Strain-driven phase separation of InAs self-assembled quantum dot's InGaAs heterocapping alloy is investigated by temperature-dependent photoluminescence (PL) spectroscopy and tuned by rapid thermal annealing (RTA) as a means to control the optical properties of such a structure. The integrated PL intensity is found to exhibit an anomalous increase with increasing temperature up to 100K. This behavior is attributed to the strain-driven phase separation-induced formation of small potential barriers surrounding the quantum dots (QDs) and supported by a rate equation model for the carrier dynamics. After RTA at 650°C during 50s, an enhancement of the integrated PL intensity, an improvement of the heterocapping alloy PL properties together with the suppression of the anomalous increase of the PL intensity with temperature has been observed. Acting as a reverse phenomenon for the strain-driven alloy decomposition, the thermal induced intermixing is expected to alter only the heterocapping alloy. However, for an annealing temperature (T_a) of 750°C, the QDs PL peak is shown to exhibit a weak blue shift and a line width narrowing indicating the beginning of In/Ga interdiffusion. At higher annealing temperature (850°C), large blue shift and broadening of the PL peak occur.