It turns out that graphene makes an ideal tunnel barrier for
establishing spin states and the resultant architecture for memory.
Step by step we are reinventing solid state chips onto graphene and
will end up with far superior hardware.
This is a revolution that just keeps on giving us rapid advances.
As I have posted before, all imagined technologies are already with
us or simply happening.
What is lagging is their adoption and acceptance although that is a
logistical problem been overcome.
Graphene tunnel barrier makes its
debut
Oct 8, 2012
Yet another use for the wonder material graphene
Researchers in the US
have found yet another use for the "wonder material"
graphene. Instead of exploiting the material's exceptional ability as
an electrical conductor, the team has found a way to use graphene as
an extremely thin "tunnel barrier" to conduction. The team
says that this new application is particularly suited to
developing spintronics – a relatively new technology that
exploits the spin of an electron as well as its charge.
Graphene is a sheet of
carbon just one atom thick and ever since the material was first
isolated in 2004, researchers have been trying to create electronics
devices that make use of its unique properties. Most of this effort
has focused on how electrons flow in the plane of the sheet – which
can behave both as a conductor and semiconductor. But now Berry
Jonker and colleagues at the US Naval Research Laboratory (NRL)
have shown that graphene can serve as an excellent tunnel barrier
when current is directed perpendicular to the plane of carbon atoms.
The spin polarization of the current is also preserved by the tunnel
barrier, a finding that could have important implications for
spintronics.
Low-energy
switching
The spin of an
electron can point in an "up" or "down" direction
and this property could be used to store and process information in
spintronics devices. Circuits that employ a spin current –
electrons with opposite spins moving in opposite directions –
could, in principle, be smaller and more efficient than conventional
electronic circuits that rely on switching charge alone. This is
because switching spins from up to down can be done using very little
energy.
Spintronics devices
are typically made from ferromagnetic materials and semiconductors.
Ferromagnetic metals, such as iron or permalloy, have intrinsically
spin-polarized electron populations – that is, different numbers of
up-spin and down-spin electrons – and thus make ideal contacts for
injecting spins into a semiconductor. However, ferromagnets and
semiconductors have a large conductivity mismatch, so spin is
injected via a tunnel barrier – an electrically insulating barrier
through which electrons tunnel quantum mechanically. The problem is
that the oxide barriers normally employed as tunnel barriers
introduce defects into the system and have resistances that are too
high – factors that adversely affect device performance.
Enter the graphene
tunnel barrier
To overcome this
problem, Jonker and colleagues decided to employ single-layer
graphene as the tunnel barrier, because the material is defect
resistant, chemically inert and stable. These properties can be
exploited to make low-resistance graphene spin contacts that are
compatible with both the ferromagnetic metal and semiconductor.
The researchers began
by mechanically transferring graphene grown by chemical vapour
deposition onto hydrogen-passivated silicon surfaces. They
achieved this by floating the graphene on the surface of water and
bringing the silicon substrate up from below. This common technique
ensures that there is no oxide layer between the silicon surface and
the graphene. The team then injected electron spins from a
ferromagnetic nickel–iron alloy into the silicon via the graphene
tunnel barrier. The voltage arising from the resulting spin
polarization in the silicon was then measured using the Hanle effect,
a method that is routinely employed by spintronics scientists.
Beyond Moore's law
"Our discovery
clears an important hurdle to the development of future semiconductor
spintronics devices – that is, devices that rely on manipulating
the electron's spin rather than just its charge for low-power,
high-speed information processing beyond the traditional size scaling
of Moore's law," Jonker says. "These results identify a new
route to making low-resistance-area spin-polarized contacts, which
are key for semiconductor spintronics devices that rely on
two-terminal magnetoresistance, including spin-based transistors,
logic and memory."
Using graphene in
spintronics structures may provide much higher values of the tunnel
spin polarization thanks to so-called spin-filtering effects that
have been predicted for selected ferromagnetic metal/graphene
structures, Jonker adds. "Such an increase would improve the
performance of semiconductor spintronics devices by providing higher
signal-to-noise ratios and corresponding operating speeds, so
advancing the technological applications of silicon spintronics,"
he says.
The work, which was
supported by programs at the NRL and the US Office of Naval Research,
is reported in Nature Nanotechnology.
About the author
Belle
Dumé is a contributing editor to nanotechweb.org
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