Researches On Ballistic Phonon Transport In A Semiconductor Nanowire

Recently, the design, manufacturing, and application of electro-mechanical devices at micron and nanometer length scales have been investigated extensively, and larger progress has been achieved. With the further development of technology, the quantum device become more and more complex. Correspondingly, we have to consider the thermal effect, which is a important factor to affect the properties of quantum device. To design a better system for heat-dissipation, it is very important for us to know the physical mechanism for thermal transport. In this thesis, we mainly investigate the ballistic phonon transmmission and thermal conductance in semiconductor nanowire.We investigate the influence of material properties of two coupled nanocavities on ballistic phonon transport in a semiconductor nanowire at low temperatures. For two identical-size nanocavities their material properties obviously affect the phonon transmission, the frequency of resonant transmission, especially the number n, and positions of the low-frequency gaps. When the two cavities are made of different materials, a new type gap appears in the spectrum. Moreover their positions depend respectively on the material parame-ters of each cavity. At lower temperatures, the thermal conductance can be enhanced by using different material parameters of cavities from those of the main nanowire.These results suggest new directions that can be explored for forming gaps and resonance phonons, for controlling thermal conductance at different temperature ranges in nanophononics.The transport properties of a converging-diverging semiconductor nanowire has investigated based on the scattering-matrix method. We find that the transport properties of phonon is affected by the narrowest width and the length of the converging-diverging region. The narrowest width is very important to transport, where the phonon transmission coefficient increases with the increase of the narrowest width. The length of the converging-diverging region affect the transport of single mode obviously. The oscillation of single mode transmission coefficient is demonstrated for the small length of the converging-diverging region. However, the oscillation is very weak for larger length of the converging-diverging region. These results may be important as basis and reference for us to design devices with special function.