Nitrogen doping in ZnSe using a new type of electron-cyclotron-resonance (ECR) source has been investigated. Selective nitrogen radical doping is performed using the ECR source with an external electrode, which can remove charged particles in the nitrogen plasma by the electrode applied electric field. Nitrogen ions are trapped by the electrode and the ion current is measured as a function of the electric field. The activation ratio of nitrogen atoms is investigated for various doping conditions by comparing the carrier profile and the atomic nitrogen profile, which are measured by capacitance–voltage techniques and secondary ion mass spectrometry. It was confirmed that nitrogen ions are removed and do not play a role asp-type dopants in ZnSe growth by molecular beam epitaxy. The activation ratio of nitrogen atoms is increased by removing the nitrogen ions. ©1997 American Institute of Physics.