Mind the gap! Semiconductor industry is relying on the wrong materials

Vienna University of Technology

The miniaturization of electronic components has been a tremendous success story, driving technological progress for decades. Work is already underway on the next revolution in computer chips: 2D materials—ultrathin layers consisting of just one or a few atomic layers—could be ideally suited for even smaller electronic structures.

However, researchers at TU Wien have now shown that many 2D materials once considered highly promising are in fact unsuitable for this purpose. It is not enough to study the properties of the material itself—interface effects must also be taken into account. When 2D materials are combined with an insulating layer, an extremely thin gap inevitably forms between them, drastically degrading their electronic properties. The good news is that this approach also allows researchers to identify which materials are not affected by this problem—potentially saving the semiconductor industry from investing billions in technologies that are fundamentally limited by the laws of physics.

It’s not just the material—it’s the interface

“For many years, researchers have quite rightly been fascinated by the remarkable electronic properties of novel 2D materials such as graphene or molybdenum disulfide,” says Prof. Mahdi Pourfath, who carried out the research together with Prof. Tibor Grasser at TU Wien’s Institute for Microelectronics. “What is often overlooked, however, is that a 2D material alone does not make an electronic device. We also need an insulating layer—usually an oxide. And this is where things become more complicated from a materials science perspective.”

The basic concept of transistors used in computer chips is simple: the conductivity of a semiconductor—this can also be an ultrathin 2D material—can be modulated between conducting and non-conducting states. Which of these states occurs is controlled by the gate, an electrode that must be separated from the active material by an insulating layer.

Mind the gap!

This insulating layer must be as thin as possible in order to allow precise control of the electric fields in the 2D material, enabling extremely small and compact devices. However, when these structures are analyzed at the atomic scale, a problem emerges that has so far received little attention.

“In many combinations of 2D materials and insulating layers, the bonding between them is relatively weak,” explains Grasser. “They are held together only by so-called van der Waals forces, which provide only a weak attraction between the semiconductor and the insulator. As a result, the two layers do not come into close contact—there is always a gap between them.”

This gap is tiny—only about 0.14 nanometers, thinner than a single sulfur atom—yet it has a major impact on electronic performance. A SARS-CoV-2 virus, for comparison, is roughly 700 times larger. “This gap weakens the capacitive coupling between the layers. No matter how good the intrinsic properties of the materials may be, the gap can become the limiting factor. As long as it exists, it imposes a fundamental limit on how far these devices can be miniaturized.”

The solution: “zipper” materials

“If the semiconductor industry wants to succeed with 2D materials, the active layer and the insulating layer must be designed together from the very beginning,” emphasizes Mahdi Pourfath. There are possible solutions: so-called “zipper materials” combine both aspects. Semiconductor and insulator interlock with each other—they are not just loosely connected by van der Waals forces, but form a stronger bond that eliminates the gap.

“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 one focuses only on the 2D materials themselves, without considering the unavoidable insulating layers from the outset, there is a risk of investing billions in an approach that simply cannot succeed for fundamental physical reasons.”