Deep-level transient spectroscopy studies of n-type InGaP/GaAs structures revealed an electron trap E1 with a thermal activation energy of 0.75 eV. From the shape of the depth profiles for this deep level an extrinsic defect can be concluded, which moves from the air exposed InGaP surface via interstitial sites into empty substitutional sites of the InGaP lattice. By comparison of InGaP samples grown at 580 degrees C with different V/III input ratios as well as at 650 degrees C, it is suggested that the most probable candidate for the E1-related defect is oxygen on phosphorus site. It is shown that this defect is due to an extrinsic defect. The concentration of the E1-related defect depends on the concentration of phosphorus vacancies, which are due to thermal degradation of the growing InGaP layer surface as a result of insufficient stabilization. The in-diffusion of oxygen is promoted by particular intrinsic defects, probably antiphase boundaries, in the InGaP layer determined by the growth condition. With a sufficiently thick GaAs cap layer or by adequately chosen growth conditions for InGaP, the surface-related defect E1 can be avoided. (c) 2005 Elsevier B.V. All rights reserved.