Elastic-phonon softening mediated ferroelectric properties in AlScN: A first-principles study

Aluminium nitride (AlN) has shown great success in device applications, particularly piezoelectric devices. However, introducing ferroelectricity into the system can add dimensions to its device applications, with an increased piezoelectric coefficient, electromechanical coupling, and greater flexibility in tailoring its properties. Scandium (Sc) doping in AlN significantly increases polarisation switching behaviour and, hence, the ferroelectric properties of AlN. This study comprehensively explores the structural, electronic, mechanical, phonon, and potential barriers for exhibiting ferroelectric properties at different Sc concentrations. The phase distortion of the wurtzite lattice and the greater ionic nature of the Sc atom led to phenomena such as elastic and phonon softening, which aid in improving the ferroelectric properties. We report that the limit of Sc alloying for improving ferroelectric properties is between 37.5% and 50%. Through this work, we highlight that the existence of a morphotropic phase boundary near 37% Sc doping can significantly improve the functional properties of AlN.