Putting electrons in a spin

Physicists from the University of Arkansas claim to have achieved the highest efficiency ever in transferring polarised electrons into a semiconductor surface.

The discovery could lead to the creation of small but powerful computational devices that may revolutionise the electronics industry.

In doing so, they also discovered some of the underlying mechanisms that prevent researchers from successfully injecting spin-polarised electrons into a semiconducting surface.

Physicists hope to harness the power of an electron’s spin to make multifunctional computational devices, where a single multifunctional device would replace hundreds of conventional devices, leading to faster, smaller electronics that consume less power.

Currently, electronic devices use an electron’s mass and charge to do the necessary work, but these devices have limitations in their size and power — researchers estimate that in about 10 years the prevalent technologies used today to make smaller, more powerful devices will reach that limit.

For about 10 years, researchers have been exploring the idea of exploiting an electron’s spin to enhance the performance of devices. Spins can rotate in a coherent manner and thus alter the resistance of a device in a controlled manner. These properties may enable greater storage capacity and information processing from spintronic devices.

Until now, however, injecting spin-polarised electrons into a semiconductor surface has not worked — a high percentage of the electrons change their spin orientation during the injection process. The highest spin efficiency recorded was 40 percent at 10 degrees Kelvin, a temperature too low for effective use in electronic devices.

Vincent LaBella and his colleagues have achieved injection efficiency of 92 percent into a gallium arsenide (GaAs 110) surface at a temperature of 100 K, which is the temperature of liquid nitrogen, a substance often used in the semiconductor industry.

The researchers used a technique that incorporates a magnetic nickel Scanning Tunnelling Microscope (STM) tip used to inject electrons that are 100 percent oriented in one direction. They can use measurements of polarisation to determine whether or not the electrons retain their spin, a technique they call spin-polarised tunnelling induced luminescence microscopy (SP-TILM).

The STM also enables the researchers to correlate surface features in the topography of the semiconductor with the degree of spin disruption.

The researchers found that areas with an atomic ‘step,’ a spot where the atoms do not form an even surface, cause spin disruption. The particular form of GaAs they used, GaAs (110) has few steps in it, accounting for the high degree of success in injecting spin-polarised electrons. The places where these steps occurred turned out to be the source of electron disruption, causing the spins to flip.

‘Until now, no one has pinned down the fact that steps scatter spins,’ LaBella said.

It takes a free electron to scatter another electron’s spin, and usually within a crystal all electrons are paired up, unless there is a broken bond. In the case of GaAs 110, all the electrons are in filled orbitals, so the spins are stable.

There are plenty of surfaces where the electronic configuration is not as smooth, implying that they would be less efficient surfaces for use in spintronics, LaBella said.

The researchers plan to study other semiconductor and ferromagnetic surfaces using the same techniques.