Band structure of strained WZ-type semiconductor NWs

Semiconductors exhibiting the cubic zinc-blende structure in bulk material may crystallize in the hexagonal wurtzite phase when grown as a nanowire. Theoretical works propose significantly altered electronic and optical properties of these wurtzite-type materials but up to now little experimental data is available due to the lack of phase pure material. Recent progress in synthesis, however, enabled the growth of phase pure wurtzite nanowires in various material systems. This proposal aims at the investigation of the band structure of silicon, germanium and gallium phosphide nanowires grown in the hexagonal crystal structure. Spectroscopic and electrical characterization will be employed in order to reveal their optical and electronic properties. These nanowires will be monolithically integrated into a micro electro mechanical device allowing for the dynamic application of high uniaxial strain levels. The distinct effects of mechanical strain on the electro-optical properties will be used to gain further insight into the band structures of this novel class of materials. The project will particularly benefit from the unique combination of knowledge from nanowire synthesis, straining techniques and spectroscopic characterization.

In this work basic optical and mechanical properties of a novel semiconductor material, wurtzite gallium phosphide, have been explored. Tensile strain, i.e. the controlled elongation of the material in one direction, was used to extend the characterization capabilities. For the realization of this project, expertise in mirco electro mechanical systems, nanowire synthesis and integration, low-temperature optical characterization, quantum physics and computational modeling was brought together to achieve an unprecedented characterization framework. The mechanism of light emission from the material was determined, contributing to a better understanding of the optical and electronic properties, to the research on related materials and to applications in the field of solid state lightning. Furthermore, the size dependence of the elastic properties of the material was characterized. The size dependence was found to be determined by the increasing contribution of surface atoms when the diameter decreases. Knowledge of these mechanical properties are prerequisite to the application, e.g. in piezoelectric sensors. Semiconductors like gallium phosphide are the basic material class used in computer chips and solid state light sources like LEDs and lasers. The material can exhibit different crystal structures, i.e. different stacking orders of the crystal planes. Semiconductors only existing in the cubic zinc-blende structure in bulk material may crystallize in the hexagonal wurtzite phase when grown as a nanowire. Nanowires are small elongated structures with a diameter on the order of 1/1000 of a human hair. Theoretical works propose significantly altered electronic and optical properties of these wurtzite-type nanowires, but up to now little experimental data has been available due to the lack of phase pure material. Recent progress in synthesis, however, enabled the growth of phase pure wurtzite nanowires in various material systems. This work focused on the investigation of the band structure of wurtzite gallium phosphide nanowires.