Analysis of Structure, Composition and Growth of Semiconductor Nanowires by Transmission Electron Microscopy

Sammanfattning: Nanowires have the potential to be a very flexible platform for the design of semiconductor devices. In nanowires it is possible to form crystal structures not found in the bulk materials under normal conditions, and to combine different III-V and group IV materials into axial or radial heterostructures. As quite complex structures can be formed, both intentionally and unintentionally, characterization of the crystal structure and composition is important. In this thesis, various transmission electron microscopy techniques are presented for this purpose. High resolution imaging can directly visualize the crystal structure, including twinning and stacking faults. The polar nature of the III-V materials leaves one more parameter to be determined. In order to determine polarity from high resolution images it is not only necessary to improve the resolution further by aberration correction, but in addition the local orientation of the sample must be determined. Convergent beam electron diffraction is an alternative method with much lower demands on the microscope and operator, and can be adapted to suit most materials and crystal structures. Transmission electron microscopy also provides several methods for determining and mapping the composition of the nanowires. It is important in all cases to avoid damaging the nanowires during the acquisition of the analytical signal. In the most commonly used method, energy dispersive X-ray spectroscopy, this can be achieved by spreading the electron dose over as large an area as possible. If there is only a single unknown parameter for the composition, alternative methods such as the shift in plasmon energy with composition can be used instead, as they have higher collection efficiencies. In order to improve the nanowires in terms of crystal structure and composition, these must be connected to the dynamic processes occurring during growth. Occasionally these processes can be inferred from the fully formed nanowires after growth, but ideally one would like to observe the growth in-situ in the microscope. This is usually possible only with highly specialized environmental microscopes. In this thesis, nanowire growth in much simpler closed cells is demonstrated. Although the growth conditions could neither be precisely measured nor controlled, the closed cells made it possible to observe for the first time growing InAs nanowires in-situ in a conventional transmission electron microscope.