Structure and interaction of point defects in transition-metal nitrides

Transition-metal nitrides (TMNs) are used in a variety of applications because of their renowned hardness and stability. Departure from the nominal 1:1 stoichiometry is often observed for TMNs and, therefore, defects have a prevalent role on TMN physical properties. Here, we report the results of first-principles calculations on nitrogen and metal point defects in the prototype system of TiN and in HfN and ZrN. We find features which are common to all systems, while we unravel also key differences. In certain cases-for example, N interstitials in TiN-the interaction between defects is attractive and it favors the formation of defect complexes. Other defects, like N interstitials in HfN and ZrN, do not pair up. We also present results on the effect of point defects on the electronic properties of TiN. Finally, we discuss pertinent experimental data and the implications of our findings on the thermal stability of TMN films.