For release: January 14, 2004
Contacts:
Michael C. Tringides,
Condensed Matter Physics, (515) 294-6439
Saren Johnston, Public Affairs, (515) 294-3474
AMES, IA – “Control is the name of the game,” said Ames
Laboratory physicist Michael C. Tringides. He was talking about the importance
of growing atomic structures and ultrathin metal films in uniform sizes and
with highly ordered geometries for technological applications that include
switches, lasing materials and semiconductors that allow computer chips to
run faster.
Exciting as the potential is for the development of these nanotechnologies
(artificially fabricated structures in the nanometer range of 0.1-100 nm)
and other microminiature
equipment, Tringides knows that realizing such applications requires laying substantial
groundwork. He and members of his research group, associate scientist Myron Hupalo
and graduate students Vincent Yeh and Michael Yakes, are doing basic research
at the U. S. Department of Energy’s Ames Laboratory to learn more about
the microscopic processes that control the growth of custom-made materials. The
work, supported by the DOE’s Office of Basic Energy Sciences, may prove
critical in the further miniaturization of silicon-based electronic devices,
a major undertaking in light of the silicon industry’s huge role in technological
innovation and production.
Tringides explained that vital to the success of these miniaturization efforts
is the ability to achieve exact control of layer thickness and atomic uniformity
of thin films and nanostructures – what he refers to as “the ‘Holy
Grail’ in nanotechnology, the next major industrial revolution.”
Noting the great demand for these materials within the silicon industry, Tringides
said, “It’s essential that these structures are grown in a robust
and reproducible way, with easy size selection. Contrary to conventional wisdom,
we’ve discovered that an intriguing type of self-organization is possible
with lead (Pb) deposited on silicon (Si) if the growth is carried out at low
temperature – around 185 Kelvin, or minus 126 degrees Fahrenheit.” He
added that in all other systems studied so far, the deposited metal atoms stack
up in islands of very wide height variation. But for Pb grown on Si (oriented
along the (111) crystal axis), he said the atoms seem to be “intelligent” and
make only one height choice.
“The selected height of these nanostructures is related to their electronic
structure,” Tringides continued. He explained that keeping electrons confined
in small metal islands
requires them to occupy sharp energy levels as dictated by the laws of quantum
mechanics. This confinement implies that the total energy of the electrons depends
strongly
on the nanostructure’s
size or shape. “This is called Quantum Size Effects, or QSE,” said
Tringides, “and a consequence of this relationship is that certain film
thicknesses are more stable than others.”
Tringides and his research group were the first to observe and monitor the highly
unusual formation of uniform-height Pb /Si islands. They observed the 7-step,
steep-edged, flat-top islands using two complementary techniques. Quantitative
electron diffraction, used by Yeh and Yakes, samples the island height uniformity
by reflecting electron waves from the surface. Scanning tunneling microscopy
(STM), used primarily by Hupalo, images the islands as they form, giving the
island size and shape. “As a result of our investigations, we have shown
that not only can QSE be observed in small objects, but QSE can dictate the island
uniformity and height,” said Hupalo.
The scientists were amazed to see this uncommon growth mode of the 7-step Pb
islands, which clearly shows that the deposited atoms seem able to “climb” and
select preferred, final positions. “No one was expecting to see the uniform-height,
self-organized growth,” said Tringides. “We couldn’t believe
how quickly the islands formed following deposition. Nature, itself, was doing
the work for us!”
Tringides explained that although QSE is the driving force for height uniformity,
one still needs to find the right temperature and surface coverage conditions
for the islands to form. These variables make it difficult to predict when the
self-organized growth is possible and explain why it has never been seen before.
“It’s necessary to study the growth as a function of the different
growth parameters, such as temperature and deposition rate, to discover when
such self-organized nanostructures form,” said Tringides. By varying these
parameters in the Pb/Si(111) system, he and his co-workers found that only odd
heights, i.e. 5-, 7-, and 9-step-high islands, are possible. Using these growth
parameters, they developed a kinetic phase diagram that serves as a guide to
select the desired island height.
But Tringides warned that island height uniformity exists only at sufficiently
low temperature. At higher temperature, the islands evolve into multiheight mounds,
limiting their potential for room-temperature applications. Working to resolve
the problem, Tringides and members of his research group have discovered that
they can “manipulate the growth” by adsorbing oxygen, which restricts
the upward motion of the Pb atoms, allowing the islands to maintain the same
height. This process extends their stability to higher temperature and their
potential for technological applications.
Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts
research into various areas of national concern, including energy resources,
high-speed computer design, environmental cleanup and restoration, and the synthesis
and study of new materials.
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Editors: Click for Image of Pb/Si(111) islands
Last revision: 1/14/04 kbg