Correlation of time-resolved electroluminescence and cathodoluminescence measurements on quantum well light emitters with varying barrier widths

Abstract

We present results of a comparative study of the time-dependent luminescence properties of multiple quantum well structures with varying barrier widths which are embedded in the active area of a light-emitting device. The carrier kinetics is investigated by different experimental approaches: Cathodoluminescence and electroluminescence experiments where excitation is on/off-modulated for the purpose of time-resolved measurements and time-resolved electroluminescence experiments in the small signal regime which allow for observation of the carrier kinetics under flatband conditions. Due to the exact determination of the excess carrier density the latter technique provides a sensitive tool for a precise estimation of the mono- and bimolecular recombination coefficients. Comparison with light output data yields radiative and nonradiative parts. We find that coupling of quantum wells dramatically favors nonradiative interface recombination as expected from a theoretical model accounting for the superlattice wavefunctions. On the other hand, the bimolecular recombination rate remains unaltered even when the barrier width is lowered from 18 to 0.9 nm. In contrast, on/off modulated experiments reveal that luminescence decay is strongly influenced by carrier drift out of the active area. A barrier width dependent carrier mobility in growth direction accounts for these results if phonon assisted hopping rather than Bloch transport is presumed. Thus, an estimation of device quality of quantum well light emitters by conventional time-resolved cathodo- (or photo-) luminescence experiments is found to be possible if internal field induced carrier drift processes are taken into account.