Ximeng Guan

First-principles investigation on bonding formation and electronic structure of metal-graphene contacts

Metal-graphene contacts play a critical role in graphene-based electronics. It is found through first-principles calculation of contacts between graphene and 12 different metals that there exist two types of contacts depending on the strength of interaction between d-orbitals in metals and pz-orbitals in graphene. Fermi level shift in the contacted graphene from the freestanding one is investigated, and the electronic structure and electrostatic potential are calculated. The carrier transport through these contacts is calculated using the extended Huckel theory-based non-equilibrium Green’s function formalism, and one type of contact is shown to have less contact resistance than the other.

Modeling and simulation of uniaxial strain effects in armchair graphene nanoribbon tunneling field effect transistors

In this paper, we perform a modeling and simulation study on strained armchair graphene nanoribbon (AGNR). Two uniaxial strain models based on a tight binding method are compared with results from first-principles calculation. Tunneling field effect transistors (TFETs) with channels made of strained AGNR of different widths are modeled and simulated by a ballistic quantum transport model based on nonequilibrium Green’s function and nonparabolic effective mass approximation. Compared with TFETs with narrow AGNR, those with strained wide AGNR can achieve better device performance.

Molecular Dynamics Study of the Switching Mechanism of Carbon-Based Resistive Memory

An electric molecular dynamics (MD) method is proposed, where an electroheat solver is introduced into a traditional MD simulation to perform a coupled calculation. The switching mechanism of carbon-based resistive random access memory is studied through this method, and the heat generation and propagation driven by an electric current pulse are simulated during the switching process. Graphitic filament breakage and growth are responsible for resistance switching. The simulation shows that a short and strong voltage pulse induces graphitic filament breakage, resulting in a high-resistance state, whereas a moderate but much longer pulse is required to enable filament growth, resulting in a low-resistance state. Key factors such as the bias condition and the power supply for such switching process are also studied. The results are quantitatively consistent with experimental measurements.

Dynamic Modeling and Atomistic Simulations of SET and RESET Operations in

In this letter, atomistic-level models and simulations of unipolar TiO₂ RRAM are addressed. A dynamic model of SET/RESET is developed based on recent experimental findings, which attributes SET to oxygen vacancy (V_O) drift and Ti₄O₇ conductive filament (CF) growth, while RESET is explained by the melting of Ti₄O₇ CF and subsequent (V_O) diffusion and recombination. Based on the model, electro-thermal and molecular dynamics simulations are carried out to reproduce the complete switching cycle at an atomistic level.

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With sub-16nm CMOS nodes looming, this work proposes a novel device structure for Gate-All-Around (GAA), Nanowire (NW) tunneling-FET (tFET), with axial heterojunction on the source-channel junction, gate-underlap on the drain end of the channel, and optimized doping levels of source and drain. This structure successfully suppresses the undesirable ambipolar transfer characteristics of conventional tFETs, while maintaining the advantage of small subthreshold swing of less than 60mV/dec. For the first time, an all-tFET inverter is demonstrated to exhibit excellent switching behaviors, out-performing both the homojunction Si NW-tFET and the conventional CMOS in inverters with the same gate length and supply voltage.

-Based Unipolar Resistive Memory

Switching processes of carbon-based resistive memory cells are simulated on a fully atomistic level by the molecular dynamics (MD) method and the Extended-Hückel-Theory-based Non Equilibrium Greens Function (EHT-NEGF) method. Graphitic filament breakage and re-growth are found to be responsible for the switching of resistance of carbon-based memory. Key parameters that affect the switching speed of a memory cell are studied and trade-off between speed and power is discussed.

Design of complementary GAA-NW tunneling-FETs of axial Si-Ge heterostructure

A comparative study of graphene nanoribbon MOSFET (GNR-FET) using the extended Huckel theory (EHT) and tight-binding (TB) is conducted within the frame of the self-consistent ballistic non-equilibrium Greenpsilas function (NEGF) formalism. The bandgap variation in armchair-edged GNR (aGNR) induced by the length of the edge bond, as well as the transport characteristics with bond length relaxation, is studied in this paper. A strong structural dependence of aGNR-FET performance on the bond length is also observed and discussed.

Simulation study of switching mechanism in carbon-based resistive memory with molecular dynamics and Extended Hückel Theory-based NEGF method

The electrostatics of InSb double-gate MOSFETs is simulated using a self-consistent solver which calculates channel bandstructure and carrier population by tight-binding (TB) approach. The Q-V g characteristic and the Quantum Confinement Stark Effect (QCSE) are evaluated. By comparing with the results from the k · p method and effective mass approach, we show that full-band approach based on TB becomes more desirable when the channel is scaled down to a low dimensional quantum well. As the consequence of narrow channel width it is observed that the density of states (DOS) near band edges is decreased.

Comparative Simulation Study of GNR-FETs using EHT- and TB-based NEGF

The thickness dependence of electron mobility in the InSb quantum well (QW) FETs are calculated based on an atomistic approach for bandstructure calculation. The electron effective mass (m*) is computed using fast yet accurate sp3d5s* tight-binding (TB) method for InSb quantum-well (QW) (or ultra-thin-body, UTB) with thickness of 3-16 nm. The m* dependence on the UTB thickness is then used in determining the electron mobility in the channel region of InSb QW-FETs. It is found that in QW-FETs, optical phonon scattering is a dominant factor, which is in turn strongly coupled to the carrier effective mass determined by channel thickness. The thickness dependence of electron mobility differs from that of the MOSFETs, where surface roughness is one of the major scattering mechanisms

A Self-Consistent Simulation of InSb Double-Gate MOSFETs Using Full-Band Tight-Binding Approach

GNR (Graphene NanoRibbon) Tunneling-FETs (GNR-TFETs) are simulated using a Non-Equilibrium Greens Function (NEGF) approach using Extended Hiickel Theory (EHT)-based Hamiltonian. By comparing performance of ribbons with different bandgaps, it is shown that reducing source/drain doping and operating voltage enables low voltage operation of GNR-TFETs with a bandgap of down to 0.5eV, while still keeping small subthreshold swing (less than 60 mV/dec) and high Ion/Ioff ratio. This also lowers the performance sensitivity on GNR width and enables the application of GNR-TFETs in low-power circuits.