Long-Lived Excitations in Wide (In,Ga)N/GaN Quantum Wells

Time-resolved photoluminescence (PL) measurements in wide (15 and 25 nm) $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{Ga}\mathrm{N}$ quantum well light-emitting diodes (LEDs) reveal that photoexcited charge carriers occupying ground states in the well can be detected up to tens of milliseconds after optical excitation. Moreover, even without photoexcitation, some residual charge carriers are present in the ground states of wide wells. We call them ``dark charge,'' since they do not recombine radiatively. However, they screen the electric field in the well and lead to the appearance of highly efficient optical transitions through excited states. The intensity of these optical transitions (PL) is sensitive to the population of dark charge. Dark charge can be depleted by short pulses of negative voltage applied to the diode and then it slowly recovers to its equilibrium value. Such long decay-regeneration times on the order of milliseconds are due to negligible overlap of electron and hole ground states in wide wells with a strong electric field. The reverse voltage applied to the LED shortens these lifetimes, since it reduces the electric field in the well. The effect of regeneration of dark charge allows us to estimate the value of electric field in the well. Furthermore, we present a simple model of the carrier dynamics in wide quantum wells.