Near-infrared reflectance spectra of 5 μm thick low-temperature (LT) GaAs films grown by molecular-beam epitaxy at different substrate temperatures TG on GaAs/AlAs Bragg reflectors (BR’s) have been studied. With decreasing TG (increasing absorption coefficient αf of the film), the reflectance of the stop band of the BR decreases monotonously, while the amplitudes of the interference fringes due to the LT-GaAs layer inside the stop band initially increase followed by a decrease for large αf values. This unusual behavior is explained by a particular optical interference effect of a thick absorbing layer on a strongly reflecting structure. This effect remarkably improves the determination of αf and enables the detection of As-antisite defects in LT-GaAs films for concentrations as low as 1×1018 cm-3, where other methods fail. © 2002 American Institute of Physics.