Hydrogen-driven boron nitride phase differentiation during the epitaxial nucleation on the diamond (001) surface

Cubic boron nitride (cBN) and diamond, sharing identical lattice structures, currently garner significant interest for next-generation high-power, high-frequency electronics. Despite successful heteroepitaxial synthesis of single-crystal cBN on diamond substrates via chemical vapor deposition (CVD), limited understanding of cBN growth blurs the origin of mixed BN phases under some synthesis conditions. Here, employing first-principles calculations based on a nanoreactor scheme, we study the cBN epitaxial nucleation on the diamond (001) surface under a limited-H condition. The discovered mechanism is initiated by inserting BN units into the surface dimer bond, leading to an island formation, which initially expands laterally - along the diamond surface - but rapidly switches to out-of-plane 3-dimensional growth. A high reaction barrier on the surface (~0.4-0.8 eV, 1300 K) aligns with challenging nucleation observed in experiments. Examining the environment hydrogen concentration effect revealed the origin of diverse BN phases experimentally, i.e., hydrogen deficiency favors amorphous BN (aBN) growth, whereas excessive hydrogen significantly raises sp2 bonding fraction, resulting in hexagonal BN (hBN) layers. Our results offer valuable guidance for the controllable synthesis of BN phases and advance research toward potential cBN electronics.