| MADISON
- University of Wisconsin-Madison researchers have
demonstrated a way to release thin membranes of semiconductors
from a substrate and transfer them to new surfaces-an
advance that could unite the properties of silicon
and many other materials, including diamond, metal
and even plastic.
Led by materials science and engineering graduate
student Michelle Roberts, the team reports in the
April 9 issue of Nature Materials that the freed
membranes, just tens of nanometers thick, retain
all the properties of silicon in wafer form. Yet,
the nanomembranes are flexible, and by varying the
thicknesses of the silicon and silicon-germanium
layers composing them, scientists can make membrane
shapes ranging from flat to curved to tubular.
Most importantly, the technique stretches the nanomembranes
in a predictable and easily controlled manner, says
materials science and engineering professor Max Lagally,
who is Roberts' advisor. In silicon that is stretched,
or under tensile strain, current flows faster-a fact
engineers already exploit to help control silicon's
conductivity and produce speedier electronics. Strain
also becomes important whenever different materials
are integrated.
The new technique makes tuning the strain of materials
simpler, while avoiding the defects that normally
result. In addition, Lagally says: "We're no longer
held to a rigid rock of material. We now have the
ability to transfer the membranes to anything we
want. So, there are some really novel things we can
do."
Potential applications, he says, include flexible
electronic devices, faster transistors, nano-size
photonic crystals that steer light, and lightweight
sensors for detecting toxins in the environment or
biological events in cells.
Although it could make controlling strain easier,
the technique is not manufacturing-ready, cautions
physics professor Mark Eriksson, because it requires
the release of nanomembranes into solution before
bonding to other materials.
"What we've done is a first demonstration," says
Eriksson. "But now that we've shown the underlying
principles are sound, we can begin taking the next
steps."
In building electronic devices, engineers routinely
layer materials with different crystal structures
on top of one another, creating strain. Larger germanium
atoms, for example, want to sit farther apart in
a crystalline lattice than do smaller atoms of silicon.
Thus, when a thin layer of silicon-germanium alloy
is bonded to a thicker silicon substrate, the silicon's
lattice structure dominates, forcing the germanium
atoms into unnaturally close proximity and compressing
the silicon-germanium.
Scientists can then use the compressive strain in
the silicon-germanium to strain a thin silicon layer
grown on top, but only if the alloy's strain is controlled.
To do so, they typically deposit many layers of silicon-germanium.
As layers are added and strain builds, "dislocations," or
breaks in the crystal lattice, naturally develop,
which give germanium atoms the extra room they need
and relax some of the strain. But the technique is
time-consuming and expensive, and the defects can
scatter current-carrying electrons and otherwise
degrade device performance.
The Wisconsin team's goal was to integrate silicon
and silicon-germanium and manage strain without having
to introduce defects. The scientists made a three-layer
nanomembrane composed of a thin silicon-germanium
layer sandwiched between two silicon layers of similar
thinness. The membrane, in turn, sat atop a silicon
dioxide layer in a silicon-on-insulator substrate.
To release the nanomembrane, the researchers etched
away the oxide layer with hydrofluoric acid.
"When we remove the membrane, the silicon-germanium
is no longer trying to fight the substrate, which
is like a big rock holding it from below. Instead,
it's just fighting the two very thin silicon layers," says
Lagally. "So the silicon-germanium expands and takes
the silicon with it."
Pulled by the silicon-germanium, the silicon now
exhibits tensile strain, which the researchers can
readily adjust by varying the thicknesses of the
layers. They call the technique "elastic strain sharing" because
in the freed membrane, strain is balanced, or shared,
between the three layers.
Levente Klein, a postdoctoral researcher working
with Eriksson, also showed that the strain produced
by the technique traps electrons in the top silicon
layer, which is the end goal for many devices that
integrate silicon and silicon-germanium, says Eriksson.
"In this research, there's a nice synergy between
the structural characteristics of the material and
the consequences for electronics," he says.
Although the Wisconsin team grew their nanomembranes
on silicon-on-insulator substrates, the method should
apply to many substances beyond semiconductors, says
Lagally, such as ferroelectric and piezoelectric
materials. All that's needed is a layer, like an
oxide, that can be removed to free the nanomembranes.
"In any application where crystallinity and strain
are important, the idea of making membranes should
be of value," says Lagally.
--Madeline Fisher; (608) 265-8592; mmfisher@engr.wisc.edu
Contact: Max Lagally
lagally@engr.wisc.edu
608-263-2078
Mark Eriksson
maeriksson@wisc.edu
608-263-6289
University of Wisconsin-Madison
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