Mapping Electronic Transport in Ge Nanowire SBFETs: From Tunneling to NDR

An in-depth discussion of the transport mechanisms in ambipolar Ge nanowire (NW) Schottky barrier (SB) field-effect transistors (SBFETs) embedded in monolithic and crystalline Al-Ge-Al heterostructures is presented. Thereto, detailed bias spectroscopy in the temperature regime between T=77.5K and 400 K was conducted. In conjunction with the obtained activation energy maps, a highly transparent contact for holes due to charge carrier injection saturation by Fermi level pinning close to the valence band and gate induced thinning of the SB width was observed. In contrast, the thermionic-and field-emission mechanism limits the overall electron conduction, indicating a distinct SB for electrons. In this regime, nanoscaled Ge distinctly differs from its bulk counterpart and delivers a strong and reproducible negative differential resistance (NDR) at room-temperature (RT) followed by a sudden current increase indicating the onset of impact ionization above a specific threshold electric field.