Ferroelectricity from Spontaneous Symmetry Breaking in Amorphous BN-Based Thin Films Grown via Atomic Layer Annealing.

Boron nitride (BN) thin film growth typically requires high temperatures (>600 °C), which limits integration with patterned Si-based electronic devices. In this work, we demonstrate that atomic layer annealing (ALA) enables the growth of amorphous BN thin films at a relatively low temperature of 350 °C. Despite BN being centrosymmetric in its most stable hexagonal phase (h-BN), spontaneous symmetry breaking in the ALA-grown films results in a clear, switchable ferroelectric polarization, as confirmed by Positive-Up Negative-Down (PUND) measurements and a piezoresponse force microscopy (PFM) DC poling response consistent with ferroelectric behavior. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) show that, while the films are predominantly composed of boron and nitrogen, minor incorporation of carbon and oxygen potentially contributes to local symmetry breaking in the BN structure. Density functional theory (DFT) calculations show that external electric fields induce atomic displacement in h-BN, breaking centro-symmetry and leading to nonzero spontaneous polarization. These results highlight ALA as a viable low-temperature route for realizing functional BN thin films exhibiting emergent ferroelectricity for electronic device applications.