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/SiO2 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 demonstrate that our PE-CVD process can be applied to reduce the growth temperature of graphene on Ge(100) down to 800°C. We observed that by increasing the growth time from minutes to hours a 2D peak appears and the D and G bands isolate indicating growth is initiated from amorphous carbon and proceed to nanocrystalline graphene. In contrast to PE-CVD growth on Cu, a significant defect peak is observed for graphene grown on Ge(100), which we attribute to the numerous nucleation sites arising from the 100 nm large terraces as reported by Luskosius et al. [3]
Fig. 1: Graphene growth in PE-CVD at 700°C on Cu foil at different growth times. a) Exemplary SEM images of the graphene flakes at growth times of 45 min (top) and 3 h (bottom). b) Average Raman spectra of the samples presented in a), extracted from the corresponding Raman maps.
References
[1] Yang F, Cong H, Yu K, Zhou L, Wang N, Liu Z, Li C, Wang Q, and Cheng B. Ultrathin Broadband Germanium–Graphene Hybrid Photodetector with High Performance. ACS Appl. Mater. Interfaces, 9 (15), 13422–13429 (2017).
[2] Chroneos A, Brach H. Diffusion of n-type Dopants in Germanium. Appl. Phys. Rev. 1, 011301 (2014).
[3] Lukosius M, Lippert G, Dabrowski J, Kitzmann J, Lisker M, Kulse P, Krüger A, Fursenki O, Costina I, Trusch A, Yamamoto Y, Wolff A, Mai A, Schroeder T, Lupina G. Graphene Synthesis and Processing on Ge Substrates. ECS Transactions 75 (8), 533–40 (2016).