The Si-doped low-temperature (LT) grown GaAs and GaAs/AlGaAs multiple quantum well(MQW) structures have been studied by photoluminescence (PL) spectroscopy. The samples were grown at 320 degrees C with different As pressures, then subjected to rapid thermal annealing from 500 to 900 degrees C. Besides the band-edge PL feature from the band-band recombination, a defect-related PL feature has also been observed in both GaAs and GaAs/AlGaAs MQW structures. Deep-level transient spectroscopy measurement shows that the As antisite-like defects exist in the as-grown and 600 degrees C-annealed Si-doped LT-grown GaAs. The different annealing temperature dependence of the band-edge PL feature in two kinds of materials is observed and discussed. The defect-related PL feature may be related to the defect complex consisting of Si and As atoms.