Figure 1
From: Spin-polarized Wide Electron Slabs in Functionally Graded Polar Oxide Heterostructures

(a) Polarization field directions, (b) schematic distribution of polarization dipole and (c) distribution of resultant carriers in the linearly graded ZnMgO/ZnO heterostructure in the [0001] orientation.
PSP and PPB represent the spontaneous and piezoelectric polarizations, respectively. The parameter c is the lattice constant, σπ is the polarization-induced charges on top of the surface and NDπ is the sheet density of positive unbalanced polarization charges.