Discovery of mechanisms and methods for surface-based defect engineering in oxide semiconductors using liquid water

Semiconducting oxides have been used in a wide range of devices, for example as photocatalysts, optoelectronics, and sensors. It is no surprise that readily-controlled manipulation of defect concentrations in oxides could enable the improvement of material properties for many applications and dramatically improve manufacturability. Defect engineering refers to material processing protocols that manipulate the type, concentration, and spatial distribution of the atomic-scale defects in materials. There are several defect control approaches, such as ion implantation and diffusion via thermal annealing, that are commonly used in silicon wafer manufacturing processes. Ion implantation has several drawbacks such as surface damage, dislocation formation, and channeling, due to implantation effects. However, the diffusion and exchange process are less expensive and do not damage the surface. This thesis focuses on elucidating thermodynamic and kinetic processes with a novel approach involving oxygen-ion diffusion in bulk and oxygen-ion exchange through the surface (injection/annihilation) in oxides under experimental annealing conditions and atmospheric air. The present work also introduces post-synthesis methods to control defect concentration and types. Semiconducting oxide synthesis by typical methods often leaves behind a significant contribution of oxygen vacancies (VO) that dominate many physical properties and mediate oxygen diffusion. However, the recent theoretical studies raised the possibility of oxygen interstitials (Oi) in binary oxides at O-rich conditions, and experimental studies observed species with much higher diffusivities and an exponential depth-profile shape that is different from the error-function shape expected for VO. Under O-rich conditions, Oi has lower formation energy together with higher mobility than VO. A linking of experimental measurements and computational simulations is still lacking in most of the literature; therefore, this thesis seeks to clarify such questions by combining isotope diffusion experiments and computational simulations. This thesis covers several domains of defect engineering from gas-solid to liquid-solid interfaces, seeks to understand the underlying injection/annihilation kinetic mechanism of Oi defects at interfaces between binary oxides (TiO2 and ZnO) and both gases and liquid water. It also introduces novel surface cleaning protocols that employ simple aqueous solutions near room temperature to control the majority exchanged species Oi. It demonstrates new methods for controlling injection rates via super-band gap illumination and pH. This approach ends up combining experiments and multiscale simulations, including first-principles density functional theory (DFT) calculations and mesoscale continuum-scale microkinetic modeling for the determination of rate constants. The present work also suggests the use of Oi to compensate adventitious donor H that exists in most oxides as a way toward developing post-synthesis protocols for fabricating p-type material.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2021

Author dc:creator

• Jeong, Heonjae

Contributors dc:contributor

• Ertekin, Elif
• Seebauer, Edmund G
• Aluru, Narayana R
• Perry, Nicola H

• defect engineering
• oxide semiconductors
• surface
• TiO2
• ZnO
• SIMS
• density functional theory calculations
• microkinetic model

Statement dc:rights

• © 2021 by Heonjae Jeong. All rights reserved.

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/110834

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/110834