Density functional methods were used to calculate binding and diffusion energies of adatoms, molecules, and small clusters on TiN͑001͒ and TiN͑111͒ surfaces in order to isolate the key atomistic processes which determine texture evolution during growth of polycrystalline TiN layers. The surface energy for nonpolar TiN͑001͒, 81 meV/Å 2 , was found to be lower than that of both Nand Ti-terminated TiN͑111͒ polar surfaces, 85 and 346 meV/Å 2. While N 2 molecules are only weakly physisorbed, Ti adatoms form strong bonds with both TiN͑001͒, 3.30 eV, and TiN͑111͒, 10.09 eV. Ti adatom diffusion is fast on ͑001͒, but slow on ͑111͒ 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 N 2 ϩ ions͒ leads to a reduced Ti diffusion length, an enhanced surface island nucleation rate, and a lower chemical potential on the ͑001͒ 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/N 2 mixtures with increasing N 2 partial pressure P N 2 and at constant P N 2 with increasing N 2 ϩ /Ti flux ratios incident at the growing film.
We use scanning tunneling microscopy to study the nucleation of homoepitaxial TiN layers grown on TiN(001) by ultrahigh vacuum reactive magnetron sputtering in pure N 2. Nucleation lengths are measured using in situ scanning tunneling microscopy as a function of temperature on two-dimensional islands as well as on large open terraces. At low growth temperatures, 500ഛ T s ഛ 865°C, nucleation is diffusion limited and we extract a surface diffusion energy of 1.4± 0.1 eV. At higher temperatures, 865ഛ T s ഛ 1010°C, nucleation is limited by the formation rate of stable clusters for which we obtain an activation energy of 2.6± 0.2 eV. Ab initio calculations combined with our experimental results suggest that the primary diffusing adspecies are TiN x molecules with x = 2 and/or 3.
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