220 | Nature | Vol 579 | 12 March 2020
Article
Achieving the growth of mono-oriented hBN triangular flakes is an
essential step towards obtaining wafer-scale single-crystal hBN. Owing
to the sixfold symmetry of Cu (111), the van der Waals registry of hBN
to Cu (111) leads to two sets of energy-minimal configurations (whose
orientation differs by 60° or 180°, an inversion) with almost degener-
ate binding energies. Thus it is commonly believed that confining hBN
flakes to mono-orientation on such a high-symmetry surface is impos-
sible^5 ,^8. Our experiments reveal that the energy degeneracy can be lifted
in the presence of spontaneously present top-layer Cu step edges. The
growth of an hBN monolayer is carried out by flowing ammonia borane
precursors onto the one-inch single-crystal Cu (111) thin film/sapphire
in a hot-wall chemical vapour deposition (CVD) furnace. An optical
micrograph of monolayer hBN triangular flakes grown on a Cu (111)
thin film with twin grains (Fig. 2a) shows that hBN flakes orient to the
same direction on one twin and to the opposite direction (or 60° in-
plane rotation along the z axis) on the counterpart twin (Extended Data
Fig. 2). Figure 2b displays optical micrographs of hBN flakes grown on
a single-crystal Cu (111) thin film without twin grains, where almost all
of the triangles are unidirectionally aligned (see also Extended Data
Fig. 3 for statistical analysis of the orientation distribution of hBN tri-
angular flakes). The observation of mono-orientation on an individual
single-crystal Cu (111) grain clearly indicates the existence of an energy-
minimized hBN–Cu (111) configuration. Therefore, eliminating the
twin grains in Cu (111) will ensure the growth of single-crystal hBN on it.
To verify the single crystallinity, we characterized the hBN mon-
olayer merged from mono-oriented triangles by microspot low-energy
electron diffraction (μ-LEED), using a probe size of around 3 μm at 80
sites across the one-inch wafer. Figure 2c displays the μ-LEED patterns
from nine randomly selected sites. All results reveal an hBN monolayer
that is unidirectionally aligned with the Cu (111) surfaces, indicating
that their single crystallinity strictly follows the Cu (111) lattices. The
atomically resolved scanning tunnelling microscopy (STM) image of
hBN on Cu (111) in Fig. 2d shows a perfect hBN lattice with a measured
lattice constant of 2.50 ± 0.1 Å, consistent with the theoretical value
of 2.5 Å. We probed more than 20 locations, and all STM images show
the same hBN lattice orientation (Extended Data Fig. 4). We did not
observe any grain boundaries formed by adjacent misoriented hBN
domains, suggesting the single-crystalline nature of hBN. We note that,
in some areas, moiré patterns arise from the lattice mismatch and/or
relatively small rotation (within 1.5°) between hBN and the underlying
Cu (111) substrate (Extended Data Fig. 5a–f ). The magnified atomic-
resolution image at the moiré boundary areas reveals that the hBNNDbcRD ND RD500 μm5 00 μm5 00 μm5 00 μma
Twinboundarywithin-planerotationby60°First-layerCu
Second-layerCu
Third-layerCu
BottomCu110001111100 μm1 00 μmFig. 1 | Cu (111) lattice orientation on c-sapphire substrates. a, Diagram
showing twinned Cu (111) (top view) with an in-plane rotation of 60°; the blue
and red triangles enclose areas with the same Cu stacking configuration in
adjacent twinned grains. The right-hand coloured triangle is the legend for the
inverse pole figure (IPF) maps at the bottom of panels b, c. b, Optical
microscope image (top) and EBSD IPF mappings (1 × 1 mm^2 ; bottom) of a Cu thin
film annealed at 1,000 °C for 1 h, where the normal direction (ND) and rolling
direction (RD) indicate the presence of twinned Cu (111) polycrystals with in-
plane misorientation by 60°. c, If the Cu thin film is annealed at 1,050 °C for 1 h,
the in-plane orientation is unified, showing the formation of single-crystal
Cu (111) without any twinned grains.