  We investigate the luminescence of Ga- and N-polar
In$_{x}$Ga$_{1-x}$N/In$_{y}$Ga$_{1-y}$N quantum wells (QWs) grown by
plasma-assisted molecular beam epitaxy on freestanding GaN as well as 6H-SiC
substrates. In striking contrast to their Ga-polar counterparts, the N-polar
QWs prepared on freestanding GaN do not exhibit any detectable
photoluminescence. Theoretical simulations of the band profiles combined with
resonant excitation of the QWs allow us to rule out carrier escape and
subsequent surface recombination as the reason for the absence of luminescence.
To explore the hypothesis of a high concentration of nonradiative defects at
the interfaces between wells and barriers, we analyze Ga- and N-polar QWs
prepared on 6H-SiC as a function of the well width. Intense luminescence is
observed for both Ga- and N polar samples. As expected, the luminescence of the
Ga-polar QWs quenches and red-shifts with increasing well width due to the
quantum confined Stark effect. In contrast, both the intensity and the energy
of the luminescence from the N-polar samples are essentially independent of
well width. Transmission electron microscopy reveals that the N-polar QWs
exhibit abrupt interfaces and homogeneous composition, excluding emission from
In-rich clusters as the reason for this anomalous behavior. The microscopic
origin of the luminescence in the N-polar QWs is elucidated using spatially
resolved cathodoluminescence spectroscopy. Regardless of well width, the
luminescence is found to not originate from the N-polar QWs, but from the
semipolar facets of v-pit defects. These results cast serious doubts on the
potential of N-polar QWs grown by plasma-assisted molecular beam epitaxy for
the development of long-wavelength light emitting diodes. What remains to be
seen is whether unconventional growth conditions may enable a significant
reduction in the concentration of nonradiative defects.
