Controllable Preparation And Properties Of Epitaxial Graphene Grown By Thermally Decomposition Of Silicon Carbide

Graphene is a kind of new two-dimensional material with a hexagonal honeycombstructure. Due to its high carrier mobility, high current density and bipolar field effect,graphene has been regarded as a promising material in microelectronic applications.Graphene grown by thermally decomposition of SiC is considered as one of the mostperspective methods due to its advantages such as large-area epitaxy, high crystallinequality, transfer free, and compatible with the microelectronic processing. The presentdissertation focuses on the epitaxial growth of graphene and its electrical properties.Firstly, we design and commit to manufacture a growth system for epitaxial graphene.Secondly, after we systematically investigate the controllable growth of epitaxialgraphene films, we propose a new raphene growth method with little perturbation oftemperature and gas flow during the growth period. Finally, the liquid-phase doping andintercalation, and the anomalous photoelectric effect of the epitaxial graphene weresystematically studied. The main results are as follows.1. We design and commit to manufacture an ultra-high vacuum （UHV） system forgrowth of epitaxial graphene. The highest working temperature and ultimate vacuum are2200℃, and3×10-6Pa, respectively. The temperature and gas flux can be accuratelycontrolled. The system can satisfy the requirements of the hydrogen etching andcontrollable graphitization process for SiC. High quality, epitaxial graphene films havebeen grown on SiC substrates by using this system.2. Growth of graphene with controllable morphology and thickness wasinvestigated. For Si-terminated SiC, the morphology and thickness of graphene can bemodulated in wide-range by varing the graphitization temperature, and the thickness ofgraphene can be further tuned a little bit by varying graphitization time, with the0.2atomic layer accuracy. So, the growth of graphene with controllabe morphology andthickness can be realized by changing the graphitization temperature and time. ForC-terminated SiC, although graphene film with thickness less than2monolayers isdifficult to be achieved by only varying the graphitization temprature and time, the thickness of graphene films can be reduced down to1monolayer by oxygen plasmaetching.3. We propose a new graphene-grown method with little perturbation oftemperature and gas flow （PTGF） during the growth period. The structure and electricalproperty of graphene prepared by PTGF method have been systematically investigated.The results reveal that by using PTGF method, the growth rate decreases and thedomain size increases significantly, and the electrical performance is improved. Themain reasons are as follows. In a relative closed growth environment, the partialpressure of silicon vapor will increase which will decrease the rate of thermaldecomposition, resulting in the decrease in growth rate of graphene or the increase inthe growth temperature. Moreover, the uniformity of temperature and homogeneity ofgas flow in a relative closed environment is improved. Lower growth rate or highergrowth temperature, and better uniformity of temperature and homogeneity of gas flow,make graphene film prepared by PTGF method with higher crystalline quality, largerdomains, and lower defect concentration, resulting in the improvement in its electricalproperties.4. Doping effect of nitric acid on properties of epitaxial graphene was investigated.The results indicate that oxidation-reduction reaction between nitric acid and epitaxialgraphene grown on C-terminated SiC results in a p-type doping and decrease in sheetresistance, similar to that of CVD graphene, while nitric acid has anomalous dopingeffect on epitaxial graphene grown on Si-terminated SiC and it leads to the n-typedoping and increase in sheet resistance. The main reasons are as follows. Due to thepresence of interfacial buffer layer between the epitaxial graphene and SiC substratem,the heat released due to the oxidation-reduction reaction of nitric acid causes thedesorption of oxygen from the dangling bonds, which will enhance the coupling effectbetween the interface and substrate and scattering centers, resulting in n-type dopingand increase in the electron concentration and sheet resistance.5. A novel liquid-phase fluorine intercalation method for Si-terminated SiC wasproposed. At room temperature and in the air, by utilizing the intercalation reactionbetween H2MoF8（produced by chemical reactions between HF, HNO3and Mo） andepitaxial graphene on Si-terminated SiC, the bonds were ruptured and fluorine atomswere intercalated into the interface. Our method is very simple and effective to decouple the graphene layer from substrate, and transform partial buffer layer to graphene layer.The mobility of the fluorine-intercalated graphene was increased due to the decouplingeffect.6. The anomalous photoelectrical properties of epitaxial graphene on Si-terminatedSiC was investigated. Photoresponse characteristics of five kinds of graphene films, i.e.epitaxial graphene grown on Si-and C-terminated SiC, CVD graphene films transferredonto SiO₂/Si, STO and Si-terminated SiC, have been studied systematically. The resultsshow that epitaxial graphene on Si-terminated SiC exhibits anomalous photoelectricalproperties. Upon illumination of xenon lamp, the photocurrent of epitaxial graphenegrown on Si-terminated SiC significantly increases by147%, while the photocurrents ofthe others slightly decrease by less than3%. The anomalous photoresponsecharacteristics result from the photo-induced desorption of oxygen on the danglingbonds of interfacial buffer layer between epitaxial graphene and Si-terminated6H-SiC.