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Growth and Characterization of the Quasi-Binary Ga2Te3-Ga2Se3 Semiconductor Alloy

Abstract

This dissertation presents an experimental investigation on the growth and characterization of the quasi-binary Ga2Te3-Ga2Se3 semiconductor alloy. Single crystal Ga2(Se1-x<\sub>Tex)3 specimens were grown via modified Bridgman methods for x = 0.60, 0.67, 0.8, 0.9, and 1.0. Preliminary structural characterization via powder x-ray diffraction showed a cubic zincblende structure. Satellite reflections discovered in the powder diffraction patterns alluded to the presence of secondary superstructures. High-resolution x-ray diffraction and x-ray absorption experiments conducted at the Advanced Photon Source, the Stanford Synchrotron Radiation Lightsource, and the Advanced Light Source confirmed the presence of ordered mesoscopic two-dimensional vacancy structures that can influence the pressure-induced amorphization behavior of Ga2SeTe2. More specifically, vacancy ordered and semi-ordered Ga2SeTe2 specimens amorphized at around 10-11 GPa as opposed to vacancy disordered specimens, which amorphized at around 8-9 GPa. X-ray absorption fine structure experiments indicated these vacancy based superstructures locally distort the lattice. However, the symmetry associated the face-centered cubic Bravais lattice is preserved. Local atomic distortions were directly observed via aberration-corrected scanning transmission electron microscopy conducted at the National Center for Electron Microscopy, where an inversion in the cation-anion orientation vector across the boundaries of the two-dimensional vacancy structures was observed. The effect of vacancy ordering on the physical properties of Ga2(Se1-x<\sub>Tex)3 was also studied. Positron annihilation spectroscopy revealed that positron lifetimes in Ga2SeTe2 are similar to that of large open-volume defects. As vacancies order, their role in positron trapping diminishes. Band gap measurements via optical absorption demonstrated that vacancy ordering in Ga2SeTe2 drives down band gap energy by approximately 0.05 eV. This observation was accompanied by Hall effect experiments, where the resistivity of vacancy ordered samples decreased by two orders of magnitude with moderate gains in charge carrier mobility.

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