Photochemical Sciences Ph.D. Dissertations

Optoelectronic Properties of Wide Band Gap Semiconductors

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Farida Selim (Advisor)

Second Advisor

Malcolm Forbes (Committee Member)

Third Advisor

Alexey Zayak (Committee Member)

Fourth Advisor

Michael Geusz (Other)

Abstract

Wide band gap oxide semiconductors exhibit a wide range of interesting electronic and optical properties that have been used in optoelectronic devices. Native defects play a significant role in oxide semiconductors and affect their optical and electrical properties. In order to use these materials in devices, understanding and controlling defects are crucial. The focus of this thesis is to study defects in two important wide band gap oxides, strontium titanate (SrTiO3) and gallium oxide (Ga2O3). A wide range of characterization techniques were employed to study the optical and electrical properties of both oxides.

The first part of this thesis focuses on enhancing the understanding of photoconductivity and luminescence phenomena in bulk SrTiO3 single crystals. The measurements show that there is a strong correlation between photoconductivity and defect concentration when sub band gap visible light is used as an illumination source. Positron annihilation spectroscopy suggested that Ti-vancacy is behind the photoconductivity phenomena in SrTiO3 single crystals.

We also observed an interesting luminescence behavior in SrTiO3 single crystals, that has never been observed in other systems. We believe that the unusual behaviors in SrTiO3 luminescence are associated with the relaxation and expansion of the lattice during phase transition of SrTiO3.

The second part of this work aims to investigate defects and electronic properties of epitaxial β-Ga2O3 thin films grown by metal organic chemical vapor deposition (MOCVD) technique on c-sapphire substrate. Thermoluminescence spectroscopy in conjunction with positron annihilation spectroscopy were used to identify the nature of defects and their activation energies. We found that annealing in various environments populate different types of defects. The measurements revealed the presence of large vacancy clusters which has been substantially increased by annealing in argon atmosphere. However, annealing in air modified the structure of defects by filling oxygen vacancies and increased the resistivity.

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