Rapid thermal annealing of GaAs in an arsine ambient has been investigated. Uncapped 2‐in. GaAs wafers were annealed in an arsine‐N2gas mixture up to annealing temperatures of 1100 °C and annealing times of 10 s. No surface decomposition occurred at an arsine partial pressure of 12.5 Torr. This capless annealing technique was employed to the activation of shallow Mg implants in GaAs. The sheet resistance of the annealed layers as a function of the annealing temperature reveals a minimum at approximately 930 °C. At higher temperatures diffusion of Mg becomes significant. A part of the Mg accumulates at the GaAs surface and diffuses out. The Mg loss due to outdiffusion can be reduced using Si3N4cap layers. The internal diffusion of Mg at high temperatures depends on the arsenic pressure during the annealing.