Plasma Enhanced CVD Growth of Graphene on Cu and Ge

Thermal chemical vapor deposition (T-CVD) is currently the process of choice to grow large area graphene with good quality on metal surfaces like copper (Cu) or nickel (Ni) due to their catalytic properties. However the typical growth temperatures are still around 1000°C, and the process cannot be easily adapted to alternative, i.e. non-metallic, substrates. Regarding these issues we used a commercially available 4” Aixtron Black Magic system for establishing a plasma enhanced CVD process (PE-CVD) for graphene growth. A pulsed DC plasma was applied to dissociate the precursor material –methane- already in the gas phase and hence reduce the growth temperature. This process facilitates graphene fabrication with good quality down to a growth temperature of 600°C on Cu substrates. By studying the growth rate for different temperatures we have been able to extract a characteristic activation energy of 1.8 eV in PE-CVD, which is reduced by 2.2 eV as compared to T-CVD owing to the dissociation of methane already in the gas phase. In order to understand the plasma enhanced growth process, we analyzed the graphene formed for different growth times at a growth temperature of 700°C. After 45 min (Fig. 1a, top) we observe amorphous carbon around high quality graphene flakes. This is supported by Raman measurements (Fig. 1b), where Raman signatures of both, amorphous carbon and crystalline graphene are found. By increasing the growth time to 3 h, a continuous graphene film is obtained (Fig. 1a, bottom). In Raman spectroscopy, the amorphous signal disappears and Raman signatures of monolayer graphene with strongly reduced defect peak are detected. This indicates that through a recrystallization process the amorphous carbon turns into a graphene film around the crystalline graphene nuclei. The fabricated graphene film has a sheet resistance down to < 0.5 kΩ/sq. when transferred onto a Si/SiO 2 substrate. Establishing a PE-CVD process at growth temperatures down to 600°C indicates that the need for catalytical substrates become superfluous. We thus adapted our approach to non-metallic substrates. As an example, we chose crystalline Ge(100), a CMOS compatible material that is used for example in broadband photodetectors in combination with graphene [1]. Graphene growth on Ge is typically performed by T-CVD at temperatures of 900°C or above. Such high temperatures might be critical for practical applications because of the temperature dependent dopant diffusion. [2] We demo