We present a systematic study of the local hydrogen bonding in hydrogenated
polymorphous silicon thin films (pm-Si:H), a heterogeneous material deposited on the edge
of crystallinity, by means of Fourier transform infrared spectroscopy. A vibrational mode at
is reported and attributed to hydrogen atoms bonded in hydrogen-rich regions present at
the interface between ordered regions and the amorphous matrix. This assignment is found
to be in good agreement with previous spectroscopic ellipsometry and Raman spectroscopy
studies. Combined with stress measurements we draw a picture of the nanostructure of
pm-Si:H films, namely a two-domain material exhibiting: (i) large fluctuations
of H content, (ii) a high mass density and (iii) the coexistence of highly strained
crystalline phases with a relaxed amorphous matrix. Unexplained optoelectronic
properties and metastability phenomena can also be accounted for by this picture.