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...[ Read more ]

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.