Graphene growth, transfer and devices fabrication

Graphene growth: The 2D structure of graphene combined with its excellent physical properties make it attractive for improving the performance of a wide range of practical applications, particularly in the fields of high frequency electronics, optoelectronics, energy harvesting, and micro/nano-electro-mechanical systems. So far, the best graphene synthesis results — in terms of structural quality, thickness, uniformity and scalability — have been achieved using chemical vapor deposition (CVD) on a copper catalyst. In order to fabricate graphene-based high- performance devices, graphene has to be transferred from the synthesis substrate onto a more device-compatible substrate. This transfer step is likely to damage the delicate single-layer which is prone to tearing, ripping and folding. Obviously, it is of first importance for devices performance and yield that graphene physical integrity is not degraded upon the transfer process. To improve graphene quality and processability, the CVD synthesis of graphene is performed onto a Cu thin film pre-deposited onto an oxidized Si wafer as catalytic substrate instead of a conventional Cu foil. Using a smooth, flat and rigid Cu film instead of a corrugated Cu foil leads to a more planar graphene film, which, in turn, causes less wrinkles and cracks after transfer. Moreover, this synthesis substrate improves graphene processability as it allows the direct processing of graphene by conventional CMOS technologies.  Conventional transfer: Up to now, numerous techniques have been reported to transfer graphene from the Cu foil to a device- compatible substrate. The application of these techniques to graphene grown on a rigid Cu film is not straightforward. These techniques, and more particularly, the supporting materials (including PMMA, PDMS and thermal release tape) — used to facilitate graphene handling during the removal of the growth substrate — are investigated and compared. Physical integrity, degree of contamination, chemical purity of the transferred graphene film as well as the presence of transfer-induced internal stress are used as figure of merit in the quest to the development of a robust, reliable and scalable transfer process. Proximity transfer: Synthesizing graphene on a Cu thin film pre-deposited onto a device-compatible substrate can be used to circumvent the delicate and cumbersome conventional transfer procedure. Using the Cu film as a sacrificial layer after the CVD process allows the direct deposition of graphene on the underlying substrate. The incomplete etching of the Cu film can be exploited to suspend graphene micro-ribbons or obtain metallic pads acting as electrodes. The key process parameters for a reliable proximity transfer consists in preventing graphene from tearing upon the Cu wet etching process step as well as enhancing the contact between graphene and the underlying so that fluid agitation cannot wash away graphene. Nano-indentation of graphene/Cu system: Recent studies demonstrated that mixing graphene flakes in a metal matrix causes a strengthening effect of the metal characterized by an enhancement of both Young’s modulus and yield strength [1]. In this work, we perform nano-indentation experiments on CVD-grown graphene on Cu film. The strengthening is investigated by measuring the response of bare Cu film and Cu/graphene stack to deformation. To better understand the mechanisms responsible for this strengthening, and more particularly how dislocations move and interact during nanoindenting Cu, TEM analyses have been performed on cross sections prepared by focused ion beam. It is found that the presence of graphene on the Cu surface reduces the plastic burst of the load-displacement curve for nanoindenting Cu. This means that dislocations nucleated in Cu are blocked at the interface between Cu and graphene and do not escape the surface. A single layer graphene acts as a very efficient and strong dislocations barrier. Energy Harvesting: In this work, we investigate the fabrication of graphene-based geometric diodes in order to develop rectenna solar cells able to harvest the energy of the electromagnetic spectrum in the teraHertz region. THz rectennas consist of micron-size antennas coupled to high-speed diodes to convert the high frequency AC field collected by the antenna to useable DC power. Geometric diodes use the ballistic transport of charge carriers in planar structures smaller than the mean free path to induce an asymmetric electrical characteristic. The 2D structure and the great charge carriers mean free path at room temperature make graphene a promising material for the realization of an efficient diode-like electrical device. The extremely low capacitance due to the planar configuration of this diode makes it suitable for high frequency operation, i.e. several tens of THz. Networks of rectennas could be used in solar cells for thermophotovoltaic energy harvesting as well as in imaging sensors, requiring high quality graphene at wafer-scale. Optoelectronics: The use of graphene as an ultra-wideband and ultrafast absorber has been suggested for photo-detectors operating above 40 GHz modulation frequency [2]. Optical interconnects using the coupling of graphene with silicon photonic waveguides have been reported for the telecommunications wavelengths using graphene flakes obtained by exfoliation [3, 4]. We present our progress on fabricating graphene photo-detectors coupled with silicon photonic waveguides using CVD graphene. [1] J. Hwang, T. Yoon, S. H. Jin, J. Lee, T.-S. Kim, S. H. Hong and S. Jeon, “Enhanced mechanical properties of graphene/copper nanocomposites using molecular-level mixing process, ”Advanced Materials, vol. 25, no. 46, pp. 6724– 6729, 2013. [2] F. Xia, T. Mueller, Y. Lin, A. Valdes-Garcia, and P. Avouris, “Ultrafast graphene photodetector,” Nature Nanotechnology, vol. 4, no. 12, pp. 839–843, 2009. [3] A. Pospischil, M. Humer, M. M. Furchi, D. Bachmann, R. Guider, T. Fromherz, and T. Mueller, “CMOS-compatible graphene photodetector covering all optical communication bands,” Nature Photonics, vol. 7, no. 11, pp. 892-896, 2013. [4] X. Gan, R.-J. Shiue, Y. Gao, I. Meric, T. F. Heinz, K. Shepard, J. Hone, S. Assefa, and D. Englund, “Chip-integrated ultrafast graphene photodetector with high responsivity,” Nature Photonics, vol. 7, no. 11, pp. 883–887, 2013.