Pathways of atomistic processes on TiN(001) and (111) surfaces during film growth: an ab initio study

Abstract

Density functional methods were used to calculate binding and diffusion energies of adatoms, molecules, and small clusters on TiN(001) and TiN(111) surfaces in order to isolate the key atomistic processes which determine texture evolution during growth of polycrystalline TiN layers. The surface energy for nonpolar TiN(001), 81 meV/Å2, was found to be lower than that of both N- and Ti-terminated TiN(111) polar surfaces, 85 and 346 meV/Å2. While N2 molecules are only weakly physisorbed, Ti adatoms form strong bonds with both TiN(001), 3.30 eV, and TiN(111), 10.09 eV. Ti adatom diffusion is fast on (001), but slow on (111) surfaces, with calculated energy barriers of 0.35 and 1.74 eV, respectively. The overall results show that growth of 111-oriented grains is favored under conditions typical for reactive sputter deposition. However, the presence of excess atomic N (due, for example, to collisionally induced dissociation of energetic N2+ ions) leads to a reduced Ti diffusion length, an enhanced surface island nucleation rate, and a lower chemical potential on the (001) surface. The combination of these effects results in preferential growth of 001 grains. Thus our results provide an atomistic explanation for the previously reported transition from 111 to 001 texture observed for sputter deposition of TiN in Ar/N2 mixtures with increasing N2 partial pressure PN2 and at constant PN2 with increasing N2+/Ti flux ratios incident at the growing film.