Hydrogen etching of chemical vapor deposition-grown graphene domains
Authors: Wang Bin  , Feng Ya-Hui  , Wang Qiu-Shi  , Zhang Wei  , Zhang Li-Na  , Ma Jin-Wen  , Zhang Hao-Ran  , Yu Guang-Hui  , Wang Gui-Qiang  ,   College of New Energy, Bohai University, Jinzhou 121013, China;,   State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Published: December 2020

In this paper we analyze the reason of the etching trenches in chemical vapor deposition (CVD) graphene domain and study the influence factor in the distribution and morphology of wrinkles.
Graphene is synthesized on Cu substrate. The Cu substrate is annealed at 1050℃ for 60 min with 1000 sccm Ar and 200 sccm H2. After annealing, 500 sccm Ar, 20 sccm H2, and 1 sccm dilute CH4 (mixed with Ar) are introduced into the CVD system for graphene growth. Hydrogen etchings of graphene are conducted with flows of 500 sccm Ar and 200 sccm H2 at atmospheric pressure, and etching are performed at 950 and 1050℃.
The striated and reticular etching trenches are observed after etching via optical microscope and scanning electron microscope. Every graphene domain is divided into island structures by these etching trenches. However, the edge of graphene domain is not etched and the size of domain is not changed. Electron backscatter diffraction (EBSD) is conducted to analyze the different morphologies of etching trenches. According to the EBSD analysis, the etching trench is closely associated with the Cu crystal orientation. Different Cu planes result in differences in mode, shape, and density of the etching trench.
We conduct a verification experiment to judge whether the etching trenches are caused by the gaps between graphene and Cu substrate or by the hydrogenation of wrinkles.
The graphene domains grown on Cu substrate with the same growth condition are etched immediately after growth without cooling process. We select graphene which grows across the Cu grain boundary, via optical microscope. A small number of regular hexagons are observed in graphene surface and the region of Cu boundary, but no etching trench is found. As the graphene growing across Cu boundary is the suspending graphene and there is no etching trench, we consider that the gap between graphene and Cu species is not a significant factor of forming etching trench. For comparison, the etching trenches are observed in the graphene domains with cooling process. Thus, the trench formation is bound up with the cooling process after growth, which can lead to the wrinkle formation on the graphene surface, giving rise to a large thermal expansion coefficient difference between the graphene and Cu species. As a major type of structural imperfection, wrinkles can show that enhanced reactivity is due to hydrogenation because of high local curvature. So we consider that the trench formation is caused by the hydrogenation of wrinkles.
Then the as-grown graphene domains are transferred to SiO2 substrate and atomic force microscope (AFM) is employed to measure the surface appearance of graphene. The AMF image shows lots of wrinkles in the graphene surface. The morphology and density of wrinkles are similar to those of the etching trenches extremely. Thus, the AFM testing result provides another evidence to prove that the etching trenches are related to the hydrogenation of wrinkles.
From the above we can draw some conclusions. Numerous trenches are observed in the graphene domains after etching, and the trench patterns are closely associated with the Cu crystal orientation. A different Cu crystal orientation leads to variations in mode, shape, and density of the etching trench. We prove that the etching trenches are caused by the hydrogenation on wrinkles formed in the cooling down process instead of the gap between Cu and graphene. This hydrogen etching technology is a convenient way to detect the distribution and morphology of wrinkles. Furthermore, it provides a reference for improving the quality of CVD graphene.