Matsuto Ogawa

Simulation of quantum transport in quantum devices with spatially varying effective mass

The authors report progress in quantum-mechanical simulation based on the Wigner function model. An exact nonlocal formulation in the Wigner representation due to a spatially varying effective mass and its discretization for numerical calculation are discussed. To verify the validity of such a formulation, the current-voltage characteristics of resonant tunneling diodes are simulated to compare with the conventional Wigner function model. The authors also point out the importance of self-consistent calculation in the electrostatic potential for precise device simulation. The emphasize that the Wigner function model is superior to the alternative method based on the transmission probability method even for the static simulation of quantum transport. >

Strain Effects on Electronic Bandstructures in Nanoscaled Silicon: From Bulk to Nanowire

In this paper, we present a comparative computational study on strain effects in Si nanostructures including bulk, thin film, and nanowire configurations. We employed a first principles calculation to identify the bandstructure parameters such as band splitting energy and transport effective mass. As a result, we found that bulk Si and Si thin film have similar strain effects on the bandstructure parameters under uniaxial lang110rang strain. Particularly, the effective mass reduction of electrons due to uniaxial lang110rang strain is expected even in Si thin film. On the other hand, Si nanowire structure with nanoscale cross section has lighter transport effective mass than the other structures, regardless of the amount of uniaxial strain.

Electron mobility calculation for graphene on substrates

By a semiclassical Monte Carlo method, the electron mobility in graphene is calculated for three different substrates: SiO2, HfO2, and hexagonal boron nitride (h-BN). The calculations account for polar and non-polar surface optical phonon (OP) scatterings induced by the substrates and charged impurity (CI) scattering, in addition to intrinsic phonon scattering in pristine graphene. It is found that HfO2 is unsuitable as a substrate, because the surface OP scattering of the substrate significantly degrades the electron mobility. The mobility on the SiO2 and h-BN substrates decreases due to CI scattering. However, the mobility on the h-BN substrate exhibits a high electron mobility of 170 000 cm2/(V·s) for electron densities less than 1012 cm−2. Therefore, h-BN should be an appealing substrate for graphene devices, as confirmed experimentally.

Quantum Transport Simulation of Silicon-Nanowire Transistors Based on Direct Solution Approach of the Wigner Transport Equation

In this paper, we present a self-consistent and 3D quantum simulator for Si-nanowire transistors based on the Wigner function model and multidimensional Schrodinger-Poisson algorithm. To achieve a sufficient numerical accuracy for calculating subthreshold current, we introduced a third-order differencing scheme for discretizing the drift term in the Wigner transport equation. By comparing with semiclassical Boltzmann and nonequilibrium Greens function approaches, the validity of the present simulator is confirmed. We also demonstrate that the source-drain tunneling is a critical physical phenomenon related to a scaling limit of nanowire devices, and the semiclassical simulation measurably underestimates a minimum gate length.

Comparisons of Performance Potentials of Silicon Nanowire and Graphene Nanoribbon MOSFETs Considering First-Principles Bandstructure Effects

In this paper, we investigate the performance potentials of silicon nanowire (SNW) and semiconducting graphene nanoribbon (GNR) MOSFETs by using first-principles bandstructures and ballistic current estimation based on the ¿top-of-the-barrier¿ model. As a result, we found that SNW-MOSFETs display a strong orientation dependence via the atomistic bandstructure effects, and SNW-MOSFETs provide smaller intrinsic device delays than Si ultrathin-body MOSFETs when the wire size is scaled smaller than 3 nm. Furthermore, GNR-MOSFETs are found to exhibit promising device performance if the ribbon width is designed to be larger than a few nanometers and a finite band gap can be established.

Theoretical performance estimation of silicene, germanene, and graphene nanoribbon field-effect transistors under ballistic transport

Silicene or germanene is a monolayer honeycomb lattice made of Si or Ge, similar to graphene made of C. In this work, we have assessed the performance potentials of silicene nanoribbon (SiNR), germanene nanoribbon (GeNR), and graphene nanoribbon (GNR), which all have a sufficient band gap to switch off, as field-effect transistor (FET) channel materials. We have demonstrated that, by comparing at the same band gap of ∼0.5 eV, the GNR FET maintains an advantage over SiNR or GeNR FETs under an ideal transport situation, but SiNR and GeNR are attractive channel materials for high-performance FETs as well.

Quantum transport simulation of ultrathin and ultrashort silicon-on-insulator metal-oxide-semiconductor field-effect transistors

The quantum transport properties of nanoscale silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) are investigated based on a quantum Monte Carlo (MC) device simulation. Quantum mechanical effects are incorporated in terms of a quantum correction of potential in well-developed particle MC computational techniques. The ellipsoidal multivalleys of the silicon conduction band are also considered in the simulation. First, the validity of the quantum MC technique is verified by comparing the simulated results with those calculated by a self-consistent Schrodinger–Poisson method at thermal equilibrium. Then, the nonequilibrium quantum transport characteristics of nanoscale SOI-MOSFETs are demonstrated. Furthermore, a quasi-ballistic behavior of ultrashort-channel devices is studied by evaluating the frequency of carrier scattering events in the channel region.

Finite-element analysis of quantum wires with arbitrary cross sections

A finite-element method is developed for the analysis of eigenstates in the valence band of quantum wires which have arbitrary potential profiles. Our method is basically based on the Galerkin procedure and triangle linear elements are used as finite elements. In our formulation the effect of the band mixing in the valence band is duly taken into account. Boundary conditions at heterointerfaces are also taken into account in the multiband envelope function space. Numerical examples are presented for circular, square, rectangular, and triangular quantum wire structures. The relation is clarified between the degeneracy in the E-ky dispersion curve and the symmetricity of the confinement potential.

The impact of increased deformation potential at MOS interface on quasi-ballistic transport in ultrathin channel MOSFETs scaled down to sub-10 nm channel length

It is a common view that ballistic transport is enhanced due to channel length scaling because of decreased scattering number. In this study, based on Monte Carlo (MC) simulation technique, we have successfully extracted quasi-ballistic transport parameters such as backscattering coefficient, by carefully monitoring particle trajectories around the potential bottleneck point. We have found that contrary to expectations, ballistic transport in ultra-scaled double-gate (DG) MOSFETs is not enhanced mainly due to intensified surface roughness (SR) scattering if the channel length reduces less than 10 nm.

Influence of Band-Gap Opening on Ballistic Electron Transport in Bilayer Graphene and Graphene Nanoribbon FETs

Although a graphene is a zero-gap semiconductor, band-gap energy values up to several hundred millielectronvolts have been introduced by utilizing quantum-mechanical confinement in nanoribbon structures or symmetry breaking between two carbon layers in bilayer graphenes (BLGs). However, the opening of a band gap causes a significant reduction in carrier velocity due to the modulation of band structures in their low-energy spectra. In this paper, we study intrinsic effects of the band-gap opening on ballistic electron transport in graphene nanoribbons (GNRs) and BLGs based on a computational approach, and discuss the ultimate device performances of FETs with those semiconducting graphene channels. We have shown that an increase in the external electric field in BLG-FETs to obtain a larger band-gap energy degrades substantially its electrical characteristics because of deacceleration of electrons due to a Mexican hat structure; therefore, GNR-FETs outperform in principle BLG-FETs.