Nickel diffused into VPE n-GaAs at 700 °C for 20 min reduces the hole diffusion length Lp from 4.3 to 1.1 μm. Deep-level transient spectroscopy (DLTS) has been used to identify energy levels in Ni-diffused GaAs at Ev+0.39 eV and Ec−0.39 eV, which have identical concentration profiles. The as-grown VPE GaAs contains traces of these levels as well as an electron trap at Ec−0.75 eV in a concentration of 1.5×1015 cm−3. Ni diffusion reduces the concentration of this level by an amount that matches the increase in concentration of each of the two Ni-related levels. Previous work has shown that the Ec−0.75 eV level is related to growth under As-rich conditions, so it may be a gallium vacancy complex. A technique for measuring minority-carrier capture cross sections has been developed, and indicates that Lp in Ni-diffused VPE n-GaAs is controlled by the Ec−0.39 eV level.