Fermi-Level Unpinning Using a Ge-Passivated Metal–Interlayer–Semiconductor Structure for Non-Alloyed Ohmic Contact of High-Electron-Mobility Transistors

We demonstrate the use of germanium passivation in conjunction with a ZnO interlayer in a metal-interlayer- semiconductor structure in a source/drain (S/D) contact. The Fermi-level pinning problem resulting in the large contact resistances in S/D contacts is effectively alleviated by inserting a thin Ge passivation layer and a ZnO interlayer, passivating the GaAs surface and reducing the metal-induced gap states on the GaAs surface, respectively. The specific contact resistivity for the Ti/ZnO/Ge/n-GaAs (~2 × 10¹⁸ cm^-3) structure exhibits a ~1660× reduction compared with that of a Ti/n-GaAs structure. These results suggest that the proposed structure shows promise as a nonalloyed ohmic contact in high-electron-mobility transistors.