In the quest for smaller, faster computer chips, researchers are increasingly
turning to quantum mechanics -- the exotic physics of the small.
The problem: the manufacturing techniques required to make quantum devices
have been equally exotic.
That is, until now.
Researchers at Ohio
State University have discovered a way to make quantum devices using technology
common to the chip-making industry today.
This work might one day enable faster, low-power computer chips. It could also
lead to high-resolution cameras for security and public safety, and cameras
that provide clear vision through bad weather.
Paul Berger, professor of electrical and computer engineering and professor
of physics at Ohio State University, and his colleagues report their findings
in an upcoming issue of IEEE Electron Device Letters.
The team fabricated a device called a tunneling diode using the most common
chip-making technique, called chemical vapor deposition.
"We wanted to do this using only the tools found in the typical chip-makers
toolbox," Berger said. "Here we have a technique that manufacturers
could potentially use to fabricate quantum devices directly on a silicon chip,
side-by-side with their regular circuits and switches."
The quantum device in question is a resonant interband tunneling diode (RITD)
-- a device that enables large amounts of current to be regulated through a
circuit, but at very low voltages. That means that such devices run on very
little power.
RITDs have been difficult to manufacture because they contain dopants -- chemical
elements -- that don't easily fit within a silicon crystal.
Atoms of the RITD dopants antimony or phosphorus, for example, are large compared
to atoms of silicon. Because they don't fit into the natural openings inside
a silicon crystal, the dopants tend to collect on the surface of a chip.
"It's like when you're playing Tetris and you have a big block raining
down, and only a small square to fit it in. The block has to sit on top,"
Berger said. "When you're building up layers of silicon, these dopants
don't readily fit in. Eventually, they clump together on top of the chip."
In the past, researchers have tried adding the dopants while growing the silicon
wafer one crystal layer at a time -- using a slow and expensive process called
molecular beam epitaxy, a method which is challenging for high-volume manufacturing.
That process also creates too many defects within the silicon.
Berger discovered that RITD dopants could be added during chemical vapor deposition,
in which a gas carries the chemical elements to the surface of a wafer many
layers at a time. The key was determining the right reactor conditions to deliver
the dopants to the silicon, he found.
"One key is hydrogen," he said. "It binds to the silicon surface
and keeps the dopants from clumping. So you don't have to grow chips at 320
degrees Celsius [approximately 600 degrees Fahrenheit] like you do when using
molecular beam epitaxy. You can actually grow them at a higher temperature like
600 degrees Celsius [more than 1100 degrees Fahrenheit] at a lower cost, and
with fewer crystal defects."
Tunneling diodes are so named because they exploit a quantum mechanical effect
known as tunneling, which lets electrons pass through thin barriers unhindered.
In theory, interband tunneling diodes could form very dense, very efficient
micro-circuits in computer chips. A large amount of data could be stored in
a small area on a chip with very little energy required.
Researchers judge the usefulness of tunneling diodes by the abrupt change in
the current densities they carry, a characteristic known as "peak-to-valley
ratio." Different ratios are appropriate for different kinds of devices.
Logic circuits such as those on a computer chip are best suited by a ratio of
about 2.
The RITDs that Berger's team fabricated had a ratio of 1.85.
"We're close, and I'm sure we can do better," he said.
He envisions his RITDs being used for ultra-low-power computer chips operating
with small voltages and producing less wasted heat.
"Chip makers today are having a great difficulty boosting performance
in each generation, so they pack chips with more and more circuitry, and end
up generating a lot of heat," Berger said. "That's why a laptop computer
is often too hot to actually sit atop your lap. Soon, their heat output will
rival that of a nuclear reactor per unit volume."
"That's why moving to quantum devices will be a game-changer."
RITDs could form high-resolution detectors for imaging devices called focal
plane arrays. These arrays operate at wavelengths beyond the human eye and can
permit detection of concealed weapons and improvised explosive devices. They
can also provide vision through rain, snow, fog, and even mild dust storms,
for improved airplane and automobile safety, Berger said. Medical imaging of
cancerous tumors is another potential application.
Posted October 15th, 2009
