Nitrogen‐Induced Microcrystalline‐to‐Nanocrystalline Structure Transition in Diamond Films Grown by Microwave Plasma Chemical Vapor Deposition: Comparison of N₂ and NH₃ Precursors

The structure and properties of polycrystalline chemical vapor deposition (CVD) diamond coatings grown by microwave plasma CVD in H 2 –CH 4 mixtures can be effectively controlled by nitrogen admixture in the process gas. Here, a comparative study of adding nitrogen from different precursors, N 2 and ammonia NH 3 , in concentrations up to 0.4%vol on grain size, texture, surface roughness, and sp 2 /sp 3 ratio of the produced films on Si substrates is performed. A transition from microcrystalline to smooth nanocrystalline diamond structure is found to occur at a certain concentration of the precursor, for NH 3 this threshold concentration being much lower, 0.02–0.1%, than for N 2 . The higher activity of ammonia is associated with easier formation of cyano radicals in the plasma as detected with optical emission spectroscopy, in accord with the lower bond‐dissociation energy for NH 3 compared to N 2 . On the contrary, a smaller addition (0.004%) of either N 2 or NH 3 promotes almost doubling of the growth rate, while preserving the microcrystalline structure of the film. The results establish ammonia as a convenient alternative precursor to commonly used N 2 added in the plasma for the diamond film structure modifications.