| dc.description.abstract | Hexagonal boron nitride is emerging as a key component in the application of graphene and other two-dimensional layered material. Hexagonal boron nitride is a good substrate for graphene because of no charge trap on the surface, insulator with bandgap 6.8ev, and the layered structure like graphene. Hexagonal boron nitride is good substrate for the 2D material to enhance the electronic property due to less substrate effect. Many researches use h-BN as substrate to do the FET and improve the electronic property obviously.
The structure of h-BN is B atom and N atom alternately arranged to form a honeycomb structure with the law of hexagonal lattice formation. Usually on Cu substrate, using CVD synthesized h-BN and the shape of h-BN is triangle because the N atom side will grow slower than B atom. In industry, they usually used ammonia borane as precursor because of its non-toxic and non-flammable properties. Because of the property of solidstate precursor, the temperature influence in the ammonia borane is significance, and different heating time cause the ammonia borane decomposition changing.
In the CVD process, the number of layers of h-BN highly depended on the substrate. Copper, gold, Nickle, and Ni-Fe can be the synthesis substrate for h-BN. The solubility of boron and nitrogen depended on the different metal or alloy. In the industry, the V monolayer h-BN can be synthesized by controlling the diffusion rate of boron and nitrogen. For copper, low solubility for boron and insolubility of nitrogen, the h-BN tends to form the monolayer in LPCVD with low source evaporation. For Ni-Fe alloy, the high solubility of boron and nitrogen, h-BN tended to form the multilayer. The h-BN nucleation density can control by the source evaporation by using different temperature or different source weight and flow rate of carrier gas.
The multilayer h-BN can be synthesized by increasing hydrogen flow rate because of hydrogen termination effect. We found that the hydrogen terminated the triangle shape h-BN edge and source will have more chance through under the monolayer to nucleate and grow the multilayer h-BN. The multilayer stacking angle is 0°, 30°, and 60°. Because the B and N are alternatively arranged to form honeycomb structure, the rotation symmetry is 60°.
The monolayer h-BN can be synthesized by controlling the precursor evaporation or adding nitrogen gas into the reaction to increase the growth speed of the nitrogen zigzag edge. However, there are some researchers used DFT to calculate the h-BN will have point defect like boron vacancy under the high nitrogen environment. The defect region will cause the electronic property of h-BN became different, and also the photophysical property. The defect h-BN will have photoluminescence peaks and the wavelength are VI decided by the type of defect, and it is widely used on researches of single photon emitter source in the quantum communication region. The defect caused electronic property changed can be observed by using angle resolved photoemission energy spectrum to measure the valence band of h-BN and found h-BN p-type behavior with nitrogen gas interacting. Because of in the defect region, there will form three electron holes around boron vacancy. In our research, we found that with nitrogen gas adding the growth speed increasing, and the whole piece copper substrate is monolayer fully covered. These result shows the concept to increase the sample yield on decrease the unwanted multilayer hBN.
In this thesis, we synthesized the multilayer and monolayer h-BN by controlling the flow rate of hydrogen and nitrogen and source temperature to observe the multilayer growing. ARPES checked the non-nitrogen, high nitrogen, and low nitrogen synthetization h-BN electronic property. The copper crystalline used EBSD to observe the annealing reconstruction structure of the sample. | en_US |