Quantitative X-ray reflectivity analysis of growing thin films

Professor J. R. Engstrom (Cornell University) and his colleagues have recently published a detailed comparative study on surface morphology obtained from in-situ, time-resolved X-ray reflectivity, which is extremely feasible as a tool for investigating surface and interfaces during thin film growth, but requires some modeling of the growth process for the interpretation. The research group prepared two sets of organic thin films, pentacene/SiO₂ and diindenoperylene SiO₂; for each system, giving a total of four films, grown to different thicknesses, under nominally identical conditions. The X-ray reflectivity data were analyzed based on three different models, and the obtained parameters were directly compared with AFM data. It was found that all models employed can give good agreement between the surface morphology obtained from fits with the actual morphology at early times. On the other hand, this agreement deteriorates at later times, once the root-mean squared (rms) film roughness exceeds about 1 monolayer. It was also found that the best fits to reflectivity data, corresponding to the lowest values of χ², do not necessarily yield the best agreement between simulated and measured surface morphologies, simply because the model reproduces all local extrema in the data. For more information, see the paper, "Quantitative modeling of in situ x-ray reflectivity during organic molecule thin film growth", A. R. Woll et al., Phys. Rev. B84, 075479 (2011).