Overlooked Effect With A Big Impact Semiconductor Industry Relies on Incorrect 2D Materials

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2D materials such as graphene or molybdenum sulphide are seen as the future for computer chips. However, a study by the Vienna University of Technology shows that some of these propagated materials are unsuitable due to an underestimated effect. But there are ways out.

The miniaturization of electronic components has been a key driver of technological progress for decades. In the context of future development steps in semiconductor technology, 2D materials, i.e. ultra-thin layers consisting of just one or a few atomic layers, are considered potential candidates for further miniaturized electronic structures.

However, research work at TU Wien shows that numerous 2D materials, which were previously considered to have high application potential, are only suitable to a limited extent. The decisive factor here is not only the investigation of the intrinsic material properties, but also the consideration of interface effects. If a 2D material is combined with an insulator layer, an extremely thin gap can form at the interface, which significantly impairs the electronic properties. At the same time, this approach enables a more targeted assessment of which material systems do not have this problem. This allows development risks in semiconductor technology to be assessed at an earlier stage. The researchers have published the results of their study in the current issue of the journal Science.

Not Only The Material, But Also The Interface is Decisive

"For many years, people have been quite rightly fascinated by the interesting electronic properties of novel 2D materials such as graphene or molybdenum disulphide," says Prof. Mahdi Pourfath, who carried out the research work together with Prof. Tibor Grasser at the Institute of Microelectronics at TU Wien. "But what is often overlooked: A 2D material alone does not make an electronic component. We also need an insulator layer, usually an oxide. And this is where things get more complicated in terms of materials science."

The basic principle of transistors in integrated circuits is to modulate the conductivity of a semiconductor, which can also include an ultra-thin 2D material, between conductive and non-conductive states. This state is controlled via the gate, i.e. an electrode that is separated from the active material by an insulating layer.

Effects of An Atomic Gap at The Interface

For effective electrostatic control, this insulator layer should be as thin as possible. This allows electric fields in the 2D material to be efficiently influenced and compact component structures to be realized. On an atomic scale, however, there is an effect that has received little attention to date.

"Many combinations of 2D materials and insulator layers show that the bond between them is not particularly strong," explains Grasser. "They are only bound to each other by so-called van der Waals forces, which only cause a very weak attraction between the semiconductor and insulator. This means that the two layers do not bond firmly to each other, leaving a gap between them."

Although this gap is extremely small at around 0.14 nanometers, it can have a significant impact on the component properties. "This gap impairs the capacitive coupling between the two layers. No matter how good the properties of the two materials may be, the gap is the limiting factor. As long as it is there, there are fundamental limits to the miniaturization of components."

Material Systems With Integrated Coupling As A Possible Solution

"If the semiconductor industry is to be successful with 2D materials, the active layer and insulator layer have to be considered together from the outset," emphasizes Mahdi Pourfath. The researchers see a possible solution in so-called "zipper materials". In such material systems, the semiconductor and insulator are structurally interlinked and are not only coupled via weak van der Waals forces, but also via a stronger bond. This means that the critical distance at the interface can be avoided.

"Our work is good news for the semiconductor industry," says Tibor Grasser. "We can predict which materials are suitable for future miniaturization steps and which are not. But if you only focus on the 2D materials without planning for the inevitable insulators from the outset, then you may end up betting billions on a horse that has no legs—and can't win for very fundamental reasons."(sg)

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