Electrical and optical investigations of GaAs/(Al,Ga)As quantum-cascade-laser structures

We have investigated the electrical and optical properties of GaAs/(Al,Ga)As quantum-cascade-laser (QCL) structures. In the case of an undoped structure, distinct current maxima followed by negative differential conductance regions appear, while the doped structure exhibits a plateau with saw-tooth-like features similar to the well-known ones for electric-field domain formation in doped, weakly coupled superlattices. The investigation of an undoped quasi-QCL structure, in which all (Al,Ga)As barriers are enlarged, shows a much sharper current maximum in the dark current and a clear plateau-like feature under illumination. The structures observed in the current–voltage characteristics are closely related to the electric-field dependence of the photoluminescence intensity in these three QCL structures.