Epitaxial transformation of hcp-fcc Ti sublattices during nitriding processes of evaporated-Ti thin films due to nitrogen-implantation

Atomistic transformation processes of Ti films due to N-implantation have been clarified through in-situ observations by using transmission electron microscope (TEM) along with molecular orbital calculations. The N ₂⁺ ions with 62 keV are implanted into as-deposited Ti films which consist of hcp-Ti and TiH_x with preferred orientations, in the 400 kV analytic high resolution TEM combined with ion accelerators. Thus, titanium nitride (TiN_y) films with preferred orientations are epitaxially formed by the inheritance of partial atomic arrangement of hcp-Ti or TiH_x in as-deposited Ti films and by the occupation of octahedral sites by N atoms, which elucidates that epitaxial transformation of hcp-fcc Ti sublattices occurs. The analysis of electronic structure of Ti films during the implantation clarifies that octahedral sites of hcp-Ti with larger space have lower electron density, which leads to the invasion of N ions into octahedral sites. Thus, the hcp-fcc transformation is induced by the shear in the <0 1·0> direction on the (0 0·1) plane, promoted by the forming of covalent bonds mainly composed of hybridized orbitals due to combination of Ti3d and N2p orbitals, and by the weakening of Ti-Ti bonds.