The Fabrication, Optimization And Quantum Coherent Transport Of The Micro-devices Based On Graphene, Bi₂Se₃ And Cd₃As₂

Recently, Dirac materials have got intense attention in condensed matter physics. The low-energy excitations of those materials do not obey the Schrodinger equation but the Dirac equation. There are several material-specific symmetries which protect the formation of the Dirac points, resulting in a gapless state. In graphene, this symmetry is a consequence of the two unequivalent honeycomb sublattices of carbon atoms. In topological insulators, it is the time-reversal symmetry. It induces the gap inverse in the band structures and thus produces the topological surface state. In three-dimensional (3D) Dirac semi-metals, time-reversal symmetry and inversion symmetry are present. They are crucial for breaking the degeneracy at the Dirac point. This is because it consists of two Weyl points with opposite chirality, which easily annihilate each other without protect. As typical two-dimensional (2D) Dirac electronic materials, Graphene and topological insulators have been hot topics in both fundamental research and application in recent years. Many novel phenomena have been reported, such as half-integer Quantum Hall effect, Quantum Spin Hall Effect and Quantum Anomalous Hall Effect and Majorana Fermion physics and Magnetic Monopole Effect etc. Besides, the most profound feature that controls the electronic transport of Dirac materials is the suppressed back-scattering due to the π Berry phase of Dirac fermion. It also makes themselves as rising stars in the spintronics and quantum computing. During my Ph.D. times, the Dirac materials is advancing fast from the Quantum spin Hall effect to graphene, then to the topological insulator, and later to the 3D Dirac semi-metals. My work therefore mainly focuses on the quantum transport properties in topological insulators, graphene and Cd3As2 films. The main results are summarized as followed:(1) We have successfully synthesized Sb doped Bi2Se3 nanoplates by the hydrothermal method. The carrier density is suppressed, enhancing the contribution of the electron transport from the surface state. The weak antilocalization and universal conductance fluctuations have been observed in the magnetic transport measurement. Based on detailed analysis, the electron-phonon interaction is suggested to play an important role in the dephasing procedure as well as the temperature dependent resistivity.(2) We set up an in situ fabrication procedure to process the CVD graphene. It reduces the irradiation damage of the electron beam during the electron beam lithography as confirmed by the absence of the D peak in Raman spectra. We in-situ exfoliate the graphene sheets by Al film. Besides, the magnetoresistance of 350% is observed in bilayer graphene. After detailed analysis, we found that the phenomenon is due to the charge impurities in bilayer graphene under the higher temperature, at which the Landau quantization is suppressed. Those charge impurities may be the Al clusters(3) We have measured electronic transport of Bi clusters decorated graphene sheets. The 2nm-thick Bi films are evaporated onto the graphene sheet. After exposing in the ambient air for 10 days, the Bi films are completely oxidized and condensed into clusters. It induces the holes doping and thus increase the carrier density. By analysis the SdH oscillation, we found the short-range scattering is enhanced. But it can be suppressed after annealing at 400K for 20 minutes.(4) The weak-antilocalizaiton effect is both observed while applying perpendicular or parallel magnetic field in a～50nm-thick Cd3As2 film. The Cd3As2 films show an insulating behavior due to quantum confinement effect. The electron-electron interaction is addressed as the source of the dephasing based on the temperature-dependent scaling behavior. The weak antilocalization can be observed while the magnetic field is parallel to the electric field, which is attributed to the strong interaction between the different conductance channels in this quasi-2D film.