Various approaches to induce a band gap in graphene based structures are theoretically investigated. The band structure and the electron transport of the proposed devices are calculated using semi-empirical extended Huckel theory (EHT) coupled

with the nonequilibrium Green's function (NEGF) formalism. We consider a stacked structure of two arm-chair nanoribbons and observe negative differential resistance (NDR) behavior in the simulated current-voltage (I - V) characteristics. The magnitude of the NDR decreases with an increase of the ribbon width. A 2D nanomesh structure of graphene patterned with a periodic array of nano holes is also investigated. The results suggest that the bandgap opening is a result of quantum confinement. However obtaining a modest bandgap in graphene often comes at the expense of strongly degraded electron mobility with lithographic difficulties. Therefore, an unconventional biasing approach of modulating the I - V characteristics without inducing any bandgap is studied. In such a scheme, NDR is observed in both single

layer and bi-layer graphene field-effect transistors. The NDR is an intrinsic property of graphene resulting from its symmetric band structure.

Experimentally, multiple layers of graphene tend to be misoriented with respect to each other. The effects of magnetic field and interlayer bias on the interlayer electron transport of large misoriented bilayer graphene nanoribbons is calculated. Edge states can result in a large peak in the transmission at the charge neutrality point that is several orders of magnitude larger than the surrounding low-energy transmission. The transmission is consistently asymmetric around the charge neutrality point for

all structures with the value differing by up to 3 orders of magnitude within 50 meV on either side of the charge neutrality point. The low-energy states exhibit a high magnetoconductance ratio, and the magnetoconductance ratio tends to increase as the width of the ribbons decrease. The maximum value of magnetoconductance ratio for the 35 nm wide bilayer ribbons at 10T is 15,000%. The effect of the bias on the transmission gives rise to non-linear I-V characteristics.

Two-dimensional materials including graphene and transition metal dichalcogenides semiconductors are of tremondous interest for potential electronic applications because of their electronic properties and rich physics. In this work, using tight binding models, first principle calculation and non equilibrium Green’s function(NEGF) simulations, interlayer resistance of misoriented MoS2, electronic properties of tetralayer graphene and edge states of trilayer graphene nanoribbons are successively studied.

In transition metal dichalcogenides like MoS2, interlayer misorientation alters the interlayer distance, the electronic bandstructure, and the vibrational modes, but, its effect on the interlayer resistance is not known. We analyze the coherent interlayer resistance of misoriented 2H-MoS2 for low energy electrons and holes as a function of the misorientation angle. The electronic interlayer resistance monotonically increases with the supercell lattice constant by several orders of magnitude similar to that of misoriented bilayer graphene. The large hole coupling gives low interlayer hole resistance that weakly depends on the misorientation angle. Interlayer rotation between an n-type region and a p-type region will suppress the electron current with little effect on the hole current. We also estimate numerical bounds and explain the results in terms of the orbital composition of the bands at high symmetry points. Density functional theory calculations provide the interlayer coupling used in both a tunneling Hamiltonian and a non-equilibrium Green function calculation of the resistivity.

In multilayer graphene, as the Fermi level and band structure are readily tunable, they constitute an ideal platform for exploring the Lifshitz transition, a change in the topology of a material’s Fermi surface. In tetralayer graphene that hosts two intersecting massive Dirac bands, we provide numerical analysis of multiple Lifshitz transitions and multiband transport, which is manifest as a nonmonotonic dependence of conductivity on the charge density n and out-of-plane electric field D, anomalous quantum Hall sequences and Landau level crossings that evolve with n, D, and B.

Lastly, due to mirror symmetry the bands of ABA stacked trilayer graphene can be identified by their parity with respect to mirror symmetry. The even parity bands exhibit gapped bilayer graphene-like dispersion, while the odd parity bands exhibit a gapped graphene-like dispersion. Using a tight binding model with Slonczewski-Weiss-McClure parameters, we look at the edge states in trilayer graphene nanoribbons in the quantum hall regime. When mirror symmetry is preserved, the system exhibits quantized longitudinal conductance at charge neutrality point, due to counterpropagating even and odd parity edge modes. We study the effects of perpendicular electric field and magnetic field and mirror symmetry breaking disorder on the band structures and longitudinal conductance.

Self-assembled nanostructures, composed of inorganic and organic materials, have multiple applications in the fields of engineering and nanotechnology. Experimental research using nanoscaled materials, such as semiconductor/metallic nanocrystals, nanowires (NW), and carbon nanotube (CNT)-molecular systems have potential applications in next generation nano electronic devices. Many of these molecular systems exhibit electronic device functionality. However, experimental analytical techniques to determine how the chemistry and geometry affects electron transport through these devices does not yet exist.

Using theory and modeling, one can approximate the chemistry and geometry at

the atomic level and also determine how the chemistry and geometry governs electron current. Nanoelectronic devices however, contain several thousand atoms which makes quantum modeling difficult. Popular atomistic modeling approaches are capable of handling small molecular systems, which are of scientific interest, but have little engineering value. The lack of large scale modeling tools has left the scientific and engineering community with a limited ability to understand, explore, and design complex systems of engineering interest.

To address these issues, I have developed a high performance general quantum charge transport model based on the non-equilibrium Green function (NEGF) formalism using density functional theory (DFT) as implemented in the FIREBALL software. FIREBALL is a quantum molecular dynamics code which has demonstrated the ability to model large molecular systems. This dissertation project of integrating NEGF into FIREBALL provides researchers with a modeling tool capable of simulating charge current in large inorganic / organic systems.

To provide theoretical support for experimental efforts, this project focused on CNT-molecular systems, which includes the discovery of a CNT-molecular resonant tunneling diode (RTD) for electronic circuit applications. This research also answers basic scientific questions regarding how the geometry and chemistry of CNT-molecular systems affects electron transport.

ABSTRACT OF THE DISSERTATION

Schottky Barrier Heights at Two-Dimensional Metallic and Semiconducting

Transition-Metal Dichalcogenide Interfaces

by

Adiba Zahin

Master, Graduate Program in Electrical and Computer Engineering

University of California, Riverside, September 2017

Dr. Roger K. Lake, Chairperson

Several advances have been made in the realization of electronic devices that utilize

atomically thin two-dimensional (2D) materials. The semiconducting transition metal

dichalcogenides in particular have been used to demonstrate a wide range of devices

which include steep tunnel-field-effect transistors [1,2], photodetectors [3,4], field-effecttransistors [5, 6] and chemical sensors [7, 8]. A variety of experimental [9, 10] and theoretical [11, 12] studies have been devoted to understand the interface formed between

the bulk metals that are deposited on the surface of the 2D transition metal dichalcognides. There is growing evidence that the Schottky-like transport behavior observed

in TMDC-metal contacts is a consequence of strong Fermi level pinning (FLP). The

origin of the Fermi level pinning in metal-TMDC interfaces has been attributed to the

formation of interface dipoles [11], defects at the metal-TMDC interface and the existence of metal-induced-gap-states (MIGS) which arise from the exponential decay of

the wavefunction of the metal Fermi level into the TMDC band gap [13, 14]. One approach to minimize the effect of Fermi level pinning would be achieving an epitaxially

clean interface between the metal and the semiconducting TMDC. Prior experimental

studies of the contact resistance between the 2H/1T polytypes of MoS2 succeeded in

idemonstrating record low contact resistance [15]. Indeed recent study shows that, FLP

The singular density of states and the two Fermi wavevectors resulting from a ring-shaped or "Mexican hat'' valence band give rise to unique trends in the charged impurity scattering rates and charged impurity limited mobilities. Ring shaped valence bands are common features of many monolayer and few-layer two-dimensional materials including the III-VI materials GaS, GaSe, InS, and InSe. The wavevector independence of the screening, calculated within the random phase approximation, is so strong that it is the dominant factor determining the overall trends of the scattering rates and mobilities with respect to temperature and hole density. Charged impurities placed on the substrate and in the 2D channel are considered. The different wavevector dependencies of the bare Coulomb potentials alter the temperature dependence of the mobilities. Moving the charged impurities 5 {\AA} from the center of the channel to the substrate increases the mobility by an order of magnitude by suppressing the large wavevector backscattering within the outer Fermi ring.

Using first-principles calculations, We investigate the cobalt (111) surface as an alternative substrate for the growth of hexagonal boron nitride (h-BN). We find the adsorption energies of B and N to be larger

on the Co(111) surface compared to the commonly used Cu(111) surface. Trace concentrations of carbon within the Co(111) substrate are found to lower the adsorption energies of B and N on Co(111). The most favorable binding sites and the migration barriers between these sites are also elucidated using nudged elastic band calculations.

Despite multiple successful applications of high-throughput computational materials design from first principles, there are a number of factors that inhibit its future adoption. Of particular importance are the limited ability to provide high fidelity in a reliable manner and the limited accessibility to non-expert users. We present example applications of a novel approach, where high-fidelity first-principles simulation techniques, density functional theory with hybrid screened exchange (HSE) and the GW approximation, are standardized and made available online in an accessible and repeatable setting. We apply this approach to extract electronic band gaps and band structures for a diverse set of 847 materials ranging from pure elements to III-V and II-VI compounds, ternary oxides and alloys. We find that for HSE and G0W0, the average relative error fits within 20%, whereas for conventional generalized gradient approximation the error is 55%. For HSE we find the average calculation time on an up-to-date server centrally available from a public cloud provider fits within a 48 hours window. This work provides a cost-effective, accessible and repeatable practical recipe for performing high-fidelity first-principles calculations of electronic materials in a high-throughput manner.

Misorientation of two layers of bilayer graphene leaves distinct signatures in the electronic properties and the phonon modes. The effect on the thermal conductivity has received the least attention and is the least well understood.

In this work, the in-plane thermal conductivity of twisted bilayer graphene (TBG) is investigated as a function of temperature and interlayer misorientation angle using nonequilibrium molecular dynamics (NEMD). The central result is that with rotation angles larger than 13 degrees, the calculated thermal conductivities decrease approximately linearly with the increasing lattice constant of the commensurate TBG unit cell. Comparisons of the phonon dispersions show that misorientation has a negligible effect on the low-energy phonon frequencies and velocities. However, the larger periodicity of TBG reduces the Brillouin zone size to the extent that the zone edge acoustic phonons are thermally populated. This allows Umklapp scattering to reduce the lifetimes of the phonons contributing to the thermal transport, and consequently, to reduce the thermal conductivity. This explanation is supported by direct calculation of reduced phonon lifetimes in TBG based on density functional theory (DFT) for larger rotation angles.

Nothing was previously known about the questions about how small twist angles (< 13 degrees) affect the thermal conductivity of TBG, and how it approaches its aligned value as the twist angle approaches 0 degrees. To provide insight into these questions, we perform large scale NEMD calculations on commensurate TBG structures with angles down to 1.87 degrees. The results show a smooth, non-monotonic behavior of the thermal conductivity with respect to the commensurate lattice constant. As the commensurate lattice constant increases, the thermal conductivity initially decreases by 50%, and then it returns to 90% of its aligned value as the angle is reduced to 1.89 degrees. These same qualitative trends are followed by the trends in the shear elastic constant, the wrinkling intensity, and the out-of-plane ZA2 phonon frequency. The picture that emerges of the physical mechanism governing the thermal conductivity is that misorientation reduces the shear elastic constant; the reduced shear elastic constant enables greater wrinkling, and the greater wrinkling reduces the thermal conductivity. The small-angle behavior of the thermal conductivity raises the question of how do response functions approach their aligned values as the twist angle approaches 0 degrees. Is the approach gradual, discontinuous, or a combination of the two?

Much attention has been given recently to the material data science. A particular emphasis is placed on low dimensional materials exhibiting novel electrical and thermal properties. An improved dimension classifier model has been created to identify the quasi-1D materials that are often classified within the 2D material family. The algorithm is based on the fact that quasi-1D materials contain different bond lengths within the unit cell. The model can identify known quasi-1D material based on the structural data from Material Project Database. Using the optimized distributed gradient boosting model (XGBoost), both the band gap and the magnetization properties can be predicted from structural and elemental features. By fitting the XGBoost model with 15,000 kinds of materials, the accuracy of the predictions on the 5000 testing samples is greater than 91%. The mean absolute error of the band gap prediction is only 0.148 eV. Additionally, 1,025 kinds of magnetic materials have been identified among 5000 kinds of materials. According to the feature importance analysis, the most correlated feature for band gap prediction is the number of the valence electrons. While, for the magnetic material classification, it is the elemental period.

One main objective of spintronics is to process and store information with the magnetic order parameters. Much of the recent attention is attracted by the magnetic skyrmions residing in magnetic materials with the antisymmetric Dzyaloshinskii-Moriya interaction. Here, we study skyrmion dynamics in two different confined geometries. We first demonstrate that single skyrmion creation and annihilation by spin waves in a crossbar geometry. A critical spin-wave frequency is required both for the creation and the annihilation of a skyrmion. The minimum frequencies for creation and annihilation are similar, but the optimum frequency for creation is below the critical frequency for skyrmion annihilation. Then we investigate the resonant modes of a single N eel type skyrmion in confined nanodisks with varying aspect ratios (AR). With the increase of disk AR, multiple new modes emerge in the power spectrum, which originate from the broken rotational symmetry of both the nanodisk and the skyrmion. Another goal of spintronics is to transport spin with minimal losses. Spin superfluid transport can be achieved in easy-plane ferromagnets and antiferromagnets by creating a non-equilibrium meta-stable state with static spin spiral textures. We show that the spin superfluid analogy can be extended to include Josephson-like oscillations of the spin current in both easy-plane ferromagnets and antiferromagnets. This spin current also has a non-linear, time-averaged component which provides a `smoking gun' signature of spin superfluidity. Furthermore, a spin oscillator device based on the spin superfluid Josephson effect is proposed.

We explore the excitonic, electronic, phononic and thermal properties of low-dimensional materials, specifically the two-dimensional and quasi-one-dimensional transition metal chalcogenides. The possibility of observing Bose-Einstein exciton condensation (BEC) in transition metal dichalcogenides (TMDs) has been analyzed at three different levels of theory. We find that, in the strong coupling regime, mean field theory with either an unscreened or screened interlayer interaction predicts a room-temperature condensate. However, intralayer interactions can essentially renormalize the quasiparticle dispersion, which can be captured by many-body GW formalism. In the strong coupling regime, the improved BEC theory predicts that intralayer interactions have a large impact on the condensate order parameter, as well as on its functional dependencies on effective mass and carrier density. We also explore the thermal properties of 2D materials, specifically in the misoriented bilayer graphene (m-BLG) system, using ab initio density functional theory (DFT) and phonon Boltzmann transport equation (BTE). we find that the lattice thermal conductivity of m-BLG reduces to almost half of its unrotated counterpart. To explain the phonon dynamics, we analyze the phonon dispersions, phonon velocity distributions, occupations, density of states and heat capacity, both before and after misorientation. Detailed calculation of the phonon-phonon scattering lifetime reveals that, the increased umklapp scattering in the acoustic and quasi-acoustic phonon branches is the main reason for the reduced thermal conductivity in m-BLG system. We also explore the thermal conductivity of quasi-1D materials, specifically TaSe3 and NbS3, using ab initio DFT and phonon BTE. We find that both materials exhibit highly anisotropic thermal transport. A thermal conductivity of 6.3 W/mK (70.6 W/mK) is observed for metallic TaSe3 (semiconducting NbS3) along the chain direction. In-depth study of velocity and lifetime distribution shows that lower scattering and higher phonon velocity in NbS3 are the reasons behind such higher thermal conductivity. The umklapp scattering process is found to be the dominant phonon scattering mechanism in this family of low-dimensional materials. We also investigate the electronic and vibrational properties of different phases of the quasi-1D material NbS3. We find that the dimerized phase NbS3-IV is a semiconductor, whereas the undimerized phase NbS3-V is a metal. Similarity between the band dispersions of phase-I and phase-IV arises from the similarity in their structures, in spite of some stacking and chiral faults. Both phase-I and phase-IV are dynamically stable, whereas the phonon dispersion in phase-V exhibits instability along the inter-chain and growth direction, indicating a possible charge density wave ground state. Finally, we explore the band alignment properties of different quasi-1D transition metal trichalcogenides (TMTs). From the DFT calculations, we can identify several TMTs as promising candidates for ohmic contacts and tunnel FET devices.

The discovery of graphene's unique electronic and thermal properties has moti-

vated the search for new two-dimensional materials. Examples of these materials

include the layered two-dimensional transition metal dichalcogenides (TMDC) and

metal mono-chalcogenides. The properties of the TMDCs (eg. MoS2, WS2, TaS2,

TaSe2) and the metal mono-chalcogenides (eg. GaSe, InSe, SnS) are diverse - ranging from semiconducting, semi-metallic and metallic. Many of these materials exhibit strongly correlated phenomena and exotic collective states such as exciton condensates, charge density waves, Lifshitz transitions and superconductivity. These properties change as the film thickness is reduced down to a few monolayers.

We use first-principles simulations to discuss changes in the electronic and the

vibrational properties of these materials as the film thickness evolves from a single atomic monolayer to the bulk limit. In the semiconducting TMDCs (MoS2, MoSe2,WS2 and WSe2) and monochalcogenides (GaS, GaSe, InS and InSe) we show confining these materials to their monolayer limit introduces large band degeneracies or non-parabolic features in the electronic structure. These changes in the electronic structure results in increases in the density of states and the number of conducting modes. Our first-principles simulations combined with a Landauer approach show these changes can lead to large enhancements up to an order of magnitude in the thermoelectric performance of these materials when compared to their bulk structure. Few monolayers of the TMDCs can be misoriented with respect to each other due to the weak van-der-Waals (vdW) force at the interface of two monolayers. Misorientation of the bilayer semiconducting TMDCs increases the interlayer van-der-Waals gap distance, reduces the interlayer coupling and leads to an increase in the magnitude

of the indirect bandgap by up to 100 meV compared to the registered bilayer.

In the semi-metallic and metallic TMDC compounds (TiSe2, TaS2, TaSe2) a phase

transition to a charge density wave (CDW) ground state occurs at a temperature that is unique to each material. Confining these materials to a single monolayer or few-monolayers can increase or decrease their CDW transition temperature and change the magnitude of the CDW energy gap. We show the low energy Raman modes observed in 1T-TaSe2 and 1T-TaS2 in their CDW ground state can emerge from zone folded phonons due to the reconstruction of the lattice in the bulk and monolayer structures. In 1T-TiSe2 the driving mechanism of the CDW is excitonic condensation.We show the excitonic gap of the monolayer and bilayer structures can increase by up to a factor of 3 compared to the excitonic gap of the bulk structure.

Interfacial coupling between the top and the bottom surface states in thin-film 3-dimensional (3D) topological insulators (TIs) destroys the momentum-spin (k-s) locking chirality, and thus prohibits the good transport properties of TI surface states. Our theoretical investigations in 3D TI thin films show that this effect only occurs near the band gaps due to the spin mismatch between the opposing surfaces. By tuning the position of the Fermi level, interfacial tunneling and in-plane semi-classical transport present unique signatures due to this mismatch. These results indicate the possibility to restore the bulk transport properties of 3D TIs in the case of thin films.

As a real-space non-trivial topological order in solid states, the skyrmion phase in B20 compounds or heavy metal interfaces is considered as a strong candidate to implement next generation storage units or spintronic devices. Using coherent transport modeling, we demonstrate that a `topological spin Hall effect' (TSHE) can be achieved in some circumstances due to individual magnetic skyrmions. In order to evaluate the device application possibilities, a topological charge analysis is carried out to identify and quantify the topological protection in each skyrmion. Based on this analysis, an on-wafer solution to individually create magnetic skyrmions is proposed. The feasibility and stability of the proposal is numerically evaluated by solving Landau-Lifshitz-Gilbert (LLG) equation.

Besides the physics that governs the operations in electronic or spintronic devices, the requirement of signal integrity gives rise to another fundamental limit to enhance the performance of next-generation devices. System designers have proposed many solutions to overcome this limit, such as differential signaling or multi-port passive networks. Characterization of these designs calls for reliable algorithms to remove the effects of access structures: the de-embedding techniques. Taking impedance mismatch as a perturbation quantity, we establish a simplified model of uniform multi-conductor transmission lines. A novel multi-port de-embedding technique is proposed theoretically and tested experimentally.