Critical review of Ohmic and Schottky contacts to β-Ga2O3

Abstract

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.

15 MeV proton damage in NiO/β-Ga2O3vertical rectifiers

“…Subsequent annealing brought a partial restoration of forward current for both fluences, but degradation of the front contact was apparent above 300 • C and prevented further improvement in device performance. This is clearly a result of the Ni/Au interacting with the NiO, since previous studies have shown the rear Ti/Au/Ga 2 O 3 contact is stable under these conditions [40,41]. Control samples that were not irradiated but annealed at the same temperatures confirm the origin of the degradation at 400 • C. The trends in forward density with fluence and annealing temperature are made clearer in the linear plot of figure 4, which shows that annealing alone degrades the forward current for 400 • C anneals.…”

Effect of Dry Etching to Improve Ohmic Contacts on Bulk, Lightly-Doped β-Ga2O3

“…In Ga 2 O 3 device fabrication, low-resistance Ohmic contacts are crucial. [1][2][3][4][5][6][7][8][9][10][11][12] High contact resistance degrades device switching speeds and exacerbates reliability concerns due to the localized heating generated at the contact interface during current flow in device operation. [2][3][4][5][6][7] Low-resistance Ohmic contacts in Ga 2 O 3 power electronics minimize power dissipation, [1][2][3] while high-resistance contacts result in increased Joule heating.…”

“…Typical Ohmic contacts on Ga 2 O 3 employ a multilayer metal contact stack comprising at least one low work function metal and one or more high work function metals. 2,3,27,28 Surface treatment prior to metallization plays a pivotal role in the barrier height and device stability. Yao et al 6 showed the impact of distinct wet chemical treatments on unpassivated surface states and bulk or nearsurface states held greater sway in dictating the electrical behavior of rectifying contacts compared to the choice of metals.…”

“…The combined application of RIE and Si ion implantation yielded a specific contact resistance of 8.1 × 10 −6 Ω cm 2 , underscoring the potential for enhancing Ohmic contact properties through the synergistic utilization of these techniques. [1][2][3][4][5][6][7][8][9][10][11] A typical metallization for n-type Ga 2 O 3 Ohmic contacts is Ti/ Au. Ti is chosen for its low work function and robust adhesion to Ga 2 O 3 , while Au possesses low resistivity and functions as a current spreading layer.…”

“…Ti is chosen for its low work function and robust adhesion to Ga 2 O 3 , while Au possesses low resistivity and functions as a current spreading layer. [3][4][5][6][7] On very heavily doped n-Ga 2 O 3 , specific contact resistivities in the range of 10 −5 -10 −6 Ω•cm 2 have been reported 1,2 but the bigger challenge is to make low resistance Ohmic contacts on lightly doped n-Ga 2 O 3 of the type encountered in transistor channels or rectifier drift regions, i.e. at the 10 17 cm −3 doping level.…”