Investigation On Fabrication And Optoelectronic Properties Of High-quality Graphene Grown By CVD

Graphene, a single atomic layer of carbon atoms, has stimulated intense researchinterest owing to its unique structure and outstanding properties. An ideal monolayer ofgraphene has a light transmittance of97.7%, with electron mobility values in excess of15000cm2V1s1at room temperature. The considerable research on graphene hasmotivated the scalable production of high-quality graphene and graphene devices.Recently, considerable research efforts focused on graphene synthesis using chemicalvapor deposition （CVD） have succeeded in large area syntheses and improved properties.In this thesis, graphene was synthesized by chemical vapor deposition usingpolystyrene as a solid carbon source. C-H bonds on polystyrene are relatively weakcompared with the widely used gaseous carbon sources. The molecules decomposedfrom polystyrene could be easily dehydrogenated into activated sp2structured carbonrelated radicals, which were helpful for promoting the formation of graphene at lowtemperature. This result also indicates that the heating temperature of solid precursor is acritical factor to control the number of graphene layers. The densities and motions of theactive radicals play a critical role in the overall synthesis of the monolayer graphene.Fabrication techniques for wafer scale graphene are already available. However, thequality of graphene in wafer scale differs from that of normal silicon as it is mainlypolycrystalline with typical grain size in the range of microns or tens of microns. Tosynthesize continuous graphene film with large grain size is still a great challenge. In thisstudy, we demonstrated a simple but efficient strategy to synthesize millimeter-sizedgraphene single crystal grains by regulating the supply of reactants in chemical vapordeposition process. The gradual increase in the temperature of carbon source and theflow rate of hydrogen were adapted to drive the continuous growth of graphene grains.Meanwhile, the nucleation density of graphene grain was controlled as lower as¹00nuclei/cm2, and the dimension of single crystal grain could grow up to¹.2mm. Fieldeffect mobility of about5000-8000cm2V^-1s^-1was obtained, suggesting that the samplessynthesized by polystyrene are of high quality. The strategy presented here provides verygood controllability and enables the possibility for large single graphene crystals, whichis of vital importance for practical applications. CVD growth of graphene is usually performed under950^-1050oC. Lowertemperature growth is obviously valuable for low cost graphene synthesis especially forlarge scale graphene films. In this thesis, we systematically studied the effect of artificialseeds containing cyclobenzene groups for graphene synthesis at low temperature byCVD. Coronene was introduced to optimize the graphene nuclei density and graphenedomain size. Moreover, a new three-step growth procedure was proposed for goodcontrol of the nucleation, domain growth and domain connection stages, which can beapplied for synthesis large area graphene at low temperature. Based on this growthprocedure, continuous graphene film （²cm×2cm） with desirable quality can be obtainedusing naphthalene as graphene precursors under400oC. Its Raman spectrum shows thehigh quality evidenced by2D peak fitted by a single Lorentzian curve with full width athalf maximum （FWHM） of³8.5cm^-1and I2D/IGof¹.3. The channel field effectmobility is⁹84cm2V^-1s^-1and it shows a weak p-type behavior, suggesting that thesamples synthesized at400oC are of reasonable quality. The low temperature graphenesynthesis may pave the way for low cost large scale graphene fabrication, and for softsubstrates, especially for polymer substrates.Chemical doping with foreign atoms is an effective method to intrinsically modifythe properties of host materials. In this thesis, a facile strategy to prepare nitrogen andboron doped monolayer graphene by using urea and boric acid as solid precursors wasused. By adjusting the doping elemental precursors, the nitrogen content could bemodulated from0.9to4.8%for nitrogen doped graphene and boron content from0.7to4.3%for boron doped graphene respectively, as estimated by X-ray photoelectronspectroscopy. The mobilities of the nitrogen and boron doped graphene-based back-gatefield-effect transistors are about350-550cm2V^-1s^-1and450-650cm2V^-1s^-1respectively.Therefore the synthesis of nitrogen and boron doped graphene sheets by solid dopingelemental precursor are considered to be an efficient approach to producing graphenewith excellent optical and electrical performance at relatively low cost.After graphene with good structure and optical performance were synthesized bysolid carbon source, silver nanoparticles were then deposited on the graphene by a citratereduction method. The interaction between graphene and silver was investigated bysuface-enhanced Raman scattering spectra and X-ray photoelectron spectroscopy. Thechange in the G band position indicates n-type doping of the graphene due to an interaction between the silver and the graphene. Silver interlayer doped four-layergraphene shows a sheet resistance of63/sq and a light transmittance of85.4%at550nm wavelength. The optical and electrical quality of the doped multilayer grapheneexceeds the minimum value of the industry standard for indium tin oxide replacementmaterials. Because of the low cost solid carbon source and Ag doping techniques, theCVD process enables inexpensive and efficient synthesis of high quality graphene filmsover large areas, which will be helpful to facilitate a wide range of cost reduction and apractical application of graphene.