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'Exotic materials' become friends with silicon

Materials that turn from isolators into conductors by mechanical stress, could lead to valuable new components in electronics. Unfortunately, this type of materials doesn’t easily match with silicon, the basic material in chip technology. This could change soon: researchers of the University of Twente demonstrate a new way of adding their ‘exotic materials’ to a silicon base. The results are in the prestigious journal Advanced Materials.

Over the past years, Gertjan Koster and his colleagues demonstrated materials with special properties. In most cases, these are layered structures: the magic happens at the interfaces. There, the material turns into a conductor or even superconductor, or it gets optically transparant. What you need as a starting point, is a perfect crystal structure. An idea might be: build these layers on top of silicon, and you can add new functionality to electronics. But silicon does not have the perfect crystal structure that is needed. And the materials placed on top of silicon, chemically react with it, potentially damaging all.

Mechanical switch

But now, there is a thin protective layer, just 4 nanometer in thickness (a nanometer is one millionth of a millimeter), that allows building on top of silicon. It is made of strontiumtitanate. As an example, the researchers now present a material that is sensitive for strain, mechanical stress. These nickelate superlattices can turn from conductor to isolator by strain. So: from ‘1’ to ‘0’ and vice versa. It can be seen as a mechanical equivalent of the transistor or a logic switch.

Strain engineering

This strain happens if two layers do fit, but there is still some tension between them. This can be undesirable, but the effect can be exploited as well. Strain is influenced on the temperature change during manufacturing. Making use of the effect in a smart way, is already called ‘strain engineering’. Piezo materials are a well-known example: if you put pressure on it, there is a change of electric voltage. Do you, in turn, change the voltage, the material will shrink or expand.

Next step: to silicon wafers

The strain-controlled material could be a valuable new component on chips, and thanks to the ultra thin protective layer that comes from IBM Research, it is now possible to build the switch on top of silicon. Of course, introducing it into chip technology requires adding new steps to a highly standardized production process, but on a small silicon surface, it already works. This is an important first step.

The research has been done in the Inorganic Materials Science group, part of UT’s MESA+ Institute, together with partners in Belgium, Switzerland, France, Slovenia and Serbia.

The paper ‘Strain-engineered metal-to-insulator transition and orbital polarization in nickelate superlattices integrated on silicon’, by Binbin Chen, Nicolas Gauquelin, Daen Jannis, Daniel Cunha, Ufuk Halisdemic, Cinthia Piamonteze, Jin Hong Lee, Jamal Belhadi, Felix Eltes, Stefan Abel, Zoran Jovanovic, Matjaz Spreitzer, Jean Fompeyrine, Johan Verbeeck, Manuel Bibes, Mark Huijben, Guus Rijnders en Gertjan Koster, is published in Advanced Materials of 11 November 2020.

ir. W.R. van der Veen (Wiebe)
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