Nobuya Mori

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.

Linear magnetoresistance due to multiple-electron scattering by low-mobility islands in an inhomogeneous conductor

Linear transverse magnetoresistance is commonly observed in many material systems including semimetals, narrow band-gap semiconductors, multi-layer graphene and topological insulators. It can originate in an inhomogeneous conductor from distortions in the current paths induced by macroscopic spatial fluctuations in the carrier mobility and it has been explained using a phenomenological semiclassical random resistor network model. However, the link between the linear magnetoresistance and the microscopic nature of the electron dynamics remains unknown. Here we demonstrate how the linear magnetoresistance arises from the stochastic behaviour of the electronic cycloidal trajectories around low-mobility islands in high-mobility inhomogeneous conductors and that this process is only weakly affected by the applied electric field strength. Also, we establish a quantitative link between the island morphology and the strength of linear magnetoresistance of relevance for future applications.

Theory of quasiballistic transport through nanocrystalline silicon dots

A model to describe the underlying physics of high-energy electron emission from a porous silicon diode is presented. The model is based on an atomistic tight-binding method combined with semiclassical Monte Carlo simulation. It well reproduces essential features of experimental findings. An initial acceleration region is shown to play a crucial role in generating quasiballistic electron emission.

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.

R-matrix theory of quantum transport and recursive propagation method for device simulations

We present a theory of quantum transport based on spectral expansion of Green’s function in an open system. In continuous models, this representation makes it possible to avoid discretization of the device area and achieve a much higher numerical accuracy with a lower computational burden compared to common grid schemes. We formulate a numerical method which enables all the observables of interest to be propagated through the device area so that the major portion of the computation time scales linearly with the device volume. As an illustration, we apply the method to quantum ballistic electron transport in model three-dimensional metal oxide semiconductor field effect transistors.

Magnetophonon resonances in quantum wires.

Magnetophonon resonances are studied in quantum wires with square confinement potential. Magnetoconductivity calculated by using the Kubo formula is found to have two components; one related to the current carried by electron hopping motion between the localized cyclotron orbits through electron-phonon interaction, and the other caused by the current carried by electron motion affected by the confinement potential. The latter, σ po, is found to be two orders of magnitude greater than the former, σ ep, around the fundamental resonance condition in the case of a GaAs quantum wire whose width is 200A and barrier height is 100meV.

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.

Study of electron-hole generation and recombination in semiconductors using the Osaka free electron laser

Band-gap luminescence from a variety of compound semiconductors has been observed under intense mid-infra-red irradiation by the free electron laser (FEL) at the Institute of Free Electron Laser, Osaka University. The FEL wavelength and power dependences of the FEL-induced luminescence intensity were measured, and the results are compared with those obtained by a full-band Monte Carlo simulation.

Reduction of acoustic-phonon deformation potential in one-dimensional array of Si quantum dot interconnected with tunnel oxides

The scattering potential for the acoustic deformation potential scattering in a one-dimensional silicon quantum dot array interconnected by thin oxide layers is theoretically investigated. One-dimensional phonon normal modes are numerically obtained using the linear atomic chain model. The strain caused by an acoustic-phonon vibration is absorbed by the oxide layers, resulting in the reduction of the strain in the Si dots. This effect eventually leads to ∼40% reduction of the scattering potential all over the structure. The amount of the reduction does not depend on the phonon energy, but rather on the ratio of the Si dot size to the oxide thickness.

Magnetophonon Resonance in Monolayer Graphene

The conductivity describing magnetophonon resonances is calculated in monolayer graphene, with the Fermi level located near the Dirac point. Intervalley scattering due to zone-edge phonons gives dominant contribution to the conductivity compared to intravalley scattering due to zone-center optical phonons mainly because of lower frequency. Resonances are classified into three types, i.e., principal, symmetric, and asymmetric transitions. The magnetophonon oscillations due to the principal and symmetric transitions are periodic in inverse magnetic field, while those due to the asymmetric transitions are not precisely periodic. The amplitude of the oscillation is shown to be weakly dependent on magnetic field.