Chasing Schottky–Mott: Metal-first non-alloyed contacts to <i>β</i>-Ga2O3 for interface quality and minimal surface modification

Metal-first non-alloyed ohmic and Schottky contacts are fabricated on β-Ga2O3 with a range of metal work functions (ϕM). The resulting ohmic contacts are of high quality with a contact resistance (Rc) as low as 0.069 ± 0.003 Ω mm. Measurements of the barrier heights (ϕB) indicate that metal-first processing, which preserves the as-grown/bare-substrate surface, also partially un-pins the Fermi-level in (010) and (2¯01) oriented Ga2O3. Depth-resolved XPS (x-ray photoelectron spectroscopy) measurements of the oxidation state throughout the contact metal at the contact–Ga2O3 interface indicate that most non-alloyed contact metals are at least partially oxidized by room temperature redox reactions with the underlying Ga2O3, with metals with a lower ϕM also demonstrating the greatest level of oxidation. As oxidation has been previously observed to enhance a metal’s work function, this may imply that to-date observations of indices of surface behavior &amp;lt;&amp;lt; 1 on β-Ga2O3, which have been attributed to severe Fermi-level pinning, may need to be corrected to account for this partial oxidation in addition to other surface modifications during device processing demonstrated in this work.