DIRECTIONAL NANOEPITAXY OF NANOWIRE ASYMMETRIC SHELL HETEROSTRUCTURES AND IMPLICATION FOR SENSORS

Abstract

Nanoscale wires, nanowires, have shown great promise for the next generation of ultra-sensitive sensors. This thesis presents a path for using directional nanoepitaxy to enable the mass-production of nanowire sensor devices. Core–shell nanowires with asymmetric shells, created using directional nanoepitaxy, exploit lattice mismatch between the core and asymmetric shell to generate strain gradients, resulting in nanowire bending. This research investigates the mechanisms behind this bending process, specifically examining the directional nanoepitaxy of GaAs–InP and GaAs–(Al,In)As core–shell nanowires by molecular beam epitaxy. Through X-ray diffraction and electron microscopy, variations in curvature and strain along these nanowires, linked to local differences in shell thickness, have been observed. Modeling using linear elastic theory indicates that these variations are due to differences in the flux contact angle along the nanowire as it bends. Nanowire growth conditions are found to lead to variations in the shell distribution around a nanowire's cross-section. Controlling temperature during the directional growth of III-V nanowires, we influence the diffusivity of group III adatoms, resulting in two possible growth modes: In diffusion-limited growth and group V-controlled growth. We employ phosphorus-controlled nanoepitaxy to grow asymmetric InP shells on GaAs cores, examining the shell distribution and twisting of the nanowires. Transmission electron microscopy analysis of the nanowire cross-sections reveals that the shell distribution is relative to the phosphorus flux. Using analytical electron tomography reconstruction techniques to determine the core-shell geometry along a nanowire, we find that twisting occurs to minimize strains energy when the bending direction is not aligned with the <11 ̅0> or <112 ̅>crystallographic directions. By synthesizing connected nanowire pairs and forming nanowire arches, we demonstrate their potential for a template to fabricate sensor-based transistor devices. The research highlights the potential of bent nanowire heterostructures for next-generation nanotechnology applications, particularly in creating innovative device geometries for bottom-up fabrication.