Abstract

Since the discovery of graphene there has been a growing interest in fabricating and studying nanometer scale graphene devices for future nanoelectronic applications.
We developed a nanoetching technique called electromigration to fabricate approximately 10 nm scale suspended and clean graphene quantum dots (QD) and ballistic transistors. Because these devices are so small, we were able to explore the fundamental quantum properties of their relativistic-like charge carriers.
Using our electromigration technique, we tailored the shape and size of suspended graphene channels, forming ultra-short devices which behave as graphene QDs if they are narrow (approximately 30 nm) or ballistic transistors if they are wider (approximately 100 nm). Our approximately 30x30 nm suspended graphene QDs are, to our knowledge, the smallest suspended graphene QDs made to date. We measured electron transport across these devices and observed a variable charging energy as a function of the charge occupation of the dot, as expected due to the chaotic billiard transport of Dirac fermions. We observed signatures of electron-vibron (e-v) coupling in our suspended QDs and measured their self-actuated out-of-plane vibron resonances (bending mode), whose frequencies range up to 100 GHz.
We used a gold film to locally gate graphene and create ultra-short p-n junctions. We fabricated approximately 20-100 nm long suspended graphene ballistic transistors and measured n-p-n junctions (down to approximately 10 nm p-n junctions). We observed coherent ballistic transport in agreement with the theory of Dirac fermions, and measured Fabry-Perot interferences in our devices. We measured coherence lengths up to approximately 700 nm in our graphene transistors, which is much longer than the length of the channels
of the transistors. This showed that the graphene contacts (gold covered) are also ballistic.
The fabrication method we developed to make ultra-short suspended graphene devices, combined with the observation of clear signatures of quantum coherent transport, opens the way to explore graphene physics and applications at the 10 nm scale.