  The photocurrent and the small-signal photoconductance of InGaN/GaN
multiple-quantum-well structures were studied at the temperature range from 10
to 300 K. The optical excitation was carried out at the quantum wells intrinsic
absorption wavelengths. Regardless of the temperature the experimental plots of
direct photocurrent vs. reverse voltage were step-like, which is related to the
sequential quantum wells passage from quasi-neutrality into the p-n-junction
space charge region. In addition, under optical excitation near the quantum
wells material absorption edge we observed the photocurrent declines with
increasing reverse bias, i.e. the negative differential photoconductance. This
phenomenon is associated with a blue shift of the InGaN quantum well absorption
edge arising due to compensation of its build-in piezoelectric field by the
p-n-junction electric field. Furthermore, it was experimentally shown that each
quantum well corresponds to two peaks in the small-signal photoconductance vs.
reverse voltage dependence. The temperature changes in the amplitude and
position of these peaks indicate that they probably related to the charge
carriers thermal emission and thermally activated tunneling from the quantum
well.
