  Boron nitride (BN) is today considered as one of the most promising materials
for many novel applications including bright single photon emission, deep UV
opto-electronics, small sized solid-state neutron detector, and
high-performance two-dimensional materials, etc. Despite the recent successful
fabrication of large-area BN single-crystals (typically <= 5 atomic layers),
the scalable growth of thicker single-crystalline BN films still constitutes a
great challenge. In this work, we demonstrate an approach to grow large-area
multilayer single-crystal BN films by chemical vapor deposition on
face-centered cubic Fe-Ni (111) single crystal alloy thin films with different
stoichiometric phases. We show that the BN growth is greatly tunable and
improved by increasing the Fe content in single-crystal Fe-Ni (111). The
formation of pyramid-shaped multilayer BN domains with aligned orientation
enables a continuous connection following a layer-by-layer,
'first-meet-first-connect', mosaic stitching mechanism. By means of selected
area electron diffraction, micro-photoluminescence spectroscopy in the deep UV
and high-resolution transmission electron microscopy, the layer-by-layer
connection mechanism is unambiguously evidenced, and the stacking order has
been verified to occur as unidirectional AB and ABC stackings, i.e., in the
Bernal and rhombohedral BN phase.
