Washington University in St. Louis
First-Principles Investigation of Doping and Alloying of β-Ga2O3
Abstract
dc:description.abstract<p><em>β</em>-Ga<sub>2</sub>O<sub>3</sub> is an emergent semiconductor for power electronics applications. It has a wide band gap of 4.8 eV and is transparent on the whole spectrum of visible light up to deep ultraviolet. It has a high Baliga figure of merit (BFOM) — a weighted numerical combination of the dielectric constant, charge carrier mobility, and critical breakdown field —, which is commonly used for a quantitative comparison of semiconductors for high-current operation and power switching applications. <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> can be grown as thin films or as large single crystals by melt growth-techniques, which is important for scalable manufacturing. However, <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> suffers from a lack of <em>p</em>-type dopants and a low thermal conductivity. Presently, all applications are based on <em>n</em>-type <em>β</em>-Ga<sub>2</sub>O<sub>3</sub>; the introduction of <em>p</em>-type <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> would enable bipolar power devices. Additionally, high-power switching results in elevated temperatures where heat retention can impede electronic performance.</p> <p>The objective of this thesis is to investigate, using first-principles density-functional-theory calculations: (1) the efficiency of doping with Bi to raise the valence band of <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> to a level at which <em>p</em>-type doping is achievable, and (2) the possibility of improving the thermal properties of <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> by alloying with the lightest Group-13 cation, B.</p> <p>We find that doping with Bi creates mid-gap states derived from the Bi 6<em>s</em> electrons at similar energy level to candidate acceptors through an anti-bonding hybridization of the Bi lone pair with the O 2<em>p</em> states. The associated states are more dispersed than the pristine valence band, as they derive from the delocalized <em>s</em>-states of Bi rather than the <em>p</em>-states of highly electronegative O, which dominate the valence band. Our calculations indicate that these intermediate states are natively filled, and at an appropriate energy level to use a co-dopant like Mg, N, or even native Ga vacancies as <em>p</em>-type dopants.</p> <p>We explored several pathways to include B as an alloy in <em>β</em>-Ga<sub>2</sub>O<sub>3</sub>, including the high-entropy alloy (HEA) approach and a variety of ordered binary and ternary alloys. Despite these efforts, we do not find a stable alloy, since the small B atoms reject the octahedral and tetrahedral coordination of the cations in <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> in favor of a flat triangular coordination, as observed in B<sub>2</sub>O<sub>3</sub>. We conclude that B is likely insoluble in <em>β</em>-Ga<sub>2</sub>O<sub>3</sub> at concentrations high enough to substantively improve its thermal conductivity.</p>
Degree
thesis:*- Name thesis:degree_name
- Master of Science (MS)
- Level thesis:degree_level
- Thesis
- Discipline thesis:degree_discipline
- Materials Science & Engineering
- Year dc:date.available
- 2020
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Tattersfield, Ben
- Contributors dc:contributor
-
- Rohan Mishra
- Katharine Flores Chuan Wang
Subjects
dc:subject × 5Rights
dc:rights- Statement dc:rights
-
- I have not registered my thesis with the U.S. Copyright Office, and do not intend to.
- Language dc:language
- English (en)
Identifiers
dc:identifier.*- OAI identifier oai:identifier
- oai:openscholarship.wustl.edu:eng_etds-1614