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Dissertation
LIGHT INDUCED EFFECTS IN OXIDE SEMICONDUCTORS
Washington State University
Doctor of Philosophy (PhD), Washington State University
01/2022
DOI:
https://doi.org/10.7273/000004588
Handle:
https://hdl.handle.net/2376/124863
Abstract
Titanium dioxide (TiO2) has many promising applications, including photocatalysis, hydrogen production, and solar cells. In this study, anatase TiO2 was annealed in a vacuum at 800 ℃, resulting in a conductive thin film. Exposure to subgap laser light caused a seven order-of-magnitude increase in resistance. Laser-irradiated regions showed an increase in optical transmission, consistent with a reduction in oxygen vacancy concentration. Results indicate that laser irradiation did not change the morphology, composition, or phase of the material.Defining the crystal phase of oxide semiconductors using subgap laser could benefit transparent electronics. This work reports laser-induced localized phase transitions of TiO2 thin films. A Raman map of the anatase Eg mode (144 cm−1) and rutile Ag mode (608 cm−1) revealed the formation of crystalline microstructures due to the laser treatment. Laser irradiation under vacuum results in an anatase-to-rutile phase transition. Irradiating the rutile region in air changes the crystal structure back to anatase, despite the thermodynamic stability of rutile. The results suggest that irradiated photons are absorbed by defects, resulting in localized electronic excitation that leads to a mixture of amorphous and crystalline regions. The phase of the crystalline regions depends strongly on the ambient conditions.
Carbon is one of the most recurring contaminants known to alter the properties of a material. The form of carbon in the sputtered TiO2 thin films appeared undetectable to Raman spectroscopy. Annealing the films under vacuum results in Raman peaks at 1350 (D) and 1585 cm-1 (G), representing disorder and graphitic C=C bonds. Secondary ion mass spectrometry results revealed a substantial amount of carbon in the sputtered films that persist after annealing. Irradiating the annealed sample with a subgap laser in air dissipates carbon locally.
Barium calcium titanate (Ba1-xCaxTiO3) annealed under a flowing humid 2% hydrogen gas mixture exhibits room temperature persistent photoconductivity (PPC) with a subtle increase in conductivity when exposed to visible light. Hydrogen in the annealing atmosphere induces PPC, implying that the process is analogous to PPC in BaTiO3. Annealing the samples in air erased the PPC effect, indicating that the process is reversible.
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Details
- Title
- LIGHT INDUCED EFFECTS IN OXIDE SEMICONDUCTORS
- Creators
- Syeed Ehsan Ahmed
- Contributors
- Matthew D McCluskey (Advisor)John S McCloy (Committee Member)Yi Gu (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Mechanical and Materials Engineering, School of
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 155
- Identifiers
- OCLC#: 1365772584; 99900898938801842
- Language
- English
- Resource Type
- Dissertation