  By means of ab-initio calculations we investigate the optical properties of
pure a-SiN$_x$ samples, with $x \in [0.4, 1.8]$, and samples embedding silicon
nanoclusters (NCs) of diameter $0.5 \leq d \leq 1.0$ nm. In the pure samples
the optical absorption gap and the radiative recombination rate vary according
to the concentration of Si-N bonds. In the presence of NCs the radiative rate
of the samples is barely affected, indicating that the intense
photoluminescence of experimental samples is mostly due to the matrix itself
rather than to the NCs. Besides, we evidence an important role of Si-N-Si bonds
at the NC/matrix interface in the observed photoluminescence trend.
