The Effect of Ion Damage and Annealing on Superconducting Transition-Metal - Nitride Compounds

Thin films of the B1 phase superconducting compounds vanadium nitride and titanium nitride have been formed. This was accomplished by heating the previously evaporated pure metal films in high purity nitrogen gas. The resistivity at room and low temperature, the superconducting transition temperature T(,C), and the upper critical field of these films were measured using a four probe d.c. resistive technique and found to be similar to those measured on bulk samples of the same compound. The films were then irradiated with nitrogen ions and the effect of lattice damage on these parameters was determined. It was found that the dependence on ion fluence of the residual resistivity and the transition temperature obeyed saturating exponential functions that could be derived from a simple defect production and annealing model. The renormalized electronic density of states N*(O) was calculated as a function of ion fluence, while the band density of states N('b)(O) was calculated using the electron lifetime model. The electron-phonon coupling constant was determined from these densities of states and from the McMillan equation for T(,C). The results do not agree and it is shown that spin fluctuations cannot be used to explain the discrepancy. It is argued that some mechanism, other than lifetime reduction of the band density of states, is responsible for the observed effects. This is in contrast to the high temperature A15 superconductors in which the electron lifetime model yields large reductions in N('b)(O). It is thought that any other mechanism present in these materials would be overshadowed by this large reduction. Subsequent annealing studies were performed on these samples. The results indicate that the radiation damage effects are, to a large extent, reversible. It is also found that annealing in vacuum at high temperatures results in loss of nitrogen and thus degradation of the properties of the material. From the ion damage and annealing results and from thermodynamic properties found in the literature it is argued that the bonding in TiN is almost three times stronger than in VN.