Optical and electronic properties of defective semiconductors from first principles calculations
Lewis, David Kirk
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Defects in semiconductors can play a vital role and even dominate the performance of optoelectronic devices. Thus, understanding the relationship between structural defects and optoelectronic properties is central to the design of new high-performance materials. In this dissertation, we apply state-of-the-art first-principles approaches based on density functional theory (DFT) and many-body perturbation theory (MBPT) to quantitatively describe trap state energies and optical excitation spectra of defective bulk gallium nitride (GaN) and monolayer germanium selenide (GeSe). GaN is a technologically important wide bandgap semiconductor used as a power electronics and blue light emitting material, and naturally contains performance-degrading defects. For GaN containing a charged nitrogen vacancy, we systematically study the trap-state energies and excitonic properties. We benchmark the accuracy of hybrid DFT by comparison to MBPT studies of defective bulk GaN and determine that the HSE functional (Heyd–Scuseria–Ernzerhof) predicts trap-state energies in excellent agreement with MBPT, and that a recently developed solid-state screened range-separated hybrid (SRSH) functional can quantitatively reproduce MBPT-predicted defect energetics, including optical excitations. Additionally, we utilize MBPT to quantify the localization of the Wannier-Mott exciton in the presence of a point defect, introducing an analysis technique of the exciton envelope and center-of-mass functions to extract the Wannier exciton Bohr radius and quantify the perturbation of the exciton wavefunction due to the defect. We then utilize (TD)SRSH to study the excited-state properties of three other important defects in GaN and predict that the carbon impurity may result in the well-known yellow luminescence in bulk GaN. Finally, we apply MBPT with the same analysis techniques developed for GaN to study the optoelectronic properties of defects in monolayer semiconducting GeSe, a material that has promising applications in next-generation optoelectronic devices; we determine that a selenium vacancy strongly modifies the optoelectronic properties of the material. Overall, this dissertation provides a recipe for performing quantitatively accurate MBPT and TDDFT calculations on defective semiconductors, with a systematic study of calculation convergence and defect-defect interactions. Additionally, by an analysis technique of the BSE-computed exciton wavefunction, we introduce a framework for describing defect-induced exciton localization that can be broadly applied to many classes of materials.