Role of confinement in chemical vapor deposition of graphene on copper thin films

The outstanding electrical, mechanical and chemical properties of graphene make it attractive for a wide range of applications, particularly in the micro electronics field. Currently, chem- ical vapor deposition on copper foils is considered as the most promising route to synthesize graphene in terms of quality, thickness, uniformity and scalability. However, the realization of high-performance graphene-based devices requires the transfer from copper foils onto specific sub- strates. This transfer step is known to cause adverse effects on graphene quality and hinders drastically its processability. In contrast, graphene growth on copper thin films predeposited on passivated substrates enables the transfer-free deposition of graphene on insulating materials by using copper as a sacrificial layer. Moreover, this approach allows the processing of graphene by conventional thin film technologies. In this study, large-area graphene films are grown by low pressure chemical vapor deposition (LPCVD) using methane as carbon source directly onto evaporated copper thin films (700 nm) pre- deposited on SiO2/Si wafers. As graphene film quality and number of layers are the primary areas of concern for high-performance electronics, the CVD parameters including temperature, pressure and hydrogen-to-methane gas ratio have been optimized to obtain uniform films. Moreover, it is found that the presence of a cover located at a maximum of a few tens of microns from the copper surface is necessary to grow graphene. The interest of confining the atmosphere near the Cu surface stands on the significant reduction of copper evaporation which allows the formation of a stable graphene film on its top. The presence of graphene and the continuity of the film have been observed by the means of scanning electron microscopy while the quality of the film has been assessed by Raman spectroscopy. The results reveal that the confinement of the atmosphere in the vicinity of the copper surface is critical for the successful growth of graphene. The use of a fused quartz substrate directly in contact to the copper thin film demonstrated the best confinement results in terms of graphene uniformity and quality. In that way, a continuous predominantly single-layer graphene film has been achieved over surface areas up to 10 cm2. The Raman measurements performed on the as- grown graphene indicate a I2D/IG ratio close to 2 and a ID/IG∼ 0 over nearly all the surface area reflecting that the film is predominately defect-free monolayer graphene.