The three-dimensional (3D) nanoscale structure of III-As nanowires is correlated with optical and electronic property measurements to deconvolve the contributions of strain, composition, and crystal structure to characteristics of interest for future electronic and optoelectronic devices. Multiple advanced two-dimensional (2D) and 3D characterization techniques are employed such as atom probe tomography, nano-probe X-ray diffraction microscopy, Bragg coherent diffraction imaging, and Bragg X-ray ptychography. Atom probe tomography is used to map the distribution of Si dopants in catalyst-free InAs nanowires at different in-situ doping conditions used during MBE growth. Doping is homogeneous in the core of the nanowires at concentrations of mid-10^18 cm^-3 regardless of the dopant flux, but an excess of Si is observed near the surface of the nanowires, overlapping with a native oxide. Measurement of carrier concentrations at each nominal doping level are in agreement with the measured chemical doping levels, suggesting nearly complete dopant activation in the nanowire core. A new approach to coherent diffraction imaging called multiangle Bragg projection ptychography is formalized and demonstrated experimentally. 3D strain and structure in InGaAs nanowires was reconstructed with better than 50 nm and 2 nm spatial resolutions respectively. Bragg coherent diffraction imaging is performed on GaAs nanowires with embedded InGaAs quantum wells (QWs) of different thickness on each facet. 3D reconstructions of in-plane strain fields reveals the likely presence of dislocations at the thicker facets (10,20 nm) and coherent growth at the smaller facets (2,4 nm). Correlative imaging is performed on InGaAs QWs in a GaAs nanowire which show a blue shift in emission toward the top of the nanowire via spatially resolved cathodoluminescence (CL). Directly correlated to CL, electron back-scatter diffraction reveals a polytypic structural change aligned with the shift in emission. X-ray nano-probe diffraction is used to investigate the QW strain along the nanowire length, but shows no variation. Atom probe tomography correlated with CL shows no change in QW morphology, but an increase in In content on the wurtzite portion of the QW is observed. Band structure calculations are used to determine the effect that each structural change has on the emission shift, revealing that composition and polytype structure both play an important role.

Creator

• Hill, Megan Olivia

DOI

• https://doi.org/10.21985/n2-vczc-n474

Subject

• Physics
• Materials Science

Language

• en

Alternate Identifier

• etdadmin_upload_771146
• http://dissertations.umi.com/northwestern:15331

Keyword

• synchrotron
• nanowires
• coherent X-rays
• III-Vs
• atom probe tomography
• correlative imaging

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright