Polysilicon films of high photoresponse, obtained by vacuum annealing of aluminum capped hydrogenated amorphous silicon

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.