Ferroelectricity and Large Piezoelectric Response of AlN/ScN Superlattice

Based on density functional theory, we investigate the ferroelectric and piezoelectric properties of the AlN/ScN superlattice, consisting of ScN and AlN buckled monolayers alternating along the crystallographic c-direction. We find that the polar wurtzite (w-ScAlN) structure is mechanically and dynamically stable and is more stable than the nonpolar hexagonal flat configuration. We show that ferroelectric polarization switching can be possible for an epitaxially tensile-strained superlattice. Because of the elastic constant C33 softening, together with an increase in e33, the piezoelectric coefficient d33 of the superlattice is doubled compared to that of pure w-AlN. The combined enhancement of Born effective charges (Z33) and sensitivity of the atomic coordinates to the external strain (∂u3∂η3) is the origin of the large piezoelectric constant e33. Moreover, we show that the epitaxial biaxial tensile strain significantly enhances the piezo-response, so that d33 becomes 7 times larger than that of w-AlN at 4% strain. The tensile strain results in a huge enhancement in e33 by increasing Z33 and ∂u3∂η3, which boost the piezoelectric.