Excitation and relaxation mechanisms in single In(Ga)As quantum dots

Spatially resolved spectroscopy is used to investigate excitation and relaxation mechanisms in individual self-assembled In(Ga)As quantum dots. For low excitation levels (∼1 e-h pair in the dot), both charged (X*) and neutral (X) single exciton species are observed simultaneously in the μ-photoluminescence (μPL) spectrum. The charge status of X* and X is unambiguously identified using μPL-excitation (μPLE) spectroscopy. At higher excitation levels, additional spectral features appear, red-shifted from X, arising from the recombination of few exciton complexes (2X, 3X). Relaxation mechanisms are investigated using μPL-excitation spectroscopy. For X, the μPLE spectrum exhibits sharp resonances, which are attributed to direct absorption of excitonic states, and broader features due to phonon-assisted absorption, whilst X* is observed only for excitation above the wetting layer energy.