The achievement of high N concentration is one necessary condition for the synthesis of crystalline C3N4 compounds. Other demands are the formation of the correct bonding configuration of carbon (sp(3)) and nitrogen (sp(2)) and the ability to produce adequate amounts of compact continuous crystalline material to allow definitive mechanical testing. However, for a number of deposition methods like ion beam assisted deposition (IBAD), reactive sputtering etc., the chemical sputtering effect, involving the formation of volatile CN species during film growth, is responsible for the limitation of the N content below 40 at.%. For low energy nitrogen irradiation ( < 100 V) using IBAD a critical arrival rate N-2(+) /C approximate to 1.8 was established above which no deposit is formed. Below this limit, the analysis of X-ray photoelectron spectra reveals a mixed CNx phase consisting of an aromatic and a non-aromatic structure, each of them containing a variety of local bonding environments. The fraction and the particular microstructure of the aromatic phase, which has a strong influence on film properties like hardness, can be adjusted as a function of N content and nitrogen ion current density. Experiments performed by evaporation of 8-aza-6-aminopurine (C4N6H4) and simultaneous irradiation by low energy nitrogen ions show that CN:H films can be grown with epsilon C/N ratio up to 1.3. Although a polymer-like phase is formed, an important aspect of this method is that chemical sputtering can be avoided by thermal condensation of a molecular precursor resulting in a significantly larger N content in the deposit. The proposed model for the evolution of the microstructure with N concentration, based on a critical discussion of our results together with a number of recent studies, can be helpful for the structural characterization of CN, films produced by other methods.