GRAPHENE-BASED TRANSISTORS AND DETECTORS: FABRICATION AND CHARACTERIZATION

Carbon and carbon-based systems have always attracted great attention thanks to the almost unlimited different structures they can be arranged in and the equally varied physical properties they own. These characteristics are mainly related to the flexibility of carbon bonding, which makes carbon an extremely versatile “building block” material. Most of the typical properties of each carbon-based system are mainly associated with the dimensionality of the structure itself. In this framework, graphene, the first two-dimensional atomic crystal available to the scientific community, has revealed to play a key role in terms of fundamental physics and potential applications, despite its short story. Its exceptional properties, like electrical and thermal conductivity, high carriers mobility and saturation velocity, mechanical strength and elasticity, wavelength-independent optical absorption coefficient and so on, suggest that graphene could replace, in the next years, more conventional materials in many fields, such as high-frequency electronics and photonics. This thesis describes the author’s work with graphene, developed throughout the PhD program. During this period, most of the research activity has been dedicated to the design, fabrication and characterization of microwave Graphene Field Effect Transistors (GFETs). In particular, three different fabrication runs have been carried out, investigating the role of different transistor layouts, of the substrate and of the dielectric material in the devices’ performance. The above-mentioned transistors have been fabricated at the Institute of Nanotechnology (INT) of the Karlsruhe Institute of Technology (KIT), Germany. Extensive GFETs characterizations, including DC/microwave, optical and thermoelectric ones, instead, have all been performed at the Laboratorio di Elettronica delle Microonde (LEM) of the University of Palermo. Concerning this last aspect, great attention has been paid to the development of a reliable, automated, multifunctional microwave/optical measurement bench through the implementation of a complete set of HTBasic software modules. Furthermore, graphene optical transparency and low resistivity have been exploited to develop a novel kind of X-ray detector based on polycrystalline grade diamond substrate and Reduced Graphene Oxide (RGO) contacts. Graphene electrodes are, in fact, basically X-ray transparent, thus introducing an almost negligible perturbation of the incoming beam. These characteristics, together with the high resistivity, high mobility, radiation hardness and high thermal conductivity shown by diamond substrates, make the RGO/diamond detector a very promising solution for in situ beam monitoring. In addition, a novel Graphene Oxide (GO) rapid thermal reduction process has been developed, combining the advantages of all typical thermal reduction processes (like the absence of toxic agents and the parallel reduction of several GO-coated substrates) with an unequalled reduction speed and without compromising the film quality. Detector design and preliminary X-ray tests showing detection capability of these devices have been performed at the SLAC National Accelerator Laboratory, Menlo Park, CA (USA), while fabrication has been carried out at the Stanford Nanofabrication Facility (SNF) and at the Stanford Nano-Center (SNC), part of the Stanford Nano Shared Facilities.