Exciton dynamics in GaAs/Ga1-xAlxAs heterojunctions and GaAs epilayers

We present a study of the exciton dynamics in modulation doped GaAs/Ga1-xAl xAs heterojunctions and GaAs epilayers. The comparison permits to identify the features characteristic for the heterojunctions. In particular, we anal yze the rise time df the transient photoluminescence (PL) intensity. In gen eral, we find a longer PL rise time for the lower-energy excitons indicatin g that the time required for the energy relaxation process increases with i ncreasing binding energy of the excitons. Moreover, the rise time of the fr ee excitons turns out to be conspicuously longer in heterojunctions than in epilayers although the time integrated PL spectra of the two systems are s imilar. From our analysis we conclude that the long rise time observed in; the heterojunctions is closely connected with the vertical drift of the pho toexcited carriers driven by the interface potential. In fact, we find that the carriers (i.e., holes in n-doped samples) photoexcited in the heteroju nctions 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).