This paper investigates the morphological evolution of hydrogenated amorphous silicon layers obtained by plasma enhanced chemical vapor deposition at different H dilutions in the regime close to the formation of the nanocrystalline phase. The role of hydrogen in the transition from the amorphous to the crystalline phase is investigated by accurate structural and chemical characterization, from the early stages of nucleation, where the nuclei present size slightly larger than the critical nucleus, i.e., about 0.8 nm in radius, up to the formation of crystalline grains larger than 30 nm in radius. A correlation between the structural characteristics of such crystalline phase and the bonding mechanism of Si with H through multiple hydrides, such as Si-H2 and Si-H3 is found, particularly the tri-hydrides are found to be directly correlated to the shape and the size of the nanocrystallites present in the films. The multiple hydrides …