Structural study of amorphous hydrogenated and unhydrogenated titanium carbide thin films by extended x-ray-absorption fine structure and extended electron-energy-loss fine structure.

Abstract

Thin films of hydrogenated and unhydrogenated titanium carbide, known to be amorphous from x-ray and electron diffraction studies, nevertheless exhibit short-range order at the level of the first-nearest-neighbor shell when studied by extended x-ray-absorption fine structure and extended electron-energy-loss fine structure. This short-range order appears to consist of the same octahedrally coordinated units present in crystalline titanium carbide (NaCl structure), where a titanium atom is surrounded by an octahedron of six carbon atoms and vice versa. The persistence of this basic structural unit is attributed to the difference in atomic radii between C and Ti and to the strong nature of the Ti-C bond. A slight relaxation in interatomic distance relative to the crystalline state, which increases upon hydrogenation, is also observed. The atomic positions in the first coordination shell in amorphous titanium carbide are slightly displaced relative to the crystalline form. Yet, upon hydrogenation, this displacement decreases in the first shell, made of carbon atoms and vacancies, surrounding a Ti atom by 40% for the 4.5 at. % H and 60% for the 6 at. % H relative to the unhydrogenated amorphous ${\mathrm{TiC}}_{\mathrm{x}}$ films. By contrast, the structural disorder in the first shell of Ti atoms surrounding a C atom is practically unaffected by the addition of hydrogen. These results imply that the hydrogen atoms, instead of occupying tetrahedral vacancies, fill carbon vacancies in the amorphous phase as they do in crystalline ${\mathrm{TiC}}_{\mathrm{x}}$, in agreement with our previous findings from static secondary-ion mass spectroscopy and x-ray photoelectron spectroscopy.