Unveiling Graphitic Carbon Nitride in Hexagonal Boron Nitride Epitaxy on Sapphire

Epitaxial growth of hexagonal boron nitride (hBN) on sapphire is of fundamental importance for the development of van der Waals heterostructures, yet the mechanism of crystallographic alignment across a weakly bonded interface remains unresolved. Here, we demonstrate that graphitic carbon nitride (g-C₃N₄), unintentionally formed during Metal-Organic Vapor Phase Epitaxy (MOVPE) from triethylboron and ammonia precursors, plays a pivotal role in transferring lattice orientation from the Al₂O₃ substrate to the overgrown hBN film. High-resolution transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, photoluminescence, and X-ray photoelectron spectroscopy consistently reveal a nanometer-thick interfacial region composed of AlN:O and g-C₃N₄. Despite its partial amorphization upon post-growth cooling, the layered structure of g-C₃N₄ provides the necessary structural flexibility to accommodate lattice mismatch and transmit crystallographic information across the interface. This finding establishes g-C₃N₄ as an effective buffer enabling epitaxial growth of two-dimensional hBN on a three-dimensional substrate. These insights suggest that the presence of carbon at the growth interface, often regarded as detrimental, can in fact be critical for achieving wafer-scale, high-quality epitaxial hBN, opening new directions for interface engineering in 2D material synthesis.