Building Highways for Electrons on Semiconductor Chips
Moore's law states that the number of transistors on a semiconductor chip doubles every 18 months. However, as the feature size of semiconductor chips shrinks to nano-meter scale, dissipation increases exponentially, disrupting the normal operation of computing devices. Solving the problem of dissipation is a grand challenge in basic and applied science. In conventional semiconductor chips, dissipation is caused by the random scattering of electrons by impurities.
With funding from the W. M. Keck Foundation, researchers at Stanford University, led by principal investigator, Professor Shoucheng Zhang, and his colleagues, Yi Cui and David Goldhaber-Gordon, researched novel topological insulator materials where electrons move like automobiles on a highway, spatially separated into different lanes, avoiding backscattering and resulting dissipation.
In 2006, Zhang's group predicted theoretically that a quantum well of the mercury telluride and cadmium telluride is a two dimensional topological insulator where the two dimensional interior region is fully insulating, and electrons can only move along the edge of the sample, where opposite spin states counter-propagate. This effect, called the quantum spin Hall effect, occurs due to large spin-orbit coupling, and only when the device geometry reaches nano-meter dimensions, where the thickness of the quantum well is about 7 nm. This theoretical prediction was soon confirmed experimentally by a group of researchers at the University of Wurzburg in Germany. That group, led by Prof. Laurens Molenkamp, grows high mobility quantum wells with precisely controlled well thickness at the nano-meter scale. They can also control the Fermi level by varying the gate voltage. For devices with quantum well thickness of about 7nm, they find that electrons indeed conduct without dissipation along the edge channel, independent of the device width and length.
Discovery of the first topological insulator material in mercury telluride has launched a world-wide competition to find other topological materials with more robust properties, with larger energy gaps so that the resulting device could function well at room temperature. Zhang’s group also predicted the three dimensional topological insulator materials in alloys of bismuth and antimony telluride. These materials can be more readily fabricated in most laboratories around the world and have sufficiently large energy gaps for robust operation at room temperature. With Keck Foundation support, the Stanford group also theoretically predicted many more novel properties of topological insulators and have fabricated functional devices. In particular, Cui’s group fabricated a nano-wire device based on topological insulators with dramatically improved performance, and Goldhaber-Gordon’s group observed the superconducting Josephson effect of topological insulators, laying the groundwork for topological quantum computing.
Zhang’s pioneering work on topological insulators has earned him a number of international prizes. He was awarded the Europhysics Prize in 2010, the Oliver Buckley Prize of the American Physical Society in 2012, the Paul Dirac Medal and Prize in 2012 and most recently, the Physics Frontiers Prize in 2013. Sharing the limelight together with the eminent physicist Stephen Hawking at the award ceremony, Zhang mused about the importance of fundamental science: “Solving the critical problems of the world today requires us to think outside of the box, and to draw inspirations from seemingly disconnected branches of knowledge. Topology started out as one of the most abstract branches of mathematics, remarkably, it has now found applications in material science and could help to solve one of the most challenging problems facing the information society today.”