Origin of the inhomogenous broadening and alloy intermixing in InAs/GaAs self-assembled quantum dots

The photoluminescence (PL) spectra of a single-layer and a multilayer sample of self-assembled InAs/GaAs quantum dots (QD's intermixed by thermal annealing at various temperatures, have been investigated. Intermixing is found to change both the optical transition energy and the intersublevel spacing of the QD energy levels. The linewidth (which is due to inhomogeneous broadening) of the PL emission peaks of both samples, are very similar, hence showing that the intermixing process is very homogeneous over 25 layers of QD's, whatever the annealing technique used. Moreover it is observed that the width of the PL peaks decreases for increasing interdiffusion down to about 5 meV for the ground-state transition of the multilayer sample. A reduction of the peak width is also observed for higher-energy states within the same ensemble of dots. The present paper shows that this effect can only be explained by some variation of the effective confining potential. Theoretical calculations have shown that the QD height, rather than the diameter, the volume, the composition or the strain, appears to be the key parameter that controls the sharpness of the PL linewidths in the investigated samples. Our model allows the identification of the main mechanisms involved in the inhomogeneous broadening of the optical transitions for the InAs/GaAs QD system.