Crystal Structures in GaAs Nanowires: Growth and Characterization

With their nanometer size cross-section and high aspect ratio, semiconducting nanowires have properties that make them promising as building blocks in future electronic and optoelectronic devices. Because of their small size, their optical and electrical properties can differ from their bulk counterparts, and their geometry allows for material combinations not accessible in thin films. Moreover, nanowires are possible to grow with crystal structures which are different from their stable phase in the bulk. The focus of this thesis is on tuning the crystal structure of Au-seeded GaAs nanowires between the zincblende (ZB) structure, which is stable in the bulk, and the metastable wurtzite (WZ) structure.
Metal-organic vapor phase epitaxy (MOVPE) has been used to demonstrate a single temperature approach to achieving high quality WZ-ZB heterostructures. To increase the complexity of the nanowires, heterostructures with controlled ZB inclusions in WZ nanowires were also grown. Detailed post-growth analysis of the seed particle composition suggested a Ga-rich growth environment for WZ, whereas it is possible to grow defect-free ZB in As-rich conditions. To achieve a sharp interface between ZB grown with As-rich conditions and a following WZ segment, the local growth environment has to be returned to Ga-rich before growth is continued. To gain further control of the growth of polytypic GaAs nanowires, the possibility of using HCl as an additive during growth has been explored.
Using in situ transmission electron microscopy, real time growth studies of GaAs WZ-ZB nanowires revealed distinct differences between the two polytypes for the growth dynamics at the seed particle-nanowire interface. By increasing or decreasing the Ga content within the Au-Ga alloy seed particle, crystal structure tuning was achieved, with the wetting angle of the seed particle as the trigger changing between growth of ZB and WZ.
The different atomic arrangements in WZ and ZB GaAs mean that the two polytypes have different electronic, optical and surface properties, even though they have the same composition. This thesis work has determined the nature of the faceting of WZ-ZB heterostructures and the lattice parameters of WZ and the higher order polytype 4H, as well as the band gap of WZ and its and band alignment with ZB.