When graduate student Pengpeng Zhang successfully
imaged a piece of silicon just 10 nanometers-or a
millionth of a centimeter-in thickness, she and her
University of Wisconsin-Madison co-researchers were
puzzled. According to established thinking, the feat
should be impossible because her microscopy method
required samples that conduct electricity.
"After she did it, we realized, 'Hey, this silicon
layer is really thin-it's much thinner than what
people normally use,'" says UW-Madison physicist
Mark Eriksson. "In fact, it's thin enough that it
should be very hard to run a current through it.
So we began asking, 'Why is this working?'"
A team led by College of Engineering professors
Paul Evans, Irena Knezevic and Max Lagally and physics
professor Eriksson has now answered that question.
Writing in the Feb. 9 issue of the journal Nature,
they have shown that when the surface of nanoscale
silicon is specially cleaned, the surface itself
facilitates current flow in thin layers that ordinarily
won't conduct. In fact, conductivity at the nanoscale
is completely independent of the added impurities,
or dopants, that usually control silicon's electrical
properties, the team reports.
"What this tells us is that if you're building nanostructures,
the surface is really important," says Evans. "If
you make silicon half as thick, you would expect
it to conduct half as well. But it turns out that
silicon conducts much worse than that if the surface
is poorly prepared and much better than that if the
surface is well prepared."
The results also mean that the powerful concepts,
methods and instruments of silicon electronics honed
by scientists and the semiconductor industry over
decades - many of which require conductive samples,
like the scanning tunneling microscopy method employed
by Zhang - can also be used to explore the nanoworld.
"We're working at the crossover between silicon
electronics and nanoelectronics," says Evans. "This
material is the same size as nanodevices like silicon
nanowires and quantum dots. But now we can use the
tools from silicon electronics we already have to
probe it."
The team studied silicon-on-insulator substrates,
in which a half-millimeter-thick silicon wafer is
covered by a much thinner layer of insulating silicon
oxide. Another silicon layer, in turn, tops the oxide
layer. In the UW-Madison investigation, this uppermost
layer was a "nanomembrane" just 10 nanometers thick.
Silicon nanomembranes could one day become the platform
for future high-speed electronics and a host of novel
sensor technologies, says Lagally. But like all silicon,
they naturally develop another unwanted layer of
oxide on top when exposed to air, resulting in an
oxide-silicon-oxide structure. And the usual means
to drive off the top oxide-by heating the material
to more than 1,200 degrees Celsius-causes nanomembranes
to ball up.
What Zhang originally developed was a method to
remove the top oxide without causing this damage.
Under ultra-high vacuum, she slowly deposited several
additional silicon or germanium layers, each just
one atom thick, at 700 degrees C.
Scanning tunneling microscopy soon revealed that
this process somehow allowed the nanomembrane to
conduct electricity. To find out why, the team analyzed
the resistance-the inverse of conductivity-of silicon
layers ranging from to 200 to 15 nanometers in thickness.
More importantly, they compared silicon's resistance
when sandwiched between two oxide layers-the usual
case-and when cleaned of the top oxide and exposed
to vacuum through Zhang's method. Knezevic then created
a model predicting resistance as a function of layer
thickness in both situations.
Knezevic's model indicates that in layers thinner
than 100 nanometers, the properties of silicon itself
become irrelevant: what matters is the surface. Even
in relatively thick layers of 200 nanometers, silicon
cleaned of the top oxide was at least 10 times more
conductive than silicon sandwiched between oxide
layers. And as layer thickness shrunk, this difference
eventually grew to six orders of magnitude.
The team has proposed that cleaning promotes conductivity
by creating new electronic states on the silicon
surface where electrons can reside. States are to
electrons what parking spaces are to cars. In silicon
sandwiched between oxide layers, every parking space-indeed,
the entire space of the lot-is jammed. With no empty
spaces to move into, electrons are trapped in position
and current can't flow.
When new states open up on the surface due to cleaning,
it's as if another level of parking spaces has been
added, and a small number of electrons jump to the
new spots. What they leave behind in the bulk silicon
are holes-empty spaces that other electrons can fill.
As electrons move into these holes, additional holes
are produced. In this way, the traffic jam breaks
up and current begins to flow-all because of the
surface.
"It's an interesting interplay," says Eriksson. "You
clean the surface so you can image it. But then the
surface ends up enabling conductivity in the entire
silicon layer."
UW-Madison College of Engineering scientist Donald
Savage; graduate students Emma Tevaarwerk and Byoung-Nam
Park; and George Celler of Soitec USA also contributed
to this work. The research was supported by the National
Science Foundation, the U.S. Department of Energy
and the U.S. Air Force Office of Scientific Research.
Madeline Fisher, 608-265-8592, mmfisher@engr.wisc.edu
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