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By means of density functional theory (DFT) calculations we havequantitatively estimated the impact of the spontaneous polarization (SP) of theSiC(0001) substrate on the electronic properties of quasi-freestanding graphene(QFG) decoupled from the SiC by H intercalation. To correctly include withinstandard DFT slab calculations the influence of the SP, which is a bulkproperty, on a surface confined property such as the graphene's doping, weattach a double gold layer at the C-terminated bottom of the slab whichintroduces a metal induced gap state that pins the chemical potential withinthe gap. Furthermore, expanding the interlayer distances at the bottom of theslab creates a local dipole moment which counters that arising from the slab'spolar character and allows to align the location of the graphene's Dirac point(DP) for cubic SiC(111) with the chemical potential. Thus, the DP shiftsobtained for other polytypes under the same slab model become an almost directmeasurement of the SP-induced doping. Our results confirm the recent proposalthat the SP induces the experimentally observed p-type doping in the graphenelayer which can achieve DP shifts of up to several hundreds of meV (orequivalently, $\sim 10^{13} e$/cm$^2$) for specific polytypes. The doping isfound to increase with the hexagonality of the polytype and its thickness. Forthe slab thickensses considered (6-12 SiC bilayers) an ample, almostcontinuous, range of doping values can be achieved by tuning the number ofstacking defects and their location with respect to the surface. The slab modelis next generalized by performing large scale DFT calculations whereself-doping is included in the QFG via point defects (vacancy plus a H atom)thus allowing to estimate the interplay between both sources of p-doping (SP-versus defect-induced) which turns out to be essentially additive.

Spontaneous polarization induced doping in quasi-free standing epitaxial graphene on silicon carbide from density functional theory