In-situ Observation of Nitriding Processes of Deposited-Ti Thin Films due to Ion Implantation in an Analytical Transmission Electron Microscope

Abstract

Both hcp-Ti and CaF2-type TiHx (x ≅ 1.5), and only hcp-Ti, with preferred orientations have been deposited on NaCl (001) substrates held at room temperature (RT) and 250°C, respectively. In order to clarify atomistic growth processes of TiNy films due to the implantation of nitrogen ions (N2+) with 62 keV into as-deposited Ti films, changes of the concentration of H and/or N atoms in Ti films, and of the crystallographic and electronic structures of the films by heating and by nitriding have been investigated by in-situ transmission electron microscope (TEM) equipped with electron energy loss spectroscopy (EELS), with the aid of molecular orbital calculations. Hydrogen atoms which constitute TiHx in the Ti films deposited at RT are released with heating, and are completely released at 350°C. The H-released unstable fcc-Ti sublattice is then transformed into hcp-Ti. The energy loss peak due to the plasmon excitation observed by EELS for TiHx shifts to the lower energy side with the decrease in the electron density in the hybridized valence band, which means the release of H atoms from TiHx during heating. On the other hand, the TiNy is epitaxially formed by the N-implantation into the hcp-Ti, through the epitaxial transformation of the hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti, and accompanying the occupation of O-sites of the transformed fcc-Ti by N atoms. The energy loss peak due to the plasmon excitation during N-implantation into the hcp-Ti films gradually shifts to the higher energy side with the increase in dose of N, which means the increase in the electron density in the hybridized valence band. By comparing the results of EELS with those of Rutherford Backscattering Spectrometry, it is clarified that the maximum concentration of N in Ti films during the N-implantation depends on the implantation temperature. The comparison between the results of EELS and those of the molecular orbital calculations elucidates that the changes of electronic structures near the Fermi level depend on the ratio of N/Ti in the films. Furthermore, taking into account the bonding interaction of Ti sublattices with ligand N atoms, the transformation mechanisms between hcp-Ti and TiNy due to the implantation of N atoms are proposed. [DOI: 10.1380/ejssnt.2011.191]