  Realization of wafer-scale single-crystal films of transition metal
dichalcogenides (TMDs) such as tungsten sulfide requires epitaxial growth and
coalescence of oriented domains to form a continuous monolayer. The domains
must be oriented in the same crystallographic direction on the substrate to
avoid the formation of metallic inversion domain boundaries (IDBs) which are a
common feature of layered chalcogenides. Here we demonstrate fully-coalesced
single orientation tungsten sulfide monolayers on 2-inch diameter c-plane
sapphire by metalorganic chemical vapor deposition using a multi-step growth
process. High growth temperatures and sulfur/metal ratios were required to
reduce domain misorientation and achieve epitaxial tungsten sulfide monolayers
with low in-plane rotational twist (0.09 deg). Transmission electron microscopy
analysis reveals that the tungsten sulfide monolayers lack IDBs but instead
have translational boundaries that arise when tungsten sulfide domains with
slightly off-set lattices merge together. By adjusting the monolayer growth
rate, the density of translational boundaries and bilayer coverage were
significantly reduced. The preferred orientation of domains is attributed to
the presence of steps on the sapphire surface coupled with growth conditions
promote surface diffusion and oriented attachment. The transferred tungsten
sulfide monolayers show neutral and charged exciton emission at 80K with
negligible defect-related luminescence. Back-gated tungsten sulfide field
effect transistors exhibited mobility of 16 cm2/Vs. The results demonstrate the
potential of achieving wafer-scale TMD monolayers free of inversion domains
with properties approaching that of exfoliated flakes.
