Enhancing light-matter interactions in oxide semiconductors

Lee, Sungki

Permalink

https://hdl.handle.net/2142/72804 Copy

Description

Title

Enhancing light-matter interactions in oxide semiconductors

Author(s)

Lee, Sungki

Issue Date

2015-01-21

Director of Research (if dissertation) or Advisor (if thesis)

Martin, Lane W.

Doctoral Committee Chair(s)

Braun, Paul V.

Committee Member(s)

• Martin, Lane W.
• Rockett, Angus A.
• Ertekin, Elif

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Oxide thin films
• Optical properties
• Hetero-junction
• Self-assembly
• photovoltaic
• photocatalytic

Abstract

Oxide semiconductors have been widely utilized because of their unique electronic properties and high stabilities in various environments for solar energy applications. Although oxide semiconductors have shown successful records as an active light-absorbing layer or transport layer in many photovoltaic and photocatalytic devices, their relatively large optical band gap and limited tunability in intrinsic electronic properties remain to be overcome for more broad range of applications. Many efforts have focused on enhancing visible-light absorption and charge carrier separation, and decreasing carrier recombination. Such approaches, however, have shown limited success in only a few oxide materials. The difficulty in adjusting intrinsic properties is originated from the relatively high defect concentration (1/1000) that typically gives rise to intrinsic defect levels and in turn determines the majority carrier type. In this regard, there have been significantly increased efforts to find the new routes, which are more versatile to a wide range of oxide semiconductors. In this work, I present my unique approaches to overcome these difficulties in various material system including oxide semiconductor, metallic oxide, and composite. This approach was achieved by combining the ability to synthesize high-quality thin-films, control the strain via substrate epitaxy, and utilize self-assembly in composite materials. First, I show the ability to control shape, structure, surface, and density of nanostructures of a model oxide semiconductor Cu2O with controlling growth kinetics and epitaxy from substrates. The primary advances herein include the characterization of surface potentials of individual nanostructure which can potentially give rise to an application of surface homojunction for the materials that have limited tunability in their properties. Second, I demonstrate an approach to utilize a significant portion of visible light spectrum by combining anatase TiO2 with anomalously high light-absorbing metallic oxides. I show the evolution of photocurrent in such heterojunction devices that originates from hot carriers transported across TiO2 simply by diffusion and drift, and its practical implications in photocatalytic devices where I observe at least an order of increased activities compared to traditional systems. Lastly, I present a new route to synthesize multilayered composite materials with which one can expect enhanced light absorption (or scattering). With model eutectic material systems including SrTiO3-TiO2, BaTiO3-TiO2, and SrMnO3-MnO2, I present systematic studies of the synthesis of a new state of matter and explore the mechanism for formation of novel, layered structures. By engineering growth kinetics and controlling composition and epitaxy, I demonstrate the synthesis of a novel, layered phase of Sr2Ti7O14 which possess unique optical, dielectric, magnetic, and thermal properties. The formation of layered phase is also observed in other analogous materials systems including BaTiO3-TiO2 but not in SrMnO3-MnOx which are also briefly discussed. My approaches in engineering the synthesis and employing thin-film epitaxy will give some new insights to the future technology of oxide semiconductors.

Graduation Semester

2014-12

Permalink

http://hdl.handle.net/2142/72804

Copyright and License Information

Copyright 2014 Sungki Lee

Owning Collections