False color scanning electron microscope image: the ‘table’ is the central green square region. The ‘pockets’ are narrowings that join to open green areas. The ‘cushion’ is the grey trench that defines the device. White scale bar = 500 nm. |
There’s
nothing worse than a shonky pool table with an unseen groove or bump
that sends your shot off course: a new study has found that the same
goes at the nano-scale, where the “billiard balls” are tiny electrons
moving across a “table” made of the semiconductor gallium arsenide.
These
tiny billiard tables are of interest towards the development of future
computing technologies. In a research paper titled “Impact of
Small-Angle Scattering on Ballistic Transport in Quantum Dots”, an
international team of physicists has shown that in this game of
“semiconductor billiards”, small bumps have an unexpectedly large effect
on the paths that electrons follow.
Better
still, the team has come up with a major redesign that allows these
bumps to be ironed out. The study, led by researchers from the UNSW
School of Physics, is published in the journal Physical Review Letters.
The team included colleagues, from the University of Oregon (US), Niels Bohr Institute (Denmark) and Cambridge University (UK).
“Scaled
down a million-fold from the local bar variety, these microscopic pool
tables are cooled to just above absolute zero to study fundamental
science, for example, how classical chaos theory works in the quantum
mechanical limit, as well as questions with useful application, such as
how the wave-like nature of the electron affects how transistors work,”
says team member Associate Professor Adam Micolich. “In doing this,
impurities and defects in the semiconductor present a serious
challenge.”
Ultra-clean
materials are used to eliminate impurities causing backscattering (akin
to leaving a glass on the billiard table) but until now has been no way
to avoid the ionized silicon atoms that supply the electrons.
“Their electrostatic effect is more subtle, essentially warping the table’s surface.” explains Micolich.
Earlier
studies assumed this warping was negligible, with the electron paths
determined only by the billiard table’s shape (e.g. square, circular,
stadium-shaped).
“We
found that we can ‘reconfigure’ the warping by warming the table up and
cooling it down again, with the electron paths changing radically in
response,” says Professor Richard Taylor from the University of Oregon.
“This shows that the warping is much more important than expected.”
Using
a new billiard design developed during PhD work at UNSW by lead author
Dr Andrew See, the silicon dopants are removed, eliminating the
associated warping, and enabling the electron paths to stay the same
each time they cool the device down for study.
“These
undoped billiard devices pinpoint the silicon dopants as the cause of
the warping. The level of improvement obtained by removing the silicon
was unexpected, earlier work on much larger devices suggested that we
wouldn’t see this level of improvement.
But
at the nanoscale, the dopant atoms really do make a really big
difference”, says Micolich, “Ultimately, our work provides important
insight into how to make better nanoscale electronic devices, ones where
the properties are both more predictable, and more consistent each time
we use them.”
Impact of Small-Angle Scattering on Ballistic Transport in Quantum Dots
Source: University of New South Wales