Si and Ge nanostructures are widely used in modern nanoelectronic and nanophotonic
devices, and fundamental understanding of phonon transport in Si and Ge nanostructures is
essential for thermal management and seeking for thermoelectric materials. Due to the strong
size effect, mode-wise phonon transport properties attract more interests in nanoscale regime
where new phonon transport phenomena and underlying mechanisms can emerge.
The first-principles calculations combining with phonon Boltzmann transport and lattice
dynamics have been proved an accurate method to obtain the mode-wise phonon transport
properties. For relatively complex Si and Ge nanostructures, the first-principles method usually
fails because of the huge computational demand. Density-functional-based tight-bind...[
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Si and Ge nanostructures are widely used in modern nanoelectronic and nanophotonic
devices, and fundamental understanding of phonon transport in Si and Ge nanostructures is
essential for thermal management and seeking for thermoelectric materials. Due to the strong
size effect, mode-wise phonon transport properties attract more interests in nanoscale regime
where new phonon transport phenomena and underlying mechanisms can emerge.
The first-principles calculations combining with phonon Boltzmann transport and lattice
dynamics have been proved an accurate method to obtain the mode-wise phonon transport
properties. For relatively complex Si and Ge nanostructures, the first-principles method usually
fails because of the huge computational demand. Density-functional-based tight-binding
(DFTB) method, together with the suitable parameters, combines the accuracy of first-principle
calculations and the efficiency of tight binding method, offering an optimal option for
investigating the mode-wise phonon transport properties in complex Si and Ge nanostructures.
In this thesis, to employ the DFTB method, the parametrizations for Si-Si, Ge-Ge and Si-Ge
interactions are developed aiming at calculating the mode-wise phonon transport properties of
Si-Ge systems. Using the new parameters, the DFTB calculations of the harmonic and
anharmonic phonon properties of 3D and 2D Si-Ge systems have been conducted to testify the
efficiency, accuracy and transferability compared to experimental measurements and first-principle
results. Also, the DFTB method with the new parameters is systematically applied to
the investigation of in-plane thermal transport properties in ultrathin silicon films of a few
nanometer thickness, and SiGe[001]
N+N superlattices with perfect interfaces and with slightly
interdiffused interfaces at short periods.
The new developed parameters for Si-Ge systems, called Therm parameter set, are able to
predict the harmonic and anharmonic phonon properties of bulk silicon, germanium, zinc-blend
SiGe, 2D silicene and germanene with good accuracy, efficiency and transferability compared
to the first-principles method and molecular dynamics simulations. The good performance of
Therm parameter set makes it an efficient choice for the studies of thermal transport in relatively
large Si-Ge systems.
Using the DFTB method with Therm parameter set, the mode-wise phonon transport in
ultrathin Si films of a few nanometer thickness (0.77 – 1.90 nm) and the effects of surface
defects are systematically investigated. The ultrathin Si films with naturally reconstructed
surfaces still show a counterintuitively high thermal conductivity (~30 W/m-K at 300 K) with
relatively weak size dependence, which demonstrates that dimensionality reduction alone
cannot suppress phonon transport in ultrathin films efficiently. The thermal conductivities of
ultrathin films are very sensitive to surface defects and a further 10-fold reduction in thermal
conductivity can be achieved using atomic-level surface defects due to the much enhanced
phonon scatterings in low frequency regime.
Using the same method, the mode-wise phonon transport properties of SiGe[001]
N+N
superlattices with perfect interfaces and with slightly interdiffused interfaces at short periods
are also directly investigated. At short periods, as the period increases, the cross-plane thermal
conductivity first decreases from a high value then comes to saturation. Detailed analysis relates
the trend of thermal conductivity with the behaviors of mode-wise phonon properties: the group
velocities of phonons with the frequency between 1 and 6 THz keep a descending trend due to
the flattening optical phonon branches, and the lifetimes of phonons below 1 THz first reduce
and then begin to increase, because of the competition between the growing number of
flattening optical phonon branches and reduced interface density on scattering phonons.
Meanwhile, the thermal conductivity of the SiGe[001]
8 with slightly interdiffused interfaces is
found to be 32.26 % lower than that of the SiGe[001]
4+4 superlattice. Compared to the
SiGe[001]
4+4 superlattices with perfect interfaces, the slightly interdiffusion of interfaces
weakens the coherent phonon transport, reduces the phonon group velocity and enhances the
phonon scattering strength below 1 THz. Mode-wise analysis also demonstrates that the
reduction of phonon group velocity is the main reason for the low thermal conductivity
compared to the reduced phonon lifetime.
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