Negative differential conductance and current bistability in undoped GaAs∕(Al,Ga)As quantum-cascade structures

Abstract

We have investigated negative differential conductivity (NDC) and laser level population in undoped GaAs∕(Al,Ga)As quantum-cascade structures using current–voltage characteristics and interband photoluminescence spectroscopy. While for both GaAs∕Al0.33Ga0.67As and GaAs∕Al0.45Ga0.55As structures a strong, bistable NDC is observed, a weaker NDC without bistability appears only in the GaAs∕Al0.45Ga0.55As structure, which is due to the resonant coupling between injector and upper laser level. Although the bistable NDC is connected with a significant population of the laser levels, it cannot be explained by resonant coupling between electronic states in the active region. We believe that the bistable NDC is caused by an interplay of resonant coupling effects within the injector with the carrier redistribution in the vicinity of the optically active region. Furthermore, a still unidentified state, which exhibits a strong photoluminescence signal, may play an important role for the bistability.