Epitaxial NaCl structure δ-TaNx(001):: Electronic transport properties, elastic modulus, and hardness versus N/Ta ratio

While metastable B1-NaCl-structure delta -TaNx is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications, it is a complex material exhibiting a wide single-phase field, x similar or equal to0.94-1.37, and little is known about its fundamental properties. Here, we report physical properties of epitaxial delta -TaNx layers grown as a function of x on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. The room-temperature resistivity (rho =225 mu Omega cm), hardness (H=30.9 GPa), and elastic modulus (E=455 GPa) of delta -TaNx(001) are independent of x over the range 0.94-1.22. However, changes in the electronic structure associated with increasing x>1.22 lead to an increase in rho with a decrease in H and E. All delta -TaNx(001) layers exhibit negative temperature coefficients of resistivity between 20 and 400 K due to weak carrier localization. delta -TaNx is superconducting with the highest critical temperature, 8.45 K, obtained for layers with the lowest N/Ta ratio, x=0.94. Based upon the above results, combined with the fact that the relaxed lattice constant a(0) shows only a very weak dependence on x, we propose that the wide phase field in delta -TaNx is due primarily to antisite substitutions of Ta on N (and N on Ta) sites, rather than to cation and anion vacancies. To first order, antisite substitutions in TaNx are isoelectronic and hence have little effect on charge carrier density. At sufficiently high N/Ta ratios, however, simple electron-counting arguments are no longer valid since large deviations from stoichiometry alter the character of the band structure itself. (C) 2001 American Institute of Physics.