We have studied the evolution of the structure of intrinsic and doped hydrogenated amorphous silicon films exposed to a hydrogen plasma. For this purpose, we combine in situ spectroscopic ellipsometry and secondary ion mass spectrometry measurements, We show that hydrogen diffuses faster in boron-doped hydrogenated amorphous silicon than in intrinsic samples, leading to a thicker subsurface layer from the early stages of hydrogen plasma exposure. At longer times. hydrogen plasma leads to the Formation of a microcrystalline layer via chemical transport, but there is no evidence for crystallization of the a-Si:II substrate, Moreover. we observe that once the microcrystalline layer is formed, hydrogen diffuses out of the sample. (C) 2005 Elsevier B.V. All rights reserved.