All ETDs from UAB

Advisory Committee Chair

Mary E Zvanut

Advisory Committee Members

Vladimir V Fedorov

Robin D Foley

Evan R Glaser

Gary M Gray

Joseph G Harrison

Document Type

Dissertation

Date of Award

2019

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

Non-uniform strain phenomena localized to Mg acceptor sites were investigated in Mg-doped GaN single crystals using electron paramagnetic resonance (EPR) spectroscopy. Samples in this study were grown either as µm thick thin film or mm thick free standing samples with the Mg concentration ranging from 3 150 x1018 cm-3. In all samples, the Mg related EPR signal was observed but the g-factor, intensity, and lineshape anisotropy characteristics varied significantly depending on the magnitude of the bi axial crystal field Δx local to the Mg acceptor. By modelling the Mg acceptor as basal atomic 2p orbitals, the angular dependent intensity could be effectively predicted in both small and large bi axial crystal field environments. Attempts at describing the Mg acceptor as being localized to an axial nitrogen neighboring the Mg resulted in incorrect EPR g factor and intensity angular dependent predictions for samples in small bi axial crystal fields. The non-uniformity of the bi axial crystal field was investigated by measuring the EPR signal over a range of frequencies between 50 130 GHz in free standing samples grown by vapor phase epitaxy (HVPE) samples. The dominant source of lineshape broadening was determined to be caused by non uniform crystal field, which resulted in a distribution of unresolved g factors, known as g strain. Other linewidth broadening mechanisms were investigated, but all results confirm that, in the ground state, the hole must reside on basal planes subjected to various bi axial strains, likely attributed to their location in the crystal. In this work, I provide experimental evidence that supports the model for the Mg acceptor as a hole on a basal site in GaN. By the application of crystal field theory, the anisotropic g factor and intensity could be effectively understood as a parameter sensitive to the magnitude of the local bi axial crystal field Δx in all GaN:Mg samples when the hole is oriented on a basal site only. Free standing ammonothermal-grown GaN:Mg samples reveal that ionized donor sites can also result in large bi axial crystal fields as evidenced by a reduced g factor anisotropy.

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