Exciton dynamics inGaAs/Ga1−xAlxAsheterojunctions and GaAs epilayers

Abstract

We present a study of the exciton dynamics in modulation doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions and GaAs epilayers. The comparison permits to identify the features characteristic for the heterojunctions. In particular, we analyze the rise time of the transient photoluminescence (PL) intensity. In general, we find a longer PL rise time for the lower-energy excitons indicating that the time required for the energy relaxation process increases with increasing binding energy of the excitons. Moreover, the rise time of the free excitons turns out to be conspicuously longer in heterojunctions than in epilayers although the time integrated PL spectra of the two systems are similar. From our analysis we conclude that the long rise time observed in the heterojunctions is closely connected with the vertical drift of the photoexcited carriers driven by the interface potential. In fact, we find that the carriers (i.e., holes in n-doped samples) photoexcited in the heterojunctions drift vertically from the heterointerface to the flat band region, where they finally recombine after forming excitons (bimolecular formation) with oppositely charged and locally excited carriers (i.e., electrons in n-doped samples).