  Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN)
has gained tremendous research interest over recent years due to its
unconventional domain growth morphology, fascinating properties and application
potentials as an excellent dielectric layer for 2D-based nano-electronics.
However, the unidirectional domain growth of h-BN thin films directly on
insulating substrates remains significantly challenging because of high-bonding
anisotropicity and complex growth kinetics than the conventional thin films
growth, thus resulting in the formation of randomly oriented domains
morphology, and hindering its usefulness in integrated nano-devices. Here,
ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically
smooth highly insulating c-plane sapphire substrates (without using any metal
catalytic layer) by pulsed laser deposition, showing remarkable unidirectional
triangular-shape domains morphology. This unidirectional domain growth is
attributed to the step-edge guided nucleation caused by reducing the
film-substrate interfacial symmetry and energy, thereby breaking the degeneracy
of nucleation sites of random domains, as revealed by the density functional
theory (DFT) calculations. Through extensive characterizations, we further
demonstrate the excellent single crystal-like functional properties of films.
Our findings might pave the way for feasible large-area direct growth of
electronic-quality h-BN thin films on insulating substrates for
high-performance 2D-electronics, and in addition would be beneficial for hetero
engineering of 2D-vdW materials with emergent phenomena.
