Graphene Synthesis By Plasma Technique Toward Transparent Conductive Film Applications

We developed the large-area microwave plasma chemical vapor deposition (CVD) of graphene for transparent electrode applications. This technique has been successfully combined with the roll-to-roll process to synthesize graphene on Cu substrates. Nucleation density of plasma CVD of graphene crystal is too high, which suppresses two-dimensional growth of graphene and leads to graphene flakes of several nanometer stacks in multiple layers. This causes low electrical conductivity of the synthesized graphene. The low concentration of carbon source is effective to reduce the nucleation density and to enlarge the crystal size. In this study, we utilize small amount of carbon delivered from Cu foil and/or the ambient in the reaction chamber as extremely low-concentration of carbon source and perform the synthesis of graphene of higher crystalline quality.

The carbon atoms precipitate on the Cu surface by the heat treatment of the Cu foil for 15 minutes at about 800°C. Then the copper foil was exposed to the hydrogen plasma for about 30 seconds under 5Pa to synthesize graphene on the foil. We did not use any carbon-contained gas such as CH4. The crystalline quality of graphene was successfully improved by using carbon atoms contained in the Cu foil and/or the ambient in the reaction chamber.

The synthesized graphene on Cu foil was transferred to the quartz or silicon substrate. After that, the van der Pauw devices for Hall Effect measurements were fabricated using conventional photolithography, metal deposition and lift-off processes. The fabricated devices show that the mobility is estimated to be more than 1000 cm²/Vs, which was dramatically improved from the previous plasma CVD using CH₄ as source gas.

This work was partially supported by the Basic research and development of high quality graphene funded by NEDO.