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 ([${\mathrm{B}}_{2}$${\mathrm{H}}_{6}$]/[${\mathrm{SiH}}_{4}$]=2 vol%) hydrogenated amorphous silicon during low-temperature (220 \ifmmode^\circ\else\textdegree\fi{}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 ${\mathrm{H}}_{2}$ 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.