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A general theoretical abrupt interface between two insulating materials is neutral and "polar"; it contains an interface charge component σ0 that emerges when the interface is forced into an insulating state by removing high energy carriers. General experimental interfaces are "polar-compensated", defects at the interface allow the ground state to be nearly neutral, nearly insulating, and to have σ0 ≈ 0.
While polar-compensated interfaces may perhaps always have greater stability than non-compensated polar interfaces, interfaces with localized free carriers, a 2D electron gas/liquid, were first constructed in 2004 between perovskite oxides SrTiO3 and LaAlO3. This was the first hint that a nominally ubiquitous interface category might actually exist. Here we use density functional theory to calculate properties of polar and compensated interfaces between semiconductors Si and ZnS. Bimodal behavior of the valence band offsets (dipole-related) combined with experimental results suggests that careful growth by molecular beam epitaxy shows some selectivity of compensating defect microstructure. We also examine the various polarization and free components of the interface monopoles that exists in finite/repeated structures. Results of the modern theory of polarization help complete the dissection of interface charge.