Heavily silicon doped GaAs grown by molecular beam epitaxy using a single gallium isotope source (69Ga) has been studied by infrared absorption to reveal localized vibrational modes (LVMs) of Si complexes. The structure observed close to 367 cm-1 is the same as that present in normal GaAs:Si spectra and does not result from mixed Ga isotopes. The electron trap Si–X gives three LVMs at 368.4, 370.0, and 399.6 cm-1, typical of second neighbor donor-acceptor pairs, but inconsistent with a previous proposal that its structure is the planar defect VGa–SiAs–AsGa. It is now suggested that the defect is a perturbed SiGa–VGa center, involving a second Si atom or a second vacancy. © 1997 American Institute of Physics.