Investigation of nanowires by x-ray diffraction methods

Semiconductor nanowires have become an important class of nanostructures, with a range of possible applications from electronics to optoelectronics. In particular, III-V compound semiconductors exhibit superior properties such as high carrier mobility and direct band gap. In nanowires, the small diameter as well as axial or radial heterostructures allow to exploit quantum mechanical confinement effects and increased density of states, to tailor band gaps and band alignments and to improve material properties compared to bulk material. In addition, the small diameter allows to epitaxially grow III-V semiconductors on a variety of substrates, including silicon, without formation of extended defects within the nanowires even for large lattice mismatches. While this is true for "simple" nanowires (one component without heterointerfaces), more complex nanowire structures as mentioned above are still prone to defect formation. The key objective of this project is to determine the structural properties of semiconductor nanowires using x-ray diffraction. In particular, we are interested in the occurrence and distribution of two particularly important types of defects in the wires: Dislocations occur to relieve strain, especially at the interface between nanowire and substrate, as well as in core-shell nanowire heterostructures with large mismatch or thermal strain. The second defect type are stacking defects of the crystal lattice, which comprises stacking faults and twin planes, but also a change in the lattice structure from cubic zinc-blende to hexagonal wurtzite, a phenomenon rather exclusive to nanowires grown along the [111]B direction. The structural data obtained from this project will be correlated to experiments on optical and electrical properties of nanowires, and will serve as input for theoretical calculations of these properties. Thus the project will advance the understanding of nanowire growth and their properties.

Within this project, the x-ray based analysis of semiconductor nanowires was further developed, and applied to different nanowire systems. The main focus was on the use of nanofocused synchrotron beams to achieve a local analysis of single wires and heterostructure as quantum dots embedded into such nanowires. Using the combination of high-resolution diffraction and elaborate finite element modelling, the distributions of strain and chemical composition in the vicinity of such heterostructure has been established. A second major achievement of the project is to establish precise and reliable structural data on hexagonal crystal polytypes, which can be fabricated so far only in nanowires. Particularly relevant in this context are investigations of hexagonal silicon, which might open a way to achieve direct-bandgap Si-based material for integration with Si electronic circuits