We investigated the PR spectra of as-grown, annealed and etched modulation-doped AlGaAs/GaAs heterostructure samples. As annealing time increases, the built-in electric fields are strengthened as a result of the diffusion of Si-Si pairs. As the etching time increases the broad signal near GaAs band-gap energy and the single peak shape (SPS) signal near AlGaAs band-gap energy change to the step-like signal and the double peak shape (DPS) signal, respectively. (C) 1997 Elsevier Science Ltd.