Abstract Ion implantation of S and Te followed by sub-second flash lamp annealing with peak temperature about 1100 °C is employed to obtain metallic n ⁺⁺-GaAs layers. The electron concentration in annealed GaAs is as high as 5 × 10¹⁹ cm⁻³, which is several times higher than the doping level achievable by alternative methods. We found that heavily doped n ⁺⁺-GaAs exhibits positive magnetoconductance in the temperature range of 3–80 K, which is attributed to the magnetic field suppressed weak localization. By fitting the magnetoconductance results with Hikami–Larkin–Nagaoka model, it is found that the phase coherence length increases with increasing carrier concentration at low temperature and is as large as 540 nm at 3 K. The temperature dependence of the phase coherence length follows l _∅ ∝ T ^η (η ∼ 0.3), indicating defect-related scattering as the dominant dephasing mechanism. In addition, the high doping level in n-type GaAs provides the possibility to use GaAs as a plasmonic material for chemical sensors operating in the infrared range.