Damage Buildup in Ultrathin Boron Nitride Under Ion Bombardment

Two-dimensional hexagonal boron nitride (2D-hBN), also known as white graphene, is an electrically insulating, wide-band-gap (~6eV) material with several emerging applications. Ion irradiation can be used to modify properites of 2D-hBN geared for specific applications. However, the behavior of multilayer 2D-hBN under ion bombardment remains essentially unexplored. Here, we study damage buildup in 2D-hBN films with thicknesses of ~50-500 nm irradiated with 500-3000 keV ions with different masses (He, Ne, Ar, and Xe) with doses up 0.5-2 displacements per atom (dpa). Experiments are complemented by molecular dynamics simulations of the formation and evolution of point defects. Results reveal a monotonic buildup of damage up to amorphization. We discuss the role of BN polymorphism and transitions between sp2 and defect BN phases under ion bombardment. A negligible dose rate effect but pronounced cascade density effects are observed, pointing to a critical role of intracascade defect processes. We compare these results for 2D-hBN films with data for bulk polycrystalline BN ceramics bombarded under the same conditions. These results help us fill in a knowledge gap in the behavior of BN in a displasive radiation environment.