Medium-range order in silicon oxycarbide glass by fluctuation electron microscopy

Silicon oxycarbide is a metastable material that has generated interest because of the great flexibility in properties that is attainable with a mixture of divalent and tetravalent anions within the network structure. In addition to the network bonding, however, these materials have also exhibited a strong propensity to include carbon-carbon bonding - so-called 'free carbon' - within the structure regardless of synthesis method. While evidence for the presence of free carbon is overwhelming, traditional diffraction characterization methods have been unable to definitively identify ordering or segregation in the material. Fluctuation electron microscopy (FEM) is a relatively new transmission electron microscopy technique that is specifically sensitive to medium-range order, which is ordered bonding on the length scale of roughly 8-50 Å. We utilize this method to identify semi-ordered bonding present in silicon oxycarbide thin films deposited by reactive rf sputtering over a wide composition range. These results indicate that the use of FEM can be extended to materials which are compositionally heterogeneous at the nano-scale. We show evidence of clusters approximately 1.8 nm in diameter that exhibit correlations similar to the bonding in turbostratic carbon.