Huge Piezoelectric Response of LaN-based Superlattices

We construct LaN-based artificial superlattices to investigate the ferroelectricity and piezoelectricity using the volume matching conditions of the parent components that soften the elastic constant C₃₃ and increase the piezoelectric modulus d₃₃. The proposed superlattice consists of LaN and YN (or LaN and ScN) buckled monolayers alternately arranged along the crystallographic c-direction. The structure of polar wurtzite (w-LaYN/w-LaScN) is both mechanically and dynamically stable, and the computed energy barrier makes the ferroelectric polarization switching possible. We show that the epitaxial strain can modify the spontaneous ferroelectric polarization as well as d₃₃. The LaN/YN superlattice exhibits a huge piezoelectric response in the unstrained state, due to their small c/a value and extremely soft C₃₃. In addition, the epitaxial strain is revealed as effective control of the nature (indirect and direct) and value of the electronic band gap.