Single and multiple quantum wells of GaNyAs1-y were grown on GaAs substrates using solid-source molecular beam epitaxy (MBE) with an RF plasma cell. Dynamic gas switching was used to control the gas composition and active nitrogen flux produced using ultra-pure Ar/N-2 gas mixtures. Real-time optical spectroscopy of the plasma during growth was used to monitor the active nitrogen flux and to optimize the growth method. High-resolution X-ray diffraction, secondary ion mass spectrometry, transmission electron microscopy and low temperature photoluminescence spectroscopy were used to study the composition, structural and optical quality of the structures. We demonstrate the exceptional sensitivity of these techniques to the nitrogen profiles in the films and the high quality of the layers produced using the Ar dilution technique. (C) 2002 Elsevier Science B.V. All rights reserved.