Atomic scale characterization of semiconductor non-planar nanostructures

Resumo

Semiconductor nanostructures are building blocks with high potential to be integrated in a wide variety of technological devices, in addition to be ideal platforms for the study of fundamental physical principles. Importantly, understanding the formation and behavior of these structures involves their characterization at atomic scale. Knowing the exact atomic arrangements is highly useful in order to face some questions related to the growth mechanisms promoting the formation of different nanostructures, and thus allowing the smart synthesis of materials with the exact sought properties. Along this thesis, we perform detailed atomic characterizations of binary compound (AB) semiconductor non-planar nanostructures, by means of (Scanning) Transmission Electron Microscopy ((S)TEM) techniques, along with computer simulations when needed, focusing on the visualization and identification of individual atomic columns. By the identification of the atomic constituents within the lattice crystal in the growing structure, we investigate polarity related issues, as the unidirectional polar growth or the polarity preservation/inversion. The system analyses at atomic scale becomes especially important when more than one material composes the nanostructures, since the matching among phases has a determining role in the system behavior. Mainly depending on the lattice misfit among the connected phases and the shape and size of the nanostructures, the mismatch strain will be released as plastic and/or elastic lattice distortions, affecting differently the performance of the system. Moreover, the combination of different materials allows the creation of smaller structures within the nanostructures, achieving quantum confinement under a certain threshold size, and known as quantum structures. The final properties of these architectures rely on the number of dimensions spatially confined, in addition to the nature of the material and their size, requiring accurate atomic characterizations. These three points are covered along this dissertation. The manuscript structure is the following: the introductory chapter (Chapter 1), giving an overview about semiconductor nanostructures, focused on nanowires, including heterostructures and quantum structures within nanowires, is followed by a chapter describing the methodology employed (Chapter 2) for the analyses performed, i.e., (S)TEM techniques, aberration correction, strain measurements and image simulation and related computing tools. The results presented are divided in four chapters: - Chapter 3, focused on the polarity issue in binary compound nanowires, and its implications on other related systems (tripods and tetrapods). - Chapter 4, which also deals with the polarity, but in 2D-like nanostructures. - In Chapter 5, the relaxation mechanisms of the mismatch strain in heterostructured nanowires are analyzed. - Chapter 6 addressed the structural and optical characterization of quantum structures within nanostructures. Finally, Chapter 7 summarizes the main conclusions of the manuscript, along with a brief outlook.