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Phononic and Photonic Semiconductor Nanostructures - Transport, Coherence, Confinement, and Dynamics of Phonons and Excitons in Single and Periodic Nanostructures
Semiconductor nanostructures form the basic building blocks for the majority of modern electronic and photonic components. The continuous reduction of their dimensions in the past decades has opened possibilities to manipulate phononic and photonic properties at the micro- and nanoscale. With decreasing structure sizes, heat in the form of atomic vibrations can drastically modify material properties and limit the performance of devices. This fact poses a major challenge for the design of nanoscale devices due to the complexity of controlling the heat flow at the nanoscale. The present work provides an overview of the experimental methods and physical processes that are relevant for thermal investigations of semiconductor nanostructures. The experimental methods dis-cussed in this work include optical techniques such as micro-Raman thermometry in its one- and two- laser version, frequency and time-domain thermoreflectance, and femtosecond pump-probe spec-troscopy based on asynchronous optical sampling, as well as contact-based techniques such as the 3-omega method, scanning thermal microscopy, and the microchip suspended platform. A comparative overview of the state-of-the-art of these techniques highlights the most suitable experimental ap-proach for materials with different structures and dimensions and identifies their main advantages and disadvantages with regard to spatial resolution and temperature sensitivity. Following the discussion of the experimental techniques, the effects of geometry and artificial perio-dicity on the thermal properties of materials with reduced dimensionality are examined. The text pro-vides an overview of phonon scattering mechanisms, phonon lifetimes, and the effects of phonon confinement on the modification of thermal properties in nanostructures. In addition, recent advances in the study of coherent and non-coherent phonon heat conduction in materials with reduced dimen-sionality are explained. These include thin films and quasi-2D membranes, nanostructures with sec-ond-order periodicity such as two-dimensional phononic crystals and superlattices, nanowires, and quantum dots. The work highlights own publications that are part of this thesis in conjunction with recent advances in the understanding and control of phonon mediated heat propagation.