dimensions are computationally unfeasible. To mitigate this issue, we have employed the empirical tight-binding mode space approximation to simulate triple heterojunction TFETs with the body thickness up to 12 nm. The triple heterojunction TFET design is optimized using the model to achieve a sub-60mV/dec transfer characteristic under realistic scattering conditions.

physics.app-ph↗

arXiv2018

Studies of two-dimensional MoS2 on enhancing the electrical performance of ultrathin copper films

Copper nanowires are widely used as on-chip interconnects due to superior conductivity. However, with aggressive Cu interconnect scaling, the diffusive surface scattering of electrons drastically increases the electrical resistivity. In this work, we studied the electrical performance of Cu thin films on different materials. By comparing the thickness dependence of Cu films resistivity on MoS2 and SiO2, we demonstrated that two-dimensional MoS2 can be used to enhance the electrical performance of ultrathin Cu films due to a partial specular surface scattering. By fitting the experimental data with the theoretical Fuchs Sondheimer model, we obtained the specularity parameter at the Cu MoS2 interface in the temperature range 2K to 300K. Furthermore, first principle calculations based on the density functional theory indicates that there are more localized states at the Cu amorphous SiO2 interface than the Cu MoS2 interface which is responsible for the higher resistivity in the Cu SiO2 heterostructure due to more severe electron scattering. Our results suggest that Cu MoS2 hybrid is a promising candidate structure for the future generations of CMOS interconnects.

physics.app-ph↗

arXiv2017

Robust Mode Space Approach for Atomistic Modeling of Realistically Large Nanowire Transistors

Atomistic quantum transport simulation of realistically large devices is computationally very demanding. The widely used mode space (MS) approach can significantly reduce the numerical cost but good MS basis is usually very hard to obtain for atomistic full-band models. In this work, a robust and parallel algorithm is developed to optimize the MS basis for atomistic nanowires. This enables tight binding non-equilibrium Green's function (NEGF) simulation of nanowire MOSFET with realistic cross section of $\rm 10nm\times10nm$ using a small computer cluster. This approach is applied to compare the performance of InGaAs and Si nanowire nMOSFETs with various channel lengths and cross sections. Simulation results with full-band accuracy indicate that InGaAs nanowire nMOSFETs have no drive current advantage over their Si counterparts for cross sections up to about $\rm 10nm\times10nm$.

cond-mat.mes-hall↗

arXiv2017

Optimization of edge state velocity in the integer quantum Hall regime

Observation of interference in the quantum Hall regime may be hampered by a small edge state velocity due to finite phase coherence time. Therefore designing two quantum point contact (QPCs) interferometers having a high edge state velocity is desirable. Here, we present a new simulation method for realistically modeling edge states near QPCs in the integer quantum Hall effect (IQHE) regime. We calculate the filling fraction in the center of the QPC and the velocity of the edge states, and predict structures with high edge state velocity. The 3D Schrödinger equation is split into 1D and 2D parts. Quasi-1D Schrödinger and Poisson equations are solved self-consistently in the IQHE regime to obtain the potential profile near the edges, and quantum transport is used to solve for the edge state wavefunctions. The velocity of edge states is found to be $\left< E \right> / B$, where $\left< E \right>$ is the expectation value of the electric field for the edge state. Anisotropically etched trench gated heterostructures with double sided delta doping have the highest edge state velocity among the structures considered.

cond-mat.mes-hall↗

arXiv2017

Grain Boundary Resistance in Copper Interconnects from an Atomistic Model to a Neural Network

Orientation effects on the resistivity of copper grain boundaries are studied systematically with two different atomistic tight binding methods. A methodology is developed to model the resistivity of grain boundaries using the Embedded Atom Model, tight binding methods and non-equilibrum Green's functions (NEGF). The methodology is validated against first principles calculations for small, ultra-thin body grain boundaries (<5nm) with 6.4% deviation in the resistivity. A statistical ensemble of 600 large, random structures with grains is studied. For structures with three grains, it is found that the distribution of resistivities is close to normal. Finally, a compact model for grain boundary resistivity is constructed based on a neural network.

cond-mat.mtrl-sci↗

arXiv2017

Control of interlayer delocalization in 2H transition metal dichalcogenides

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers - depending on the balance between spin-orbit inter- action and interlayer hopping. This balance depends on layer thickness, momentum space symmetry points and applied gate fields. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as major source for the observed photoluminesence.

cond-mat.mes-hall↗

arXiv2017

A Multiscale Modeling of Triple-Heterojunction Tunneling FETs

A high performance triple-heterojunction (3HJ) design has been previously proposed for tunneling FETs (TFETs). Compared with single heterojunction (HJ) TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission resonances that significantly improve the ON-state current ($I_{\rm{ON}}$). Coherent quantum transport simulation predicts, that $I_{\rm{ON}}=460\rm{μA/μm}$ can be achieved at gate length $Lg=15\rm{nm}$, supply voltage $V_{\rm{DD}}=0.3\rm{V}$, and OFF-state current $I_{\rm{OFF}}=1\rm{nA/μm}$. However, strong electron-phonon and electron-electron scattering in the heavily doped leads implies, that the 3HJ devices operate far from the ideal coherent limit. In this study, such scattering effects are assessed by a newly developed multiscale transport model, which combines the ballistic non-equilibrium Green's function method for the channel and the drift-diffusion scattering method for the leads. Simulation results show that the thermalizing scattering in the leads both degrades the 3HJ TFET's subthreshold swing through scattering induced leakage and reduces the turn-on current through the access resistance. Assuming bulk scattering rates and carrier mobilities, the $I_{\rm{ON}}$ is dropped from $460\rm{μA/μm}$ down to $254\rm{μA/μm}$, which is still much larger than the single HJ TFET case.

cond-mat.mes-hall↗

arXiv2017

Combination of equilibrium and non-equilibrium carrier statistics into an atomistic quantum transport model for tunneling hetero-junctions

Tunneling hetero-junctions (THJs) usually induce confined states at the regions close to the tunnel junction which significantly affect their transport properties. Accurate numerical modeling of such effects requires combining the non-equilibrium coherent quantum transport through tunnel junction, as well as the quasi-equilibrium statistics arising from the strong scattering in the induced quantum wells. In this work, a novel atomistic model is proposed to include both effects: the strong scattering in the regions around THJ and the coherent tunneling. The new model matches reasonably well with experimental measurements of Nitride THJ and provides an efficient engineering tool for performance prediction and design of THJ based devices.

cond-mat.mes-hall↗

arXiv2016

Performance degradation of superlattice MOSFETs due to scattering in the contacts

Ideal, completely coherent quantum transport calculations had predicted that superlattice MOSFETs may offer steep subthreshold swing performance below 60mV/dec to around 39mV/dec. However, the high carrier density in the superlattice source suggest that scattering may significantly degrade the ideal device performance. Such effects of electron scattering and decoherence in the contacts of superlattice MOSFETs are examined through a multiscale quantum transport model developed in NEMO5. This model couples NEGF-based quantum ballistic transport in the channel to a quantum mechanical density of states dominated reservoir, which is thermalized through strong scattering with local quasi-Fermi levels determined by drift-diffusion transport. The simulations show that scattering increases the electron transmission in the nominally forbidden minigap therefore degrading the subthreshold swing (S.S.) and the ON/OFF DC current ratio. This degradation varies with both the scattering rate and the length of the scattering dominated regions. Different superlattice MOSFET designs are explored to mitigate the effects of such deleterious scattering. Specifically, shortening the spacer region between the superlattice and the channel from 3.5 nm to 0 nm improves the simulated S.S. from 51mV/dec. to 40mV/dec. I. INTRODUCTION

cond-mat.mes-hall↗

arXiv2016

Scalable GaSb/InAs tunnel FETs with non-uniform body thickness

GaSb/InAs heterojunction tunnel field-effect transistors are strong candidates in building future low-power integrated circuits, as they could provide both steep subthreshold swing and large ON-state current ($I_{\rm{ON}}$). However, at short channel lengths they suffer from large tunneling leakage originating from the small band gap and small effective masses of the InAs channel. As proposed in this article, this problem can be significantly mitigated by reducing the channel thickness meanwhile retaining a thick source-channel tunnel junction, thus forming a design with a non-uniform body thickness. Because of the quantum confinement, the thin InAs channel offers a large band gap and large effective masses, reducing the ambipolar and source-to-drain tunneling leakage at OFF state. The thick GaSb/InAs tunnel junction, instead, offers a low tunnel barrier and small effective masses, allowing a large tunnel probability at ON state. In addition, the confinement induced band discontinuity enhances the tunnel electric field and creates a resonant state, further improving $I_{\rm{ON}}$. Atomistic quantum transport simulations show that ballistic $I_{\rm{ON}}=284$A/m is obtained at 15nm channel length, $I_{\rm{OFF}}=1\times10^{-3}$A/m, and $V_{\rm{DD}}=0.3$V. While with uniform body thickness, the largest achievable $I_{\rm{ON}}$ is only 25A/m. Simulations also indicate that this design is scalable to sub-10nm channel length.

cond-mat.mes-hall↗

arXiv2016

Transferable tight binding model for strained group IV and III-V materials and heterostructures

It is critical to capture the effect due to strain and material interface for device level transistor modeling. We introduced a transferable sp3d5s* tight binding model with nearest neighbor interactions for arbitrarily strained group IV and III-V materials. The tight binding model is parameterized with respect to Hybrid functional(HSE06) calculations for varieties of strained systems. The tight binding calculations of ultra small superlattices formed by group IV and group III-V materials show good agreement with the corresponding HSE06 calculations. The application of tight binding model to superlattices demonstrates that transferable tight binding model with nearest neighbor interactions can be obtained for group IV and III-V materials.

cond-mat.mtrl-sci↗

arXiv2016

P-Type Tunnel FETs With Triple Heterojunctions

A triple-heterojunction (3HJ) design is employed to improve p-type InAs/GaSb heterojunction (HJ) tunnel FETs. The added two HJs (AlInAsSb/InAs in the source and GaSb/AlSb in the channel) significantly shorten the tunnel distance and create two resonant states, greatly improving the ON state tunneling probability. Moreover, the source Fermi degeneracy is reduced by the increased source (AlInAsSb) density of states and the OFF state leakage is reduced by the heavier channel (AlSb) hole effective masses. Quantum ballistic transport simulations show, that with V_{DD} = 0.3V and I_{OFF} = 10^{-3}A/m, I_{ON} of 582A=m (488A=m) is obtained at 30nm (15nm) channel length, which is comparable to n-type 3HJ counterpart and significantly exceeding p-type silicon MOSFET. Simultaneously, the nonlinear turn on and delayed saturation in the output characteristics are also greatly improved.

cond-mat.mes-hall↗

arXiv2016

High-Performance Complementary III-V Tunnel FETs with Strain Engineering

Strain engineering has recently been explored to improve tunnel field-effect transistors (TFETs). Here, we report design and performance of strained ultra-thin-body (UTB) III-V TFETs by quantum transport simulations. It is found that for an InAs UTB confined in [001] orientation, uniaxial compressive strain in [100] or [110] orientation shrinks the band gap meanwhile reduces (increases) transport (transverse) effective masses. Thus it improves the ON state current of both n-type and p-type UTB InAs TFETs without lowering the source density of states. Applying the strain locally in the source region makes further improvements by suppressing the OFF state leakage. For p-type TFETs, the locally strained area can be extended into the channel to form a quantum well, giving rise to even larger ON state current that is comparable to the n-type ones. Therefore strain engineering is a promising option for improving complementary circuits based on UTB III-V TFETs.

cond-mat.mes-hall↗

arXiv2015

Quantum Transport Simulation of III-V TFETs with Reduced-Order K.P Method

III-V tunneling field-effect transistors (TFETs) offer great potentials in future low-power electronics application due to their steep subthreshold slope and large "on" current. Their 3D quantum transport study using non-equilibrium Green's function method is computationally very intensive, in particular when combined with multiband approaches such as the eight-band K.P method. To reduce the numerical cost, an efficient reduced-order method is developed in this article and applied to study homojunction InAs and heterojunction GaSb-InAs nanowire TFETs. Device performances are obtained for various channel widths, channel lengths, crystal orientations, doping densities, source pocket lengths, and strain conditions.

cond-mat.mes-hall↗

arXiv2015

NEMO5: Achieving High-end Internode Communication for Performance Projection Beyond Moore's Law

Electronic performance predictions of modern nanotransistors require nonequilibrium Green's functions including incoherent scattering on phonons as well as inclusion of random alloy disorder and surface roughness effects. The solution of all these effects is numerically extremely expensive and has to be done on the world's largest supercomputers due to the large memory requirement and the high performance demands on the communication network between the compute nodes. In this work, it is shown that NEMO5 covers all required physical effects and their combination. Furthermore, it is also shown that NEMO5's implementation of the algorithm scales very well up to about 178176CPUs with a sustained performance of about 857 TFLOPS. Therefore, NEMO5 is ready to simulate future nanotransistors.

cs.DC↗

arXiv2015

Surface Passivation in Empirical Tight Binding

Empirical Tight Binding (TB) methods are widely used in atomistic device simulations. Existing TB methods to passivate dangling bonds fall into two categories: 1) Method that explicitly includes passivation atoms is limited to passivation with atoms and small molecules only. 2) Method that implicitly incorporates passivation does not distinguish passivation atom types. This work introduces an implicit passivation method that is applicable to any passivation scenario with appropriate parameters. This method is applied to a Si quantum well and a Si ultra-thin body transistor oxidized with SiO2 in several oxidation configurations. Comparison with ab-initio results and experiments verifies the presented method. Oxidation configurations that severely hamper the transistor performance are identified. It is also shown that the commonly used implicit H atom passivation overestimates the transistor performance.

cond-mat.mtrl-sci↗