A new, capless annealing technique for GaAs, which utilizes a controlled-excess arsenic vapor pressure has been studied. The excess arsenic vapor is provided by the thermal decomposition of InAs and is about 100 times larger than the arsenic pressure of thermally decomposing GaAs for temperatures of 800-900°C. This excess pressure forces Si implants to activate predominantly on donor sites for doses of (2-50)×1012 cm -2 studied thus far. A SiF+ dose of 5×1012 cm -2 and implant energy of 250 keV annealed at 900°C for 30 min produced a sheet electron concentration of 3.5×1012 cm -2 and mobility of 4400 cm2 V-1 sec -1. Photoluminescence studies show that the annealed surfaces must be proximate during anneal to avoid buildup of impurities at the surface. A model based on GaAs evaporation and buildup of bulk impurities on the surface is proposed to explain the results.