In-plane ferroelectricity and enhanced piezoelectricity in ultrathin AlN (101̄0) surfaces

Using density functional theory, we investigate the in-plane ferroelectricity and piezoelectricity of the w-AlN (101̄0) surface in ultrathin films composed of 2-5 w-AlN layers. We find that these few-layer structures are stable in both a ferroelectric wurtzite phase-with large positive and negative in-plane polarizations reaching approximately 2.265 C m^-2-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.