arXiv subjects

Leonard F. Register

Publications and source records attributed to Leonard F. Register.

30 records · Page 2Linked to original sources

arXiv2015

Quantum transport simulation of exciton condensate transport physics in a double layer graphene system

Spatially indirect electron-hole exciton condensates stabilized by interlayer Fock exchange interactions have been predicted in systems containing a pair of two-dimensional semiconductor or semimetal layers separated by a thin tunnel dielectric. The layer degree of freedom in these systems can be described as a pseudospin. Condensation is then analogous to ferromagnetism, and the interplay between collective and quasiparticle contributions to transport is analogous to phenomena that are heavily studied in spintronics. These phenomena are the basis for pseudospintronic device proposals based on possible low-voltage switching between high (nearly shorted) and low interlayer conductance states and on near perfect Coulomb drag-counterflow current along the layers. In this work, a quantum transport simulator incorporating a non-local Fock exchange interaction is presented, and used to model the essential transport physics in the bilayer graphene system. Finite size effects, Coulomb drag-counterflow current, critical interlayer currents beyond which interlayer DC conductance collapses at sub-thermal voltages, non-local coupling between interlayer critical currents in multiple lead devices, and an Andreev-like reflection process are illustrated.

cond-mat.mes-hall↗

arXiv2014

Gate-Tunable Resonant Tunneling in Double Bilayer Graphene Heterostructures

We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron-nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

cond-mat.mes-hall↗

arXiv2014

Atomistic simulation of the electronic states of adatoms in monolayer MoS2

Using an ab initio density functional theory (DFT) based electronic structure method, we study the effects of adatoms on the electronic properties of monolayer transition metal dichalcogenide (TMD) Molybdenum-disulfide (MoS2). We consider the 1st (Li, Na, K) and 7th (F, Cl, Br) column atoms and metals (Sc, Ti, Ta, Mo, Pd, Pt, Ag, Au). Three high symmetry sites for the adatom on the surface of monolayer MoS2 are examined as starting points to search for the most energetically stable configuration for each adatom-monolayer MoS2 system, as well as the type of associated bonding. For the most stable adatom positions, we characterize the emergence of adatom-induced electronic states including any dopant states.

cond-mat.mes-hall↗

arXiv2014

Ballistic performance comparison of monolayer transition metal dichalcogenide MX2 (M = Mo, W; X = S, Se, Te) MOSFETs

We study the transport properties of monolayer MX2 (M = Mo, W; X = S, Se, Te) n- and p- channel metal-oxide-semiconductor field effect transistors (MOSFETs) using full-band ballistic non-equilibrium Green's function simulations with an atomistic tight-binding Hamiltonian with hopping potentials obtained from density functional theory. We discuss the subthreshold slope, drain-induced barrier lowering (DIBL), as well as gate-induced drain leakage (GIDL) for different monolayer MX2 MOSFETs. We also report the possibility of negative differential resistance behavior in the output characteristics of nanoscale monolayer MX2 MOSFETs.

cond-mat.mes-hall↗

arXiv2013

Atomistic Full-Band Simulations of Monolayer MoS2 Transistors

We study the transport properties of deeply scaled monolayer MoS2 n-channel metal-oxide-semiconductor field effect transistors (MOSFETs) using full-band ballistic quantum transport simulations with an atomistic tight-binding Hamiltonian obtained from density functional theory. Our simulations suggest that monolayer MoS2 MOSFETs can provide near-ideal subthreshold slope, and suppression of drain-induced barrier lowering (DIBL) and gate-induced drain leakage (GIDL). However, these full-band simulations also exhibit limited transconductance. These ballistic simulations also exhibit negative differential resistance (NDR) in the output characteristics associated with the narrow width in energy of the lowest conduction band, but this NDR may be substantially reduced or eliminated by scattering in MoS2.

cond-mat.mes-hall↗

arXiv2013

Theory and synthesis of bilayer graphene intercalated with ICl and IBr for low power device applications

Graphene intercalation materials are potentially promising for the implementation of the ultra-low power, excitonic-condensate-based Bilayer pseudoSpin Field-Effect Transistor (BiSFET) concept, as well as other novel device concepts requiring a graphene interlayer dielectric. Using density functional theory (DFT) we study the structural and electronic properties of bilayer graphene intercalated with iodine monochloride (ICl) and iodine monobromide (IBr). We determine the structural configuration of ICl and IBr graphene intercalation compounds (GICs). We also conduct an in-depth exploration of inter-layer electronic coupling, using \textit{ab initio} calculations. The presence of intercalants dopes the graphene layer. It also reduces, but does not eliminate, the electronic coupling between graphene layers, which may enable BiSFET operation. In addition, we present experimental results for ICl-GIC synthesis and characterization.

cond-mat.mes-hall↗

arXiv2012

Topological insulator Bi2Se3 thin films as an alternative channel material in MOSFETs

Three-dimensional (3-D) topological insulators (TI) are characterized by the presence of metallic surface states and a bulk band gap. Recently theoretical and experimental studies have shown an induced gap in the surface state bands of TI thin films. The gap results from interaction of conduction band (CB) and valence band (VB) surface states from the opposite surfaces of a thin film, and its size is determined by the film thickness. This gap formation could open the possibility of thin-film TI-based metal-oxide-semiconductor field-effect transistors (MOSFETs). Here we explore the performance of MOSFETs based on TI thin films, specifically Bi2Se3, using quantum ballistic transport simulations with the tight-binding Hamiltonian in the atomic orbital basis. Our simulations indicate that Bi2Se3 MOSFET will be vulnerable to short-channel effects due to the high relative dielectric constant of Bi2Se3(~100)despite its expected excellent electrostatic integrity inherent in a two-dimensional system, and will have other limitations as compared to silicon-based MOSFETs. However, Bi2Se3 MOSFETs, and presumably other TI-based MOSFETs, appear to provide reasonable performance that perhaps could provide novel device opportunities when combined with novel TI properties such as spin-polarized surface states.

cond-mat.mes-hall↗

arXiv2012

Density functional theory studies of interactions of graphene with its environment: substrate, gate dielectric and edge effects

This paper reviews the theoretical work undertaken using density functional theory (DFT) to explore graphene's interactions with its surroundings. We look at the impact of substrates, gate dielectrics and edge effects on the properties of graphene. In particular, we focus on graphene-on-quartz and graphene-on-alumina systems, exploring their energy spectrum and charge distribution. Silicon-terminated quartz is found to not perturb the linear graphene spectrum. On the other hand, oxygen-terminated quartz and both terminations of alumina bond with graphene, leading to the opening of a band gap. Significant charge transfer is seen between the graphene layer and the oxide in the latter cases. Additionally, we review the work of others regarding the effect of various substrates on the electronic properties of graphene. Confining graphene to form nanoribbons also results in the opening of a band gap. The value of the gap is dependent on the edge properties as well as width of the nanoribbon.

cond-mat.mes-hall↗

arXiv2011

Dielectric capping effects on binary and ternary topological insulator surface states

Using a density functional based electronic structure method, we study the effect of crystalline dielectrics on the metallic surface states of Bismuth- and chalcogen-based binary and ternary three dimensional topological insulator (TI) thin films. Crystalline quartz (SiO2) and boron nitride (BN) dielectrics were considered. Crystalline approximation to the amorphous quartz allows to study the effect of oxygen coverage or environmental effects on the surface states degradation which has gained attention recently in the experimental community. We considered both symmetric and asymmetric dielectric cappings to the sufaces of TI thin films. Our studies suggest that BN and quartz cappings have negligible effects on the Dirac cone surface states of both binary and ternary TIs, except in the case of an oxygen-terminated quartz surface. Dangling bond states of oxygens in oxygen-terminated quartz dominate the region close to Fermi level, thereby distorting the TI Dirac cone feature and burying the Dirac point in the quartz valence band region. Passivating the oxygen-terminated surface with atomic hydrogen removes these dangling bond states from the Fermi surface region, and consequently the clear Dirac cone is recovered. Our results are consistent with recent experimental studies of TI surface degradation in the presence of oxygen coverage.

cond-mat.mtrl-sci↗

arXiv2011

Density Functional Study of Ternary Topological Insulator Thin Films

Using an ab-initio density functional theory based electronic structure method with a semi-local density approximation, we study thin-film electronic properties of two topological insulators based on ternary compounds of Tl (Thallium) and Bi (Bismuth). We consider TlBiX$_2$ (X=Se, Te) and Bi$_2$$X$_2$Y (X,Y= Se,Te) compounds which provide better Dirac cones, compared to the model binary compounds Bi$_2$X$_3$ (X=Se, Te). With this property in combination with a structurally perfect bulk crystal, the latter ternary compound has been found to have improved surface electronic transport in recent experiments. In this article, we discuss the nature of surface states, their locations in the Brillouin zone and their interactions within the bulk region. Our calculations suggest a critical thin film thickness to maintain the Dirac cone which is significantly smaller than that in binary Bi-based compounds. Atomic relaxations or rearrangements are found to affect the Dirac cone in some of these compounds. And with the help of layer-projected surface charge densities, we discuss the penetration depth of the surface states into the bulk region. The electronic spectrum of these ternary compounds agrees very well with the available experimental results.

cond-mat.mtrl-sci↗

arXiv2011

Density functional theory based study of graphene and dielectric oxide interfaces

We study the effects of insulating oxides in their crystalline forms on the energy band structure of monolayer and bilayer graphene using a \textit{first principles} density functional theory based electronic structure method and a local density approximation. We consider the dielectric oxides, SiO$_{2}$ ($α$-quartz) and Al$_{2}$O$_{3}$ (alumina or $α$-sapphire) each with two surface terminations. Our study suggests that atomic relaxations and resulting equilibrium separations play a critical role in perturbing the linear band structure of graphene in contrast to the less critical role played by dangling bonds that result from cleaving the crystal in a particular direction. We also see that with the addition of a second graphene layer, the Dirac cone is restored for the quartz surface terminations. Alumina needs more than two graphene layers to preserve the Dirac cone. Our results are at best semi-quantitative for the common amorphous forms of the oxides considered. However, crystalline oxides for which our results are quantitative provide an interesting option for graphene based electronics, particularly in light of recent experiments on graphene with crystalline dielectrics (hexagonal BN) that find considerable improvements in transport properties as compared to the those with amorphous dielectrics.

cond-mat.mtrl-sci↗

arXiv2010

Intrinsic and extrinsic perturbations on the topological insulator Bi2Se3 surface states

Using a density functional based electronic structure method, we study the effect of perturbations on the surface state Dirac cone of a strong topological insulator Bi$_2$Se$_3$ from both the intrinsic and extrinsic sources. We consider atomic relaxations, and film thickness as intrinsic and interfacial thin dielectric films as an extrinsic source of perturbation to the surface states. We find that atomic relaxations has no effect on the degeneracy of the Dirac cone whereas film thickness has considerable effect on the surface states inducing a gap which increases monotonically with decrease in film thickness. We consider two insulating substrates BN and quartz as dielectric films and show that surface terminations of quartz with or without passivation plays critical role in preserving Dirac cone degeneracy whereas BN is more inert to the TI surface states. The relative orbital contribution with respect to bulk is mapped out using a simple algorithm, and with the help of it we demonstrate the bulk band inversion when spin-orbit coupling is switched on. The layer projected charge density distributions of the surface states shows that these states are not strictly confined to the surface. The spatial confinement of these states extends up to two to three quintuple layers, a quintuple layer consists of five atomic layers of Bi and Se

cond-mat.mtrl-sci↗