In the three-terminal device measurement geometry, a current is applied to contacts 1 and 2, and a voltage is measured across contacts 2 and 3. Electrical spin injection produces spin accumulation in the transport channel under the magnetic contact 2 and a corresponding output voltage. When a magnetic field Bz is applied, the injected spins precess and dephase, and the spin accumulation decreases to zero. Credit: US Naval Research Laboratory |
Researchers
in the Materials Science and Technology division of the Naval Research
Laboratory have recently demonstrated electrical injection, detection
and precession of spin accumulation in silicon, the cornerstone material
of modern device technology, at temperatures up to 225 degrees Celsius.
These results provide the first demonstration that spin accumulation in
Si is viable as a basis for practical devices which meet the operating
temperatures specified for commercial (85?C), industrial (100?C) and
military (125?C) applications.
This
is a key enabling step for developing devices which rely on electron
spin rather than electron charge, an approach known as semiconductor
spintronics that is expected to provide devices with higher performance,
lower power consumption and less heat dissipation.
The
electron possesses an internal angular momentum called the spin. The
International Technology Roadmap for Semiconductors has identified the
electron’s spin as a new state variable that should be explored as an
alternative to the electron’s charge for use beyond Moore’s Law, a
projection named after Intel co-founder Gordon E. Moore. Moore predicted
in 1965 that the number of transistors per unit area in an integrated
circuit would double approximately every two years as advances in
fabrication technology enabled the devices to be made smaller.
Although
this approach has been remarkably successful, critical device
dimensions now approach atomic length scales, so that further size
scaling becomes untenable.
“Researchers
have been forced to look beyond the simple reduction of size to develop
future generations of electronic devices,” states NRL senior scientist
Dr. Berry Jonker. “Electrical generation, manipulation and detection of
significant spin polarization in silicon at temperatures that meet
commercial and military requirements are essential to validate spin as
an alternative to charge for a device technology beyond Moore’s Law.”
Using
ferromagnetic metal / silicon dioxide contacts on silicon, NRL
scientists Connie Li, Olaf van ‘t Erve and Jonker electrically generate
and detect spin accumulation and precession in the silicon transport
channel at temperatures up to 225?C, and conclude that the spin
information can be transported in the silicon over distances readily
compatible with existing fabrication technology. They thus overcome a
major obstacle in achieving control of the spin variable at temperatures
required for practical applications in the most widely utilized
semiconductor.
To
make a semiconductor spintronic device, one needs contacts that can
both generate a current of spin-polarized electrons (called a spin
injector), and detect the spin polarization of the electrons (spin
detector) in the semiconductor. Because the magnetic contact interface
is likely to introduce additional scattering and spin relaxation
mechanisms not present in the silicon bulk, the region of the
semiconductor directly beneath the contact is expected to be a critical
factor in the development of any future spin technology.
The
NRL scientists probe the spin environment directly under the magnetic
metal / silicon dioxide contact using the three terminal geometry
illustrated in the accompanying figure. Demonstration of spin precession
and dephasing in a magnetic field transverse to the injected spin
orientation, known as the Hanle effect, is conclusive evidence of spin
accumulation, and enables a direct measure of the spin lifetime, a
critical parameter for device operation. The NRL researchers observed
Hanle precession of the electron spin accumulation in the silicon
channel under the contact for biases corresponding to both spin
injection and extraction, and determine the corresponding spin
lifetimes.
Electronic
states can form at the contact interface and introduce deleterious
effects for both charge and spin transport. These undesirable states can
serve as traps which prevent propagation of either charge or spin in
the silicon channel. In bulk silicon, the spin lifetime is known to
depend upon the carrier density, and generally decreases as the electron
density increases.
“In
this study we show that the spin lifetime determined from our
measurements changes systematically as one changes carrier concentration
of the particular silicon sample used,” adds Jonker. “Our results were
obtained for a number of different carrier densities and show this
trend, thus making it very clear that we obtain spin injection and
accumulation in the silicon itself rather than in interface defect
states.”
The
result of this research rules out spin accumulation in interface states
and demonstrates spin injection, accumulation and precession in the
silicon channel.
The
complete findings of this study titled, “Electrical injection and
detection of spin accumulation in silicon at 500K with magnetic metal /
silicon dioxide contacts” are published in the 22 March 2011 issue of Nature Communications
