Metal-Semiconductor Interfaces and Ohmic Contacts on Gallium Oxide
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Updating global power infrastructure demands high-performance, high voltage power electronics. Ultrawide bandgap semiconductors such as β-Ga2O3 outperform conventional semiconductors such as silicon, SiC, and GaN due to their large critical electric field, enabling superior voltage handling in smaller devices. Kilovolt-class β-Ga2O3 transistors have already demonstrated exceptional capabilities, making reliability and contact formation key next steps in device development. This dissertation first explores enhancement-mode devices for normally-off fail-safe operation by threshold voltage modulation using fin-channel lateral field-effect transistors. These devices achieve a shift from negative to positive threshold voltages while identifying significant device processing challenges, including high on-resistance from alloyed ohmic contacts and large interface trap densities. To address these challenges, a metal-first contact process is developed, which achieves ultra-low contact resistances (∼0.05 Ω-mm, or 3.9 × 10−7 Ω-cm2) without alloying and improves contact repeatability. Scanning transmission electron microscopy and x-ray photoelectron spectroscopy indicate that surface contamination from conventional liftoff processing accounts for prior observations of contact inconsistency, but atomic Ga-flux polishing effectively repairs the surface. Further contact improvements using oxygen plasma and UV-ozone treatments are presented. Improved ohmic contact processing using liftoff is also demonstrated with the addition of oxygen treatments. The metal-first process is also applied to a range of ohmic and Schottky metals, enabling Schottky barrier height extraction via a refined thermionic field emission model that incorporates degenerate doping effects. This process mitigates Fermi-level pinning, resulting in a surface index of ∼ 0.7 on (010) and (-201) surfaces. Depth-resolved XPS identifies room-temperature oxidation at the metal-semiconductor interface as a contributor to residual pinning. This work highlights the need for precise fabrication techniques to fully leverage β-Ga2O3’s potential in ultrawide bandgap power electronics, paving the way for next-generation devices.