The surface layer formed during chemical vapour deposition (CVD) of tungsten trioxide thin films was studied by means of atomic force microscopy (AFM), scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE). Films were deposited at atmospheric pressure by pyrolytic decomposition of tungsten hexacarbonyl (W(CO)(6)) and were annealed at 400-500 degreesC. Data from SE experiments and theoretical simulations showed that a layer forms at the surface of the WO3 film that has a different structure and composition from the bulk film. This surface layer becomes thicker with increasing oxygen flow rate during film deposition. This layer was predominantly amorphous for as-deposited films and predominantly crystalline after annealing. Root mean squared (rms) roughness values were calculated from AFM images of the surface layer. A high degree of surface roughness was revealed after deposition and annealing (similar to40 nm), and the roughness value increased after additional annealing to 470 degreesC in similar to10(-1) Pa of O-2. (C) 2003 Elsevier Science B.V. All rights reserved.