Structural changes in boron-doped hydrogenated amorphous silicon during long-time annealing below the deposition temperature

A large change in the number of Si-H bonds in boron-doped ([B2H6]/[SiH4]=2 vol%) hydrogenated amorphous silicon during low-temperature (220°C) annealing has been observed. The number of Si-H bonds as determined by Raman scattering, infrared spectroscopy, and from the optical gap showed a logarithmic decrease with anneal time to less than half after a 100-h anneal. It is shown that during long-anneal sequences hydrogen is transferred irreversibly from a bonded state into molecular H2 trapped in the samples. This has important implications for the determination of H-diffusion coefficients by nuclear techniques such as secondary-ion mass spectroscopy or elastic-recoil detection analysis.