Rethinking how band gap changes in nitride alloys enable design of wurtzite ferroelectrics

Wide- and ultrawide-band gap wurtzite III nitrides are commonly alloyed with group-3 elements such as Sc, Y, and Gd to reduce the polarization switching coercive field ( Ec E c ) for ferroelectrics. Because group-3 nitrides typically adopt the rocksalt structure with relatively small band gaps, alloying is generally expected to reduce both the band gap ( Eg E g ) and dielectric breakdown field ( Eb E b ). However, recent reports of band gap widening in Ga ₁₋ₓ 1 - x Sc ₓ x N and In ₁₋ₓ 1 - x Sc ₓ x N challenge this assumption. Using DFT calculations, we show that band gaps in heterostructural III-nitride alloys are governed not only by the ground-state polymorph but also by structurally relevant polymorphs of the alloying end member. Extending the analysis to AlN-, GaN-, and InN-based alloys with Sc, Y, La, and B, we identify three distinct band gap regimes that follow wurtzite-, hexagonal-BN-, or rocksalt-like trends. These behaviors are governed by bond ionicity and structural distortion, providing design guidelines for engineering Eg E g and implication for Eb E b in wurtzite III-nitride alloys.