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Putting the insulator back into topological insulators

Combining two common topological insulators together reduces the number of defects to produce true insulating topological insulator behaviour

 A team of researchers from the UK has shown that anti-site defects are the key to controlling the unwanted bulk conduction that has plagued a new form of material known as a topological insulator.

Touted as the next silicon, topological insulators are unusual materials which, as the name suggests, should be insulating.

However, unlike familiar insulators such as diamond or quartz, they have surfaces which conduct electricity without the scattering that is a limiting factor in the performance of current electronic devices. This could one day lead to unprecedented advances in computing technology, and the realisation of an entirely new form of electronics known as spintronics.

However, most current examples of this exotic state of matter don’t display the insulating bulk suggested in their name.

This causes problems not only for trying to study their intrinsic properties, but also for building a device that exploits the special surface current that sets these materials apart from normal semiconductors.

 Figure 1:Schematic of the layered tetradymite structure, with Bismuth (red), and the inner and outer chalcogenides marked as green and blue spheres respectively. This schematic indicates a Bismuth on the outer chalcogenide antiste defect, and a Bismuth vacancy

Now, a team of researchers from University College London and the Universities of St. Andrews and Warwick, all in the UK, have found the imperfections in these materials responsible for their unintentional conductivity, and show ways to manage these in order to realise true topological insulators.

The scientists used theoretical calculations to understand what happens when the crystals grown don’t have the perfect textbook structure. They found that the most likely structural imperfections are also ones that cause the high, undesired bulk conduction that is commonly observed in experiment.

However, by combining two common compounds together, these defects can be carefully balanced to encourage the material to become insulating.

 Figure 2: Angle-resolved photoemission measurements confirm that alloying Bi2Se3 with Bi2Te3 is an effective way to tune bulk conductivity in topological insulators, reaching the desired regime where only the special topological surface state (V-shaped feature) crosses the Fermi level

 The researchers show this experimentally, demonstrating an efficient way to tune the topological insulators into exactly the regime desired for exotic future applications. This borrows ideas commonly used to modify conventional semiconductors like Silicon or GaAs.

Applying such tricks to topological insulators could ultimately lead to these semiconductors being superseded in the never ending hunt for faster and smaller electronic devices.

Further details of this work have been published in the paper, "Controlling Bulk Conductivity in Topological Insulators: Key Role of Anti-Site Defects" by D.O. Scanlon et al in Advanced Materials, published online on 19th March, 2012, DOI: 10.1002/adma.201200187.
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