James A. Jensen

Tailored Organometallics as Low-Temperature CVD Precursors to Thin Films

We have been investigating the use of “tailored” organo-metallic precursors for the chemical vapor deposition (CVD) of thin films in an effort to open up new approaches to the low-temperature synthesis of ceramic, electronic, and optical materials that contain transition metals. Following our approach to the successful deposition of high-quality titanium carbide thin films at 150–250°C from tetraneopentyltitanium, we have extended this technique to the preparation of other carbides, such as ZrC, HfC, TaC, and WC, at temperatures some 1000°C lower than previous CVD methods from the metal halides and methane. The details of the thermolysis of the organometalico precursors are most consistent with a mechanism involving homolysis of M-C bonds to give alkyl radicals followed by C-C and C-H bond activation steps. Metal diboride thin films such as TiB 2 , ZrB 2 , and HfB 2 can be prepared at ca. 250 °C by CVD of suitable tetrahydroborate precursors: Ti(BH 4 ) 3 -(MeOCH 2 CH 2 OMe), Zr(BH 4 ) 4 , and Hf(BH 4 ) 4 . In these cases, the principal byproducts are hydrogen and diborane. Mirror-bright, adhesive thin films up to 2 μm thick can be grown on a variety of substrates including glass, steel, aluminum, copper, silicon, and graphite. Despite the amorphous nature of the films, their electrical resistivities are essentially identical with those of the bulk polycrystalline material.

Transition metal tetrahydridoborates as models of methane activation: synthesis and structure of Ti(BH4)3(PMe3)2

The X-ray crystal structure of Ti(BH4)3(PMe3)2 shows that two of the BH4– groups possess unusual geometries involving interaction of the titanium centre with one B–H bond in a ‘side-on’ manner; this geometry resembles one proposed transition state for the approach of a molecule of methane to a transition metal leading to C–H bond cleavage.