In-plane ferroelectricity and enhanced piezoelectricity in ultrathin AlN (1010) surfaces

-and a paraelectric hexagonal phase with zero polarization, forming a characteristic triple-well energy landscape with substantial switching barriers. Substituting Sc at the Al sites provides an effective means to tune these barriers, significantly lowering the energy cost of polarization reversal and enabling controllable ferroelectric performance for targeted applications. Furthermore, the ultrathin films exhibit strong in-plane piezoelectric responses, which are further enhanced by Sc alloying. These results highlight Sc-doped w-AlN layers as a promising platform for low-dimensional ferroelectric and piezoelectric devices.