We describe a 750‐keV, 5–10×1012 cm−2 dose, 40Ar implant pretreatment for semi‐insulating (SI) GaAs substrates allowing generation of 28Si implanted n layers with higher mobility and/or activation efficiency. This effect was observed in Bridgman Cr‐doped, Bridgman Cr‐O‐doped, liquid encapsulated Czochralski (LEC) Cr‐doped, and LEC undoped substrates. Pretreated and control samples were studied by carrier profiling, van der Pauw measurements, and secondary ion mass spectrometry (SIMS). The compensation ratio (ϑ = NA/ND) of n layers in pretreated substrates was lower than in controls. Probable mechanisms of improvement in ϑ are (1) reduced surface Cr concentration due to pretreatment, (2) preferential incorporation of 28Si on Ga sites due to pretreatment‐created stoichiometry imbalance, and (3) a combination thereof.