The impact of H$_2$ and N$_2$ on the material properties and secondary electron yield of sputtered amorphous carbon films for anti-multipacting applications

Amorphous carbon thin films were prepared by direct current hollow cathode sputter deposition in Ar discharge with the injection of small amounts of H$_2$ and/or N$_2$. The influence of these additives on the film properties with particular focus on the application as a coating for electron cloud mitigation in particle accelerators is characterized by optical spectroscopy, X-ray photoelectron spectroscopy, and secondary electron yield (SEY) measurements. The SEY maximum increased from initially 0.98 with pure Ar in the gas discharge up to 1.38 at 0.5% H$_2$ while the addition of 1% of pure N$_2$ enabled to reduce it to 0.88. The simultaneous addition of N$_2$ to the H$_2$ containing discharge allowed an average SEY maximum reduction of 20%. The optical bandgap revealed a correlation between the increase/decrease of the bandgap and the SEY increment/reduction for H$_2$/N$_2$ addition. The surface composition changes and the resulting modification of the sp$^2$ /sp$^3$ ratio correlate with the changes in SEY and optical properties. The obtained results highlight the potential of intentionally injected N$_2$ to counteract the detrimental effect of the inevitable H$_2$ partial pressure in the coating systems during the production of amorphous carbon thin films for anti-multipacting applications in particle accelerators.