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 embedded in Schottky-barrier FETs. Key for the fabrication of these devices is the unique selective and controllable transformation of Si and Ge nanowires into Al, which enables high-quality monolithic and single-crystalline Al contacts, fulfilling compatibility with modern CMOS fabrication. TEM and EDX confirmed both the composition and crystalline nature of the presented heterostructures, with no intermetallic phases formed during the exchange process. 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 Schottky barrier height for electrons and holes in both material systems. The Al-Si material system revealed highly symmetric effective Schottky barriers, which is highly 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 Schottky barrier, while thermionic and field emission mechanism limited the overall electron conduction, indicating a distinct Schottky barrier for electrons. In this regime, nanometer scale Ge departs from its bulk counterpart and delivers a strong and reproducible negative differential resistance (NDR) followed by a sudden current increase indicating the onset of impact ionization above a certain threshold electric field. Importantly, embedding the proposed Al-Ge heterojunctions into a three-gate FET architecture provides a unique fusion of the concept of reconfiguration and NDR embedded in a universal adaptive transistor that may enable energy efficient programmable circuits with multi-valued operability that are inherent components of artificial intelligence electronics. 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 Schottky barrier FETs, 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
Masiar Sistani
1
, Raphael Behrle
1
, Sven Barth
2
, Corban G.E. Murphey
3
, James F. Cahoon
3
,
Martien I. den Hertog
4
, Zahra S. Momtaz
4
, Walter M. Weber
1
1
Institute of Solid State Electronics, Technische Universität 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
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
embedded in Schottky-barrier FETs. Key for the fabrication of these devices is the unique selective and
controllable transformation of Si and Ge nanowires into Al, which enables high-quality monolithic and
single-crystalline Al contacts, fulfilling compatibility with modern CMOS fabrication. TEM and EDX
confirmed both the composition and crystalline nature of the presented heterostructures, with no
intermetallic phases formed during the exchange process. 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 Schottky barrier height for electrons and holes in
both material systems. The Al-Si material system revealed highly symmetric effective Schottky barriers,
which is highly 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 Schottky barrier, while thermionic and field emission
mechanism limited the overall electron conduction, indicating a distinct Schottky barrier for electrons.
In this regime, nanometer scale Ge departs from its bulk counterpart and delivers a strong and
reproducible negative differential resistance (NDR) followed by a sudden current increase indicating
the onset of impact ionization above a certain threshold electric field. Importantly, embedding the
proposed Al-Ge heterojunctions into a three-gate FET architecture provides a unique fusion of the
concept of reconfiguration and NDR embedded in a universal adaptive transistor that may enable
energy efficient programmable circuits with multi-valued operability that are inherent components of
artificial intelligence electronics. 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 Schottky barrier FETs, which are highly anticipated for the
implementation of electronic device functionalities beyond the capabilities of conventional FETs.