In situ reflection anisotropy spectroscopy (RAS) during GaAs(0 0 1) surface nitridation and the chemical beam epitaxy growth of GaInNAs/GaAs(0 0 1) films containing variable levels of N, is correlated with post-deposition scanning transmission electron microscope (STEM) energy dispersive X-ray chemical composition analysis from cross-section samples. For low N fluxes, below the solubility limit of N in InGaAs, InGaAsN behaves much the same as InGaAs during growth. However for higher N fluxes, during the growth of InGaAsN/GaAs the RAS shows the surface permanently loses its ability to form an As-dimer reconstructed surface, and bulk optical anisotropy of the order 1% develops. Post-deposition STEM from cross-sections shows that the low-N InGaAsN films are initially homogeneous and of uniform thickness, whereas the high-N InGaAsN films grow with a segregated microstructure containing InGaAs(N) and GaN-like phases. The segregated microstructure is anisotropic along the [1 1 0] and [-1 1 0] directions, which explains the observed RAS bulk optical anisotropy. Any nitridation of the GaAs by the N-atom flux prior to growth promotes the growth of islands rather than smooth homogeneous films of InGaAsN. (C) 2002 Elsevier Science Ltd. All rights reserved.