Nanowires under tension create the basis for ultrafast transistors


Date:
February 7, 2022
Source:
Helmholtz-Zentrum Dresden-Rossendorf
Summary:
Nanowires have a unique property: These ultra-thin wires can
sustain very high elastic strains without damaging the crystal
structure of the material. A team of researchers has now succeeded
in experimentally demonstrating that electron mobility in nanowires
is remarkably enhanced when the shell places the wire core under
tensile strain.



FULL STORY ========================================================================== Smaller chips, faster computers, less energy consumption. Novel concepts
based on semiconductor nanowires are expected to make transistors in microelectronic circuits better and more efficient. Electron mobility
plays a key role in this: The faster electrons can accelerate in
these tiny wires, the faster a transistor can switch and the less
energy it requires. A team of researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the TU Dresden and NaMLab has now succeeded
in experimentally demonstrating that electron mobility in nanowires is remarkably enhanced when the shell places the wire core under tensile
strain. This phenomenon offers novel opportunities for the development
of ultrafast transistors.


========================================================================== Nanowires have a unique property: These ultra-thin wires can sustain
very high elastic strains without damaging the crystal structure of the material. And yet the materials themselves are not unusual. Gallium
arsenide, for example, is widely used in industrial manufacturing,
and is known to have a high intrinsic electron mobility.

Tension creates speed To further enhance this mobility, the Dresden
researchers produced nanowires consisting of a gallium arsenide core
and an indium aluminum arsenide shell.

The different chemical ingredients result in the crystal structures in the shell and the core having slightly different lattice spacings. This causes
the shell to exert a high mechanical strain on the much thinner core. The gallium arsenide in the core changes its electronic properties. "We
influence the effective mass of electrons in the core. The electrons
become lighter, so to speak, which makes them more mobile," explained
Dr. Emmanouil Dimakis, scientist at the HZDR's Institute of Ion Beam
Physics and Materials Research and initiator of the recently published
study.

What started out as a theoretical prediction has now been proven
experimentally by the researchers in the recently published study. "We
knew that the electrons in the core ought to be even more mobile in
the tensile-strained crystal structure. But what we did not know was
the extent to which the wire shell would affect electron mobility in
the core. The core is extremely thin, allowing electrons to interact
with the shell and be scattered by it," remarked Dimakis. A series of measurements and tests demonstrated this effect: Despite interaction with
the shell, electrons in the core of the wires under investigation moved approximately thirty percent faster at room temperature than electrons
in comparable nanowires that were strain-free or in bulk gallium arsenide.

Revealing the core The researchers measured electron mobility by
applying contactless optical spectroscopy: Using an optical laser
pulse, they set electrons free inside the material. The scientists
selected the light-pulse energy such that the shell seems practically transparent to the light, and free electrons are only produced in the
wire core. Subsequent high-frequency terahertz pulses caused the free
electrons to oscillate. "We practically give the electrons a kick and
they start oscillating in the wire," explained PD Dr. Alexej Pashkin,
who optimized the measurements for testing the core-shell nanowires
under investigation in collaboration with his team at the HZDR.

Comparing the results with models reveals how the electrons move: The
higher their speed and the fewer obstacles they encounter, the longer
the oscillation lasts. "This is actually a standard technique. But this
time we did not measure the whole wire -- comprising the core and the
shell -- but only the tiny core.

This was a new challenge for us. The core accounts for around one
percent of the material. In other words, we excite about a hundred
times fewer electrons and get a signal that is a hundred times weaker,"
stated Pashkin.

Consequently, the choice of sample was also a critical step. A typical
sample contains an average of around 20,000 to 100,000 nanowires on
a piece of substrate measuring roughly one square millimeter. If the
wires are spaced even closer together on the sample, an undesirable
effect can occur: Neighboring wires interact with each other, creating
a signal similar to that of a single, thicker wire, and distorting the measurements. If this effect is not detected, the electron velocity
obtained is too low. To rule out such interference, the Dresden research
team carried out additional modelling as well as a series of measurements
for nanowires with different densities.

Prototypes for fast transistors Trends in microelectronics and the semiconductor industry increasingly demand smaller transistors that
switch ever faster. Experts anticipate that novel nanowire concepts
for transistors will also make inroads into industrial production over
the next few years. The development achieved in Dresden is particularly promising for ultra-fast transistors. The researchers' next step will be
to develop the first prototypes based on the studied nanowires and to
test their suitability for use. To do this, they intend to apply, test
and enhance metallic contacts on the nanowires, as well as testing the
doping of nanowires with silicon and optimizing manufacturing processes.

========================================================================== Story Source: Materials provided by
Helmholtz-Zentrum_Dresden-Rossendorf. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Leila Balaghi, Si Shan, Ivan Fotev, Finn Moebus, Rakesh Rana,
Tommaso
Venanzi, Rene' Hu"bner, Thomas Mikolajick, Harald Schneider,
Manfred Helm, Alexej Pashkin, Emmanouil Dimakis. High electron
mobility in strained GaAs nanowires. Nature Communications, 2021;
12 (1) DOI: 10.1038/s41467-021-27006-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220207112656.htm

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