Graphene spin communication reaches unprecedented lengths
Researchers have made use of commercially available CVD graphene from Graphenea on standard SiO2/Si substrates to produce spin transport devices with spin communication over lengths up to 45 micrometers, significantly higher than any previous research has shown. The achievement was reached by engineering metal contacts and graphene channel length to optimize for spin transport. A report on the research was just accepted in the journal ACS Nano.
Spintronics is a branch of technology that utilizes electron spin for computing, instead of the traditionally employed charge manipulation. Spintronics promises microchips that are not only faster than today’s technology, but also compatible with the emerging platform of quantum computing as well as having lower energy consumption that today’s microchips.
Graphene is an excellent channel for on-chip spin-based communication, primarily due to its weak interactions with electron spin and favorably high carrier mobility. Electrons in a defined spin state can travel far in graphene without losing that state through collision with carbon atoms in the material or other processes.
Image: Spin transport in graphene. T. Ghiasi/University of Groningen. Original article: Leutenantsmeyer et al, PRL 2018.
The last decade has brought major advances in practical use of graphene as a spin transport channel. Top research reported spin diffusion lengths on the order of 10 micrometers, with graphene carefully deposited on a flat hexagonal boron nitride substrate to avoid substrate-induced spin relaxation. A more practical approach is to use graphene grown by chemical vapor deposition (CVD) on technologically relevant silicon-based substrates. However, that approach has resulted in spin diffusion lengths only on the order of 2-6 micrometers at room temperature, due to natural grain boundaries, intrinsic defects, and fabrication-induced impurities in graphene. The current research overcomes these barriers by engineering electrical contacts to avoid strong interactions with the graphene channel, and by using extremely long spin communication channels.
To get the best performance out of their devices, the researchers applied ferromagnetic tunnel contacts, minimizing contact resistance. The carrier mobility in the commercial-grade CVD graphene was 2000-3000 cm2V-1s-1, consistent across devices. Suggesting that electrical contacts locally modify the Fermi level of graphene, the researchers propose that making long transport channels actually supports long-range spin transport. The results indicate a 3-fold increase in spin communication compared to previous state-of-the-art, and a record-breaking spin diffusion length of 9 micrometers, which is remarkably achieved in devices that are of an industrial grade, in a system that is compatible with mass production. Spin propagation underneath the metal contacts is found to be more than ten times worse than in open graphene channels.
It is noteworthy that the quality of contact-to-graphene interfaces is shown to be of the highest quality, which, coupled with industrial-grade CVD production processes and spin transport that is nearly unaffected by ambient conditions, implies high industrial readiness of these spin communication channels for use in realistic large-scale spin computing circuits.