  While the properties of wurtzite GaAs have been extensively studied during
the past decade, little is known about the influence of the crystal polytype on
ternary (In,Ga)As quantum well structures. We address this question with a
unique combination of correlated, spatially-resolved measurement techniques on
core-shell nanowires that contain extended segments of both the zincblende and
wurtzite polytypes. Cathodoluminescence hyperspectral imaging reveals a
blueshift of the quantum well emission energy by $75\pm15$ meV in the wurtzite
polytype segment. Nanoprobe x-ray diffraction and atom probe tomography enable
$\mathbf{k}\cdot\mathbf{p}$ calculations for the specific sample geometry to
reveal two comparable contributions to this shift. First, there is a 30% drop
in In mole fraction going from the zincblende to the wurtzite segment. Second,
the quantum well is under compressive strain, which has a much stronger impact
on the hole ground state in the wurtzite than in the zincblende segment. Our
results highlight the role of the crystal structure in tuning the emission of
(In,Ga)As quantum wells and pave the way to exploit the possibilities of
three-dimensional bandgap engineering in core-shell nanowire heterostructures.
At the same time, we have demonstrated an advanced characterization toolkit for
the investigation of semiconductor nanostructures.
