The commonly used method to produce graphene is by repeatedly cleaving thin graphite layers using a piece of sticky tape. Graphene flakes produced by this method, the so-called scotch tape method, usually have an area of less then 100 μm2. Though the electronic quality of such graphene is high, the method of mechanical exfoliation is not scalable and results in randomly distributed flakes on a wafer, making it unsuitable for industry applications.
A way of producing large area graphene is by epitaxial growth on a silicon carbide (SiC) substrate. Using controlled sublimation of silicon from the SiC crystal at temperatures around 1500°C it is possible to produce large area graphene. This graphene is 1-2 atomic monolayers thick and can completely cover a wafer of several cm2 big. The epitaxial growth process can take place at the silicon terminated (0001) face of the SiC crystal as well as the carbon terminated (000 ) face.
Epitaxial graphene has somewhat different structural and electronic properties than exfoliated graphene, also depending on the face it was grown on. Si-face grown graphene for instance has an electronically inactive buffer layer between graphene and substrate. C-face graphene grows in randomly rotated layers which are strongly decoupled due to the random orientation. Therefore the electronic structure of the layers resembles the electronic structure of exfoliated single layer graphene. Epitaxial graphene is reported to have fairly high mobilities, ranging from ~2.000 cm2 V-1 s-1 when grown on the Si-face of the SiC crystal up to even several ten thousands cm2 V-1 s-1 when grown on the C-face.
We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and performed nonlocal spin transport measurements. We observed a very long spin relaxation time of τs = 1.3 ns at room tempeature, while the spin diffusion coefficient Ds was strongly reduced compared to typical results on exfoliated graphene. Also we observed different values for the diffusion coefficient measured in charge and spin transport measurements.
These effects can be explained by the influence of localized states arising from the buffer layer at the interface between the graphene and the SiC surface. We developed a spin transport model for a diffusive channel with coupled localized states that result in an effective increase of spin precession frequencies and a reduction of spin relaxation times in the system. Combined with newly performed measurements on quasi-free-standing monolayer epitaxial graphene on SiC(0001) our analysis shows that the buffer layer indeed has a large effect on spin transport in the adjacent graphene channel.
We are currently developing the technology to create top-gated graphene spin valve devices with the use of optical and electron beam lithography.
T. Maassen, J.J. van den Berg, E.H. Huisman, H. Dijkstra, F. Fromm, T. Seyller and B.J. van Wees, "Localized States Influence Spin Transport in Epitaxial Graphene" Phys. Rev. Lett. 110, 067209 (2013)
Thomas Maassen, J. Jasper van den Berg, Natasja IJbema, Felix Fromm, Thomas Seyller, Rositza Yakimova and Bart J. van Wees, "Long Spin Relaxation Times in Wafer Scale Epitaxial Graphene on SiC(0001)", Nano Letters 12 (3), pp 1498-1502 (2012)
Also check out our recent publication in Nederlands Tijdschrift voor Natuurkunde.
Bachelor/Master projectsThere are a couple of projects available concerning this topic. Please contact one of the people involved in case you are interested.
People currently involved in this project:
Jasper van den Berg (PhD)
Eek Huisman (Postdoc)
Bart van Wees (Groupleader)
For information about our collaborators please visit the website of the ConceptGraphene European consortium
|Last modified:||09 July 2015 10.59 a.m.|