COUPLED SPIN-CHARGE TRANSPORT IN DOPED-GRAPHENE

Graphene, a single sheet of carbon atoms, is an attractive two-dimensional material due to electronic characters described with massless Dirac equation and has been widely studied in various field, such as semiconductor, photonics, and biotechnology. In particular, graphene has emerged as a leading candidate for electronic and spintronic device application because of the high electron mobility and very weak spin-orbit coupling. For electronics, graphene can replace silicon-based transistor due to high mobility, structural stability, and outstanding thermal conductivity if the band gap of graphene can be engineered. Graphene is also promising spin transporting channel because it has a long spin relaxation length and time. In addition, the disordered graphene with vacancy, adatom, and proximity effect by substrate can have unique electronic characteristics, such as a strong localization, a magnetic ordering, a large spin-orbit coupling, and topological edge state. These properties can lead to unprecedented spindependent transport features. Therefore, understanding spin and charge transport properties in disordered graphene is very important for the graphene spinrtonics and electronics. In this thesis, I studied the spin and charge transport properties in graphene with adatoms and focusing on the spin Hall effect originated from the implanted spin-orbit interaction. In Au-clustered graphene, the dominance of the spin Hall effect which induced nonlocal resistance was observed at the particular carrier concentration. The presence of the spin-Hall induced nonlocal resistance was further confirmed through the Hanle curve and its temperature dependence. The behavior of spin relaxation time obtained from Hanle curves is consistent with the determined spin-orbit coupling symmetry, which is asymmetric near the Dirac point and symmetric at higher concentrations. Based on these results, I suggest that spinvalley relation in Au-decorated graphene can cause the gate-dependent spin current from the spin Hall effect with the enhanced spin-orbit coupling. Also, I researched localization and quantum edge state of disordered graphene prepared with doping with magnetic impurity Fe. The disordered graphene displayed strong localization and quantum spin edge state with splitting of Landau zeroth level. These phenomena could be understood with topological Anderson insulator state. This thesis is organized as follows: In chapter I, II and III, I reviewed a basic knowledge for graphene, graphene spintronics and spin Hall effect from the text book and related papers for transport study in doped graphene. In chapter IV, I described fabrication of the graphene Hall bar device together with graphene growth, wet-transfer, e-beam and photo lithography, and measurement methods. In chapter V, I discussed gate dependent nonlocal spin Hall resistance in Au-clustered graphene and discussed the behavior of nonlocal resistance and magneto-conductivity in the sample. Finally, I presented metal insulator transition and quantum Hall edge state based on the gate voltahe dependent resistivity magneto-transport properties, especially at charge neutral point in Al2O3 / Fe / graphene system and discussed the results in chapter VI.