Origin of the Double Polarization Mechanism in Aluminum-Oxide-passivated Quasi-free-standing Epitaxial Graphene on 6H-SiC(0001)

In this work, we synthesize a 43 nm thick layer of amorphous Al2O3 on transfer-free p-type hydrogen-intercalated quasi-free-standing (QFS) epitaxial chemical vapor deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001), using trimethylaluminum as the source of aluminum and deionized water or oxygen plasma as the source of oxygen. With a combination of surface-sensitive X-ray reflectometry (angular range of 0-6 degrees), spectroscopic ellipsometry (angle of incidence of 49 degrees and lambda = 490 nm), and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, we draw depth profiles of the physical density, refractive index, and elemental composition to reveal that the physical properties of the oxide are inhomogeneous. Its density increases from the graphene interface (rho = similar to 1.5 g/cm(3)) toward the surface (rho = similar to 3.0 g/cm(3)). Characteristically, it takes 26 nm for the oxide density to equal its reference value on a test silicon substrate. We demonstrate that an additional postdeposition annealing step in an oxygen-radical-reach environment densifies the oxide and elevates its oxygen content, thus suggesting that the a-Al2O3 is oxygen deficient. We reason that the structural inhomogeneity induces the negative polarization effect and the associated reduction of intrinsic hole density observed in a-Al2O3-passivated p-type QFS graphene on hexagonal SiC(0001).