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Leonard F. Register

Publications and source records attributed to Leonard F. Register.

At least 19 recordsLinked to original sources
arXiv2024

Engineering Si-Qubit MOSFETs: A Phase-Field Modeling Approach Integrating Quantum-Electrostatics at Cryogenic Temperatures

This study employs advanced phase-field modeling to investigate Si-based qubit MOSFETs, integrating electrostatics and quantum mechanical effects. We adopt a comprehensive modeling approach, utilizing full-wave treatment of the Schrodinger equation solutions, coupled with the Poisson equation at cryogenic temperatures. Our analysis explores the influence of interface traps on quantum dot (QD) barrier heights, affecting coupling due to tunneling. A wider trap distribution leads to the decoupling of quantum dots. Furthermore, the oscillations in the transmission and reflection coefficients increase as the plunger/barrier gate length increases, reducing the coupling between the QDs. By optimizing plunger and barrier gate dimensions, spacer configurations, and gap oxide lengths, we enhance control over quantum well depths and minimize unwanted wave function leakage. The modeling algorithm is also validated against the experimental data and can accurately capture the oscillations in the Id Vgs caused by the Coulomb blockade at cryogenic temperature

quant-ph↗
arXiv2022

Band Alignment in Black Phosphorus/Transition Metal Dichalcogenide Heterolayers: Impact of Charge Redistribution, Electric Field, Strain and Layer Engineering

The objective of this work is to study the effects of charge redistribution, applied layer-normal electric fields, applied strain, and layer engineering on the band alignment of Black Phosphorus (BP)/Molybdenum disulphide (MoS2) heterostructure through Density Functional Theory (DFT) simulations. Black phosphorus works as a p-type material with high mobility, mechanical flexibility, and sensitivity to number of layers. Combining it with the more electronegative material, MoS2 results in strong carrier confinement and a Type II heterostructure. Charge redistribution among the layers shifts the band alignment expected from the Electron Affinity Rule. Applied external fields, strain and multiple BP layers provide band-alignment tunability within the Type II range and/or, transition to Type I and Type III heterostructures. The tunability in BP/MoS2 heterostructure may be useful as tunnel field effect transistors, rectifier diodes with tunable barrier height, reconfigurable FETs, and electro-optical modulators. Furthermore, considering heterostructures of monolayer BP with other monolayer Transitional Metal Dichalcogenides (TMD) suggests the ability to achieve different band alignment types. In our simulations, a Type I alignment is found with Tungsten diselenide (WSe2), Molybdenum diselenide (MoSe2), and Tungsten disulphide (WS2), and a Type III for Hafnium disulphide (HfS2) and Hafnium diselenide (HfSe2).

cond-mat.mtrl-sci↗
arXiv2021

Quantum Lifetime Spectroscopy and Magnetotunneling in Double Bilayer Graphene Heterostructures

We describe a tunneling spectroscopy technique in a double bilayer graphene heterostructure where momentum-conserving tunneling between different energy bands serves as an energy filter for the tunneling carriers, and allows a measurement of the quasi-particle state broadening at well defined energies. The broadening increases linearly with the excited state energy with respect to the Fermi level, and is weakly dependent on temperature. In-plane magnetotunneling reveals a high degree of rotational alignment between the graphene bilayers, and an absence of momentum randomizing processes.

cond-mat.mes-hall↗
arXiv2020

Large spin Hall effect in 5d-transition metal anti-perovskites

The spin Hall effect (SHE) is highly promising for spintronic applications, and the design of materials with large SHE can enable ultra-low power memory technology. Recently, 5d-transition metal oxides have been shown to demonstrate a large SHE. Here we report large values of SHE in four 5d-transition metal anti-perovskites which makes these anti-perovskites promising spintronic materials. We demonstrate that these effects originate in the mixing of dx2-y2 and dxy orbitals caused by spin orbit coupling.

cond-mat.mtrl-sci↗
arXiv2019

Semi-Classical Monte Carlo Simulation of Contact Geometry, Orientation, and Ideality on Nano-scale Si and III-V n-channel FinFETs in the Quasi-Ballistic Limit

The effects of contact geometry and ideality on InGaAs and Si nano-scale n-channel FinFET performance are studied using a quantum-corrected semi-classical Monte Carlo method. Illustrative end, saddle/slot, and raised source/drain contacts were modeled, and with ideal transmissivity and reduced transmissivity more consistent with experimental contact resistivities. Far-from-equilibrium degenerate statistics, quantum-confinement effects on carrier distributions in real-space and among energy valleys, quasi-ballistic transport inaccessible through drift-diffusion and hydrodynamic simulations, and scattering mechanisms and contact geometries not readily accessible through non-equilibrium Green's function simulation are addressed. Silicon $\langle \hbox{110} \rangle$ channel devices, Si $\langle \hbox{100} \rangle$ channel devices, multi-valley (MV) InGaAs devices with conventionally-reported energy valley offsets, and idealized $Γ$-valley only $\left( Γ\right)$ InGaAs devices are modeled. Simulated silicon devices exhibited relatively limited degradation in performance due to non-ideal contact transmissivities, more limited sensitivity to contact geometry with non-ideal contact transmissivities, and some contact-related advantage for Si $\langle \hbox{110} \rangle$ channel devices. In contrast, simulated InGaAs devices were highly sensitive to contact geometry and ideality and the peripheral valley's energy offset. It is illustrative of this latter sensitivity that simulated $Γ$-InGaAs device outperformed all others by a factor of two or more in terms of peak transconductance with perfectly transmitting reference end contacts, while silicon devices outperformed $Γ$-InGaAs for all contact geometries with non-ideal transmissivities, and MV-InGaAs devices performed the poorest under all simulation scenarios.

cond-mat.mes-hall↗
arXiv2018

Spin-Conserving Resonant Tunneling in Twist-Controlled WSe2-hBN-WSe2 Heterostructures

We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe2) monolayers with controlled rotational alignment, and separated by hexagonal boron nitride. In samples where the two WSe2 monolayers are rotationally aligned we observe resonant tunneling, manifested by a large conductance and negative differential resistance in the vicinity of zero interlayer bias, which stem from energy- and momentum-conserving tunneling. Because the spin-orbit coupling leads to coupled spin-valley degrees of freedom, the twist between the two WSe2 monolayers allows us to probe the conservation of spin-valley degree of freedom in tunneling. In heterostructures where the two WSe2 monolayers have a 180° relative twist, such that the Brillouin zone of one layer is aligned with the time-reversed Brillouin zone of the opposite layer, the resonant tunneling between the layers is suppressed. These findings provide evidence that in addition to momentum, the spin-valley degree of freedom is also conserved in vertical transport.

cond-mat.mes-hall↗
arXiv2018

Strongly enhanced tunneling at total charge neutrality in double bilayer graphene-WSe$_2$ heterostructures

We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe$_2$ barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics well, and suggests the emergence of a many-body state with condensed interbilayer excitons when electrons and holes of equal densities populate the two layers.

cond-mat.mes-hall↗
arXiv2017

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides. Here, the phenomenon of oxygen impurities trapping photo-excited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for the carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in transition metal dichalcogenides, and facilitates defect engineering in relevant material and device applications.

cond-mat.mtrl-sci↗
arXiv2017

Coherent Interlayer Tunneling and Negative Differential Resistance with High Current Density in Double Bilayer Graphene-WSe2 Heterostructures

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 uA/um2 and 2.5 uA/um2, and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 K and 300 K, respectively.

cond-mat.mes-hall↗
arXiv2017

Detection of current induced spin polarization in epitaxial Bi$_2$Te$_3$ thin film

We electrically detect charge current induced spin polarization on the surface of molecular beam epitaxy grown Bi$_2$Te$_3$ thin film in a two-terminal device with a ferromagnetic MgO/Fe and a nonmagnetic Ti/Au contact. The two-point resistance, measured in an applied magnetic field, shows a hysteresis tracking the magnetization of the Fe. A theoretical estimate is obtained for the change in resistance on reversing the magnetization direction of Fe from coupled spin-charge transport equations based on quantum kinetic theory. The order of magnitude and the sign of the hysteresis is consistent with spin-polarized surface state of Bi$_2$Te$_3$.

cond-mat.mes-hall↗
arXiv2016

DFT Simulations of Inter-Graphene-Layer Coupling with Rotationally Misaligned hBN Tunnel Barriers in Graphene/hBN/Graphene Tunnel FETs

Van der Waal's heterostrucutures allow for novel devices such as two-dimensional-to-two-dimensional tunnel devices, exemplified by interlayer tunnel FETs. These devices employ channel/tunnel-barrier/channel geometries. However, during layer-by-layer exfoliation of these multi-layer materials, rotational misalignment is the norm and may substantially affect device characteristics. In this work, by using density functional theory methods, we consider a reduction in tunneling due to weakened coupling across the rotationally misaligned interface between the channel layers and the tunnel barrier. As a prototypical system, we simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a graphene/hBN/graphene system. We find that rotational misalignment between the channel layers and the tunnel barrier in this van der Waal's heterostructure can significantly reduce coupling between the channels by reducing, specifically, coupling across the interface between the channels and the tunnel barrier. This weakened coupling in graphene/hBN/graphene with hBN misalignment may be relevant to all such van der Waal's heterostructures.

cond-mat.mes-hall↗
arXiv2016

Theoretical and experimental investigation of vacancy-based doping of monolayer MoS$_2$ on oxide

Monolayer transition metal dichalcogenides are novel, gapped two-dimensional materials. Toward device applications, we consider MoS$_2$ layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional based simulations, we consider the effects of near-interface O vacancies in the oxide slab, and Mo or S vacancies in the MoS$_2$ layer. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS$_2$-dielectrics system and for isolated dielectric layers for reference. Among our results, we find that with O vacancies, both the Hf-terminated HfO$_2$-MoS$_2$ system, and the O-terminated and H-passivated Al$_2$O$_3$-MoS$_2$ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS$_2$, in manner analogous to modulation doping. The n-type doping of monolayer MoS$_2$ by high-k oxides with oxygen vacancies then is experimentally demonstrated by electrically and spectroscopically characterizing back-gated monolayer MoS$_2$ field effect transistors encapsulated by oxygen deficient alumina and hafnia.

cond-mat.mes-hall↗
arXiv2016

Air Stable Doping and Intrinsic Mobility Enhancement in Monolayer $MoS_{2}$ by Amorphous $TiO_{x}$ Encapsulation

To reduce Schottky-barrier-induced contact and access resistance, and the impact of charged impurity and phonon scattering on mobility in devices based on 2D transition metal dichalcogenides (TMDs), considerable effort has been put into exploring various doping techniques and dielectric engineering using $high-κ$ oxides, respectively. The goal of this work is to demonstrate a $high-κ$ dielectric that serves as an effective n-type charge transfer dopant on monolayer (ML) molybdenum disulfide ($MoS_{2}$). Utilizing amorphous titanium suboxide (ATO) as the '$high-κ$ dopant', we achieved a contact resistance of ~ $180$ $Ω.μm$ which is the lowest reported value for ML $MoS_{2}$. An ON current as high as $240$ $μA/μm$ and field effect mobility as high as $83$ $cm^2/V-s$ were realized using this doping technique. Moreover, intrinsic mobility as high as $102$ $cm^2/V-s$ at $300$ $K$ and $501$ $cm^2/V-s$ at $77$ $K$ were achieved after ATO encapsulation which are among the highest mobility values reported on ML $MoS_{2}$. We also analyzed the doping effect of ATO films on ML $MoS_{2}$, a phenomenon which is absent when stoichiometric $TiO_{2}$ is used, using ab initio density functional theory (DFT) calculations which shows excellent agreement with our experimental findings. Based on the interfacial-oxygen-vacancy mediated doping as seen in the case of $high-κ$ ATO - ML $MoS_{2}$, we propose a mechanism for the mobility enhancement effect observed in TMD-based devices after encapsulation in a $high-κ$ dielectric environment.

cond-mat.mes-hall↗
arXiv2016

Ensemble Monte Carlo for III-V and Si n-channel FinFETs considering non-equilibrium degenerate statistics and quantum-confined scattering

Particle-based ensemble semi-classical Monte Carlo (MC) methods employ quantum corrections (QCs) to address quantum confinement and degenerate carrier populations to model tomorrow's ultra-scaled MOSFETs. Here we present new approaches to quantum confinement and carrier degeneracy effects in a three-dimensional (3D) MC device simulator, and illustrate their significance through simulation of n-channel Si and III-V FinFETs. Original contributions include our treatment of far-from-equilibrium degenerate statistics and QC-based modeling of surface-roughness scattering, as well as considering quantum-confined phonon and impurity scattering in 3D. Typical MC simulations approximate degenerate carrier populations as Fermi distributions to model the Pauli-blocking (PB) of scattering to occupied final states. To allow for increasingly far-from-equilibrium non-Fermi carrier distributions in ultra-scaled devices, we instead generate the final-state occupation probabilities used for PB by sampling the local carrier populations as a function of energy and energy valley. This process is aided by the use of fractional carriers or sub-carriers, which minimizes classical carrier-carrier scattering. Quantum confinement effects are addressed through quantum-correction potentials (QCPs) generated from Schrödinger-Poisson solvers, as commonly done. However, we use our valley- and orientation-dependent QCPs not just to redistribute carriers in real space, or even among energy valleys, but also to calculate confinement-dependent phonon, impurity, and surface-roughness scattering rates. FinFET simulations are used to illustrate how, collectively, these quantum effects can substantially reduce and even eliminate otherwise expected benefits of In$_{\text{0.53}}$Ga$_{\text{0.47}}$As FinFETs over otherwise identical Si FinFETs, despite higher thermal velocities in In$_{\text{0.53}}$Ga$_{\text{0.47}}$As.

cond-mat.mes-hall↗
arXiv2016

Write error rate of spin-transfer-torque random access memory including micromagnetic effects using rare event enhancement

Spin-transfer-torque random access memory (STT-RAM) is a promising candidate for the next-generation of random-access-memory due to improved scalability, read-write speeds and endurance. However, the write pulse duration must be long enough to ensure a low write error rate (WER), the probability that a bit will remain unswitched after the write pulse is turned off, in the presence of stochastic thermal effects. WERs on the scale of 10$^{-9}$ or lower are desired. Within a macrospin approximation, WERs can be calculated analytically using the Fokker-Planck method to this point and beyond. However, dynamic micromagnetic effects within the bit can affect and lead to faster switching. Such micromagnetic effects can be addressed via numerical solution of the stochastic Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, determining WERs approaching 10$^{-9}$ would require well over 10$^{9}$ such independent simulations, which is infeasible. In this work, we explore calculation of WER using "rare event enhancement" (REE), an approach that has been used for Monte Carlo simulation of other systems where rare events nevertheless remain important. Using a prototype REE approach tailored to the STT-RAM switching physics, we demonstrate reliable calculation of a WER to 10$^{-9}$ with sets of only approximately 10$^{3}$ ongoing stochastic LLGS simulations, and the apparent ability to go further.

physics.data-an↗
arXiv2016

Voltage-Controlled Low-Energy Switching of Nanomagnets through Ruderman-Kittel-Kasuya-Yosida Interactions for Magnetoelectric Device Applications

In this letter, we consider through simulation Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between nanomagnets sitting on a conductive surface, and voltage-controlled gating thereof for low-energy switching of nanomagnets for possible memory and nonvolatile logic applications. For specificity, we consider nanomagnets with perpendicular anisotropy on a three-dimensional topological insulator. We model the possibility and dynamics of RKKY-based switching of one nanomagnet by coupling to one or more nanomagnets of set orientation. Applications for both memory and nonvolatile logic are considered, with follower, inverter and majority gate functionality shown. Sub-attojoule switching energies, far below conventional spin transfer torque (STT)-based memories and even below CMOS logic appear possible. Switching times on the order of a few nanoseconds, comparable to times for STT switching, are estimated for ferromagnetic nanomagnets.

cond-mat.mes-hall↗
arXiv2016

Voltage Controlled Magnetic Anisotropy Based Low Energy Switching of a Ferromagnet on a Topological Insulator

We present a novel memory device that consists of a thin ferromagnetic layer of Fe deposited on topological insulator thin film, Bi2Se3. The ferromagnetic layer has perpendicular anisotropy, due to MgO deposited on the top surface of Fe. When current is passed on the surface of Bi2Se3, the surface of the Bi2Se3 becomes spin polarized and strong exchange interaction occurs between the d electrons in the ferromagnet and the electrons conducting the current on the surface of the Bi2Se3. Part of the current is shunted through the ferromagnet which generates spin transfer torque in the ferromagnet. The combination of the spin transfer torque and exchange interaction torque along with voltage-controlled magnetic anisotropy (VCMA) allows ultralow-energy switching of the ferromagnet. We perform micromagnetic simulations and predict switching time of the order of 2.5 ns and switching energy of the order of 0.45fJ for a ferromagnetic bit with thermal stability of 43kBT. Such ultralow-energy and high-speed VCMA-induced switching of a perpendicular anisotropy ferromagnet on a topological insulator could be utilized for energy-efficient memory design.

cond-mat.mes-hall↗