No strain, no piezoelectric gain: pushing piezoelectric boundaries via composition-dependent strain engineering in wurtzite scandium-doped aluminum nitride

Abstract

This study presents an investigation of the structural and piezoelectric properties of scandium-doped aluminum nitride (Al_1−xSc_xN) alloys over a wide composition range, using first-principles calculations. We explore compositions ranging from pure AlN to Al_0.5Sc_0.5N under various biaxial strain conditions, revealing the complex interplay between composition, strain, and piezoelectric response. Our findings demonstrate that the piezoelectric coefficient d₃₃ of unstrained Al_1−xSc_xN reaches a maximum at x = 0.4375, significantly surpassing that of pure AlN. Remarkably, we show that applying biaxial strain dramatically enhances d₃₃, with values peaking at compositional-dependent critical strains. These peak values, ranging from 519.03 pC N⁻¹ for strained AlN to an extraordinary 5121.58 pC N⁻¹ for strained Al_0.625Sc_0.375N, represent improvements of over two orders of magnitude compared to their unstrained counterparts. We attribute this enhancement to a strain-induced phase transition from wurtzite to non-polar hexagonal layered structures, similar to the structural change observed with enhanced Sc concentration, accompanied by significant changes in elastic and piezoelectric constants. Notably, we demonstrate that this phenomenon can be exploited through changes in both tensile strain in Al-rich compositions and compressive strain in Sc-rich compositions, highlighting the exceptional tunability of Al_1−xSc_xN. These insights provide a fundamental understanding of Al_1−xSc_xN behavior and offer valuable guidance for optimizing its properties in next-generation piezoelectric devices.