Discrimination of non-radiative recombination centers in Si-doped GaAs/AlGaAs quantum well structures has been achieved by means of a two wavelength excited photoluminescence technique. In a crystal grown at 700 degrees C, traps were found neither in GaAs wells nor in Al0.2Ga0.8As barriers, but inside external Al0.4Ga0.6As layers which originally were left out of the doping region. These traps were classified as a pair: one originating in a lattice native defect or a residual impurity; and the other by Si atoms (possibly a DX center) diffused from the doped region. In contrast, traps were found even inside the GaAs wells in case of samples grown at 600 degrees C, reflecting deteriorated crystal quality.