FOR RELEASE: March 27 1997
Contact: Larry Bernard
Office: (607) 255-3651
Internet: [email protected]
Compuserve: Larry Bernard 72650,565
http://www.news.cornell.edu
PHOTOS AVAILABLE
ITHACA, N.Y. -- Cornell University scientists have achieved a "Holy Grail"
of materials science -- pure, single crystal growth of any film on a
semiconductor substrate, a technique that holds promise to revolutionize
electronics.
"This is preliminary work, but if it truly works -- and we think it will --
it definitely could revolutionize the microelectronics industry," said
Yu-Hwa Lo, Cornell associate professor of electrical engineering who led
the work. "This is a whole new way of doing things. The potential is very
great, unimaginable really, but we don't know yet if it will actually
happen. We have demonstrated that it is possible."
Lo and his doctoral student, Felix E. Ejeckam, and colleagues have for the
first time demonstrated a "universal substrate" on which a crystal of any
material can be grown. The technique opens the door for manufacturing
whole new classes of devices in optoelectronics and microelectronics, for
such items as new lasers, detectors, sensors, imaging systems, signal
processing and computer chips, compact discs, data storage and dozens of
other examples. The ability to grow single crystals of any material on
silicon, for example, may breed an entire new generation of electronics.
Their research paper, "Lattice engineered compliant substrate for
defect-free heteroepitaxial growth," is scheduled to appear in Applied
Physics Letters (March 31, 1997, vol. 70, 13), by Lo, Ejeckam, Shanthi
Subramanian, former Cornell doctoral student in materials science and
engineering now at Exxon Research and Engineering Co.; and Hong Q. Hou and
B.E. Hammons of Sandia National Laboratories.
The Cornell team also presented their work at the Institute of Electrical
and Electronics Engineers Lasers and Electro-Optics Society annual meeting
in November 1996 in Boston, by Lo, Ejeckam and Matthew Seaford, Cornell
doctoral student also affiliated with the Air Force Wright Patterson
Laboratory where he did this research, and the Sandia researchers. And it
will be presented in May at the International Conference on Indium
Phosphide and Related Materials in Hyannis, Mass., and the Materials
Research Society meeting in San Francisco on March 31.
The research was funded by the Office of Naval Research; the Defense
Advanced Research Projects Agency; the Materials Science Center at Cornell,
which is funded by the National Science Foundation (NSF); and the Cornell
Nanofabrication Facility, which also is NSF-funded.
Semiconductors are tiny crystals, such as silicon or germanium, that
conduct electricity. They are the heart of all integrated circuits, which
power everything from computers to cellular telephones to fiber
communication networks. A major obstacle to the manufacture of various
semiconductors is that the single-crystal semiconductor thin films must be
deposited on a crystal of the same structure. For example, a
light-emitting gallium arsenide thin film must be deposited on a gallium
arsenide bulk substrate, or else defects will result and the semiconductor
cannot be used.
Each single crystal is characterized by its lattice structure and lattice
constant. When a crystal layer is grown on a bulk crystal substrate, even
a mismatch of 1 percent in their lattice constants causes problems. But
the Cornell technique, for which Cornell has applied for a patent, shows
that a mismatch of 15 percent can be overcome -- a feat previously
unachievable.
The Cornell team solved that problem by what might be called a simple twist
of fate. By rotating a thin film slightly and bonding it to a substrate,
the surface of this new substrate becomes flexible, or compliant, and a
crystal of any material can grow on its surface. They call it a twist
boundary, in which the crystal materials are bonded by angular
misalignment; and they call the new substrate, a new compliant substrate.
"It's ingenious," said Stephen Sass, Cornell professor of materials science
and engineering, who was the Ph.D. thesis adviser for Subramanian and has
been following the work through discussions with her, Ejeckam and Lo.
"I've studied twist boundaries for years and I still don't understand how
the boundary/thin film substrate combination is able to accommodate such
large mismatches in atomic plane spacing. The results are striking, almost
too good to be true. It's a very exciting discovery and I'm eager to look
into the science underlying it."
The Cornell team demonstrated the technique with thick, pure crystal layers
of indium gallium phosphide, gallium antimonide and indium antimonide, with
mismatches as high as 15 percent. They successfully grew these crystals on
a gallium arsenide wafer that had a flexible layer thin film. With
traditional methods, it would not have been possible.
The Cornell team, in collaboration with researchers from the Wright
Patterson Laboratory and the Sandia Laboratory, has demonstrated that the
defect density in an indium antimonide layer has been reduced by at least
100,000 times with the new method, compared to the conventional method.
This means that indium antimonide crystals can be grown on gallium
arsenide, to form the basis for infrared detection and a Hall sensor -- a
sensor that has been used in airplanes and soon will be used in automobiles.
If this can be done for another compound semiconductor, gallium nitride,
which has a lattice mismatch of about 20 percent, then high-quality blue
and ultraviolet lasers as well as high-temperature, high-power electronic
circuits can be fabricated. Blue lasers, rather than red, will be used in
the next generation of compact discs, for example, because the shorter
wavelength stores more data. High-powered electronic circuits that can
also withstand high temperatures are used in automobile, aerospace,
communication and power industries. The Cornell team expects it can be
done with this technique, and they are in the process of trying it.
The researchers also believe that the crystals can be grown on silicon
wafers, opening the door for computers that, for example, could have
different types of semiconductors operating at the same time on the same
motherboard.
In microelectronics, the next generation ULSI (Ultra Large Scale
Integrated) circuits can be made using this technique. If the technique is
successful on a silicon substrate, then no other substrate material would
be needed, the researchers say. Since this lattice engineering technique
is a structural phenomenon, there is no reason why it should not work with
silicon.
"This new concept and technique enables the growth of numerous
high-quality, new compound semiconductors that cannot otherwise be achieved
due to lack of adequate substrates," the authors write. "With further
research, this work will lead to the production of 'universal substrates'
on which compound semiconductors of nearly any lattice constant and crystal
structure can be grown without defects."
"This is a nagging problem in the semiconductor industry," said Ejeckam,
who expects to receive his Cornell Ph.D. in June and who contributed to the
work as part of his doctoral thesis. "You can't grow gallium arsenide
crystals on silicon. People are always trying different techniques, but
they all had a major problem. We were looking for something universal that
would be far-reaching with wide impact. It also had to be easy to
implement. And we found it."
Ejeckam was pleased to be part of the research team. "It's rare that one
gets to be present at the embryonic stages of a new field," he said. "We
are going to be able to make whole new devices in optoelectronics and
microelectronics. This is the seed for a whole new area, and it has an
extremely high potential to change the field. Whether it truly can be
sustained over decades, we can't tell. But the applications are
wide-ranging and they span so many areas in materials science and
electrical engineering. It's universal."
He added: "No one has ever seen anything like this before. And this is
just the outer layer of the onion."
-30-
EDITORS: High-resolution photos of the researchers and transmission
electron microscope images that demonstrate the technique are available via
the Web at: http://www.news.cornell.edu/science/March97/substrate.lb.html
RSS