EXCITON DYNAMICS IN THIN ALGAAS GAAS QUANTUM-WELLS GROWN BY MBE/

We report high-resolution, picosecond laser spectroscopy measurements of the relaxation of lowest-energy heavy-hole excitons in GaAs quantum wells grown with and without growth interruptions. In both cases exci tons relax by losing potential energy in a diffusion motion driven by potential fluctuations in the quantum-well plane. In the quantum wells grown without growth interruptions the low-energy shift of exciton li nes is comparable with the width of an inhomogeneously broadened line and shows two-step decay with a slow exponential component characteris tic for exciton localization at well interfaces. In the growth-interru pted quantum wells in which the size of islands with constant well thi ckness is large compared with exciton diameter we observe splitting of the heavy-hole transitions into the multiplets of narrow lines corres ponding to one monolayer difference in the well width. The energy shif ts of each line in this case amount to only a fraction of the width of individual lines (or there is no shift at all) suggesting the interis land migration of excitons mediated by acoustic phonon scattering as b eing responsible for exciton relaxation. Again a two-step decay of the luminescence is observed at low temperatures (2 K). Temperature-depen dent measurements show that at higher temperatures luminescence decay becomes governed by a single exponential as expected for delocalized e xcitons (no energy shift is observed during the exciton decay time). T his allowed us to study directly intrinsic properties of excitons in q uantum wells, i.e, to determine the lifetime of K-parallel to=0 excito ns, which is a fundamental parameter of the system and has been assess ed by many theories. The experimentally determined, from the present w ork, values of the radiative lifetime an 24.4 ps for a 13 ML thick wel l and 21.8 ps for a 17 ML well. These values agree very well with theo retical estimates of Andreani ct al. (Solid State Cornmun., 77 (1991) 641). The effective lifetimes measured as a time to decay to lie of th e value of the maximum of PL intensity are considerably longer than th e radiative lifetime, since in thermal equilibrium only a small fracti on of excitons occupy the states with k(1) < k(0) which can decay radi atively.