Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Dr. Ambrose Seo

Abstract

Hydrogen exposure of insulating d0-titanates, such as SrTiO3 (STO), has displayed the formation of intriguing conducting states. These conducting states form through the use of forming gas (N2/H2) annealing or hydrogen plasma exposure, where hydrogen gas is exposed to high energy microwaves. The exposure of STO to hydrogen causes metallic conductivity due to the introduction of hydrogen cations on some of the oxygen sites. However, the optical properties of this hydrogen-exposed STO have not been well-studied. Further, Ba0.5Sr0.5TiO3 (BST), an insulating dielectric, also shows changes in its conductivity upon hydrogen exposure. Unlike STO where the conductivity of the hydrogen-exposed material has been characterized, the optical, electronic, and transport properties of hydrogen exposed BST have not been studied. Thus, by studying hydrogen-exposed BST and STO, our understanding of the effects of hydrogen on insulators can be enhanced.

In the first study, the effects of the exposure of insulating dielectric BST thin films to a hydrogen plasma is presented. These BST thin films are deposited on GdScO3 (GSO) substrates via Pulsed Laser Deposition (PLD). After deposition, the thin films are exposed to a hydrogen plasma. Just five minutes of hydrogen plasma exposure is enough to induce conductivity in the BST thin film. This conducting state is dominated by the interplay of disorder and strong electron correlations introduced during hydrogen exposure. Further, the optical properties indicate the formation of a transparent conductor, as the introduction of disorder and strong correlations has not changed the optical properties of the BST thin film in the visible spectrum. BST demonstrates an example of a new type of transparent conductor that utilizes large effective mass carriers to generate conductivity.

In the second study, the effects of hydrogen doping on the surface of STO is explored. The conducting heterointerface that forms between PLD-deposited thin films of LaAlO3 (LAO) on STO is used as the standard to explore this hydrogen surface doping. The optical, electronic, and transport properties of water-leached and buffered hydrofluoric acid (BHF) etched heterointerfaces are characterized and compared. The recently developed water-leaching method is compared with the well-known BHF etching method, which has been shown to unintentionally dope the STO surface with fluorine and hydrogen. Both methods generate single-terminated atomically flat STO substrate surfaces that are ideal for heterointerface formation. After deposition, the optical, electronic, and transport properties of both the water-leached and BHF-etched heterointerfaces show no meaningful difference, demonstrating that water-leaching may also unintentionally dope the STO substrate surface with hydrogen. However, these results confirm that water-leaching generates a high-quality conducting heterointerface without the safety concerns of BHF.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.197

Available for download on Thursday, November 14, 2019

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