Epitaxial growth and characterisation of GaAs nanowires on Si for optoelectronic device applications
Creators: name: Kang, Jung-Hyun
Publisher: name: The Australian National University
Year: 2012
Subjects: subject: TK7874.85.K36 2012
subject: Nanowires.
subject: Optoelectronic devices.
subject: Molecular beam epitaxy.

description: This thesis examines the Au-assisted growth of GaAs nanowires by MOCVD, and how this growth process can be tailored to produce well-aligned nanowires on Si suitable for applications in electronics and optoelectronics. i) Improving the morphology of GaAs nanowires on Si: Straight, vertically aligned GaAs nanowires were grown on Si (111) substrates coated with thin GaAs buffer layers. V/III precursor ratio and growth temperature are crucial factors influencing the morphology and quality of buffer layers. A double layer structure, consisting of a thin initial layer grown at low V/III ratio and low temperature, followed by a layer grown at high V/III ratio and high temperature, was crucial for achieving straight, vertically aligned GaAs nanowires on Si (111) substrates. An in-situ annealing step at high temperature after buffer layer growth improved the surface and structural properties of the buffer layer, which further improved the morphology of GaAs nanowire growth. ii) Improving the crystal structure of GaAs nanowires on Si: Defect-free GaAs nanowires were grown on Si by using either a two-temperature growth mode consisting of a short initial nucleation step under higher temperature followed by subsequent growth under lower temperature or a rapid growth rate mode with high source flow rate. These two growth modes not only eliminated planar crystallographic defects but also significantly reduced tapering. Core-shell GaAs-AlGaAs nanowires grown by two-temperature growth mode showed improved optical properties with strong photoluminescence and long carrier life times. iii) Optimizing the growth conditions for perfect GaAs nanowires on Si: By systematically manipulating the arsine (group-V) and triethylgallium (group-III) precursor flow rates, it was found that TMGa flow rate has the most significant effect on nanowire quality. Defect-free GaAs nanowires with minimal tapering and long exciton lifetimes were obtained at the highest TMGa flow rates. It was observed that Ga adatom concentration significantly affacts the growth of GaAs nanowires. iv) Band-gap engineering for GaAs nanowires on Si: Based on the defect-free GaAs nanowires grown by the two-temperature growth mode, highly strained core-shell nanowires of excellent optical quality were grown with GaAs cores and GaP shells. Photoluminescence from these nanowires was observed at energies dramatically blue-shifted from the unstrained GaAs free exciton emission energy. Using Raman scattering, it was possible to separately measure the degree of compressive and shear strain of the GaAs core and the Raman response of the GaP shell was related to tensile strain. This work presents significant advances in the growth of exceptionally high quality GaAs nanowires on Si, and reveals intriguing behaviour in the growth of nanoscale materials. These findings will greatly assist the development of future GaAs nanowire-based electronic and optoelectronic devices, and are expected to be more broadly relevant to the rational synthesis of other III-V nanowire materials on Si substrate and ultimate integration of III-V semiconductor optoelectronics and Si based microelectronics. -- provided by Candidate.