Resonant spectroscopy of II-VI self-assembled quantum dots: Excited states and exciton–longitudinal optical phonon coupling

Abstract

Using resonantly excited photoluminescence (PL) along with photoluminescence excitation (PLE) spectroscopies, we study the carrier excitation processes in $\mathrm{CdTe}∕\mathrm{ZnTe}$ and $\mathrm{CdSe}∕\mathrm{ZnSe}$ self-assembled quantum dots (QD's). PLE spectra of single $\mathrm{CdTe}$ 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 $\mathrm{ZnTe}$ 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 $\mathrm{CdTe}$ 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 $100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. This large splitting between excited and ground states in $\mathrm{CdTe}$ QD's suggests strong spatial confinement. In contrast, the LO phonon-assisted absorption shows significant size selectivity. In the case of $\mathrm{CdTe}$ dots the exciton-LO phonon coupling is strongly enhanced for smaller-sized dots which have higher emission energies. In contrast, for $\mathrm{CdSe}$ 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 $\mathrm{CdTe}$ 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 $\mathrm{CdSe}$ QD's.