Microdefects in Cr-doped SI LEC (001) GaAs wafers which were implanted with Zn+ or As /Zn and capless-annealed in an As ambient have been studied by means of transmission electron microscopy. Most of the microdefects in Zn +- implanted GaAs specimens were identified as precipitates and stacking fault tetrahedra (SFTs). Every SFT was accompanied by a precipitate at the apex. Most of the precipitates were distributed from Rp to Rp + 2∆Rp. Two types (α and β) of SFTs were differentiated by the arrangement of atoms in the core of the stair-rod partial dislocations bounding the periphery of the SFTs in a polar Frystal. β-SFTs were, however, predominantly formed in Zn+ implanted GaAs specimens. Dual implantation of As+ and Zn+ suppressed the formation of SFTs, but not that of precipitates. The formation of SFTs was found to be influenced by the deviation in stoichiometry.