Hydrogen Effusion From Tritiated Amorphous Silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300 degrees C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600 h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15 h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04 eV. The second process, slow tritium outgassing with a time constant of 200-300 h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60 years. (C) 2008 American Institute of Physics.