Hydrogen diffusion is a crucial step in film growth by chemical vapor deposition of both hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (mu-Si:H) materials. To gain an insight into the correlation between hydrogen diffusion and the amorphous to microcrystalline transition, we have exposed freshly deposited intrinsic, boron- and phosphorus-doped a-Si:H thin films to hydrogen (or deuterium) plasma in conditions of mu c-Si:H deposition by chemical transport. Using both in-situ and ex-situ characterizations techniques, we examined the kinetics of hydrogen excess evolution during the plasma exposure. Solution of the partial differential equation for the diffusion of mobile H atoms with a specific boundary condition that accounts for the reduction of atomic H flux with the growth of the mu c-Si:H layer supports the theory that the out-diffusion is a consequence of the growth of the mu c-Si:H layer. (C) 2009 Elsevier B.V. All rights reserved.