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Quantum Transport Beyond the Ballistic Limit
von Reto RhynerIn 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.
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.