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Hydrogenated amorphous silicon (a‐Si:H) films having a thickness of 7 μm were deposited on molybdenum by dc glow discharge and then capped with a 0.1‐μm‐thick aluminum (Al) layer by thermal evaporation. Subsequent vacuum annealing at 500–725 °C resulted in the formation of crystalline Si, as observed by Raman scattering and x‐ray diffraction. This was in contrast to the uncapped a‐Si:H films which were still amorphous at the same annealing temperatures, except at ≳700 °C. That the Al capped films were crystalline caused a ten‐fold increase in the dark conductivity in comparison to the uncapped film annealed at the same temperature. The capped films annealed at 500 °C showed a photoresponse (the ratio of the photoconductivity to dark conductivity) of 30, a photoconductivity of 2×10-4 (Ω cm)-1, and a carrier diffusion length of 5.3 μm—values much higher than those of the uncapped films (heated or not). This was due to a large grain size combined with the retention of hydrogen, which passivated the grain boundaries. On the other hand, a significant loss of hydrogen from the capped film at 580 °C, as observed by in situ evolved gas analysis, resulted in the diffusion length reducing to 1.8 μm and ten‐fold decreases of the photoconductivity and photoresponse. All three parameters increased slightly with a further increase in the annealing temperature from 600 to 680 °C, due to the increase in the grain size. At 695 °C, further hydrogen evolution caused a sharp decrease in the values of these parameters. © 1995 American Institute of Physics.