Optical investigations of low-dimensional semiconductor structures

This thesis discusses two optical emitters, gallium arsenide (GaAs) nano-wires (NWs) and 2D molybdenum disulfide (MoS2) films, which have the potential to be integrated into silicon (Si) and graphene based electronics. Optical properties of these systems are studied using a combination of micro-photoluminescence spectroscopy (μ-PL) and microscopy techniques to understand the effects of structure and environment on light emission. Firstly, it is demonstrated that GaAs NWs can be grown directly on Si using molecular beam epitaxy (MBE). By applying a capping material to the NW surface, in this case GaAsP, we achieve an enhancement of emission yield of up to 104, as well as a method of controlling emission wavelength through the application of lattice strain. The second part of this thesis concerns 2D sheets of MoS2 under 5 atomic layers thick, a direct bandgap semiconductor which can be integrated into graphene electronics. A method for producing these films is discussed which utilises breaking of Van der Waals forces between atomic planes using the mechanical cleavage technique. In this work we show that the shape of PL emission from MoS2 is heavily effected by the level of doping in the film, which is in turn influenced by interactions with dielectric environments. In the final section of this thesis the problem of irregular emission spectra is addressed and reproducibility of emission properties is found to increase with the deposition of a dielectric capping layer on the MoS2 surface. By utilising the subsurface microscopy technique Ultrasonic Force Microscopy, we show this improvement occurs due to increased mechanical bonding between MoS2 and the SiO2 substrate, which increases the stability of the charge environment.