Oct 5, 2012
Graphene tunnel barrier makes its debut
Until now, researchers have mainly focused on using graphene (a 2D sheet of carbon atoms) as a conductor or semiconductor, where current flows in plane, parallel to the carbon sheet. In contrast to this work, researchers at the US Naval Research Laboratory have now 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 in the tunnel barrier, a finding that could have important implications for spintronics – a relatively new technology that exploits the spin of an electron as well as its charge.
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 using such a spin current (which consists of electrons with opposite spins moving in opposite directions) would be smaller and more efficient than conventional electronic circuits that rely on switching charge alone, 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, ferromagnetics and semiconductors have a large conductivity mismatch, so a tunnel barrier (an electrically insulating barrier between two conducting materials through which electrons tunnel quantum mechanically) is thus needed to allow for spin injection. The problem is that the oxide barriers normally employed as tunnel barriers today, such as Al2O3 and MgO, introduce defects into the system and have resistances that are too high – factors that adversely affect device performance.
Enter graphene tunnel barrier
To overcome this problem, Berry 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 – a well known technique that ensures that there is no oxide layer between the silicon surface and the graphene. The team then injected electron spins from ferromagnetic NiFe into the silicon via the graphene tunnel barrier, and directly measured a voltage arising from the resulting spin polarization in the silicon using the Hanle effect, a method 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 told nanotechweb.org. "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 band structure derived spin-filtering effects that have been predicted for selected ferromagnetic metal/graphene structures, he 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."
The work, which was supported by programs at NRL and the US Office of Naval Research, is reported in Nature Nanotechnology.
About the author
Belle Dumé is contributing editor at nanotechweb.org.