Healing the Achilles heel of 2D transistors

Stability is one of the key characteristics of consumer electronics. Our devices must operate reliably throughout their lifetime to become suitable for everyday applications. And this is, precisely, the Achilles heel of transistors based on two-dimensional materials, which typically show much worse stability than silicon devices. Now, a team of researchers from Graphene Flagship partners TU Wien, Austria, AMO GmbH and RWTH Aachen University, Germany, and collaborators at Wuppertal University, Germany have now demonstrated a novel engineering approach to enhance the electrical stability of two-dimensional transistors by carefully tuning the Fermi energy. The results, part of the 2D-Experimental Pilot Line (2D-EPL) project within the Graphene Flagship, have been reported in Nature Electronics.

Today, there is little doubt that devices based on graphene and other two-dimensional (2D) materials can exceed the state of the art for certain applications, thanks to their intrinsic properties. Two-dimensional materials are also seen as some of the most promising candidates for realizing ultimately scaled transistors at the end of the roadmap of silicon technology.  However, devices based on 2D materials often show poor electrical stability, meaning that their behavior changes depending on their operation history.

“Component reliability is one aspect that is often neglected in research. This is precisely where we have been working for several years, because it is of central importance for applications." explains Max Lemme, from Graphene Flagship and 2D-EPL partner AMO GmbH. The instability is not only caused by 2D materials themselves, but mostly by charges trapped into the oxide-insulator used to fabricate the transistors. "Ideally, one would like to use a different insulator with fewer charge traps," says Lemme, "but there are no scalable solutions for this yet. In our work, we have shown instead that it is possible to use a standard insulator such as aluminum oxide and to significantly suppress the adverse effects of the charge traps in the oxide, by adjusting the charge carrier density in the 2D material."

The work combines a thorough theoretical analysis of the novel approach – dubbed by the authors ‘stability-based design’ – and a proof of principle demonstration of the concept, performed by measuring different types of graphene-based FETs. The key idea of the approach is to try to engineer the combination 2D-material/insulator in such a way that the energy of the charge traps in the insulator is as different as possible from the one of the charge carriers in the 2D material. Lemme explains: “Graphene based FETs were the ideal test bed for our approach, as it is relatively easy to tune the energy of charge carriers in graphene. The approach, however, is applicable to all FETs based on 2D semiconductors”.  These results represent a major step forward towards stable and reliable 2D materials transistors to be integrated in semiconductor technology.

Bibliographic information:

T. Knobloch, B. Uzlu, Y. Yu. I.llarionov, Z. Wang, M. Otto, L. Filipovic, M. Waltl, D. Neumaier, M. C. Lemme, T. Grasser, Improving stability in two-dimensional transistors with amorphous gate oxides by Fermi-level tuning, Nature Electronics (2022) – Open Access
DOI: 10.1038/s41928-022-00768-0