Building Transistors from Graphene Nanoribbons

The microscopic ribbons lie criss-crossed on the gold substrate. (Image courtesy of EMPA.)

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the nanoelectronics of the future: while graphene is a conductive material, it can become a semiconductor in the form of nanoribbons.

This means that it has a sufficiently large energy or band gap in which no electron states can exist that it may become a key component of nanotransistors. However, the smallest details in the atomic structure of these graphene bands have massive effects on the size of the energy gap and thus on the suitability of nanoribbons as components of transistors.

On the one hand, the gap depends on the width of the graphene ribbons, while on the other hand it depends on the structure of the edges. Since graphene consists of equilateral carbon hexagons, the border may have a zigzag or a so-called armchair shape, depending on the orientation of the ribbons. While bands with a zigzag edge are conductive, those with the armchair edge become semiconductors.

This poses a major challenge for the production of nanoribbons—either by cutting them from a layer of graphene or by cutting carbon nanotubes—since the edges may be irregular and thus the graphene ribbons may not exhibit the desired electrical properties.

Researchers from the Swiss Federal Laboratories for Materials Science and Technology (EMPA) in collaboration with the Max Planck Institute for Polymer Research in Mainz and the University of California at Berkeley have now succeeded in growing ribbons exactly nine atoms wide with a regular armchair edge from precursor molecules. Their research is published in Nature Communications.

The specially prepared molecules are evaporated in an ultra-high vacuum for this purpose. After several process steps, they are combined like puzzle pieces on a gold base to form the desired nanoribbons of about one nanometer in width and up to 50 nanometers in length.

These structures, which can only be seen with a scanning tunneling microscope, now have a relatively large and, above all, precisely defined energy gap. This enabled the researchers to go one step further and integrate the graphene ribbons into nanotransistors.

Empa researcher Gabriela Borin Barin evaporates specially prepared molecules in high vacuum to grow graphene nanoribbons. (Image courtesy of EMPA.)

Initially, however, the first attempts were not very successful: measurements showed that the difference in the current flow between the ON state and the OFF state was far too small. The problem was the dielectric layer of silicon oxide, which connects the semiconducting layers to the electrical switch contact. In order to have the desired properties, it needed to be 50 nanometers thick, which in turn influenced the behavior of the electrons.

However, the researchers subsequently succeeded in massively reducing this layer by using hafnium oxide (HfO2) instead of silicon oxide as their dielectric material. The layer is therefore now only 1.5 nanometers thin and the "ON"-current is orders of magnitudes higher.

Another problem involved the incorporation of graphene ribbons into the transistor. In the future, the ribbons should no longer be located criss-cross on the transistor substrate, but rather aligned exactly along the transistor channel. This would significantly reduce the currently high level of non-functioning nanotransistors.

For more news from the world of cutting-edge electronics research, find out why Even Simple Electronics are Subject to Chaotic Oscillations.

Source: EMPA