Material properties of gallium oxides
: material integration, thermal anisotropy, irradiation effects

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Gallium oxide (Ga2O3) is an ultra-wide band gap semiconductor with potential applications in power electronics due to its high predicted breakdown field (significantly higher than in materials such as SiC or GaN). One of Ga2O3’s main drawbacks is its low and anisotropic thermal conductivity, which can be a limiting factor in thermal management for potential devices. Furthermore, low hole mobility makes impractical the use of any potential p-type Ga2O3. Because of this, the incorporation of Ga2O3 with other materials for improved thermal management, as well as designing well performing p-n junctions, is important for the further development of Ga2O3 technology.
While different types of adhesion of Ga2O3 to higher thermal conductivity substrates have been demonstrated, the effects of anisotropic thermal transport on the potential interface thermal boundary resistances has not been well studied. It is also notable that multiple Ga2O3 polymorphs exist (although the β phase is the most stable and most well studied). These polymorphs are known to have varying electronic properties, which could be tied back to the varying local coordination environments. This implies that as a result of irradiation damage or ion doping, induced structural defects could affect Ga2O3’s electronic properties, or possibly lead to a partial or full polymorph transition. Therefore, understanding the structural changes in Ga2O3 as a result of ion implantation/irradiation is important.
In this thesis, we review several methods for adhesion of Ga2O3 to higher thermal conductivity substrates, discussing their benefits and drawbacks. We simulate the thermal boundary resistance (TBR) between β-Ga2O3 of different crystallographic orientations and (100) diamond with a Van der Waals bonded interface. We find said TBR to vary by as much as 70% depending on the β-Ga2O3 crystal face used at the interface, lowest for (100) β-Ga2O3 as 48.6±0.3 m2KGW−1.
We further estimate that the TBR between β-Ga2O3 and diamond can be reduced by at least 3.6 times when the bonding is realised through a 10 nm amorphous Al2O3 interlayer, compared to a direct Van der Waals interface. We further investigate a method for thin film (ranging between 8 and 30 nm in thickness) Ga2O3 deposition from oxidised liquid gallium. We measure the valence band offset of the thin film to an SiO2 substrate using x-ray photoelectron spectroscopy to be 0.1 eV and comment on what this implies for a potential interface with Si and diamond. We also measure the thin film’s out of plane thermal conductivity as 3±0.5 Wm−1K−1 using transient thermoreflectance.
Results from an in situ ion irradiation experiment on β-Ga2O3 using 200 keV Ar ions are discussed. We observe an anisotropic shrinking of the material’s lattice dimensions with increasing irradiation dose. While the material’s structure remains in the β polymorph, extra reflections appear in the material’s diffraction pattern above an irradiation dose of 2 displacements per atom. We discuss the possible source of these. We also note a different mode for damage formation at higher incident ion energies, showing the absence of some of the complex defects (seen under 200 keV Ar ion irradiation) when irradiating β-Ga2O3 with Ar ions of 2 MeV incident energy. We propose a cellular automaton model of the ion irradiation process in β-Ga2O3, which suggests a significant decrease in the average local Ga-ion coordination number, which is expected to lead to changes in the material’s electronic properties.
Date of Award9 May 2023
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMartin H H Kuball (Supervisor) & James W Pomeroy (Supervisor)

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