  Hexagonal boron nitride is a promising material for many applications ranging
from deep UV emission to an ideal substrate for other two dimensional crystals.
Although efforts towards the growth of wafer-scale, high quality material
strongly increased in recent years, the understanding of the actual growth
mechanism still remains fragmentary and premature. Here, we unveil fundamental
growth mechanisms by investigating the growth of hBN by metalorganic vapor
phase epitaxy (MOVPE) in a wide range of growth conditions. The obtained
results contradict the widespread opinion about the importance of parasitic
gas-phase reactions decreasing the growth efficiency. Two different growth
mechanisms that depend on ammonia flow and reactor pressure can be
distinguished. Both mechanisms are effective in the case of polycrystalline
growth, but the growth of highly ordered, flat layers, is strongly hindered.
The problem is caused by a low efficiency of boron chemisorption on
N-terminated edges of sp2-BN sheets forming the atomic steps on the surface of
the layer. Two-dimensional growth can be activated and sustained by the flow
modulation epitaxy (FME) method, an alternate switching of ammonia and TEB
flows. The success of the FME method is explained in terms of periodic changes
between N- and B-terminated reconstructions at the edges of sp2- BN sheets,
which restore boron chemisorption. The presented results identify the
fundamental growth mechanisms, which is the prerequisite for any further
deterministic development of efficient, high-quality, large-scale hBN growth
with MOVPE.
