POLYSILICON FILMS OF HIGH PHOTORESPONSE, OBTAINED BY VACUUM ANNEALINGOF ALUMINUM CAPPED HYDROGENATED AMORPHOUS-SILICON

Hydrogenated amorphous silicon (a-Si:H) films having a thickness of 7 mu m were deposited on molybdenum by de glow discharge and then capped with a 0.1-mu m-thick aluminum (Al) layer by thermal evaporation. Sub sequent vacuum annealing at 500-725 degrees C resulted in the formatio n of crystalline Si, as observed by Raman scattering and x-ray diffrac tion. This was in contrast to the uncapped a-Si:H films which were sti ll amorphous at the same annealing temperatures, except at >700 degree s C. That the Al capped films were crystalline caused a ten-fold incre ase in the dark conductivity in comparison to the uncapped film anneal ed at the same temperature. The capped films annealed at 500 degrees C showed a photoresponse (the ratio of the photoconductivity to dark co nductivity) of 30, a photoconductivity of 2x10(-4) (Ohm cm)(-1), and a carrier diffusion length of 5.3 mu m-values much higher than those of the uncapped films (heated or not). This was due to a large grain siz e 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 degrees C, as observed by in situ evolved gas anal ysis, resulted in the diffusion length reducing to 1.8 mu m and ten-fo ld decreases of the photoconductivity and photoresponse. All three par ameters increased slightly with a further increase in the annealing te mperature from 600 to 680 degrees C, due to the increase in the grain size. At 695 degrees C, further hydrogen evolution caused a sharp decr ease in the values of these parameters. (C) 1995 American Institute of Physics.