Princeton,
N.J. -- In a stride that could hasten the development
of computer chips that both calculate and store data,
a team of Princeton scientists has turned semiconductors
into magnets by the precise placement of metal atoms
within a material from which chips are made.
The effort marks the first time that scientists
have achieved this degree of control over the atomic-level
structure of a semiconductor, a goal that has eluded
researchers for many years. The team used this unique
capability to make a semiconductor magnetic, one
atom at a time. Team leader Ali Yazdani said that
manipulating semiconductors could eventually revolutionize
computers by exploiting not just the flow of electrons
but also their quantum property, called spin, for
computation.
"Using the tip of a scanning tunneling microscope,
we can take out a single atom from the base material
and replace it with a single metal that gives the
semiconductor its magnetic properties," said Yazdani,
a Princeton professor of physics. "The ability to
tailor semiconductors on the atomic scale is the
holy grail of electronics, and this method may be
the approach that is needed."
The team, which also includes scientists from the
University of Illinois at Urbana-Champaign and the
University of Iowa as well as Princeton, will publish
their results as the cover article of the July 27
issue of the scientific journal, Nature.
By incorporating manganese atoms into the gallium
arsenide semiconductor, the team has created an atomic-scale
laboratory that can reveal what researchers have
sought for decades: the precise interactions among
atoms and electrons in chip material. The team used
their new technique to find the optimal arrangements
for manganese atoms that can enhance the magnetic
properties of gallium arsenide. Implementation of
their findings within the chip manufacturing process
could result in a major advance in the use of both
the magnetic "spin" as well as electric charge for
computation.
"Chips might take on many new capabilities once
such 'spintronic' technology is perfected," Yazdani
said. "One thing we might be able to do is make chips
that can both manipulate data and store it as well,
which right now generally requires two separate parts
of a computer working together."
Computers use two different kinds of technology
to calculate results and store data. While semiconductor
chips -- often based on silicon or more advanced
materials such as gallium arsenide -- do the calculating,
data storage has generally been accomplished with
magnetic materials within floppy disks or reels of
tape. Combining these functions into a single device
could reduce the size and energy requirements of
computer hardware, a perennial goal of the industry.
Although gallium arsenide "doped" with manganese
has been a promising candidate material for such
dual-function chips for a decade, working with the
material has proven frustrating for a number of reasons.
One difficulty is that researchers have not been
able to engineer the material with optimal magnetic
properties.
"Up until now, we have not had a way to control
how the manganese sits in the gallium arsenide substrate," Yazdani
said. "We could not specify, for example, how large
the bits of manganese would be, or how far apart
they would be located. And because we couldn't study
how changing these variables affected the semiconductor's
performance, it was hard to know what its ideal specifications
should be. For the most part, we had to just crystallize
the material -- with the dopant arranged more or
less randomly -- and hope."
Dale Kitchen, a reasearcher in Yazdani's lab and
first author of the Nature paper, hit upon a solution
while working with a high-tech tool used to explore
complex materials called a scanning tunneling microscope,
which operates very differently than a desktop optical
microscope. The device has a finely-pointed electrical
probe that passes over a surface in order to detect
variations with a weak electric field. The team,
however, found that the charged tip could also be
used to eject a single gallium atom from the surface,
replacing it with one of manganese that was waiting
nearby.
"The important thing technically was that we could
incorporate the manganese into the underlying crystal
lattice," Yazdani said. "If you want to study how
the semiconductor functions, it would not have been
enough merely to deposit the manganese on the surface.
They needed to become a single integrated material."
Using their new technique, the team was able to
find the precise arrangements of manganese atoms
that exhibited magnetic properties, the important
factor in developing spin-based electronics. The
experimental data agreed with theoretical work that
had been performed by Michael Flatté and his
group at the University of Iowa, which had anticipated
the atomic arrangement that optimized magnetism in
the experiments.
Yazdani cautioned that his team's technique would
not translate immediately into new chip technology
but would benefit fundamental research by providing
a testbed for exploring magnetism in other semiconductors.
"We can now ask questions about these magnetic atoms
and get answers," he said. "How does it affect the
semiconductors' performance when you change their
orientation, for example, or their distance from
one another? Answers to these questions may allow
us to link the electric current and magnetic spin
within these new semiconductors, and that's a goal
the field has been seeking for many years."
###
This research was funded in part by the National
Science Foundation and the U.S. Army Research Office.
Abstract
Atom-by-Atom Substitution of Mn in GaAs and Visualization
of Their Hole-Mediated Interactions
by D. Kitchen, A. Richardella, J.-M. Tang, M. E.
Flatté, A. Yazdani
The discovery of ferromagnetism in Mn doped GaAs1
has ignited interest in the development of semiconductor
technologies based on electron spin and has led to
several proof-of-concept spintronic devices2-4. A
major hurdle for realistic applications of Ga1-xMnxAs,
or other dilute magnetic semiconductors, remains
their below room-temperature ferromagnetic transition
temperature. Enhancing ferromagnetism in semiconductors
requires understanding the mechanisms for interaction
between magnetic dopants, such as Mn, and identifying
the circumstances in which ferromagnetic interactions
are maximized5. Here we report the use of a novel
atom-by-atom substitution technique with the scanning
tunnelling microscope (STM) to perform the first
controlled atomic scale study of the interactions
between isolated Mn acceptors mediated by the electronic
states of GaAs. High-resolution STM measurements
are used to visualize the GaAs electronic states
that participate in the Mn-Mn interaction and to
quantify the interaction strengths as a function
of relative position and orientation. Our experimental
findings, which can be explained using tight-binding
model calculations, reveal a strong dependence of
ferromagnetic interaction on crystallographic orientation.
This anisotropic interaction can potentially be exploited
by growing oriented Ga1-xMnxAs structures to enhance
the ferromagnetic transition temperature beyond that
achieved in randomly doped samples. Our experimental
methods also provide a realistic approach to create
precise arrangements of single spins as coupled quantum
bits for memory or information processing purposes. |
