  We explore and systematically compare the morphological, structural and
optical properties of GaN/(Al,Ga)N multiple quantum wells (MQWs) grown by
plasma-assisted molecular beam epitaxy (PA-MBE) on freestanding GaN$(0001)$ and
GaN$(000\bar{1})$ substrates. Samples of different polarity are found to be
comparable in terms of their morphological and structural perfection and
exhibit essentially identical quantum well widths and Al content. Regardless of
the crystal orientation, the exciton decay in the MQWs at 10 K is dominantly
radiative and the photoluminescence (PL) energy follows the quantum confined
Stark effect (QCSE) for different quantum well widths. A prominent
free-to-bound transition involving interface shallow donors is, however,
visible for the N-polar MQWs. At room-temperature, in contrast, the exciton
decay in all samples is dominated by nonradiative recombination taking place at
point defects, presumably Ca or V N located at the bottom QW interface.
Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay
times than the Ga-polar MQWs at room-temperature. This improved internal
quantum efficiency is attributed to the beneficial orientation of the internal
electric field that effectively reduces the capture rate of minority carriers
by interface trap states.
