Coupling and Strain Effects in Vertically Stacked Double InAs/GaAs Quantum Dots: Tight-Binding Approach

The empirical tight-binding approach is used to study atomic-scale effects on electronic coupling in vertically stacked, self-assembled InAs/GaAs quantum dots. A model with unstrained dots is first studied to isolate the atomistic coupling effects from the strain effects. The strain effects are next considered by means of the valence force field method. Electron levels in coupled quantum dots follow closely the simple analogy of coupled dots as artificial molecules. The electron ground state of double dot has always bonding-like character. The coupling of hole states is more complicated because the coupling depends both of the hole envelope function and the atomic character of the hole state. It is shown that the character of the hole ground state of double dot changes from antibonding to bonding-like, when the distance between the dots decreases. It reorders hole levels, changes state symmetries, and makes changes in optical spectra. The calculated red-shift of the lowest transition for closely-spaced dots agrees well with experimental data. We present also some preliminary results on strain effects in such nanocrystals.