Subband population and current instabilities in GaAs/(Al,Ga)As quantum-cascade structures

We have investigated the transport and population properties of an undoped GaAs/(Al,Ga)As quantum-cascade structure (QCS) below and above the designed threshold field strength Fcrit using current–voltage characteristics and photoluminescence (PL) spectroscopy. The device exhibits a current instability at Fcrit, which is correlated with an abrupt change in the field-dependent PL spectra. Below Fcrit, the PL spectra are dominated by the emission from the lowest subband of the QCS exhibiting the usual Stark shift. Above Fcrit, a significant emission from the lower laser level as well as, though much weaker, from the upper laser level is observed. This current instability may be due to the occupation of the laser levels and/or of additional barrier states, which open transport channels for the electrons within the (Al,Ga)As barriers.