Aromatic-doped boron carbide films formed by PECVD of metacarborane and aniline or pyridine: Chemical and electronic structure

Closo-1, 7-dicarbadodecaborane (metacarborane) and aniline or pyridine have been used to fabricate semiconducting composite films by plasma enhanced chemical vapor deposition (PECVD). Chemical and electronic structures of films with aniline/metacarborane or pyridine/metacarborane ratios of ~ 3:1 were characterized by X-ray photoelectron spectroscopy (XPS), variable angle spectroscopic ellipsometry (VASE), and first-principles density functional theory (DFT)-based electronic structure calculations. Aniline moieties are coordinated to carborane icosahedra by CB bond formation, whereas NB and possibly CB bonding was observed for pyridine/metacarborane films. The aniline/metacarborane film displayed indirect and direct band gaps of 1.0 eV and 2.9 eV, respectively, with corresponding values for the pyridine/metacarborane film of 1.4 eV and 3.3 eV. In contrast, the metacarborane film without aromatic doping exhibits indirect and direct band gaps of 2.6 and 4.9 eV, respectively. DFT results indicate that in both aniline/metacarborane and pyridine/metacarborane films, states near the valence band maximum are associated with the aromatic species, while states near the bottom of the conduction band are associated with either the aromatic or metacarborane moiety. These results indicate that aromatically-doped boron carbide films derived from metacarborane isomers exhibit very similar structures and characteristics to those formed by orthocarborane isomers. Due to the n-type (p-type) nature of boron carbide films formed by PECVD metacarborane (orthocarborane), the results reported here suggest that aromatic doped boron carbide p-n junctions are possible for neutron detection or other applications.