The major secondary defect in post-annealed 5 x 1014/cm2 As+ implanted (111) silicon is microtwins. This was shown to be true for rapid thermal annealing similar to conventional furnace annealing. However, the twin density after rapid thermal annealing to 1100°C for 10 seconds was found to be greater than that in a comparable specimen furnace annealed at 1000°C for 10 minutes. Sheet resistivity after annealing was found to be closely related to the twin density of the specimen. Thermal stress was shown not to be the cause of this increase in twin growth. The higher average crystal regrowth temperature appears to be responsible for growth of more large area twin. Twins of small size were shown to anneal out at high temperature but large area twins did not. Twin free specimens were obtained by regrowth of the amorphous layer at low temperature which formed only small area twins followed by a high temperature anneal which annihilated all the small twins.