Abstract
We have investigated the transport and population properties of undoped GaAs/(Al,Ga)As quantum-cascade structures (QCS) below and above the designed threshold field strength Fcrit using current–voltage characteristics and photoluminescence (PL) spectroscopy. The devices exhibit a negative differential conductance (NDC) at Fcrit. The NDC is correlated with an abrupt change of intensity and energy positions in the field-dependent PL spectra. Below Fcrit, the PL spectra show only emission from the lowest interband transition in the QCS exhibiting the usual Stark shift. Above Fcrit, a significant population of the laser levels is observed. The switching behaviour is tentatively attributed to an abrupt spatial redistribution of the carriers at Fcrit mediated by additional states within the (Al,Ga)As barriers.
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