Zinc oxide (ZnO) is a prototypical wide-band-gap semiconductor whose $n$-type conductivity can be tailored through extrinsic donor doping. However, the roles of defect association, compensation, and dopant mobility remain incompletely understood. We investigate Cl- and F-related defects in ZnO using screened hybrid density-functional calculations, with emphasis on substitutional donors, defect complexes, vacancy-mediated migration, and self-consistent carrier statistics. Substitutional chlorine and fluorine on the oxygen site, ${\mathrm{Cl}}_{\mathrm{O}}$ and ${\mathrm{F}}_{\mathrm{O}}$, are predicted to be shallow donors, with (+/0) transition levels above the conduction-band minimum and low formation energies under O-poor conditions. Halogen-native-defect complexes can be thermodynamically bound and provide possible compensation or passivation channels, but their impact on the carrier concentration depends on their equilibrium abundance and charge-state distribution. Self-consistent charge-neutrality calculations show that both Cl and F substitutional donors can sustain $n$-type conditions under the freeze-in treatment considered here, with the charge balance dominated by ionized substitutional donors and free electrons. Large vacancy-mediated migration barriers further indicate that substitutional halogen donors are kinetically stable after incorporation. These results provide a microscopic description of the thermodynamic, electronic, and kinetic factors governing Cl and F doping in ZnO.

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• Dopants
• Electronic structure
• Point defects
• Doped semiconductors
• Semiconductors
• Wide band gap systems
• Density functional calculations