Lattice distortion in thin films of IVB metal (Ti, Zr, Hr) nitrides

Nitride films of IVB metals (titanium, zirconium and hafnium) exhibit non-conventional lattice distortion which is displayed in the expansion of the lattice parameter calculated from the (111) diffraction peak. It is commonly assumed that this phenomenon may be explained in terms of rhombohedral distortion of the cubic lattice. However, our experimental data do not agree with the shift and broadening of the peak that are characteristic of rhombohedral distortion. We propose an alternative model for the observed expansion which is based on selective trapping of interstitial atoms in various crystallographic planes. It is shown that entrapment of interstitial atoms in the (111) plane is favorable, in comparison with the (100) plane. Entrapped atoms cause hydrostatic lattice expansion which varies with the different orientations of the grains. Non-uniform lattice expansion seems to be the main source of intrinsic microstrains and macrostrains usually observed in thin sputtered films.