Dilute nitride semiconductors of composition GaAs1-xNx (0.0017 < x < 0.0115) are grown by plasma-assisted molecular beam epitaxy with a helical resonator plasma source for active nitrogen. The plasma source is self-starting at the operating pressure and can be operated at two different frequencies for which the emission spectrum is dominated by N-2 molecules or by N atoms. For the same power the molecular-rich mode is found to produce a higher flux of active nitrogen. After extended operation the plasma tube becomes contaminated with As which reduces the flux of active nitrogen and creates a below band gap emission band in the photoluminescence of the GaAs1-xNx. For the clean discharge tube no difference is observed in the photoluminescence for samples grown in the molecule-rich or atom-rich mode. (c) 2007 American Vacuum Society.