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The advantages of the InGaAsN/GaAs system comprising the active layers of single- and multi-quantum well laser diodes (emission wavelengths in the second and third optical windows, AlGaAs reflectors etc.), in comparison with other materials, make this quaternary material an interesting field for study. The monoatomic nitrogen species required for the growth of InGaAsN layers by molecular beam epitaxy, using a radiofrequency power source, are mixed in the plasma with ionised species, among others. The authors present a detailed characterisation of the plasma in the vicinity of the growing surface by measuring its I-V characteristics. A magnetic field was used to deflect ions and their effect on the properties of InGaAsN quantum wells was observed. These ionised species were observed to damage the surface, introducing nonradiative centres. As observed by photoluminescence experiments, the optical quality is improved as the density of ions impinging on the surface is reduced. Rapid thermal annealing experiments were also carried out, showing that the observed PL intensity improvement is related to the ion concentration in the quantum wells.