Boras, Georgios; (2022) Structural and Optical Characterization of III-V Nanostructures Monolithically Grown on Si Substrates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Group III-V semiconductor nanostructures have emerged as an important material platform over the past decades for wide-range device implementation in the field of electronics and optoelectronics. Among them, nanowires (NWs) are particularly attractive owing to the elastic strain relaxation through their sidewall facets which allows for the combination of lattice mismatched materials. Hence, optically active III-V materials become compatible with the mature and prevalent Si platform. Moreover, NWs are ideal for hosting quantum dots (QDs) ensuring their deterministic positioning and uniformity. This configuration opens the route for sophisticated applications including single photon emission, a crucial function in quantum information processing. In addition, another type of nanostructures that has attracted attention is the two-dimensional nanosheets, whose principal benefit is the band structure tuning from bulk to 2D by modulation of their thickness. Consequently, they are established as promising blocks for various optoelectronic devices and applications. In the current thesis, we reported the growth of self-catalysed AlGaAs NWs monolithically on Si (111) substrates via solid-source molecular beam epitaxy (MBE). The self-formation of an Al-rich shell is exhibited, which is thicker at the base and thins down towards the NW tip, while it demonstrates wide alloy fluctuations. The predominantly ZB structure presents twin defects and occasional WZ insertions, further increasing the intricacy of the NWs. The optical probing via photoluminescence reveals fully tuneable emission with the Al content of the alloy. Among the morphological variations of AlGaAs NWs, the branched NWs are of unique interest. The branching events increase with Al content, while the branches are confirmed to grow on the NW trunks epitaxially. In addition, complex compositional distribution in the branches is presented, as Ga-rich stripes along the growth direction of the branches, attributed to the different nucleation energies on different faces at the liquid/solid interface of the branch, intersect with Ga-rich stripes perpendicular to them, deriving from the rotation of the sample during growth. Moreover, self-catalysed, single GaAs/AlGaAs dot-in-wire structures have been designed and grown by inserting a short GaAs segment in each AlGaAs NW. The exhaustive optical probing reveals centrally localized peaks, with a decently narrow linewidth of roughly 490 μeV. The QD emission is comprised of an exciton and a biexciton transition, while a high degree of polarization is noticed when compared to the AlGaAs NW-related emission. The above characteristics are important steps towards achieving single photon emission. Finally, we optically inspect InAs nanosheets grown via MBE via photoluminescence measurements. Pristine nanosheets exhibit surface charge via carrier trapping mechanisms at the surface states, which is suggestive of the “memory effect”. The impact of sulphur passivation and core/shell configuration on the optical response of the nanosheets is evaluated. In addition, we fabricated an optoelectronic memory unit based on pristine InAs nanosheets, adopting a field-effect transistor configuration, which demonstrates negative photoresponse with good reproducibility and ultra-low power consumption.

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