Nanoscale carbon structures, including graphene and carbon nanotubes, has drawn intense interest for study and for applications over a broad range of industries and fields due to the highly unusual structure and properties of the materials. Graphene consists of a single flat layer of carbon atoms, densely bound in a honeycomb structure. While discovered earlier, it is perhaps easiest to describe carbon nanotubes as a layer of graphene rolled into a cylinder. Graphene has been shown to be highly conductive, durable, flexible, and impermeable. Likewise, carbon nanotubes demonstrate exceptional strength and conductivity.
Both graphene and carbon nanotubes have been fabricated by several methods. Among them, vacuum growth techniques, including sublimation (for graphene) and chemical vapor deposition (for both) hold the greatest promise for scalable, controlled production of high quality materials. Sublimation and chemical vapor deposition (CVD) have been used to produce single and multi-layer graphene films. We are developing an array of processes and process specific tools for graphene manufacturing on a variety of substrates. In this presentation, we review results we have achieved using several of these techniques, and compare reactor designs and process tradeoffs with the different tools. The earliest technique for graphene formation on large scale substrates was silicon sublimation from SiC, based upon graphitization of SiC surfaces, a well known phenomenon. Sublimation formation of graphene on the surface of a SiC wafer has been performed in a vertical reactor under ultra high vacuum (UHV) and in an Ar atmosphere at 1300° to 1650°C. Dramatically different growth characteristics are seen for the C face and the Si face of SiC, including growth rate and self-limiting characteristics. CVD of graphene has been achieved on a variety of substrates including dielectrics and metals, with different substrates requiring different process conditions. CVD on metal has been widely demonstrated using copper, nickel, and iridium substrates at temperature of 900-1000°C. Recently, the Cornell research team has demonstrated graphene CVD on a wide range of dielectrics as well, including: 6H-SiC (both Si-face and C-face), SiO2, and sapphire in the 1300°C to 1650°C range. In contrast to the sublimation approach with SiC, this carbon addition technique allows multi-layer graphene formation with controlled thickness (3 – 100 monolayers). Carbon nanotubes can be grown utilizing the same tools at 600 – 900°C using ethylene or methane for multi-wall or single wall nanotubes, respectively. Our team is refining process tools for the different growth regimes as well as for large area graphene production. We will present an overview of these approaches, the results, and directions being pursued.