An Ammonization-Based Transformation of Hexagonal Boron Nitride on Ir(111) from Surface to Near-Surface Regions

Epitaxial growth of thin films on solid substrates significantly depends on the surface structure, including outmost and near-surface reconstruction. Here, using in situ surface measurements, we show an ammonization-based hexagonal boron nitride (h-BN) transformation on the Ir(111) substrate, from surface to near-surface regions. Near-surface B dopants are driven to segregate onto the surface induced by N atoms from NH₃ dehydrogenation; thus, B species react with N species to form h-BN overlayers on the surface, i.e., the ammonization growth route. Further, growth dynamics in near-surface-dependent ammonization and normal chemical vapor deposition growth are systematically investigated. The activation energy of the former is 0.53 eV higher than the latter. The difference in growth barriers is attributed to the diffusion of B species from near-surface to surface regions, causing the growth mechanism changes from reaction-limited aggregation to diffusion-limited aggregation. In addition, h-BN overlayers are converted into boron oxide (B₂O₃) in an O₂ atmosphere and reverse back to h-BN by reacting with NH₃ on the surface; thus, a reversible structural transformation on the surface was achieved by successive oxidation and ammonization. This work gains a deep understanding of the h-BN conversion from surface to near-surface regions and provides valuable insights into their evolution dynamics.