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S
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Series in
Microelectronics
edited by
Qiuting
Huang
Andreas Schenk
Mathieu Maurice Luisier
Bernd
Witzigmann
Reto Rhyner,
Quantum Transport Beyond the Ballistic Limit.
2015. XIV, 148 pages.
ISBN 978-3-86628-545-3
Abstract:
In this thesis an existing
full-band and atomistic quantum transport simulator based on the nonequilibrium Green’s function (NEGF) formalism is
extended to model fully coupled electron-phonon transport. Within this new
approach the electrothermal properties of nanoscale
devices are computed and investigated.
To model the electron properties the semi-empirical sp3d5s∗ tightbinding (TB) method is
utilized, while lattice oscillations (phonons) are described based on the
valence-force-field (VFF) formalism. Both methods
reproduce the full bulk dispersion relations of a wide range of materials, they
provide an atomistic resolution of the simulation domain and they can be
extended to nanostructures in a relatively straightforward manner.
Full-band and atomistic quantum transport models are
necessary to correctly capture all the quantum mechanical effects and to
properly resolve the atomic granularity of novel nanoelectronic
devices. Such approaches are very accurate, but computationally very demanding,
especially in the presence of electron-phonon scattering. It is therefore
necessary to introduce physical and numerical approximations to make
dissipative quantum transport simulations computational feasible. These
approximations are validated by calculating the phonon-limited low-field
mobility of various semiconductors and comparing them with the exact solution
of the linearized Boltzmann transport equation.
As a next step fully coupled electron-phonon transport
simulations are performed in the NEGF formalism. The required scattering selfenergies are derived and discussed. They drive both the
electron and the phonon populations out of equilibrium; energy is exchanged
between them, while the total energy remains conserved. This gives rise to
local variations of the lattice temperature and the formation of hot spots and
self-heating effects. The electrothermal properties
of ultra-scaled silicon gate-all-around nanowire field-effect transistors (Si
GAA NWFETs) are then investigated with the newly developed capabilities. It is
found that the resulting self-heating effects strongly increase the
electron-phonon scattering strength and lead to a significant reduction of the
device ON-current.
Finally, anharmonic
phonon-phonon interactions are incorporated into the fully coupled electrothermal quantum transport approach through an
additional scattering self-energy. The anharmonic
phonon decay process allows to soften the observed
artificial accumulation of high energy phonons in the Si GAA NWFETs. As a
consequence more realistic electrothermal transport
simulations become feasible.
About the Author:
Reto Rhyner
was born in Glarus, Switzerland, on April 8, 1983. He enjoyed his childhood in Linthal. From 2003 to 2008 he studied Electrical
Engineering and Information Technology at the ETH Zurich where he received his
Dipl. Ing. Degree (MSc) in 2008.
In the same year he started working at Air-On AG in Cham, Switzerland as a
R&D Engineer. He supported the development of novel air conditioning
systems by thermodynamic modeling and simulation. In late 2010, Reto came back to ETH Zurich and joined the Integrated
Systems Laboratory as a research and teaching assistant. His research interests
focus on numerical simulation of quantum transport and scattering mechanisms in
nanoscale devices.
Keywords: Quantum
Transport, Scattering, Atomistic, Nanoscale Devices, high-performance
computing, Computational Physics, Self-heating, Fully-coupled Electron-Phonon
Transport, Nonequilibrium Green's Function (NEGF), Electrothermal Properties.
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