Investigation of carrier transport and carrier distribution in GaAs/(Al,Ga)As quantum-cascade structures

Abstract

We have investigated the current density–electric field (j-F) characteristics and photoluminescence (PL) spectra of several GaAs/(Al,Ga)As quantum-cascade structures (QCS’s) up to the threshold field strength to obtain information on the carrier distribution and field inhomogeneities. Due to the complex structure of each single period of the cascaded device, the dark j-F characteristics of undoped QCS’s exhibit distinct current maxima, resulting in regions of negative differential conductivity, while a plateaulike feature appears under illumination. For doped QCS’s, a plateau with sawtoothlike structures appears, which are well known from doped, weakly coupled superlattices under electric-field-domain formation. However, in contrast to the splitting of the PL line that is observed for doped, weakly coupled superlattices, no splitting has been observed in the doped QCS’s. The j-F characteristic of an undoped QCS with thicker barriers than the original QCS exhibits a much more pronounced current maximum in the dark. Under illumination, the j-F characteristic of this structure shows a clear plateaulike feature, which contains additional fine structure indicating the existence of electric-field inhomogeneities within each period due to the separation of electrons and holes. The PL spectra of the undoped and doped QCS’s show emission from only the widest quantum well (QW) because the holes accumulate very quickly in this QW resulting from the strong coupling within the entire QCS. However, in the PL spectra of the undoped QCS with thicker barriers, PL lines from several quantum wells can be identified. The carrier distribution process in QCS’s is also discussed as a function of the applied electric field and compared for the different structures.