Date of Degree

6-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Mim Lal Nakarmi

Committee Members

Nicolas Giovambattista

Vladimir V. Chaldyshev

Bruce Kim

Yuhang Ren

Subject Categories

Condensed Matter Physics

Keywords

wide bandgap, photoluminescence, optical studies, hexagonal boron nitride, atomic-like emission, defects

Abstract

In this dissertation work, optical properties of wide bandgap materials such as hexagonal Boron Nitride (h-BN) and Zinc Oxide (ZnO) have been studied. Deep UV photoluminescence spectroscopy was employed to study the optical properties of bulk h-BN and powder crystals using a laser of wavelength 200 nm, which is the fourth harmonic of Ti:Sapphire laser as excitation source. The properties and chemical compositions of annealed and unannealed bulk h-BN were investigated. The PL spectra from h-BN samples annealed at 900 ºC in ambient air, had strong phonon assisted band edge emissions along with a sharp atomic-like emission line at 4.09 eV, and its phonon replicas at 3.89 and 3.69 eV. The sharp emission line and phonon replicas were not observed in unannealed sample. The peak energy of the sharp emission line at 4.09 eV is robust based on power and temperature dependent PL measurements. We found the optimized condition of annealing to generate the 4.09 eV line is around 800-900 ºC for 1 hour. Comparative chemical composition analysis of the residual impurities was carried out using X-ray photoelectron spectroscopy (XPS) in order to identify the impurities related to the sharp transition. They revealed N-C and B-C bonding related peaks in the high resolution XPS spectra of samples that have strong 4.09 eV emission, indicating CB and CN are present in those samples. Hence, carbon related defect could be responsible for this emission of 4.09 eV line. Our experiment on XPS measurement after sputtering the sample also showed the defect related to localized states could be residing on the surface region. We also studied the optical properties of h-BN crystal powders using the deep UV PL spectroscopy. We observed similar effect of annealing on the PL emissions from the powder samples. In contrast, annealed powder samples contain sharp atomic-like PL emission lines in the UV spectral region not only at 4.10 eV but also, at 4.12 eV along with their longitudinal optical phonon replicas.

Mesoscopic Zinc oxide (ZnO) particles annealed at 650 ºC for 2 hours have been investigated for optical properties using the third harmonic laser (260 nm) generated from a Ti: sapphire laser for optical excitation. PL spectrum measured at low temperature has dominant emission peak at around 3.308 eV due to the transitions of free electrons from conduction band to holes bound to acceptor state (FA) along with longitudinal optical (LO) phonon replicas. It also revealed excitonic emissions at 3.359 and 3.371 eV due to donor bound exciton (DºX) and free exciton (FX) transitions respectively. From the excitation power dependent PL measurement at low temperature, we observed a redshift of FX and DºX emission peaks showing bandgap renormalization. This power dependent PL data shows consistency with Vashista and Kalia model. We also studied the effect of lysozyme in the optical properties of ZnO nanoparticles. Using the third harmonic laser as excitation source, we observed the band edge emissions at 3.318, 3.359, and 3.374 eV which are related to FA, donor bound exciton (DºX) and free exciton (FX) in the low temperature PL measurement. In addition, a dominant emission peak around 2.4 eV donor-acceptor-pair (DAP) transition was confirmed. Based on these emissions peaks, energy level diagram was constructed.

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