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 SiO and hexagonal Boron-Nitride (hBN) 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 a found for quartz or pyrex. In the remote regime, we investigate the etching of single layer and bilayer graphene on SiO and hBN substrates. We find isotropic etching for single layer graphene on SiO, whereas we observe highly anisotropic etching for graphene on a hBN 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.
Graphene nanoribbons (GNRs) have emerged as a promising platform for graphene nano devices, including a range of intriguing quantum phenomena beyond opening of a confinement induced band gapFujita; Nakada; Son_2006; Son; Trauzettel. In armchair GNRs, giant Rashba spin-orbit coupling can be induced with nanomagnets, leading to helical modes and spin filteringKlinovaja. Further, Majorana fermions localized at the ends of the ribbon were predicted in proximity of an s-wave superconductorKlinovaja. Zigzag ribbons, on the other hand, were proposed as a promising system for spin filtersSon_2006. Theory showed that electronic states in zigzag ribbons are strongly confined to the edgeFujita; Nakada; Son_2006, recently observed in experimentsTao; Pan_2012; Zhang_2013; wang2016giant. Further, edge magnetism was predicted to emerge at low temperaturesFujita; Nakada; Son; YazyevRPP, with opposite GNR edges magnetized in opposite directions. High quality, crystallographic edges are very important here, since edge disorder suppresses magnetic correlationsYazyevRPP and tends to cause electron localization, inhibiting transport studies. GNRs fabricated with standard electron beam lithography (EBL) and Ar/O etching typically exhibit pronounced disorder Han_2007; MuccioloPRB79; OostingaPRB81; StampferPRL; GallagherPRB81_2010; LiuPRB80; MolitorSST, complicating transport studies.
Fabrication methods creating ribbons with clean crystallographic edges were recently developed, including carbon nanotube unzipping Jiao; Kosynkin_2009, ultrasonication of intercalated graphite Li, chemical bottom up approaches Cai; ruffieux2016surface, anisotropic etching by nickel nanoparticles campos2009anisotropic or carbothermal etching of graphene sheets Nemes; Krauss. Here, we use a hydrogen (H) plasma etching technique McCarroll; Yang; Shi; Xie because it allows precise, top-down and on-demand positioning and tailoring of graphene nanostructures. Such nanostructures can easily be designed to spread out into larger graphene areas incorporated into the same graphene sheet, thus providing for a relatively easy way to make electrical contacts.
In this work, we investigate the anisotropic H plasma etching of graphite surfaces in dependence of the gas pressure and the sample - plasma distance. We find that the etching characteristics can be divided into a direct and a remote plasma regime. In the direct plasma regime, the sample is placed within the glowing plasma, and surfaces show many hexagons of various sizes indicating a continuous defect induction throughout the etching process. In the remote plasma regime, on the other hand, the sample is placed downstream of the glowing plasma, and etching occurs only from preexisting defects which makes the fabrication of well defined graphene nanostructures possible. Further, we have prepared single layer (SL) and bilayer (BL) graphene flakes on SiO and hexagonal boron nitride (hBN) substrates and exposed them to the remote H plasma. We observe a strong dependence of the anisotropy of the etch on the substrate material. SL graphene on SiO is etched isotropically, confirming previous findingsShi; Diankov, whereas we observe highly anisotropic etching of SL graphene on hBN wang2016patterning, producing very regular and symmetric hexagonal etch pits. Anisotropic etching of SL graphene on hBN offers the possibility to fabricate diverse graphene nanostructure with well defined edges (e.g. GNRs) and allows investigation of their intrinsic electronic transport properties.
A pure H plasma was created in a quartz tube through a matching network by a MHz radio frequency (RF) generator at a typical power of W. This RF power was capacitively coupled to the mm diameter tube by an outer electrode acting as a surfatronMoisan. The pressure was regulated using a needle valve for 20 SCCM H gas flow of purity 6N. The sample was placed at a distance from the end of the surfatron, was electrically floating and a three-zone furnace controlled the temperature . See supplementary online materials (SOM) for additional information. Ion impact energy is roughly the difference between the plasma potential and the floating potential and is around eV with an average ion mass of 2 amu. We estimate the ion flux to be significantly lower than ions/cms measured for a similar plasma setup but at lower pressureEren. In order to characterize and optimize the anisotropic etching process, we studied the influence of pressure, distance, and temperature on the etching process, generally finding good repeatability. We first investigated graphite flakes, allowing for rather simple and fast processing. The graphite specimenNGS were cleaned by peeling with scotch tape and subsequently exposed for one hour to a pure H plasma at