Tomofumi Zushi

Nano-device simulation from an atomistic view

Fluctuations of device characteristics due to random discrete dopant (RDD) distribution are numerically investigated in ultra-small Si nanowire transistors. Kinetic Monte Carlo process simulation is performed to obtain realistic RDD distributions, whose effects on the transport characteristics are then analyzed by using a non-equilibrium Greens function (NEGF) method. Fluctuations due to atomic disorder near the Si/SiO2 interface are also investigated by performing molecular dynamics oxidation simulation for realistic atomic structure models and NEGF device simulation for transport characteristics.

Molecular Dynamics Simulation of Heat Transport in Silicon Nano-structures Covered with Oxide Films

We perform a series of molecular dynamics (MD) simulations to investigate the heat transport in Si nano-structures, while explicitly including oxide cover layers in the simulation system for the first time. The dependences of thermal diffusion velocity on the thicknesses of the SiO2 film and Si lattice are investigated. The results show that thermal diffusion velocity decreases with Si lattice thickness and does not depend on SiO2 film thickness.

Coupled Monte Carlo simulation of transient electron-phonon transport in nanoscale devices

Using a coupled Monte Carlo method for solving both electron and phonon Boltzmann transport equations, the transient electrothermal behaviors of nanoscale Si n-i-n device are simulated. The nonequilibrium optical phonon distribution is characterized by a temperature different from that of the acoustic phonons, and these two temperatures show different characteristics not only in the steady state, but also in transient conditions. It has been also suggested that the simulated transient response of the phonon temperatures can be practically described by the equivalent thermal circuit model, which is useful for, e.g., projecting the NBTI lifetime during the realistic circuit operations.

Molecular dynamics simulation on longitudinal optical phonon mode decay and heat transport in a silicon nano-structure covered with oxide films

A series of molecular dynamics (MD) simulations have been conducted to investigate the heat transport in terms of the phonon dynamics in nanoscale silicon (Si). This work is motivated by a concern over the stagnation of heat at the drain region of nanoscopic transistors, owing to this, a large amount of optical phonons with a low group velocity are emitted from hot electrons, which are ballistically transferred through channel region. The point of this work is the explicit inclusion of the SiO2 film in the MD simulation of the Si lattice. The calculation results show that longitudinal optical (LO) phonons decay faster as Si lattice thickness decreases and turn into acoustic phonons. In contrast, thermal diffusion rate decreases with Si lattice thickness. Both the decay rate of LO phonons and thermal diffusion rate are not governed by oxide thickness. These results imply that the phonon scattering at the SiO2/Si interface is enhanced by thinning the Si layer. In nanoscopic devices, a thin Si layer is effective in diminishing the optical phonons with a low group velocity, but it hinders the subsequent heat transport.

Effects of atomic disorder on carrier transport in Si nanowire transistors

Effects of oxidation-process-induced atomic disorder on extended electronic states in the channel region of narrow Si nanowire (NW) field-effect-transistors (FETs) are theoretically investigated by using the molecular dynamics, empirical tight-binding, and non-equilibrium Greens function methods. Simulation results show that the injection velocity in n-type Si NW FETs is less affected by the disorder compared to p-type devices, which can be attributed to differences in the in-plane carrier profile.

Impact of Self-Heating Effect on the Electrical Characteristics of Nanoscale Devices

Hot phonon generation and its impact on the current conduction in a nanoscale Si-device are investigated using a Monte Carlo simulation technique. In the quasi-ballistic transport regime, electrons injected from the source lose their energies mainly by emitting optical phonons in the drain. Due to the slow group velocity of the optical phonons, the efficiency of the heat dissipation is so poor that a region with a nonequilibrium phonon distribution, i.e., a hot spot, is created. In this study, we have implemented the hot phonon effect in an ensemble Monte Carlo simulator for the electron transport, and carried out the steady state simulations. Although it is confirmed that the optical phonon temperature in the hot spot is larger than that of acoustic phonons by > 100 K, the electron current density is not significantly affected. The local heating would degrade the hot electron cooling efficiency and the parasitic resistance in the drain, but they have a minor impact on the quasi-ballistic electron transport from the source to the drain.

Molecular dynamics simulation on LO phonon mode decay in Si nano-structure covered with oxide films

A series of molecular dynamics (MD) simulations is conducted to investigate the dynamics of longitudinal optical (LO) phonon in Si nano-structure confined with oxide films. This work is motivated by heat issues in nanoscopic devices; it is considered that the LO phonons with low group velocity are accumulated in the nanoscopic device and the electric property deteriorates. We estimate the relaxation time of the LO phonon and investigate its dependency on the oxide thickness. The calculation results show that the LO phonon decays faster as the oxide thickness increases and turns into acoustic phonon. The result indicates that the presence of SiO2 films promotes the scattering of the phonon and this is effective to diminish the optical phonon.

Numerical simulation of transient heat conduction in nanoscale Si devices

Two numerical simulation techniques are presented to investigate the heating issues in nanoscale Si devices. The first one is the Monte Carlo simulation for both electron and phonon transport, and the transient electrothermal analysis is carrier out in n+-n-n+ device with the n-layer length of 10 nm. The second is the molecular dynamics approach for simulating the atomic thermal vibration in the nanoscale Si/SiO2 systems. It is shown that the lattice temperature at the drain edge is raised by the hot electron injection from the source after turning on the device, and the impact of this phenomenon becomes more significant in the smaller devices due to the worse heat conductivity.