The integration of graphene into CMOS compatible Ge technology is in particular attractive for optoelectronic devices in the infrared spectral range. Since graphene transfer from metal substrates has detrimental effects on the electrical properties of the graphene film and moreover, leads to severe contamination issues, direct growth of graphene on Ge is highly desirable. In this work, we present recipes for a direct growth of graphene on Ge via thermal chemical vapor deposition (TCVD) and plasma-enhanced chemical vapor deposition (PECVD). We demonstrate that the growth temperature can be reduced by about 200 °C in PECVD with respect to TCVD, where usually growth occurs close to the melting point of Ge. For both, TCVD and PECVD, hexagonal and elongated morphology is observed on Ge(100) and Ge(110), respectively, indicating the dominant role of substrate orientation on the shape of graphene grains. Interestingly, Raman data indicate a compressive strain of ca. − 0.4% of the graphene film fabricated by TCVD, whereas a tensile strain of up to + 1.2% is determined for graphene synthesized via PECVD, regardless the substrate orientation. Supported by Kelvin probe force measurements, we suggest a mechanism that is responsible for graphene formation on Ge and the resulting strain in TCVD and PECVD.

The chemical vapor deposition (CVD) growth of graphene on copper is controlled by a complex interplay of substrate preparation, substrate temperature, pressure and flow of reactive gases. A large variety of recipes have been suggested in literature, often quite specific to the reactor, which is being used. Here, we report on a relation between growth rate and quality of graphene grown in a scalable 4″ CVD reactor. The growth rate is varied by substrate pre-treatment, chamber pressure, and methane to hydrogen (CH:H) ratio, respectively. We found that at lower growth rates graphene grains become hexagonal rather than randomly shaped, which leads to a reduced defect density and a sheet resistance down to 268 Ω/sq.

Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I /I < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq.

We report on the time-dependent influence of atmospheric species on the electrical properties of functionalized graphene sheets (FGSs).

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/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). Figure 1