Abstract
Using resonantly excited photoluminescence (PL) along with photoluminescence excitation (PLE) spectroscopies, we study the carrier excitation processes in and self-assembled quantum dots (QD’s). PLE spectra of single QD’s reflect two major mechanisms for carrier excitation: The first, associated with the presence of sharp and intense lines in the spectrum, is a direct excited-state–ground-state transition. The second, associated with the appearance of up to four much broader excitation lines, is a longitudinal optical (LO) phonon-assisted absorption directly into the QD ground states. LO phonons with energies of both QD’s and barrier material are identified in the PLE spectra. Resonantly excited PL measurements for the QD ensemble as a function of excitation energy makes it possible to separate the contributions of these two mechanisms. We find that for QD’s the distribution of excited states coupled to the ground states reflects the energy distribution of the QD emission, but shifted up in energy by . This large splitting between excited and ground states in QD’s suggests strong spatial confinement. In contrast, the LO phonon-assisted absorption shows significant size selectivity. In the case of dots the exciton-LO phonon coupling is strongly enhanced for smaller-sized dots which have higher emission energies. In contrast, for QD’s the strength of exciton-LO phonon coupling is nearly uniform over the whole ensemble—that is, the dot energy distribution determines the intensities of LO phonon replicas. We show that for QD’s after annealing, that is, after an increase in the average dot size, the exciton-LO phonon interaction reflects the dot energy distribution, as observed for QD’s.
- Received 4 September 2003
DOI:https://doi.org/10.1103/PhysRevB.70.125306
©2004 American Physical Society