Superconductivity and structural changes of nitrogen-ion implanted Mo thin films

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Abstract
Sputtered Mo thin films were implanted with 125 keV N2+ ions at room temperature to various doses ranging from 1*1017 ions/cm2 to 16*1017 ions/cm2. The X-ray diffraction studies showed that phase transformation occurred from the cubic gamma -Mo2N to a B1-type MoN phase, and finally to the hexagonal delta -MoN phase. The superconducting transition temperature of the implanted films increased initially with nitrogen-ion dose, and reached a maximum value of Tc=6.8 K for 5*1017 ions/cm2. The X-ray photoelectron spectra of the core-electron levels revealed that during this stage electron charge transfer proceeded from the Mo atoms to the N atoms. Thus, the phase transformation of cubic gamma -Mo2N to B1-MoN was considered to occur continuously with increasing nitrogen dose by filling up vacant sites of the nitrogen atom sublattice in the B1 structure. The change of Tc and the corresponding hardness changes observed were discussed qualitatively in terms of the effect of nitrogen vacancies on the electronic structure and the bonding character of the sub-stoichiometric or stoichiometric MoN compound.