Optoelectronic properties of semiconductor nanowires in the infrared regime
Keywords: Semiconductor
THz spectroscopy
Infrared Regime
Nanowires
Photoluminescence spectroscopy

Semiconducting nanowires are promising elements for potential use in nanoscale devices. Due to their versatility, they can be utilised for a wide range of applications. To correlate the nanowires with their best possible device applications, a powerful all-optical platform was developed to characterise both their electrical and optical properties by combining terahertz spectroscopy and photoluminescence spectroscopy. The focus will be on three different material systems (gallium phosphide, silicongermanium, indium arsenide) and their potential use for optoelectronic devices in the infrared region of the electromagnetic spectrum.
Theory has predicted a direct bandgap for silicon-germanium alloys grown in a hexagonal crystal phase, but it has not been experimentally proven so far. This thesis demonstrates the realization of hexagonal silicon and silicon-germanium via template-assisted nanowire growth. The characterisation of these hexagonal nanowires is depicted via a series of terahertz and photoluminescence spectroscopy measurements to study their electrical and optical properties and therefore confirm information about their bandgap. Firstly, hexagonal gallium phosphide nanowires, including the effect of the different growth recipes and defects, are studied as they form the template (nanowire core) for later shell growth of hexagonal silicon and silicon-germanium. The structural flaws in gallium phosphide nanowires under different growth conditions are qualitatively examined; these defects can be further transferred to the shell growth, and thus determine the shell quality. In the following, the core-shell nanowires, including hexagonal silicon shell and hexagonal silicongermanium shell (with varying germanium contents), are investigated. Defect density is found to correlate strongly with the photoconductivity lifetime. For the first time, the surface recombination velocity of hexagonal silicon is determined. Meanwhile indications of a direct bandgap of hexagonal silicon are observed and elaborated on. Theoretical calculations are used to support the rare case of a direct bandgap in silicon. It is anticipated that such silicon nanowires could be designed as high-emission-efficiency nanolasers, which could revolutionize the field of silicon-based technology.
With the same characterisation tools, a second semiconducting nanowire system — indium arsenide and its compound indium arsenide antimonide — is investigated. Such a type of material has naturally a direct bandgap, which can be tuned from 0.1 eV to 0.4 eV (by changing its composition). Therefore, the study of these indium-arsenide based nanowires aids in the design of higher-performance devices. To give a thorough overview, nanowires grown via metal organic vapour phase epitaxy and molecular beam epitaxy are compared. They are also passivated with Al2O3 because these nanowires are surface sensitive. The influence of diameter, crystal structure and passivation layer on their electrical and optical properties are systemically studied. Furthermore, the bandgap engineering of indium arsenide antimonide and the benefits of doping are discussed. Finally, indium arsenide nanomembranes are proposed as an alternative to the nanowires for the optimal design of future nanoscale optoelectronic device systems.