Polar Wurtzite NbAlN: A Transition-Metal Nitride Alloy for Polarization-Engineered GaN Heterostructures.

The discovery of ferroelectric ScAlN sparked a search for new polar wurtzite alloys; however, incorporating transition metals remains a formidable challenge. Here, we demonstrate the synthesis of wurtzite-phase NbAlN, a polar transition-metal nitride alloy that is highly compatible with GaN-based heterostructures. Despite the strong metallic character of elemental Nb, we achieved coherent epitaxial NbAlN films that preserve the wurtzite crystal structure and metal polarity of the underlying GaN, as directly visualized by annular bright-field scanning transmission electron microscopy. Nb incorporation induces a monotonic lattice expansion and produces coherent Nb-enriched nanoscale regions that are accommodated within the continuous wurtzite framework. Functionally, NbAlN barriers induce a substantial enhancement in the two-dimensional electron gas density at the AlN/GaN heterointerface, consistent with polarization-related carrier modulation by the metal-polar NbAlN barrier. This study demonstrates that d-electron transition metals can be integrated into polar nitride systems without compromising their functional asymmetry, making NbAlN a promising building block for polar nitride heterostructures and carrier-density-engineered GaN electronics.