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 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 , which boost the piezoelectric.