Alleviation of Fermi-level pinning effect on metal/germanium interface by insertion of an ultrathin aluminum oxide

Due to the strong Fermi-level pinning close to the germanium (Ge) valence band edge, all metal/p-type Ge contacts show Ohmic characteristics, while metal/n-type Ge contacts exhibit rectifying behaviors. In this paper, we report a simple method to alleviate this Fermi-level pinning effect by inserting a thin layer of aluminum oxide (Al2O3), formed by oxidation of aluminum (Al), between the metal/Ge interface. The effective Schottky barrier heights of nickel (Ni)/n-type Ge, cobalt (Co)/n-type Ge, and iron (Fe)/n-type Ge decrease from 0.54, 0.62, and 0.61 eV to 0.39, 0.23, and 0.18 eV, respectively, with this thin layer of Al2O3. The tunneling oxide significantly suppresses the Fermi-level pinning, and yet does not restrict the current density. This method seems promising to realize low resistance metal contact to n-type Ge, which is essential to realize n-channel Ge complementary metal-oxide-semiconductor field-effect transistor with metal source and drain.