  Core-shell nanowire heterostructures form the basis for many innovative
devices. When compound nanowire shells are grown by directional deposition
techniques, the azimuthal position of the sources for the different
constituents in the growth reactor, substrate rotation, and nanowire
self-shadowing inevitably lead to sequential deposition. Here, we uncover for
In$_{0.15}$Ga$_{0.85}$As/GaAs shell quantum wells grown by molecular beam
epitaxy a drastic impact of this sequentiality on the luminescence efficiency.
The photoluminescence intensity of shell quantum wells grown with a flux
sequence corresponding to migration enhanced epitaxy, i. e. when As and the
group-III metals essentially do not impinge at the same time, is more than two
orders of magnitude higher than for shell quantum wells prepared with
substantially overlapping fluxes. Transmission electron microscopy does not
reveal any extended defects explaining this difference. Our analysis of
photoluminescence transients shows that co-deposition has two detrimental
microscopic effects. First, a higher density of electrically active point
defects leads to internal electric fields reducing the electron-hole wave
function overlap. Second, more point defects form that act as nonradiative
recombination centers. Our study demonstrates that the source arrangement of
the growth reactor, which is of mere technical relevance for planar structures,
can have drastic consequences for the materials properties of nanowire shells.
We expect that this finding holds also for other alloy nanowire shells.
