Effective Schottky Barrier Height Lowering of Metal/n-Ge with a TiO₂/GeO₂ Interlayer Stack

A perfect ohmic contact formation technique for low-resistance source/drain (S/D) contact of germanium (Ge) n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) is developed. A metal-interlayer-semiconductor (M-I-S) structure with an ultrathin TiO₂/GeO₂ interlayer stack is introduced into the contact scheme to alleviate Fermi-level pinning (FLP), and reduce the electron Schottky barrier height (SBH). The TiO₂ interlayer can alleviate FLP by preventing formation of metal-induced gap states (MIGS) with its very low tunneling resistance and series resistance and can provide very small electron energy barrier at the metal/TiO₂ interface. The GeO₂ layer can induce further alleviation of FLP by reducing interface state density (D_it) on Ge which is one of main causes of FLP. Moreover, the proposed TiO₂/GeO₂ stack can minimize interface dipole formation which induces the SBH increase. The M-I-S structure incorporating the TiO₂/GeO₂ interlayer stack achieves a perfect ohmic characteristic, which has proved unattainable with a single interlayer. FLP can be perfectly alleviated, and the SBH of the metal/n-Ge can be tremendously reduced. The proposed structure (Ti/TiO₂/GeO₂/n-Ge) exhibits 0.193 eV of effective electron SBH which achieves 0.36 eV of SBH reduction from that of the Ti/n-Ge structure. The proposed M-I-S structure can be suggested as a promising S/D contact technique for nanoscale Ge n-channel transistors to overcome the large electron SBH problem caused by severe FLP.