Using graphene – either as an alternative to, or most likely as a complementary material with – silicon, offers the promise of much faster future electronics, along with several other advantages over the commonly used semiconductor.
- The graphene grain boundaries may make the mass application of graphene in microelectronics very difficult.
Graphene grain boundary (Source: Beckman Institute)
However, creating the one-atom thick sheets of carbon known as graphene in a way that could be easily integrated into mass production methods has proven difficult.
When graphene is grown, lattices of the carbon grains are formed randomly, linked together at different angles of orientation in a hexagonal network. However, when those orientations become misaligned during the growth process, defects called grain boundaries (GBs) form. These boundaries scatter the flow of electrons in graphene, a fact that is detrimental to its successful electronic performance.
Beckman Institute researchers Joe Lyding and Eric Pop and their research groups have now given new insight into the electronics behavior of graphene with grain boundaries that could guide fabrication methods toward lessening their effect. The researchers grew polycrystalline graphene by chemical vapor deposition (CVD), using scanning tunneling microscopy (STM) and spectroscopy for analysis, to examine at the atomic scale grain boundaries on a silicon wafer. They reported their results in the journal ACS Nano.
Lyding compared graphene lattices made with the CVD method to pieces of a cyclone fence.
The research involved Pop’s group, led by Beckman Fellow Josh Wood, growing the graphene at the Micro and Nanotechnology Lab, and transferring the thin films to a silicon (Si02) wafer. They then used the STM at Beckman developed by Lyding for analysis, led by first author Justin Koepke of Lyding’s group.
Their analysis showed that when the electrons’ itinerary takes them to a grain boundary, it is like, Lyding said, hitting a hill.
In the paper, the researchers were able to report on their analysis of the orientation angles between pieces of graphene as they grew together, and found “no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and Si02 topography.” They reported that analysis of those patterns “indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.”
Lyding said that the relationship between the orientation angle of the pieces of graphene and the wavelength of an electron impinges on the electron’s movement at the grain boundary, leading to variations in their scattering.
The researchers work is aimed not just at understanding, but also at controlling grain boundaries. One of their findings – that GBs are aperiodic – replicated other work and could have implications for controlling them, as they wrote in the paper: “Combining the spectroscopic and scattering results suggest that GBs that are more periodic and well-ordered lead to reduced scattering from the GBs.”