Single-Source Precursors to Diborides of Zirconium and Hafnium

Stable scalable syntheses of precursors will be developed for the chemical vapor deposition (CVD) of ZrB2 and HfB2, both of which are ultrahigh temperature ceramics. Precursors will derive from decaborane, which readily forms stable metal complexes when deprotonated. Present CVD technology relies on the zirconium or hafnium halides and halogenated boranes or diborane as precursors, with evolution of corrosive hydrogen halide byproducts; the films can also contaminated with halogen. A few reports describe CVD with the homoleptic borohydrides Zr(BH4)4 and Hf(BH4)4, but these are pyrophoric solids that degrade upon standing (even in inert atmosphere) and are impractical to transport. Moreover, the deposited films are overstoichiometric in boron. The innovation of this work is the use of decaborane as boron reservoir in single-source CVD precursors. We expect that decaborane will support safer, more thermally stable precursors, in scalable syntheses. Their character as single-source reagents confers cost advantages: cleanup and by-products are minimized. The proposed precursors are halogen and oxygen-free. Zirconium and hafnium complexes incorporating the [B10H12]2– dianion will be synthesized, with chemical and spectroscopic characterization, at Case Western Reserve University in the laboratory of Professor Thomas Gray, and scaled up in the laboratory of Boron Specialties, LLC.