We have investigated the optical and phototransport properties of GaN/AlGaN
quantum wells by photoluminescence and photovoltage spectroscopy. We show
that the internal piezoelectric and spontaneous polarization fields cause a
strong red-shift of the ground level energy of the quantum wells. Furtherm
ore, identical quantum wells grown on different buffer layers (GaN or AlGaN
) exhibit different emission energies, but similar well-width dependence of
the n = 1 state, due to the different distribution of strain between well
and barrier. The built-in field also causes a strong reduction of the excit
on oscillator strength, which is not observable in photovoltage spectra. A
long-living (thousands of seconds) charge storage effect is observed in the
phototransport spectra due to the presence of point defects, presumably as
sociated to Ga vacancies.