Heatpulse rapid thermal annealing was used to activate Si implants of 3.5 × 1012cm−2 at 100 keV and 1.0 × 1013 and 1.0 × 1014cm−2 at 200 keV into semi-insulating GaAs. The effects of Si3N4 encapsulation, anneal temperature and time, and substrate Cr-doping level were investigated. The annealed samples were characterized with C-V, Van der Pauw, differential Hall, and SIMS measurements. Conventional furnace anneals were carried out for comparison, and in all cases, Heatpulse anneals produced sharper carrier concentration profiles. 84% electrical activation was obtained for the 200 keV, 1.0 × 1013 cm−2 implant after a 950°C, 5 sec. Heatpulse anneal. Capped Heatpulse anneals produced less Cr depletion from the implanted region than furnace anneals.