Thermal and Plasma Enhanced Chemical Vapor Deposition of Graphene on Copper and Germanium
Graphene is a one atom thick layer of graphite, which has a hexagonal lattice formed by carbon atoms. It is a transparent, exible and highly conductive material, which is utilized in many applications such as transparent electrodes, photodetectors, modulators, etc. Chemical vapor deposition (CVD) has become the method of choice to fabricate large area graphene films on catalytically active copper (Cu) and germanium (Ge) substrates. These are employed in literature to fabricate monolayer graphene with high quality close to the melting temperature of the substrate. While a Cu foil is a promising candidate to fabricate graphene in roll-to-roll systems, the strained copper foil may melt down easily at the typical synthesis temperatures of graphene. Direct growth of graphene on Ge at elevated temperatures is also challenging, since in CMOS technology Ge is often deposited onto silicon (Si). During graphene synthesis the Si atoms may diffuse towards the surface and form carbides with the carbon atoms. Furthermore, due to the diffusion within the Ge film the dopand profile may change altering the final device properties. The aim of this work was to reduce the graphene growth temperature on Cu foils and Ge substrates. As a first step, a pre-treatment method of the Cu foil was developed to flatten the surface. Then, the mechanism of the thermal graphene growth was studied on Cu foil by varying the process parameters systematically. The quality of the fabricated graphene was examined by Raman spectroscopy and conductivity measurements. By increasing the chamber pressure, the diffusion of the precursor molecules through the boundary layer was reduced and hexagonal and defect-free graphene flakes were fabricated at atmospheric pressure. Hexagonal grains with a grain size up to 17 µm were realized, when the amount of hydrogen flow was increased. As a result of the large flake size and small amount of defects, the sheet resistance was reduced down to 268 ohm/sq. and a Hall mobility up to 1853 cm2V-1s-1 was determined. By inducing a plasma in the same chamber, almost defect-free graphene was fabricated on Cu foils at temperatures as low as 600°C. In order to ensure two-dimensional graphene growth, a sacrificial foil was employed to screen the electrical field induced by the plasma. At the early stage of growth a defective carbon film was observed around highly crystalline graphene flakes. With increasing growth time, the defective regions were found to recrystallize into graphene and defect-free graphene films with a sheet resistance down to 468 ohm/sq. were obtained at low synthesis temperatures. Direct growth of graphene was established on Ge(100) and Ge(110) first at elevated temperatures. By thermal CVD growth monolayer graphene was fabricated with a sheet resistance of 2.5 kohm/sq. agreeing with literature values. In presence of a plasma, the growth temperature was reduced by almost 200°C down to 757°C, which is the lowest graphene growth temperature reported hitherto. Akin to plasma enhanced growth on Cu, a defective carbon film was observed on Ge wafers. The samples, which were fabricated by thermal and plasma enhanced CVD growth, were analyzed by Raman spectroscopy and compressive and tensile strain were determined, respectively. Based on these strain properties, a growth model was suggested for the graphene synthesis on Ge by thermal and plasma enhanced CVD.
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