First-principles study of structural, electronic, optical, mechanical, piezoelectric, and ferroelectric properties in...

Scandium-doped wurtzite AlN (AlScN) has emerged as a highly promising wide-bandgap semiconductor. However, a complete understanding of the atomistic origins of its simultaneously enhanced electronic, optical, elastic, piezoelectric, and ferroelectric properties remains elusive. Here, using density functional theory (DFT) calculations, we elucidate how Sc doping governs the hierarchical structure-property relationships in AlN. Sc substitution induces anisotropic lattice expansion, softens the phonon spectrum, and promotes strong Sc-d/Al-p/N-p orbital hybridization, leading to band gap reduction and Fermi-surface reconstruction. The resulting narrower band gap, red-shifted absorption edge, and enhanced dielectric polarizability are quantitatively linked to orbital-resolved transition probabilities. Mechanically, continuous Sc incorporation induces shear softening that reduces hardness while retaining intrinsic stiffness anisotropy, thereby circumventing the conventional rigidity-toughness trade-off. Piezoelectrically, the longitudinal strain coefficient (d₃₃) increases by up to sixfold at 50% Sc concentration. Ferroelectric calculations further reveal a 50% reduction in the polarization switching barrier for Al_0.667Sc_0.333N, consistent with experimental observations of room-temperature ferroelectricity in AlScN. By integrating these multiphysics insights, we propose a theoretical framework that correlates Sc-induced orbital hybridization with the simultaneous modulation of electronic, piezoelectric, and ferroelectric properties, providing quantitative guidelines for engineering AlScN-based multifunctional materials.