We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled In0.5Ga0.5As/AlxGa1−xAs quantum dot (QD) structures, grown by atomic layer molecular beam epitaxy. A blue shift of the energy transition of both the QD and the wetting layer (WL) is found when increasing the Al content in the barrier. Nevertheless, the energy separation between 0D and 2D levels increases with x and, therefore, the coupling between the fundamental QD state and the WL electronic level can be tuned by increasing the barrier band gap. Since the WL state acts as the intermediate level on which escape and thermalization occur, we found that the larger separation between QD and WL states reduces the QD thermal quenching and enhances the QD radiative efficiency at high temperature.