Ferroelectric Fractals: Switching Mechanism of Wurtzite AlN

Abstract: The advent of wurtzite ferroelectrics is enabling a new generation of ferroelectric devices for computer memory that has the potential to bypass the von Neumann bottleneck, due to their robust polarization and silicon compatibility. However, the microscopic switching mechanism of wurtzites is still undetermined due to the limitations of density functional theory simulation size and experimental temporal and spatial resolution. Thus, physics-informed materials engineering to reduce coercive field and breakdown in these devices has been limited. Here, the atomistic mechanism of domain wall migration and domain growth in wurtzites is uncovered using molecular dynamics and Monte Carlo simulations of aluminum nitride. We reveal the anomalous switching mechanism of fast 1D single columns of atoms propagating from a slow-moving 2D fractal-like domain wall. We find that the critical nucleus in wurtzites is a single aluminum ion that breaks its bond with one nitrogen and bonds to another nitrogen; this creates a cascade that only flips atoms directly in the same column, due to the extreme locality (sharpness) of the domain walls in wurtzites. We further show how the fractal shape of the domain wall in the 2D plane breaks assumptions in the KAI model and leads to the anomalously fast switching in wurtzite structured ferroelectrics.