All ETDs from UAB

Advisory Committee Chair

Mary Ellen Zvanut

Advisory Committee Members

Joseph G Harrison

David J Hilton

Minseo Park

Yogesh K Vohra

Document Type

Dissertation

Date of Award

2019

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

Charge transfer occurring in semi-insulating GaN crystals grown by the high nitrogen pressure solution (HNPS) method was investigated using electron paramagnetic resonance (EPR) spectroscopy. Samples were semi-insulating due to compensation of residual shallow oxygen donors with Mg and/or Be acceptor impurities. EPR was detected after illumination with photon energies greater than 2.7 eV in samples containing at least $10^{17} \textnormal{ cm\textsuperscript{-3}}$ beryllium atoms. The resonance consisted of two Gaussian lines. One was stable at temperatures less than 25 K with isotropic $g=1.989$ and was attributed to a paramagnetic neutral acceptor state. This resonance was quenched with photon energies between 0.5 eV and 2.7 eV, and warming the sample to temperatures greater than 50 K also decreased the EPR intensity. The other resonance with $g_\parallel=1.945,g_\perp=1.941$ was only detected while the sample was under high intensity illumination and was attributed to a paramagnetic state associated with neutral shallow O donors. EPR photo-excitation ($A^- + h\nu \rightarrow A^0 + e^-$) and photo-quenching ($A^0 + h\nu \rightarrow A^- + h^+$) processes were systematically observed by monitoring the intensity of the $g=1.989$ resonance during and after sample illumination with various photon energies in the range $0.5 \textnormal{ eV} \leq h\nu \leq 3.2 \textnormal{ eV}$. The intensity of this resonance was proportional to the concentration of neutral acceptors, allowing charge transfer rates to be directly observed with EPR. Charge transfer was modeled with a set of rate equations that tracked neutral acceptor, neutral donor, and charge carrier concentrations. Optical cross sections were extracted from best fits that used a pattern search algorithm for parameter optimization. The cross sections were analyzed using an expression for absorption by defects with strong electron-phonon coupling. The zero-phonon line for photo-excitation and photo-quenching was 2.5 eV and 0.80 eV, respectively, in good agreement with predictions for C\textsubscript{N}. Thermal quenching of C\textsubscript{N}-related EPR can only be explained by electron emission from a beryllium-related trap level close to the conduction band. The analysis shows that unintentional carbon impurities in semi-insulating HNPS GaN substrates play a key role in charge transfer as C\textsubscript{N}, and the work presents a significant advancement in the interpretation of time-dependent photo-EPR measurements.

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