Calcium fluoride is a crystalline insulator with a well-defined surface. Therefore, it is ideally suited for manufacturing extremely small transistors. (Image courtesy of the researchers)

For a long time, something important has been regularly neglected in electronics. If you want to make electronic components smaller and smaller, you also need the right insulator materials. This is why so-called 2D materials are considered to be the great hope: they are as thin as a material can possibly be — in extreme cases they consist of only a single layer of atoms. This makes it possible to produce novel electronic components with tiny dimensions, high speed, and optimal efficiency.

However, there is one problem: electronic components always consist of more than one material. 2D materials can only be used effectively if they can be combined with suitable material systems, such as special insulating crystals. If this is not considered, the advantage that 2D materials should offer is nullified. A team from the Faculty of Electrical Engineering at TU Wien (Vienna) is now presenting their findings on this issue.

"The semiconductor industry today is mostly based on silicon and silicon oxide," said Professor Tibor Grasser. "These are materials with very good electronic properties. Ever thinner layers of these materials have been used to miniaturize electronic components; and that worked well for a long time, but at some point, we reach a natural limit." When the silicon layer is only a few nanometers thick, so that it consists of only a few atomic layers, the electronic properties of the material deteriorate very significantly. "The surface of a material behaves differently from the bulk of the material, so if the entire object is practically only made up of surfaces and no longer has any bulk, it can have completely different material properties."

One has to therefore switch to other materials in order to create ultra-thin electronic components. This is where 2D materials come into play: they combine excellent electronic properties with minimal thickness. "As it turns out, these 2D materials are only the first half of the story, however," said Grasser. "The materials have to be placed on the appropriate substrate, and an insulator layer is also needed on top of it. This insulator also has to be extremely thin and of extremely good quality, otherwise you have gained nothing from the 2D materials. It's like driving a Ferrari on muddy ground and wondering why you don't set a speed record."

The TU Wien research team has analyzed this problem. "Silicon dioxide, which is normally used in industry as an insulator, is not suitable in this case," said Grasser. "It has a very disordered surface and many free, unsaturated bonds that interfere with the electronic properties of the 2D material," so it is better to look for a well-ordered structure. The team has achieved excellent results with fluorides — a special class of crystals. A transistor prototype with a calcium fluoride insulator has provided convincing data, and other materials are still being analyzed.

"New 2D materials are currently being discovered. That's nice, but with our results we want to show that this alone is not enough," said Grasser. "New semiconducting 2D materials must also be combined with new types of insulators. Only then can we really succeed in producing a new generation of efficient and powerful electronic components in miniature format."