Chemical and electronic structure of composite films deposited by plasma-enhanced chemical vapor deposition from orthocarborane and pyridine source compounds

Semiconducting boron carbide films, with aromatic moieties, were formed by plasma-enhanced chemical vapor deposition (PECVD) using pyridine (C5H5N) and closo-1,2 dicarbadodecaborane (1,2-B10C2H12; ortho-carborane) as source compounds. X-ray photoelectron spectroscopy (XPS), in conjunction with pyridine/orthcarborane cluster density functional theory (DFT) calculations, indicates that such films consist of partially dehydrogenated orthocarboranes coordinated to pyridine moieties at icosahedral B sites. The XPS data also suggest a constant average coordination number of 3-4 pyridines per icosahedral carborane, indicating that higher pyridine content in the film results in greater areas of plasma-polymerized pyridine. Ultraviolet photoelectron spectroscopy (UPS) results and DFT cluster calculations indicate that electronic states near the valence band maximum are associated with pyridine moieties, while states near the conduction band minimum are associated with either carborane or pyridine moieties. Variable angle spectroscopic ellipsometry (VASE), the UPS data and theoretical results also indicate only gradual changes in indirect band gap energies with changing pyridine/orthocarborane stoichiometry, in agreement with previously reported results. The results presented here are consistent with and provide additional insight regarding recent neutron voltaic and photoluminescence results for these materials.