Coulomb interactions in single charged self-assembled quantum dots: Radiative lifetime and recombination energy

We present results on the charge dependence of the radiative recombination lifetime, τ, and the emission energy of excitons confined to single self-assembled InGaAs quantum dots. There are significant dot-to-dot fluctuations in the lifetimes for a particular emission energy. To reach general conclusions, we present the statistical behavior by analyzing data recorded on a large number of individual quantum dots. Exciton charge is controlled with extremely high fidelity through an n-type field effect structure, which provides access to the neutral exciton (X⁰), the biexciton (2X⁰), and the positively (X¹⁺) and negatively (X^1-) charged excitons. We find significant differences in the recombination lifetime of each exciton such that, on average, τ(X^1-)/τ(X⁰)=1.25, τ(X¹⁺)/τ(X⁰)=1.58, and τ(2X⁰)/τ(X⁰)=0.65. We attribute the change in lifetime to significant changes in the single particle hole wave function on charging the dot, an effect more pronounced on charging X⁰ with a single hole than with a single electron. We verify this interpretation by recasting the experimental data on exciton energies in terms of Coulomb energies. We directly show that the electron-hole Coulomb energy is charge dependent, reducing in value by 5%--10% in the presence of an additional electron, and that the electron-electron and hole-hole Coulomb energies are almost equal.