Building a Framework for Understanding Amorphous Hydrogenated Boron Carbide as a Novel Semi-Insulating Electronic Material

Boron carbide (BC) possesses a number of unique characteristics that render it suitable for specialized device applications, including neutron detectors and low-k dielectrics. Since 10B is one of few isotopes with a high neutron capture cross section, boron carbide is one of a handful of boron-rich semiconductors under consideration for direct-conversion solid-state neutron detection. Further, with such a low average atomic number, BC has the potential to exhibit extremely low polarizability and therefore, a low dielectric constant, while also retaining chemical, thermal, and mechanical robustness, which positions well for low-k dielectric uses. For both of these applications, low leakage currents are required, and traditional semiconducting polycrystalline BxC (x ~ 4-11) is therefore unsuitable. A unique BC variant, amorphous hydrogenated boron carbide (a-BxC:Hy), fabricated by plasma-enhanced chemical vapor deposition from carborane precursors, has been found to possess the necessary semi-insulating properties for these applications. Known since the early 90's, carborane-based a-BxC:Hy has been the subject of some physical and electronic structure studies, but a lot remains to be done to thoroughly characterize its materials properties, as well as their tunability. Due to the amorphous nature of the solid, much of this characterization is nontrivial. Although it possesses a relatively similar stoichiometry to traditional BxC, a-BxC:Hy is a very different material, both in terms of atomic structure and material properties. Further, due to its complex bonding, it does not fall into a conventional framework for analysis, such as that developed for the classes of amorphous inorganic tetrahedral semiconductors or organic semiconductors. This contribution will describe recent efforts toward understanding a-BxC:Hy, including its local physical structure and the role of carbon and hydrogen, the nature of its electronic structure and disorder parameters such as Urbach energy, and their relationship to optical and electrical transport properties.