Low resistivity contact on n-type Ge using low work-function Yb with a thin TiO2 interfacial layer

This work demonstrates the benefit of a lower contact barrier height, and hence reduced contact resistivity (ρc), using a low work-function metal (Yb) in unpinned metal-interfacial layer-semiconductor (MIS) contacts on n-type Ge. Fermi-level unpinning in MIS contacts on n-Ge is first established by introducing a 2 nm TiO2−x interfacial layer between various contact metals (Yb, Ti, Ni, Pt) and n-Ge. Further, Yb/TiO2−x/n-Ge MIS contact diodes exhibit higher current densities (up to 100×) and lower effective contact barrier height (up to 30%) versus Ti/TiO2−x control devices over a wide range of TiO2−x thickness (1–5 nm). Finally, low work-function Yb combined with doped TiO2−x having a low conduction band offset with Ge and high substrate doping (n+-Ge: 2.5×1019 cm−3) is shown to result in an ultra-low ρc value of 1.4 × 10−8 Ω cm2, 10 × lower than Ti/TiO2−x control devices.