Growth, characterization, and fabrication of GaAs core/shell and axial nanowire devices

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Tunnel diodes
Surface passivation
Solar cell

Semiconductor nanowires are promising candidates for the emerging nano-scale optoelectronics. They provide opportunities for novel axial and lateral designs with the possibility of improvement in the device performance and reduction in the size. An essential requirement for this research and development is the fundamental understanding of the electronic, electrical and optical properties of semiconductor nanowires. This thesis aims to address several critical factors that limit commercial integration of GaAs nanowire devices. The latter includes investigation of novel nanowire growth methods and understanding the charge transport properties in axial and radial structures. I grew gold-catalyzed GaAs nanowires via the vapour-liquid-solid mechanism using the metalorganic chemical vapor deposition technique. A thin GaP shell was used to passivate the sidewall surface states in GaAs nanowires. Electrical and optical measurements were carried out on the core/shell GaAs/GaP nanowires to demonstrate unpinning of the Fermi level by improvement in the nanowire resistivity and photoluminescence, respectively. Control of the surface recombination velocity in GaAs nanowires was also achieved using a thin lattice-matched InGaP passivating shell. This was determined through an enhancement of the minority carrier diffusion lengths in GaAs/InGaP nanowires measured using electron beam induced current technique. In addition, axial GaAs nanowire p-n junctions were fabricated to demonstrate a free-standing single nanowire photodetector. The degree of the p-n junction abruptness and the impact of the Au reservoir effect was studied by a numerical modeling of the corresponding electrostatic potential. This model was further verified using electron holography measurements. Radial GaAs nanowire p-n junctions combined with a novel growth technique lead to development of GaAs homostructure radial tunnel diodes. A lithography-free growth method took advantage of an array of Ga2O3 coated GaAs pedestals to electrically isolate nanowire devices from the substrate. Nano-probe measurements of radial GaAs nanowire p-n junctions indicated clear tunneling current-voltage properties.

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Simon Watkins
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.