Critical review of Ohmic and Schottky contacts to β-Ga2O3

Over the last decade, beta-phase gallium oxide (β-Ga2O3) has developed an extensive interest for applications such as high-power electronics. Due to its ultrawide bandgap of ∼4.8 eV and predicted breakdown field of ∼8 MV/cm along with its ability to be grown from the melt, this material demonstrates immense promise for high-voltage switching. The pace of development for β-Ga2O3 over these past ten years has been rapid, and ample new information has been generated on metal/β-Ga2O3 interfaces. With the advent of high-quality melt-grown substrates and epitaxial layers, low ideality factors (<1.1) and high Schottky barrier heights (>2.2 eV) have been demonstrated for Schottky contacts. For Ohmic contacts, specific contact resistivities of the common Ti/Au metallization are routinely reported in the 10−5–10−6 Ω cm2 range. This critical and comprehensive review focuses on the fundamental physics of Ohmic and Schottky contacts to bulk and epitaxial β-Ga2O3 in the published literature. It centers on the influence of surface treatments and defects on electrical contacts, Ohmic contacts, and Schottky contacts to β-Ga2O3. Native upward band bending is observed on β-Ga2O3, and the influence of wet and dry etching on band bending along with Schottky barrier height and ideality factor is discussed. Work on Ohmic contacts concentrates on the conventional Ti/Au anneal but additional Ohmic metallizations such as conductive oxides and others are treated as well. Schottky contacts are examined with specific focus on Fermi level pinning, thermal stability of Schottky metallizations, and Schottky barrier inhomogeneity.