EXCITED-STATES AND ENERGY RELAXATION IN STACKED INAS GAAS QUANTUM DOTS/

Excited states and energy relaxation processes are studied for stacked InAs/GaAs QD's with GaAs cap Layers grown by migration enhanced epita xy. Photoluminescence excitation (PLE) spectra reveal the excited stat e spectrum as a function of size for self-assembled InAs QD's in multi layered samples with 36-ML spacers. The observed energy shifts and spl ittings are consistent with those of hole states numerically calculate d for pyramidal QD's supporting assignment to the transition between t he electron ground \000] and the \001] excited hole state. The optical results suggest the island shape uniformity to improve in multilayere d samples, which is attributed to the contribution of the buried islan ds to the surface strain altering the island formation kinetics and en ergetics that also underlie vertical self-organization. Time-resolved photoluminescence (TRPL) results yield a lifetime of 40 ps for thr fir st excited \001] hole state, attributed to multiphonon relaxation proc esses bridging the approximately 100 meV level separation, and ground- state lifetimes around 700 ps independent of the detection energy. At high excitation densities saturation of QD states leads to long-living excited-state PL and up to 1 ns delay in the ground-state PL decay, s howing radiative decay to be the dominant recombination process in the QD's. The results presented contribute to the understanding of PLE sp ectra of an inhomogeneous QD ensemble, which is argued to be sensitive to the shape uniformity, the excited-state spectrum, and competing re combination processes. [S0163-1829(98)03115-4].