Fabrication Of Graphene Field Effect Devices And Their Electronic Transport Propeties

Graphene, as the firstly discovered two-dimensional material, has attracted worldwide interests and became the frontier and research focus of today’s national defense edge due to its special linear bandstructure and outstanding properties in mechanics, thermotics, electronics and optics with the constant emerging of new physical phenomena and new graphene-based devices. This work began with the preparation of graphene material and then studied the approaching and technology of the fabrication of graphene field effect devices(GFEDs) based on a dual beam(focused electron beam & ion beam) system. After that, the electronic transport, the impact of electrode contact and the effects under high electric field were studied deeply. Moreover, a kind of all-carbon based graphene transistor has been proposed. The interlayer coupling of the double-layer and multi-layer graphene were also studied for the further research of the heterostructures based-on two dimensional materials. The main achievements are summarized as follows:1. The basic theory of the GFED has been summed up and deduced. Beginning with the tight-binding model, the graphene’s bandstructure, density of states, quantum capacitance, carrier density and the final field effect model has been deduced in succession, which are the foundations of device fabrication and effect analysis.2. The graphene have been prepared successfully by two methods respectively: the chemical vapor deposition(CVD) and mechanical exfoliation(or so-called “scotch-tape”). The millimeter-size single crystal graphene, which were confirmed by lots of inspections including the Raman spectrum, the optical transmission, the scanning electron microscope(SEM), the transmission electron microscope(TEM) and so on, were obtained by a circumfluence CVD method.3. Two kinds of fabrication-technologies of GFED based on dual beam system have been developed. One is the in situ fabrication and characterization using the focused-ion-beam(FIB) etching and the focused-electron-beam(FEB) induced deposition. Another is the conventional semi-conductor technique of electron-beam lithography(EBL) with the assistance of axygen plasma etching and metal-deposition. The EBL process was independently developed by us on the dual beam system and the resolution of 10 nm, the alignment precision of 50 nm were achieved.4. The electronic transports including the impacts of charge puddles and electrode contact were detailed studied in different GFEDs. An effect of current-induced doping in graphene transistors with asymmetrical metal/graphene barriers between the source and drain electrodes was found. The current-induced doping means that the carrier concentration and even the charge type in graphene could be manipulated repeatedly and reversibly by the current flowing through the graphene device. With the electrode/graphene contact getting worse and worse, the electric transport property of the metal- oxide-graphene heterostructure, a kind of insulating contact between the electrodes and graphene, was studied and the conductive mechanism was found being dominated by the space-charge-limited(SCL) transport with charge traps.5. A self-amplification effect of current was found in high-field transport, which means that the current in graphene transistor with fixed bias and zero gate voltage could increase with time and finally reaches up to the breakdown threshold. The current self-amplification effect is very important to the safe reliability of GFEDs.6. A so called all-carbon based graphene field effect transistor(GFET) in which the electrodes are also carbonaceous has been fabricated by one-step e-beam direct writing(EBDW). In conventional GEFDs, metal/graphene contacts for source and drain electrodes are inevitable, however, such metal contacts could seriously hinder the performances of graphene transistors. Raman scattering and high-resolution transmission electron microscopy(HRTEM) measurements revealed that the carbonaceous and conductive electrodes are composed by graphite nano-sheets with the electric resistivity of ~4.8×10-3?cm. The electrical measurements unambiguously demonstrated that the performances of such all-carbon based GFET are comparable to those of the devices with the metallic electrodes. It was also found that the graphite nano-sheets in electrodes are perpendicular to the channel graphene, which provides a potential strategy for improving the electrode/graphene contact in the graphene-based devices. The one-step fabrication of GFET is more convenient and lower-cost comparing to the conventional GFEDs with metal electrodes due to the metal deposition and lift-off processes, as well as the development of EBL, are not required.7. The impacts of the interlayer coupling to the optical properties of the double-layer and multi-layer graphene were studied. Firstly, the bandstructures and linear optical absorptions of double-layer graphene with different interlayer spacing were calculated based on the first-principles theoretical calculations. The results indicated that the AB stacked double-layer graphene could greatly increase the optical absorption with the decrease of the interlayer spacing. Secondly, the third-order nonlinear optical properties of the coupling and decoupling graphene layers were studied in experiment using the Z-scan technique. It was found that the coupling of layers has adverse effect upon the nonlinear optical properties. The study manifests that the interlayer coupling plays an important role to the properties of two dimensional materials, which may have great implications for the research of van der Waals heterostructure based devices.