Molecular orbital studies of titanium nitride chemical vapor deposition: Gas phase complex formation, ligand exchange, and elimination reactions

The chemical vapor deposition (CVD) of titanium nitride can be carried out with TiCl4 or Ti(NR2)(4) and NH3. The present study uses molecular orbital methods to examine complexes of NH3 with TiCl4 and Ti(NH2)(4) and the subse quent reaction paths for ligand exchange and elimination reactions which ma y occur in the gas phase. Geometry optimizations were carried out at the B3 LYP/6-311G(d) level of density functional theory, and energies were calcula ted using a variety of levels of theory, up to G2 for systems with five or fewer heavy atoms. The TiCl4. NH3, TiCl4.(NH3)(2), and Ti(NH2)(4). NH3 comp lexes are bound by 14.9, 30.9, and 7.9 kcal/mel, respectively. The barrier for TiCl4 + NH3 -> TiCl3NH2 + HCl is 18.4 kcal/mol and is lowered by 23.1 k cal/mol with the introduction of a second NH3. The computed barrier height of 8.4 kcal/mol for the Ti(NH2)(4) + NH3 ligand exchange reaction is in ver y good agreement with the experimental activation energy of 8 kcal/mol for Ti(NMe2)(4) + NH3 ligand exchange. The barrier for formation of Ti(NH2)(2)N H by elimination from Ti(NH2)(4) is 33.5 kcal/mol and is reduced by 10 kcal /mol when assisted by an additional NH3. However, examination of the free e nergies at CVD conditions indicates that the reactions without catalysis by an extra NH3 are favored. Further elimination of NH3 from Ti(NH2)(2)NH can yield a diimido product, Ti(NH)(2), or a nitrido product, Ti(NH2)N, but th e barriers and heats of reaction are sufficiently high to make these reacti ons unlikely in the gas phase during the CVD process.