Interactions of ion beams with surfaces: Dynamics of the reaction of N2+ with rhenium

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Abstract
Reactions of N2+ ion beams with the surface of polycrystalline rhenium foil over the range 150–3 000 eV have been studied by the techniques of x‐ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS). The reactions produce a nitride layer of the type ReNx, 0.21⩽x⩽0.40, with x varying as a function of ion kinetic energy and depth into the surface. The nitride layer extends from the surface down to the penetration depth of the atoms, which varies from ∼15 Å at 0.3 keV to ∼76 Å at 2.5 keV ion energy. The product nitride and chemisorbed nitrogen on Re are distinctly different, exhibiting N1s binding energies of 397.9 and 396.8 eV, respectively, and TDS maxima near 425 and 600 °C, respectively. The value of x in ReNx increases linearly with the nitrogen ion flux and reaches a steady state condition at a dose of 9.5 ×1016 ions/cm2 which is determined by the opposing rate of nitride formation and the sputtering rate by impinging N2+ ions. An expression describing the rate of nitration as a function of the reaction cross section σr and the sputtering cross section σs is derived. The simplified expressions for the limiting cases of initial rates (t→0) and the steady state (t→∞) product concentration, which are used to interpret the experimental results, yield σr≃3×10−18 cm2 for the N2++Re(metal)→ReNx reaction at an ion impact energy of 500 eV.