Fermi-Level Depinning in Germanium Using Black Phosphorus as an Interfacial Layer

On substrates such as Ge, GaAs, GaSb, InP and a few other materials, Fermi-level pinning (FLP) is a major concern as it inhibits barrier height modulation (BHM) with different work function metals. We demonstrate for the first time a novel and simple mechanical exfoliation of multilayer black phosphorus (BP) as an interfacial layer (IL) to depin the Fermi level on Ge substrate. Multilayer-BP IL was able to alleviate the critical issue of FLP, as it leads to (i) an increase in reverse current by more than two and a half orders on n-Ge (quasi-Ohmic current-voltage(I-V) characteristics) with a decrease in Schottky barrier height (SBH) of 0.14 eV, and (ii) a decrease in reverse current by two orders on p-Ge (Schottky I-V characteristics) with an increase in SBH of 0.52 eV as compared to devices without IL. The device with multilayer-WSe2 IL, having a high conduction band offset (CBO), leads to poor electrical performance compared to a multilayer-BP IL with low CBO. Based on this study, multilayer-BP can be considered as an ideal candidate for an IL material to resolve FLP issue and achieve BHM for various other pinned semiconductors as well.