When 12:00 PM - 2:00 PM Aug 23, 2017
Where 3158 HH Dow
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Compositional Inhomogeneity and Defects in Nanomaterials for Optoelectronics

Lifan Yan
Thesis Defense

Semiconductor nanostructures are used in various optoelectronic applications such as solar cells, lasers and quantum computing. As the size shrinks and the complexity of these nanostructures grows, it is crucial to understand how compositional inhomogeneities occur within these structures. This work focuses on compositional inhomogeneities within nanostructures for a number of III-V material systems, and how it affects the optical properties of them.
This thesis is organized in two parts: the first part concentrates on comparing and contrasting the application of Atom Probe Tomography(APT) and Scanning Transmission Electron Microscopy(STEM) in investigating composition inhomogeneity in III-V semiconductor alloys. 2D layers, such as InGaN/GaN and GaAsBi/GaAs, and buried Quantum Dots(QDs), such as Germanium QDs in AlAs and GaSb QDs in GaAs are examined. InGaN bulk layer only present random alloy composition fluctuation, while QWs less than 1 nm thick shows nano-clustering. For GaAsBi bulk layer, it is found that surface droplets can induce composition inhomogeneities since Ga droplets can enhance Bi incorportation. Although samples without any surface droplets are more uniform, lateral composition modulation, pores, as well as GaAsBi precipitates can occur. Surface roughness is proposed to be another cause for non-uniform Bi incorporation. Ge QDs were induced by high temperature annealing and form to reduce interfacial energy. A combined APT and STEM study shows that the formation mechanism for QD depends strongly on the growth method. For instance, Droplet Epitaxy method results in the formation of smaller and more dilute GaSb QD pairs compared to the more typical Stranski-Kranstanov method. Nonetheless, both methods result in similar compositional profiles along the wetting layers.
The second part discusses the role of compositional inhomogeneity and defects play in optical properties of InGaN heterostructure nanowires. For InGaN heterostructure nanowires, compositional inhomogeneities and defects present themselves in various aspects and affect the optical property. A six-fold lateral branching structure is also observed, and nanowires with lateral branches are optically inactive under room temperature. Higher strained InN heterostructure nanowires have more pronounced composition induced changes. These nanowires exhibit crystal shape changes, but also more complicated strain relaxation mechanisms.



Prof. Joanna Millunchick, MSE
Assoc. Prof. Emmanuelle Marquis, MSE
Assoc. Research Scientist, Kai Sun, MSE
Prof. Pallab Bhattacharya, ECE