Researchers at the National Physical Laboratory (NPL) used local scanning electrical techniques to examine the local nanoscale electronic properties of epitaxial graphene, in particular the differences between the edges and central parts of graphene Hall bar devices.
The researchers found that the central part of the graphene channel demonstrated electron conduction (n-doped), whereas the edges demonstrated hole conduction (p-doped). They were also able to precisely tune the conduction along the edges of the graphene devices using side-gates, without affecting the conductive properties at the centre.
At a smaller scale, these effects become more acute; when working at the submicron level, the altered properties may affect up to 25% of the material. Although both n- and p-type semiconductors conduct electricity, different types of conduction need to be acknowledged in the development of any devices.
The inversion effects were greatest just after the graphene had been cleaned, indicating that the carrier inversion was caused by defects at the channel edge introduced by the plasma etching process used to form the graphene devices. By contrast, a few hours after cleaning, the inversion effects were reduced as airborne molecules had adsorbed onto the uncoupled bonds at the edges of the graphene.