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. We find that the polar wurzite (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 epitaxially tensile strained superlattice. Due to the elastic constant C33 softening along with an increase in e33, the piezoelectric coefficient d33 of the superlattice is doubled compared to pure w-AlN. The combined enhancement of Born effective charges (Z33) and the sensitivity of the atomic co-ordinates to external strain (\frac{\partial u_{3}}{\partial\eta_{3}}) is the origin of large piezoelectric constant e33. Moreover, we show that epitaxial biaxial tensile strain significantly enhances the piezo-response, so that d33 is seven times larger than that of w-AlN at 4% strain. The tensile strain results in a huge enhancement in e33by increasing Z33 and \frac{\partial u_{3}}{\partial\eta_{3}}, which boosts the piezoelectric coefficient. As both superlattice growth and epitaxial strain are already experimentally demonstrated in wurzite nitrides, our results show a new, more controlled approach to significantly enhance and tune the piezoelectric response of w-AlN materials.