Enhancing the piezoelectric performance of nitride thin films through interfacial engineering

Abstract

Scandium-doped aluminum nitride (ScAlN) with a wurtzite crystal structure exhibits piezoelectricity and ferroelectricity, and its application potential in micro-electromechanical devices is actively being investigated. One strategy to improve the piezoelectric/ferroelectric properties is to extend the solubility of Sc in wurtzite-type ScAlN. Herein, we demonstrated that introducing a lutetium (Lu) buffer layer with a hexagonal close-packed structure improved the crystallinity and c-axis orientation in wurtzite-type Sc_xAl_1-xN thin films containing x = 0.508. The stabilization of the wurtzite phase was attributed to the epitaxial strain caused by the lattice matching between the Lu buffer layer and the ScAlN layer, resulting in an unprecedented piezoelectric constant of 35.5 pC/N, surpassing the previous 31.6 pC/N for x = 0.410. This value is an extension of the Sc concentration dependence predicted by first-principles calculations, suggesting that supersaturated Sc doping caused further elastic softening. Our results highlight interfacial engineering with lattice-compatible buffer layers as a straightforward and effective strategy to unlock the piezoelectric performance of ScAlN.