Anisotropic etching of graphite and graphene in a remote hydrogen plasma
We investigate the etching of a pure hydrogen plasma on graphite samples and graphene flakes on SiO2 and hexagonal boron-nitride substrates. The pressure and distance dependence of the graphite exposure experiments reveals the existence of two distinct plasma regimes: the direct and the remote plasma regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit numerous etch pits of various size and depth, indicating continuous defect creation throughout the etching process. In contrast, anisotropic etching forming regular and symmetric hexagons starting only from preexisting defects and edges is seen in the remote plasma regime, where the sample is located downstream, outside of the glowing plasma. This regime is possible in a narrow window of parameters where essentially all ions have already recombined, yet a flux of H-radicals performing anisotropic etching is still present. At the required process pressures, the radicals can recombine only on surfaces, not in the gas itself. Thus, the tube material needs to exhibit a sufficiently low H radical recombination coefficient, such as found for quartz or pyrex. In the remote regime, we investigate the etching of single layer and bilayer graphene on SiO2 and hexagonal boron-nitride substrates. We find isotropic etching for single layer graphene on SiO2, whereas we observe highly anisotropic etching for graphene on a hexagonal boron-nitride substrate. For bilayer graphene, anisotropic etching is observed on both substrates. Finally, we demonstrate the use of artificial defects to create well defined graphene nanostructures with clean crystallographic edges.Nanofabrication: remote etching of graphene on hBN is highly anisotropicEtching of graphite with hydrogen plasma gives rise to two distinct etching regimes, governed by the degree of direct plasma exposure. A team led by D. M. Zumbühl at the University of Basel investigated the interplay between graphite exposure to pure hydrogen plasma and the resulting etching features. When the sample surface is in direct contact with the plasma, a defect-rich etching regime occurs, resulting in the formation of etch pits with variable size. However, when graphite is away from the plasma source, a remote anisotropic etching takes place, giving rise to regular hexagons originating from localized defects and edges. In the latter regime, the authors found that in the case of graphene, isotropic etching is obtained using SiO2 as substrate, whereas highly anisotropic etching occurs when graphene is placed on hBN.