Semiconductor Nanowires: Epitaxy and Applications

Sammanfattning: Semiconductor nanowires are nanoscale objects formed by bottom-up synthesis. In recent years their unique properties have been exploited in fields such as electronics, photonics, sensors and the life sciences. In this work, the epitaxial growth of nanowires and their applications were studied. Heteroepitaxial growth of III-V nanowires on silicon substrates was demonstrated. This may enable direct band gap materials for optoelectronic devices, as well as high-mobility, low-contact resistance materials for electronics, to be integrated directly on the Si platform. Furthermore, gold-free nanowire synthesis on Si was demonstrated, which offers an advantage in terms of compatibility with established Si processing. Controlled nanowire synthesis by employing lithography was demonstrated. This combination of established "top-down" planar processing, and "bottom-up" nanowire growth, enables deterministic synthesis with individual nanowire site control. The process was first demonstrated with electron beam lithography and later extended to nanoimprint lithography, which is a parallel, high-throughput method, suitable for commercial volumes. Nanowire applications were demonstrated by three examples: (i) Vertical light-emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates. LED functionality was established on both kinds of substrates. This provided a direct demonstration of light-emitting devices on Si made possible by heteroepitaxial III-V nanowire growth on Si. (ii) A single-electron transistor constructed from a heterostructured nanowire with an InAs island sandwiched between two InP barriers. The narrow diameter of the nanowire provides the lateral confinement, and the tunnel barrier resistances are tunable by varying the InP barrier thickness. Coulomb oscillations and Coulomb blockade with a charging energy of approx. 4 meV were observed. (iii) Sensory nerve cell interactions with nanowires. Substrates covered with 2.5 um long and 50 nm diameter nanowires supported cell adhesion and axonal outgrowth. The cells interacted closely with the nanostructures, and viable cells penetrated by wires were observed, as well as wire bending due to forces exerted by the cells.