Why graphene may be the substrate for the next generation of computer chips

Sandwiching individual graphene sheets between insulating layers to produce electrical devices with unique new properties could open up a new dimension of physics research
July 31, 2012

Cross-sectional TEM image of graphene–hBN heterostructures. (a) Schematic of one of University of Manchester’s devices: two graphene monolayers (dark grey) are interlaid with hBN crystals (blue) and have separate electrical contacts (yellow). (b) Optical image of the device. The graphene Hall bar is highlighted with thin black lines. The black rectangle (1:8 m4:4 m) indicates the area enlarged in the SEM micrograph (c), in which the yellow rectangle marks the region chosen for a study. (d) Low-magnification SEM image of the final cross-sectional slice for TEM imaging. (e) High-resolution bright-field aberration-corrected STEM image of the graphene–hBN heterostructure in the circled region. (f) Averaged line scan of the intensity along the dashed black line in e with a schematic of the atomic layer sequence for this particular device shown on the right, in which carbon, boron and nitrogen atoms are represented as black, blue and red respectively. (Credit: S. J. Haigh et al./Nature Materials)

Wonder material graphene is a two-dimensional material consisting of a single layer of carbon atoms arranged in a honeycomb or chicken wire structure. It is the thinnest material in the world and yet is also one of the strongest. It conducts electricity as efficiently as copper and outperforms all other materials as a conductor of heat.

Now University of Manchester scientists have shown that a new side-view imaging technique can be used to visualize the individual atomic layers of graphene within the devices they have built. They found that the structures were almost perfect — even when more than 10 different layers were used to build the stack.

This surprising result indicates that the latest techniques of isolating graphene could be a huge leap forward for engineering at the atomic level, the scientists say, and gives more weight to graphene’s suitability as a major component in the next generation of computer chips.

The scientists note that the field has expanded beyond studying graphene as isolated 2D crystals. There is a rapidly growing interest in atomic-scale heterostructures made from a combination of alternating layers of graphene, hexagonal boron-nitride (hBN), MoS2, and so on. Such heterostructures provide a higher electronic quality for lateral graphene devices and also allow a conceptually new degree of flexibility in designing electronic, optoelectronic,  micromechanical and other devices..

Side-view imaging

The researchers’ side-view imaging approach works by first extracting a thin slice from the center of the device. This is similar to cutting through a rock to reveal the geological layers or slicing into a chocolate cake to reveal the individual layers of icing.

The scientists used a beam of ions to cut into the surface of the graphene and dig a trench on either side of the section they wanted to isolate. They then removed a thin slice of the device.

“The difference is that our slices are only around 100 atoms thick and this allows us to visualize the individual atomic layers of graphene in projection,” said Dr. Sarah Haigh from The University of Manchester’s School of Materials.

“We have found that the observed roughness of the graphene is correlated with their conductivity. Of course we have to make all our electrical measurements before cutting into the device. We were also able to observe that the layers were perfectly clean and that any debris left over from production segregated into isolated pockets and so did not affect device performance.

“We plan to use this new side view imaging approach to improve the performance of our graphene devices.”

Demonstrating its remarkable properties won Professor Andre Geim and Professor Kostya Novoselov the Nobel prize for Physics in 2010. The University of Manchester is building a state-of-the-art National Graphene Institute to continue to lead the way in graphene research.