The formation of a-Si:H-a-SiN(x) and p-doped a-Si:H (p)-intrinsic (i) interfaces has been investigated in situ by spectroscopic phase modulated ellipsometry (PME) from UV to IR. The thin films are deposited by plasma enhanced chemical vapor deposition (PECVD). In both cases, the nature of the interface is found to be influenced by the deposition sequence affecting the corresponding devices’ behaviour. The a-SiN(x)/a-Si:H and p/i interfaces are found to be atomically sharp. A different behaviour is observed when p-doped a-Si:H or a-SiN(x) is deposited first. In the latter case, the ellipsometric measurements clearly reveal a nitrogen incorporation in the first monolayers of a-Si:H together with an increase of intensity of the SiH bonds at the interface. Likewise, a hydrogen accumulation, at a level of a few monolayers, is observed at the i/p interface. More generally, the monolayer sensitivity of IRPME to vibrational absorption is emphasized.