Novel Indium Arsenide double gate and gate all around nanowire MOSFETs for diminishing the exchange correlation effect: A quantum study

In this paper we present novel double gate (DG) metal oxide semiconductor field effect transistor (MOSFET) and gate all around (GAA) nanowire metal oxide semiconductor field effect transistor (NWT) with a diminished exchange-correlation (Ex-Corr) effect. The key idea in this work is to use Indium Arsenide (InAs) semiconductor instead of Si. We have evaluated and compared different parameters of DG-MOSFET and GAA-NWTs such as threshold voltage, sub-threshold slope, drain induced barrier lowering and ON and OFF state currents from quantum view. Quantum mechanical transport approach based on non-equilibrium green’s function (NEGF) has been performed in the frame work of effective mass theory in consideration with Ex-Corr effect. This simulation method consists of three dimensional Poisson’s equation in which a Schrodinger equation is first solved in each slice of the device to find Eigen energies and Eigen functions. Then, a transport equation of electrons moving in the sub-bands is solved. This fully quantum method treats such effects as source-to-drain tunneling, ballistic transport, and quantum confinement on equal footing. The results show that only a few lowest Eigen sub-bands are occupied and the upper sub-bands can be safely neglected. Also, the interaction between electrons and Ex-Corr effect is diminished in the proposed structure.     

Accuracy of the Frensley inflow boundary condition for Wigner equations in simulating resonant tunneling diodes

In this paper, the accuracy of the Frensley inflow boundary condition of the Wigner equation is analyzed in computing the I–V characteristics of a resonant tunneling diode (RTD). It is found that the Frensley inflow boundary condition for incoming electrons holds only exactly infinite away from the active device region and its accuracy depends on the length of contacts included in the simulation. For this study, the non-equilibrium Green’s function (NEGF) with a Dirichlet to Neumann mapping boundary condition is used for comparison. The I–V characteristics of the RTD are found to agree between self-consistent NEGF and Wigner methods at low bias potentials with sufficiently large GaAs contact lengths. Finally, the relation between the negative differential conductance (NDC) of the RTD and the sizes of contact and buffer in the RTD is investigated using both methods.     

Spin-polarized current generated by magneto-electrical gating

We theoretically study spin-polarized current through a single electron tunneling transistor (SETT), in which a quantum dot (QD) is coupled to non-magnetic source and drain electrodes via tunnel junctions, and gated by a ferromagnetic (FM) electrode. The I–V characteristics of the device are investigated for both spin and charge currents, based on the non-equilibrium Green's function formalism. The FM electrode generates a magnetic field, which causes a Zeeman spin-splitting of the energy levels in the QD. By tuning the size of the Zeeman splitting and the source–drain bias, a fully spin-polarized current is generated. Additionally, by modulating the electrical gate bias, one can effect a complete switch of the polarization of the tunneling current from spin-up to spin-down current, or vice versa.     

Improved double-gate armchair silicene nanoribbon field-effect-transistor at large transport bandgap

The electrical characteristics of a double-gate armchair silicene nanoribbon field-effect-transistor(DG ASi NR FET)are thoroughly investigated by using a ballistic quantum transport model based on non-equilibrium Green's function(NEGF) approach self-consistently coupled with a three-dimensional(3D) Poisson equation. We evaluate the influence of variation in uniaxial tensile strain, ribbon temperature and oxide thickness on the on-off current ratio, subthreshold swing, transconductance and the delay time of a 12-nm-length ultranarrow ASi NR FET. A novel two-parameter strain magnitude and temperature-dependent model is presented for designing an optimized device possessing balanced amelioration of all the electrical parameters. We demonstrate that employing Hf O2 as the gate insulator can be a favorable choice and simultaneous use of it with proper combination of temperature and strain magnitude can achieve better device performance.Furthermore, a general model power(GMP) is derived which explicitly provides the electron effective mass as a function of the bandgap of a hydrogen passivated ASi NR under strain.     

3D quantum mechanical simulation of square nanowire MOSFETs by using NEGF method

In order to investigate the specifications of nanoscale transistors, we have used a three dimensional (3D) quantum mechanical approach to simulate square cross section silicon nanowire (SNW) MOSFETs. A three dimensional simulation of silicon nanowire MOSFET based on self consistent solution of Poisson-Schrödinger equations is implemented. The quantum mechanical transport model of this work uses the non-equilibrium Green’s function (NEGF) formalism. First, we simulate a double-gate (DG) silicon nanowire MOSFET and compare the results with those obtained from nanoMOS simulation. We understand that when the transverse dimension of a DG nanowire is reduced to a few nanometers, quantum confinement in that direction becomes important and 3D Schrödinger equation must be solved. Second, we simulate gate-all-around (GAA) silicon nanowire MOSFETs with different shapes of gate. We have investigated GAA-SNW-MOSFET with an octagonal gate around the wire and found out it is more suitable than a conventional GAA MOSFET for its more I _on /I _off , less Drain-Induced-Barrier-Lowering (DIBL) and less subthreshold slope.     

Orientation effect on the electronic transport properties of C24 fullerene molecule

The transport properties of the cage-like molecule depend on its orientation between the electrodes, but the investigation on the mechanism has not been found. Using first-principle density-functional theory (DFT) and non-equilibrium Green’s function (NEGF) formalism for quantum transport calculation, we study the electronic transport properties of C₂₄ fullerene molecule with different orientations in Au–C₂₄–Au two-probe system. The effects of k-point sampling on the Brillouin zone are explored. Our results show that the negative differential resistance of C₂₄ molecule is found in such a system and can be tuned by the molecule's orientation in the two-probe system. We also proposed a mechanism for it. The I–V characteristic under bias voltage is determined. The present findings could be helpful for the application of the C₂₄ molecule in the field of single molecular devices or nanometer electronics.     

Spin-flip effects in a parallel-coupled double quantum dot molecule

We investigate theoretically the electronic transport through a parallel-coupled double quantum dot (DQD) molecule attached to metallic electrodes, in which the spin-flip scattering on each quantum dot is considered. Special attention is paid to the effects of the intradot spin-flip processes on the linear conductance by using the equation of motion approach for Green’s functions. When a weak spin-flip scattering on each quantum dot is present, the single Fano peak splits into two Fano peaks, and the Breit–Wigner resonance may be suppressed slightly. When the spin-flip scattering strength on each quantum dot becomes strong, the linear conductance spectrum consists of two Breit–Wigner peaks and two Fano peaks due to the quantum interference effects. The positions and shapes of these resonant peaks can be controlled by using the magnetic flux through the quantum device.     

Quantum electronic transport: Linear and nonlinear conductance from the Keldysh approach

This is a review of the derivation of the Landauer conductance using the Keldysh non-equilibrium Green's function (NEGF) formalism and the equations-of-motion (EOM) method. We consider the elastic quantum electronic transport through a multi-lead device and treat the conductor in the mean-field approximation. This is suitable for open quantum dots as well as for several molecular systems where charging effects are negligible. The focus of the presentation is to unveil the technical issues involved in the formalism. We show how the Landauer conductance emerges as a linear term in the current-voltage I-V characteristics and indicate how to go beyond this regime. We address the connection of the NEGF approach to recent developments in molecular transport and discuss the problems that arise when one tries to include interaction effects beyond the mean field.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

Exact integral operator form of the Wigner distribution-function equation in many-body quantum transport theory

A formal derivation of a generalized equation of a Wigner distribution function including all many-body effects and all scattering mechanisms is given. The result is given in integral operator form suitable for application to the numerical modeling of quantum tunneling and quantum interference solid state devices. In the absence of scattering and many-body effects, the result reduces to the noninteracting-particle Wigner distribution function equation, often used to simulate resonant tunneling devices. The derivation uses a Weyl transform technique which can easily incorporate Bloch electrons. Weyl transforms of self-energies are derived. Various simplifications of a general quantum transport equation for semiconductor device analysis and self-consistent numerical simulation of a quantum distribution function in the phase-space/frequency-time domain are discussed. Recent attempts to include collisions in the Wigner distribution-function approach to the numerical simulation of tunneling devices are clearly shown to be non-self-consistent and inaccurate; more accurate numerical simulation is needed for a deeper understanding of the effects of collision and scattering.     

Nanoscale device modeling: the Green’s function method

The non-equilibrium Green’s function (NEGF) formalism provides a sound conceptual basis for the devlopment of atomic-level quantum mechanical simulators that will be needed for nanoscale devices of the future. However, this formalism is based on concepts that are unfamiliar to most device physicists and chemists and as such remains relatively obscure. In this paper we try to achieve two objectives: (1) explain the central concepts that define the ‘language’ of quantum transport, and (2) illustrate the NEGF formalism with simple examples that interested readers can easily duplicate on their PCs. These examples all involve a short n ^+  + – n ⁺ – n ^+  + resistor whose physics is easily understood. However, the basic formulation is quite general and can even be applied to something as different as a nanotube or a molecular wire, once a suitable Hamiltonian has been identified. These examples also underscore the importance of performing self-consistent calculations that include the Poisson equation. The I–V characteristics of nanoscale structures is determined by an interesting interplay between twentieth century physics (quantum transport) and nineteenth century physics (electrostatics) and there is a tendency to emphasize one or the other depending on one’s background. However, it is important to do justice to both aspects in order to derive real insights.     

Single-electron transport through quantum point contact

Here, we employ a numerical approach to investigate the transport and conductance characteristics of a quantum point contact. A quantum point contact is a narrow constriction of a width comparable to the electron wavelength defined in a two-dimensional electron gas (2DEG) by means of split-gate or etching technique. Their properties have been widely investigated in the experiments. In our study, we define a quantum Hall based split-gate quantum point contact with standard gate geometry. Firstly, we obtain the spatial distribution of incompressible strips (current channels) by applying a self consistent Thomas-Fermi method to a realistic heterostructure under quantized Hall conditions. Later, time-dependent Schrödinger equation is solved for electrons injected in the current channels. The transport characteristics and time-evolutions are analyzed in the integer filling factor regime (ν = 1) with the single electron density. The results confirm that the current direction in a realistic quantum point contact can be controllable with the external interventions.     

Suppression of the tunneling effect by shallow junctions in field-effect transistor

We study the quantum wave transport in nanoscale field-effect transistors. It has been shown that the tunneling effect between the source and the drain in an ultra-short channel transistor significantly degrades the control of the drain current by the gate. However, the tunneling effect is suppressed by reducing the depth of the source and drain junctions which is designated to suppress the short-channel effects concerning the cut-off characteristics of the field-effect transistor. The reduced junction depth confines the carriers in the direction (y -direction) perpendicular to the transport direction (x -direction). The matching of y -direction wavefunctions at regional boundaries suppresses the tunneling effect and normal FET current–voltage characteristics has been obtained, which explains theoretically the successful fabrication of nanoscale field-effect transistors.     

Advances in the Spherical Harmonic–Boltzmann–Wigner approach to device simulation

Recent advances in the Spherical Harmonic Boltzmann method of device modeling are presented. A new surface scattering model and improved numerical interpolation schemes have been developed. The method is shown to be capable of calibrating an entire deep submicron process to provide I–V characteristics and substrate current self-consistently. Substrate currents agree with experiment over a complete process without any fitting parameters. Applications to a 50 nm MOSFET predict well-behaved device operation. The method requires approximately ten minutes to self-consistently calculate a MOSFET bias point and provide the device distribution function. The spherical harmonic method has been extended to account for quantum mechanical effects by applying it to the Wigner equation. We treat the Wigner equation as a quantum correction to the Boltzmann equation thereby making the spherical harmonic approach a natural method of solution.     

A two-dimensional domain decomposition technique for the simulation of quantum-scale devices

The simulation of realistically sized devices under the Non-Equilibrium Greens Function (NEGF) formalism typically requires prohibitive amounts of memory and computation time. In order to meet the rising computational challenges associated with quantum-scale device simulation we offer a 2-D domain decomposition technique. This technique is applicable to a large class of atomistic and spatial simulation problems. Considering a decomposition along both the cross section and length of the device, the framework presented in this work ensures efficient distribution of both memory and computation based upon the underlying device structure. As an illustration we stably generate the density of states and transmission, under the NEGF formalism, for the atomistic-based simulation of square 5 nm cross section silicon nanowires consisting of over one million atomic orbitals.     

Nonlinear conductivity of quasi-one-dimensional layered compound TiS3

The crystal structure and transport properties of TiS₃ whiskers in the plane of layers (ab) have been studied. Maxima of the logarithmic derivative of resistance, dln R/d(1/T), are observed at 17, 60 and 120 K both along and across the chains. Strong nonlinearity of the current–voltage characteristics has been revealed in both directions. Nonlinear conductivity along the chains is observed up to T=60 K, while in the transverse direction it is observed up to T=130 K. The results indicate possible phase transitions of electrons to collective states, probably, charge density waves.     

Spin transport properties in a non-uniform quantum wire modulated by both Rashba and Dresselhaus spin–orbit couplings

Spin transport properties in a non-uniform quantum wire (QW) in the presence of both the Rashba and Dresselhaus spin–orbit couplings (SOCs) is investigated by using the non-equilibrium Green's function (NEGF) method combined with the Landauer Büttiker formalism. It is found that such a non-uniform quantum wire exhibits considerable spin polarization in its conductance in the influence of both the Rashba and Dresselhaus SOCs, and that the two SOCs' strengths strongly affect both the magnitude and sign of the electron spin polarization. Interestingly, the Rashba and Dresselhaus SOCs play the same modulating role in the electron spin polarization. The proposed nanostructure can potentially be utilized to devise an all-electrical spintronic device.     

Interacting quantum wires: A possible explanation for the 0.7 anomalous conductance

We investigate an effective one-dimensional conducting channel considering both the contact umklapp and the Coulomb electron-electron interaction. We show that, at low electronic density, the proximity to the Wigner crystal reproduces the anomaly in conductance at 0.7G₀. The crucial ingredient of our theory is the fact that the gate voltage acts as a bias controlling the intensity of the umklapp term. At large gate voltages, the umklapp vanishes and we obtain a conducting quantum wire with a perfect conductance. At low gate voltages, the Wigner crystal is pinned by the umklapp term, giving rise to an insulating behavior with vanishing conductance. This crossover pattern has a transition point which can be identified with the anomalous conductance around 0.7G₀. This picture is obtained within the framework of a renormalization group calculation. The conductance static regime is achieved by taking first the limit of finite length and then the limit of zero frequency.