Magnetotransport behavior of epitaxial graphene inhomogeneously doped by Bi(110) islands

The concept of proximity coupling is a promising approach to specifically modify the properties of epitaxial graphene layers. In order to introduce spin-orbit coupling into graphene, we have deposited Bi with an average thickness of up to 3.6 bilayers (BL) on epitaxial monolayer graphene (MLG) on SiC(0001) and perform magnetotransport measurements in magnetic fields up to 4 T. Upon adsorption, epitaxial Bi(110) islands are formed, which change the initial n-type doping of MLG locally. The formation of inhomogeneous carrier concentration profiles on MLG results in a positive and linear magnetoresistivity effect with increasing Bi coverage. Along with this, the slope of the Hall resistivity decreases, suggesting a rise of the carrier concentration. However, by extracting the carrier concentration from the Shubnikov--de Haas oscillations, we confirm that the carrier concentration in the uncovered regions remains constant and that the change of the Hall slope is solely an effect of the inhomogeneity. Also, the conductivity of MLG has not changed drastically and even at coverages as high as 2.4 BL the mobility in the noncovered region is reduced by only about 15% of its original value. Moreover, the signatures of weak localization in the magnetoresistivity vanish with increasing Bi coverage, while no signs of weak antilocalization were found at all. Apparently, the proximitized Bi islands induce a well-defined lateral doping profile so that the electrons are not penetrating into the areas of the Bi islands, thus mimicking antidots, but are reflected at their edges. This scattering process seems to be phase breaking, thus suppressing the weak localization effect. Our results show clearly that both the coupling but also the homogeneity at the interface is crucial for proximity coupling.