Characterization of Interfaces between Chemically Cleaned or Thermally Oxidized Germanium and Metals

Germanium (Ge) is a promising channel material for next-generation metal-insulator-semiconductor field-effect transistors (MISFETs) because it has higher carrier mobility than c-Si. To get a better understanding of Fermi-level pinning near the valence band edge of Ge at metal/Ge junctions, the surface band bending of Ge substrates and chemical bonding features of interfaces between chemically cleaned or thermally oxidized Ge and metals were investigated by X-ray photoelectron spectroscopy (XPS). Three different metal films (Al, Pt, and Au) were formed on surface-cleaned Ge and thermally grown GeO2/Ge. With metal deposition on surface-cleaned Ge, evidence of Ge atom diffusion into the metal layer, such as formation of metal-Ge bonds, was clearly observed. This Ge atom diffusion was suppressed by inserting an ultrathin GeO2 layer at the metal/Ge interface. Al deposition on GeO2/Ge indicated that the GeO2 layer had reduced in thickness by around 1 nm. By inserting a GeO2 layer more than 2 nm thick into the Al/p-type Ge interface, the junction properties changed from Ohmic to Schottky-type, as observed from both XPS analysis and I-V characteristics. However, for Au/Ge and Pt/Ge, we confirmed that the Ge atoms diffused into the metal layer, and that it was possible to suppress this diffusion by the insertion a GeO2 layer. However, no significant change was observed in the Schottky barrier height before or after the GeO2 layer was inserted.