Electrical Transport in Monolithic Al-Si/Al-Ge Heterojunction based Nanowire Schottky Barrier Field-Effect Transistors

Abstract

Overcoming the difficulty in reproducibility and deterministically defining the metal phase of metal-semiconductor heterojunctions is among the key prerequisites to enable next-generation nanoelectronic, optoelectronic and quantum devices. In this respect, a comprehensive understanding of the charge carrier injection and the electronic conduction mechanisms, which are distinctly different from conventional MOSFETs, are necessary. Here, we provide an in-depth discussion of the transport mechanisms in Si and Ge nanowires (NWs) embedded in Schottky barrier (SB) FETs (SBFETs). Key for the fabrication of these devices is the selective and controllable transformation of Si and Ge NWs into Al, which enables high-quality monolithic and single-crystalline Al contacts, fulfilling compatibility with modern CMOS fabrication. To investigate the transport in Al-Si and Al-Ge heterostructures, detailed and systematic electrical characterizations carried out by bias spectroscopy in the temperature regime between T = 77.5 K and 400 K. Thereof, activation energy maps have been extracted to evaluate the effective SB height for electrons and holes in both material systems. The Al-Si material system revealed symmetric effective SBs, which is interesting for reconfigurable electronics relying on reproducible nanojunctions with equal injection capabilities for electrons and holes. In stark contrast, the Al-Ge material system revealed a highly transparent contact for holes due to Fermi level pinning close the valance band and charge carrier injection saturation by a thinned SB, while thermionic and field emission mechanism limit the overall electron conduction. In this regime, nanometer scale Ge departs from its bulk counterpart and delivers a strong and reproducible negative differential resistance followed by a sudden current increase indicating the onset of impact ionization above a certain threshold electric field. Most importantly, the presented description of the temperature dependent transport mechanisms in Al-Si and Al-Ge nanojunctions contributes to a better understanding of metal-group-IV based SBFETs, which are highly anticipated for the implementation of electronic device functionalities beyond the capabilities of conventional FETs.

Full text

Electrical Transport in Monolithic Al-Si/Al-Ge Heterojunction based
Nanowire Schottky Barrier Field-Effect Transistors
R. Behrle1*, M. Sistani1, S. Barth2, C.G.E. Murphey3, J.F. Cahoon3, M.I. den Hertog4, Z.S. Momtaz4,
W.M. Weber1
1 Institute of Solid State Electronics, TU Wien, Vienna, Austria
2 Physikalisches Institut, Goethe Universität Frankfurt, Frankfurt am Main, Germany
3 Department of Chemistry, University of North Carolina, Chapel Hill, United States
4 Institut Néel, CNRS, Université Grenoble-Alpes, Grenoble, France
*raphael.behrle@tuwien.ac.at
Overcoming the difficulty in reproducibility and deterministically defining the metal phase of metal-
semiconductor heterojunctions is among the key prerequisites to enable next-generation
nanoelectronic, optoelectronic and quantum devices. In this respect, a comprehensive understanding
of the charge carrier injection and the electronic conduction mechanisms, which are distinctly different
from conventional MOSFETs, are necessary. Here, we provide an in-depth discussion of the transport
mechanisms in Si and Ge nanowires (NWs) embedded in Schottky barrier (SB) FETs (SBFETs). Key for the
fabrication of these devices is the selective and controllable transformation of Si and Ge NWs into Al,
which enables high-quality monolithic and single-crystalline Al contacts, fulfilling compatibility with
modern CMOS fabrication. To investigate the transport in Al-Si and Al-Ge heterostructures, detailed and
systematic electrical characterizations carried out by bias spectroscopy in the temperature regime
between T = 77.5 K and 400 K (see Figure 1). Thereof, activation energy maps have been extracted to
evaluate the effective SB height for electrons and holes in both material systems. The Al-Si material
system revealed symmetric effective SBs, which is interesting for reconfigurable electronics relying on
reproducible nanojunctions with equal injection capabilities for electrons and holes. In stark contrast,
the Al-Ge material system revealed a highly transparent contact for holes due to Fermi level pinning
close the valance band and charge carrier injection saturation by a thinned SB, while thermionic and
field emission mechanism limit the overall electron conduction. In this regime, nanometer scale Ge
departs from its bulk counterpart and delivers a strong and reproducible negative differential resistance
followed by a sudden current increase indicating the onset of impact ionization above a certain
threshold electric field. Most importantly, the presented description of the temperature dependent
transport mechanisms in Al-Si and Al-Ge nanojunctions contributes to a better understanding of metal-
group-IV based SBFETs, which are highly anticipated for the implementation of electronic device
functionalities beyond the capabilities of conventional FETs.
Figure 1. TEM images of the Al-Si and Al-Ge interface in the upper panel prove the abrupt metal-semiconductor interface.
Utilizing bias spectroscopy transfer characteristics in the lower panel visualize transport regimes over temperature.
Additionally, the transfer characteristic at T = 295 K is shown (right y-axis).