When 2:00 PM - 4:30 PM Jun 21, 2018
Where 2166 H.H. Dow
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"Growth of and Compositional Inhomogeneities in III-IV Bi Alloys"


Christopher Ryan Tait
PhD Dissertation Defense

Solubility dynamics in materials science drives discovery and novel material development. Bi has garnered interest in III-V semiconductors because of its impact in the electronic properties including large bandgap reduction per percentage Bi, high spin orbit coupling, and preserved electron mobility. The endpoints of III-Bi binary compounds are either unstable, like GaBi, or have low melting point, like InBi. Finding good conditions in which there is appreciable Bi incorporation is difficult.  Growths often result in droplets on the surface. This dissertation explores the phenomena surrounding incorporation of Bi including development of computational tools for investigation of Bi impact on electronic structure, a new incorporation model taking Bi surface buildup into account, an investigation into the inhomogeneities that appear along with Ga droplets, and the inhomogeneities that appears in growths with a clean surface.  A new kinetic model is constructed to account for Bi buildup on the surface building on previous models. Results of experiments in GaSbBi are utilized to confirm trends predicted in the model.  Bi droplets are found to reduce Bi incorporation by becoming a kinetic sink. A series of growths in GaAsBi with varying As:Ga ratio shows that Ga droplets contributes to Bi inhomogeneity in the bulk characterized by x-ray diffraction, transmission electron microscopy, and atom probe tomography.  The mechanism associated with this phenomenon is non-uniform Ga availability at the growth surface due to droplet wicking. Growths with clean surfaces are also shown to exhibit inhomogeneities including Bi clusters, lateral composition modulation, and nanopores at growth temperatures below 325°C.  These phenomena are explained by a destabilization in the growth mode due to differences in surface diffusivity of As and Bi. Ultimately, homogeneous incorporation of Bi requires growth with surfaces that are smooth and free of droplets limiting the available parameter space and maximal incorporation for Bi.