Atomistic Investigation of the Ferroelectric and Dielectric Properties of AlN-Based Films

Wurtzite aluminum nitride (AlN) thin films are a highly promising for developing next-generation nonvolatile ferroelectric memory, as they exhibit robust ferroelectricity while avoiding the performance-limiting charged oxygen vacancies prevalent in oxide ferroelectrics. This study employs first-principles density functional theory (DFT) calculations to investigate how scandium (Sc) and tantalum (Ta) substitution modulates the key properties of AlN for such applications. Our calculations on Al0.75Sc0.25N and Al0.75Ta0.25N reveal distinct structural responses: Sc incorporation causes isotropic lattice expansion, while Ta doping induces anisotropic distortion. Electronically, both dopants significantly reduce the band gap, but crucially, Ta-substituted AlN exhibits twofold increased dielectric permittivity compared to pure AlN. This permittivity engineering causes a prominent reduction in the coercive field. A semiempirical analysis estimates the coercive field for Al0.75Ta0.25N at ~4.9 MV/cm, which is nearly half that of pure and Sc-substituted AlN (~10.4 and 8.4 MV/cm, respectively). While the remanent polarization in Ta-doped AlN is slightly lower than in pure AlN, it is higher than in the Al0.75Sc0.25N. This optimal combination of a large polarization and a significantly reduced coercive field establishes Ta-substituted AlN as a superior material for low-voltage ferroelectric memory devices.