Ark. -- University of Arkansas researchers have witnessed
the birth of a quantum dot and learned more about
how such atomic islands form and grow, using the ultrahigh
vacuum facility on campus. This information will help
researchers better understand and use materials that
could lead to small, efficient and powerful computers,
communication devices and scientific instruments.
Seongho Cho, Zhiming Wang,
and Gregory Salamo report their findings in the upcoming
issue of the journal Applied Physics Letters.
"We have changed the way
people have to think about how nanostructures grow
on a surface," said Salamo, University Professor
of physics. "People had a different idea of how
these islands formed, but until now there was not
The researchers combined the
molecular-beam epitaxy machine, which creates material
atom by atom, with scanning tunneling microscopy,
which can observe the atoms, to witness the creation
of quantum dots, or atomic islands, of indium gallium
arsenide (InGaAs) atoms atop a gallium arsenide (GaAs)
surface. InGaAs is a material of electronic and optical
interest for properties that could enhance communications
equipment, computers and electronics.
At the atomic level, a surface
is characterized by small monolayer "steps."
Until now, researchers believed that the first atom
of a quantum dot would land at the base of the step,
rather than further out towards the edge of the step.
The work of Cho, Wang and Salamo shows instead that
the first atom lands at the step's edge.
"An island growing from
below the step edge must first build up to a height
equal to the step. This is unnecessary since it could
more easily just start from the top of the step,"
said Wang, a research professor working with Salamo.
The researchers found that
the first atoms of InGaAs land side by side atop the
GaAs surface and experience a strain, much like a
person trying to squeeze into an already crowded line.
Therefore, after a short time, it becomes easier for
an InGaAs atom to land atop other InGaAs atoms instead
of on the initial surface. Also, fewer atoms land
on a layer as the layers build up, allowing the atoms
to have more space and experience less strain. The
researchers witnessed this sequential, upward, narrowing
growth as they studied the formation of the InGaAs
quantum dots, which ended by forming a pyramid-like
This observation also is significant
because it may offer a more general explanation of
how other semiconductor materials behave at the nanoscale,
"It was predicted by previous
theory independent of materials, but wasn't observed
for InGaAs islands before," he said.
"We do not yet have a
complete picture of how these quantum dots grow,"
Salamo said. "But we have added to the picture."
This picture has implications
that extend beyond semiconductors, he added.
"How these small structures
grow and how they behave tells us about the rules
that govern small structures in general," Salamo
said. "Cells are small. DNA is small. Everything
is composed of small structures. When you understand
how things go together, they supply a library for
looking at other things in science."
Contact: Zhiming M. Wang, research
professor of microelectronics-photonics, (479) 575-4217,
Gregory J. Salamo, University Professor of physics,
Fulbright College, (479) 575-5931, firstname.lastname@example.org
Melissa Lutz Blouin, science and research communications
manager, (479) 575-5555, email@example.com