In a European collaboration within the project ConceptGraphene, researchers at Chalmers University of Technology in Sweden have developed a method for fast and inexpensive quality control of graphene grown on silicon carbide (SiC). The technique, based on optical microscopy, is used to understand the effect of the silicon carbide substrate on the quality of the graphene layer and paves the way for optical microscopy as an industrial quality control tool of epitaxial graphene on silicon carbide.
The unique and superior properties of graphene (a single layer of carbon atoms) promise a revolution in electronics. Several processes have been developed to produce graphene on a large scale, and a strong contending technology is the growth of graphene on the surface of silicon carbide at high temperature. Also called epitaxial graphene on silicon carbide, this material often requires quality control, because the surface of the silicon carbide reconstructs during graphene growth. Steps and defects in the substrate may lead to the appearance of stepped terraces and the formation of areas with many layers instead of a single layer. These unwanted features limit the performance of the electronic devices and large-scale integration.
Now, in collaboration with colleagues at Sweden's Link?ping University and the UK's National Physical Laboratory (NPL), researchers at Chalmers have shown that graphene on silicon carbide can be quality-controlled at the nanoscale by using simple inspection with an optical microscope (published online on 13 August in Nano Letters; DOI: 10.1021/nl402347g). Their technique is said to allow simple and accurate identification of even a single graphene layer (0.3nm thick) and nanometer-scale steps on silicon carbide.
Until now, it was believed that graphene on silicon carbide has too little contrast to be observed directly by optical microscopy. But, by cross-checking optical data with electrical measurements, it was found that a single layer of graphene on silicon carbide reflects about 1.3% of light - faint but sufficient to be detected in an optical microscope.
As an illustration of the power of the technique, researchers have applied it to fabricate graphene devices on specific parts of the substrate, identified using optical microscopy. It is shown that stepped terraces on silicon carbide are not as detrimental as multilayer-graphene domains. Also, they show that, positioned on monolayer areas, the devices have characteristics that are truly given by the unique properties of graphene. Apart from facilitating fundamental studies, the technique can also have an industrial impact.
"Our findings will help the advancement of epitaxial graphene research and technology on the way towards wafer-scale production and achieving control of graphene quality at the nanoscale," believes professor Sergey Kubatkin about the usefulness of the method. "Optical microscopy of epitaxial graphene delivers similar information to that obtained by other well-established but more complex techniques such as low-energy electron microscopy (LEEM) and scanning probe microscopy (SPM), not only in a much faster and economic way but also without perturbing the graphene layer," he adds.
The optical microscopy technique is already used as a tool in producing epitaxial graphene at Link?ping University spin-off Graphensic AB, of which professor Yakimova (co-author of the study) is co-founder & CEO. "It is a fantastic tool for production evaluation of our material," says Mikael Syv?j?rvi, who works with production protocols at Graphensic.
The research work was supported by ConceptGraphene (a collaborative project funded by the European Union within its 7th Framework Program), the Swedish Research Council and Foundation for Strategic Research, The Knut and Alice Wallenberg Foundation (KAW), and the IRD Graphene Project of the UK's National Measurement Office.
