We report here a photoluminescence (PL), high-resolution transmission electron microscopy (HRTEM), and secondary ion mass spectrometry (SIMS) study of highly Si-doped InGaAs/AlAsSb single quantum wells (SQWs) that are lattice-matched to InP substrates grown by molecular beam epitaxy (MBE). High doping is shown to markedly influence the PL spectra of the SQWs. HRTEM shows the bowing and composition grading of the interfaces. SIMS depth profiles confirm the out-diffusion of In and Ga from the InGaAs well to the AlAsSb barriers and,Al incorporation into the InGaAs well. These group-III species interdiffusion combined with the exchange reaction between As and Sb are confirmed to be the origin of the extraordinarily broad PL spectra. Modulation doping combined with interface termination is shown to be effective in producing a sharp line-shape in the PL spectra, even for heavily doped SQWs.