Graphene grown by chemical vapor deposition on evaporated copper thin films

Abstract: Graphene is a thin atomic layer of carbon atoms which has a hexagonal lattice structure. Due to its exceptional properties such as high electrical conductivity, high carrier mobility, high thermal conductivity, high optical transparency and super hydrophobicity, graphene is expected to play an important role in future nanoscience and nanotechnology. Chemical vapor deposition (CVD) is a novel technique proposed recently to synthesis large area high quality graphene, which was not possible by the conventional mechanical exfoliation method of graphite. However, though a lot of progresses have been made using Cu foil as catalyst in CVD, this thesis will focus mainly on evaporated Cu thin film grown graphene, because for industrial application it is more convenient, which is largely unexploited in literature. Cu was opt here because the solubility of C in Cu is extremely small, making it easier to achieve uniformly monolayer graphene, which is very difficult otherwise, e.g. using Ni as the catalyst. Thus, investigation of graphene’s quality on top of Cu thin film by tuning the thickness of Cu also alteration of other parameters such as temperature, hydrogen concentration to optimize the graphene growth condition and finally fabricating suspended graphene devices are the goal of this project. Suspended graphene draw attention because in suspended graphene devices carriers will have relatively high mobility with less scattering which is very promising for different applications. Most importantly, suspended graphene can vibrate, whose resonance frequency is sensitive to the mass of absorbed particles, making it promising for future ultrasensitive mass sensors. In this work, after CVD synthesis, transfer of graphene onto foreign Si substrates with 300 nm SiO2 was used to inspect the graphene. We stress that this complex transfer technique is not easy to be industrialized, and is only used here to estimate the quality of graphene because the graphene is otherwise invisible on Cu. In the final devices, this transfer step is unnecessary, as the graphene can be suspended simply by locally removing the Cu underneath the graphene channel, rendering a fully semiconductor industry compatible process. The graphene on SiO2/Si samples were briefly characterized by scanning electron microscopy, conductance measurements and Raman spectroscopy. It was found that using 600 nm thick Cu catalyst with annealing in nominal 750oC for 5 minutes continued by 5 minutes growth in 20 sccm H2, 30 sccm pre-diluted methane(5% in argon) and 1000 sccm argon gas flow was the optimal CVD condition. Subsequently, suspended graphene channel devices were fabricated by two step lithography, together with techniques such as wet etching, lift off and critical point drying, etc. Nevertheless, despite some devices appear very successful in microscope, they do not show the expected good enough electrical performances, even though the Raman spectroscopy indicates decent quality graphene. The reason is ascribed to the polycrystalline nature of the graphene, which means the effect from the grain boundaries need to be considered in future studies. Under the background that arrays of suspended graphene devices have never been reported using a scalable and transfer-free technique, and even the CVD using evaporated Cu thin film catalyst has not been thoroughly investigated in literature, our proof-of-principle results indicates a promising future of this research direction, at least good enough for continued improvements.