MOVPE-grown n-type GaAs/InGaP/GaAs structures with different GaAs cap layer thicknesses were studied by deep-level transient spectroscopy. An electron trap El with a thermal activation energy of 0.75eV is formed in lattice matched InGaP after the MOVPE growth, if is not capped by sufficiently thick GaAs. The lattice mismatch of the InGaP layer influences the thermal activation energy of the deep defect, but not its occurrence or concentration. The starting surface concentration of the deep level defect as well as its diffusion at room temperature into the InGaP are apparently promoted by intrinsic defects in the InGaP layer determined by its growth condition. The shape of the depth profiles suggests that an extrinsic defect moves via interstitial sites into empty substitutional sites of the InGaP lattice. Oxygen atoms on phosphorus sites are probable candidates for the El-related defects. (C) 2004 Elsevier B.V. All rights reserved.