There is no doubt that graphene is the key to the future of electronics. It is the most significant material for developing new types of electronic devices because of its many extraordinary properties, such as extraordinary strength (it is about 200 times stronger than steel by weight), almost transparent nature and conductivity of heat and electricity with great efficiency. However, in order to use graphene in high-performance semiconductor electronics ultra-narrow strips of graphene are needed and scientists have struggled to create them. Until now.
Scientists at University of Wisconsin-Madison have now discovered a method to grow these ultra-narrow strips, called nanoribbons, with desirable semiconducting properties directly on a conventional germanium semiconductor wafer. This discovery is aimed at allowing manufacturers of electronics to develop the next-generation of electronic devices that will have much greater performance.
This technology is also likely to find applications in other industries as well, such as military, used in sensors that detect specific chemical and biological species and photonic devices that manipulate light. Furthermore, this method of producing nanoribbons is not overly complicated – it is scalable and is compatible with current equipment used in semiconductor processing.
In fact, it is hard to put into words how significant this achievement is. Professor Michael Arnold, one of the authors of the study, said – “Graphene nanoribbons that can be grown directly on the surface of a semiconductor like germanium are more compatible with planar processing that’s used in the semiconductor industry, and so there would be less of a barrier to integrating these really excellent materials into electronics in the future”.
Where graphene could be in the future, now mostly silicon is used, which is not as efficient in conducting electricity and dissipating heat. However, to use graphene in such applications is not easy and that is why nanoribbons are needed. They have to be extraordinary narrow – they need to be less than 10 nanometres wide. They also must have smooth, well-defined “armchair” edges in which the carbon-carbon bonds are parallel to the length of the ribbon. Such nanoribbons can be manufactured by cutting larger sheets of graphene into ribbons. But this technique is not perfect as produced ribbons have very rough edges.
These graphene ribbons can also be produced by surface-assisted organic synthesis, where molecular precursors react on a surface to polymerize nanoribbons. But resulting ribbon, although with smooth edges, is far too short for use in electronics. But now scientists found a way to manufacture ultra-narrow nanoribbons with smooth, straight edges directly on germanium wafers. As scientists describe it, they are growing graphene in this shape via process called chemical vapour deposition. Although described as a rather simple method, it is hard to explain it in a commonly understandable way.
In this process scientists start with methane, which adsorbs to the germanium surface and decomposes to form various hydrocarbons. Then these hydrocarbons react with each other and form graphene on surface of the germanium wafer. Team of researchers made this discovery when they were exploring dramatically slowing the growth rate of the graphene crystals by decreasing the amount of methane in the chemical vapour deposition chamber.
Scientists found that at a very slow growth rate graphene naturally grows into long nanoribbons on a specific crystal facet of germanium and researchers only need to control this process to produce nanoribbons less than 10 nanometres wide. Moreover, these strips of graphene have very smooth, armchair edges and can be very narrow and very long, all of which is needed for future generations of electronics.
However, there are still some problems left to solve. Using this process, graphene grows at completely random spots on the germanium wafer. Furthermore, strips are oriented in two different directions on the surface. So now scientists will try to find a way to control the place where graphene starts growing and to align the nanoribbons to the same direction.