We studied the effect of the annealing temperatures (in the 1100-1200 C temperature range) on the electrical activation of ion implanted Si and Mg in GaN. Si+ ions were implanted at multiple energies (from 80keV to 180keV) and with a total fluence up to 2.7× 1014 cm-2 in heteroepitaxial GaN films on sapphire. Some samples were subjected to conventional low ramp rate processes at 1100 C and 1200 C in N2 ambient. A fast ramp rate pre-annealing in N2 ambient was carried out on other samples before those low-ramp rate processes at 1100 C and 1200 C. The calibrated Scanning Capacitance Microscopy (SCM) was applied to determine the active (ie substitutional) Si depth profile. The rapid pre-annealing process leads to an increase from 36% to 63% in the substitutional Si dose. The Mg ions were implanted with energy of 50keV and fluence of 5× 1014 cm-2 in n-GaN layer on sapphire. Two fast ramp rate …