Hexagonal boron nitride (h-BN), an insulating two-dimensional layered material, has recently attracted a great attention due to its fascinating optical, electrical, and thermal properties, and promising applications across the fields of photonics, quantum optics, and electronics. However, mechanically exfoliated bulk h-BN and h-BN films grown on catalytic metal substrates have been mainly used to study the fundamental properties, lacking in scalability for practical implementation of h-BN.

Here, we exploit the scalable approach to grow high-quality h-BN on Si-based nano-trenches and epitaxial gallium nitride (GaN) substrates by using metal-organic chemical vapor deposition (MOCVD). Firstly, the conformal growth of sp2hybridized few-layer h-BN over an array of Si-based nanotrenches with 45 nm pitch and the aspect ratio of ~ 7:1 was successfully accomplished by using pulsed-mode MOCVD. Surface-sensitive near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory calculations reveal that the B-O bonds formed on the non-catalytic SiO2 surface act as nucleation sites for the formation of mixed sp2-and sp3-hybridized BON2 and BN3 at the very initial stage of the pulsed-mode injection of MOCVD precursors, enables the conformal growth of few-layer sp2-hybridized h-BN with an excellent step coverage. We believe that these results can provide a broad avenue for the implementation of fascinating 2D materials for current state-of-the-art 3D Si-based architectures, overcoming the down-scaling limit. Secondly, it was found that under specific MOCVD growth conditions, a unique few-layer h-BN film can be grown on GaN substrates, in which the few-layer h-BN film is suspended on GaN nanoneedles. The combination of state-of-the-art microscopic and spectroscopic analyses, which includes fifth-order aberration-corrected scanning transmission electron microscopy, second harmonic generation, second-order resonant Raman scattering, and photoluminescence spectroscopy in the deep-ultraviolet range, revealed that the suspended h-BN films exhibit unprecedented atomic stacking. The mechanism underlying the formation of unique atomic stacking will be investigated through structural and electrical characterizations, as well as theoretical modeling. Our findings unveil new perspectives for the scalable synthesis of engineered h-BN polytypes.