  Transition metal dichalcogenides (TMDs) are materials that can exhibit
intriguing optical properties like a change of the bandgap from indirect to
direct when being thinned down to a monolayer. Well-resolved narrow excitonic
resonances can be observed for such monolayers, however only for materials of
sufficient crystalline quality, so far mostly available in the form of
micrometer-sized flakes. A further significant improvement of optical and
electrical properties can be achieved by transferring the TMD on hexagonal
boron nitride (hBN). To exploit the full potential of TMDs in future
applications, epitaxial techniques have to be developed that not only allow to
growlarge-scale, high-quality TMD monolayers, but allow to perform the growth
directly on large-scale epitaxial hBN. In this work we address this problem and
demonstrate that MoSe2 of high optical quality can be directly grown on
epitaxial hBN on an entire two-inch wafer. We developed a combined growth theme
for which hBN is first synthesized at high temperature by Metal Organic Vapor
Phase Epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by
Molecular Beam Epitaxy (MBE) at much lower temperatures. We show that this
structure exhibits excellent optical properties, manifested by narrow excitonic
lines in the photoluminescence spectra. Moreover, the material is homogeneous
on the area of the whole two-inch wafer, with only +/-0.14 meV deviation of
excitonic energy. Our mixed growth technique may guide the way for future
large-scale production of high quality TMD/hBN heterostructures.
