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 subsequent reaction paths for ligand exchange and elimination reactions which may occur in the gas phase. Geometry optimizations were carried out at the B3LYP/6-311G(d) level of density functional theory, and energies were calculated 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 complexes 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 kcal/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 very 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)NH 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 energies 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 the barriers and heats of reaction are sufficiently high to make these reactions unlikely in the gas phase during the CVD process.