The variation of the composition and structural properties of plasma enhanced chemical vapour deposited hydrogenated amorphous silicon-nitrogen alloys (a-SiNx:H-y) are studied as a function of the feed gas flow ratio. A capacitively coupled parallel plate chemical vapour deposition reactor (with R = NH3/SiH4, ratios of 0.2 to 200) was used to deposit the a-SiNx:H-y thin films. Rutherford backscattering spectrometry and elastic recoil detection were used to analyse the composition of the films. The Si content in the films decreased in a logarithmic manner for 0 < R < 4 and saturated for higher R values at similar to 27 at.%. The N content increased logarithmically with R and saturated at a maximum asymptotic value of similar to 49 at.% for R > 4. A turning point for the experimentally measured properties was observed at R approximate to 4 for the specified deposition conditions. This corresponds to a N/Si ratio of about 1.45 +/- 0.15 with a hydrogen content of 22 at.% for the deposited a-SiNx:H-y films. Below this pivotal ratio, the films have high growth rates, a refractive index between 1.9 and 2.7, a N/Si ratio between 0.5 and 1.5 and moderate values of compressive stress (similar to 0.7 GPa). While, above this pivotal ratio of R approximate to 4, growth rates become significantly lower, the refractive index minimises to 1.8, N and Si concentrations in the films saturate and the compressive stress rapidly increases.