Update: Plastic-GaAs spintronic device revealed

Ohio State University researchers have combined traditional inorganic semiconductors with organic spintronics, in a device claimed to be the first of its kind.

For the demonstration, the scientists used an organic magnet made from vanadium tetracyanoethylene, a polymer developed by Professor Arthur Epstein at Ohio State with Professor Joel Miller of the University of Utah - a material in which spintronic data storage and retrieval was achieved last year.

Now an Ohio State team led by Dr Ezekiel Johnston-Halperin has incorporated the polymer into a GaAs device.

"In order to build a practical spintronic device, you need a material that is both semiconducting and magnetic at room temperature. To my knowledge, Art's organic materials are the only ones that do that," said Johnston-Halperin.

"The polymer is a ferromagnet, and a massively interesting thing on its own. Even without a magnetic field, the polymer is still magnetic - just like iron or cobalt," he told Electronics Weekly.
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The experimental device is essentially an LED whose current has to flow through the polymer.

It exploits 'spin orbit coupling' where the spin of an electron creating a photon in GaAs or other direct bandgap semiconductor is transferred to the angular momentum of the photon, which is seen at right or left circular polarisation.

"The spintronics and interesting physics occurs between polymer and top, n-doped, layer," said Johnston-Halperin. "The only purpose of LED is to detect spin injection. It could be replaced with a transistor or other kind of electronic device, but with other devices, it is not very easy to tell whether spin-injection is happening."

Light leaving the LED is indeed polarised - indicating that spintronic action is occurring - and the direction of polarisation shows that electrons with spin that makes their magnetisation parallel to the polymer's own magnetic moment experience lower electrical resistance in the polymer than electrons magnetised at anti-parallel.

Essentially electrons with a certain spin state pass through preferentially.

Baulked electrons pass through later after their spin state flips spontaneously.

"The fact that they were able to measure the electrons' polarisation with the LED also suggests that other researchers can use this same technique to test spin in other organic systems," added Ohio State University.

Johnston-Halperin built the device as a science experiment: to examine the fundamental spin physics of the polymer, using the well-understood science of spin orbit coupling in GaAs as a tool to get quantative results.

"From a technology angle, it opens the door for hybrid material systems where you can play with the polymer or the GaAs," he said. "Hybrid structures promise functionality that no other materials, neither organic nor inorganic, can currently achieve alone."

The polymer cannot stand high temperatures or lithography solvents, so fabrication involved building the GaAs part in a conventional fab, then adding the organic layer at lower temperature, followed by deposition of the aluminium and gold contacts through a shadow mask to avoid etching.

"You could ask, why didn't we go with all organics, then?" asked Johnston-Halperin. "Well, the reality is that industry already knows how to make devices out of inorganic materials. That expertise and equipment is already in place. If we can just get organic and inorganic materials to work together, then we can take advantage of that existing infrastructure to move spintronics forward right away."