Process and characterization of ohmic contacts for beta-phase gallium oxide

beta-Ga2O3 is a promising material for next-generation power devices because of its ultra-wide bandgap, the commercial availability of bulk substrates, epitaxial growth, and ease of n-type doping. To fully exploit its potential, it is critical to establish fabrication processes to form low-resistance ohmic contacts with excellent long-term stability. Due to upward band bending and unavoidable redox reactions occurring at the contact interface, making a good ohmic contact to gallium oxide can be challenging. Herein, we use a process-structure-property approach to systematically review the reported processes for ohmic contact formation on gallium oxide, the contact microstructure, and the resulting electrical properties including charge transport physics. Furthermore, we describe the present evidence for ohmic contact stability under accelerated aging. Using thermodynamic assessment, we propose alternate ohmic contact materials candidates. Finally, we identify gaps in the scientific knowledge on ohmic contacts to Ga2O3 and highlight opportunities for future investigations.