Portland, Ore. - Spin-dependent transistors could one day harness electron spin to encode up to 10 states (as opposed to binary logic's mere two) in single-electron devices-enabling chips with a million times the density of today's memories, according to the University of Arkansas.
A precise understanding of how an electron's spin rotates as it moves through a material could enable such single-electron, spin-dependent devices.
Unfortunately, there has been no instrument available to measure electron spin, so even the first step toward a spin-dependent computer has not been taken-that is, to carefully characterize the properties of a spin-dependent transistor.
To perform the world's first careful characterization of a spin-dependent transistor, Paul Thibado, associate professor of physics at the University of Arkansas, won a National Science Foundation grant of $760,000, with which he has already perfected the two-tipped scanning-tunneling microscope (STM). Now, Thibado has received a $370,000 NSF grant to perform the characterization of a spin-based transistor.
"Others have built two-tip STMs, but the tips were side-by-side and with all the mounting hardware you couldn't get them close enough together. With our instrument we can get them to within 10 nanometers by opposing the tips at 90 degrees to each other, so their mounting hardware is out of the way," Thibado said.
Spin-dependent devices use magnetizable metals like iron and nickel instead of aluminum, because the magnetic spin direction of electrons in those materials retains their orientation. For instance, all the spins can be made to point up in iron by merely applying an external magnetic field, whereas in aluminum the spins remain randomized. To that end, the tips of Thibado's twin-tip STM are made of nickel.
"The first thing we discovered is you need a vacuum barrier between the metal tip and the semiconductor to prevent them from forming an interface. For instance, if you put iron directly on gallium arsenide the spins will be randomized at the interface," said Thibado.
The vacuum barrier enables one tip to inject spin-polarized electrons into the material. The other tip can measure the device's response without interfering with the flow of spin-polarized electrons. With the appropriate externally applied magnetic field-acting as a gate-the two-tip STM can temporarily create a spin-dependent transistor on any semiconductor surface. This trick will enable the researchers to characterize not only the transistor, but also compare many different materials as candidates for future spin-dependent computers.
"We will begin with gallium arsenide, but after characterizing a spin-dependent transistor in that material, we hope to try out other promising semiconductors," said Thibado.
Using a computer-operated nanoscale-precision positioning system to move the dual STM tips, the University of Arkansas researchers will carefully measure the current and voltage properties of the spin-dependent transistor, at various temperatures and at various distances between the source and drain. Thibado's team will seek to characterize the material carefully enough so that future spin-dependent computer architects can pick and choose from among their discoveries. They will also characterize several materials for use as STM tips.