We investigate the hydrogen related bonding structure in hydrogenated polymorphous silicon films (pm-Si:H) and hydrogenated microcrystalline silicon. Infra-red spectra reveal some new features for both kinds of films. namely new modes appearing in the stretching band. We propose that this peculiar hydrogen bonding occurs at the surface of crystallites in a platelet-like configuration. Increasing the size of the crystallites increases the size of the platelets so that the compactness of the resulting films decreases as shown by mass density measurements. We show that this peculiar hydrogen bonding is responsible for the low temperature (LT) effusion peak. These results point towards a metastable nature of the crystallites contributing to the growth of pm-Si:H films. () 2002 Elsevier Science B.V. All rights reserved.