AR--- April 04, 2005--- 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
Cho, Zhiming Wang, and Gregory Salamo report their
findings in the upcoming issue of the journal Applied
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 direct evidence."
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.
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.
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.
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 structure.
observation also is significant because it may offer
a more general explanation of how other semiconductor
materials behave at the nanoscale, Wang said.
was predicted by previous theory independent of materials,
but wasn't observed for InGaAs islands before,"
do not yet have a complete picture of how these quantum
dots grow," Salamo said. "But we have added
to the picture."
picture has implications that extend beyond semiconductors,
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."
Zhiming M. Wang
Research professor of microelectronics-photonics
University Professor of physics
Science and research communications manager
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