Hydrogen solubility and network stability in amorphous silicon

In order to investigate the role of hydrogen in amorphous silicon (a-Si), hydrogenated amorphous silicon layers have been prepared by ion implantation at different H concentrations. Implanted samples have been characterized before and after annealing up to 550 °C by small-angle x-ray scattering, secondary-ion[-mass spectrometry, and infrared spectroscopy. The study of the evolution of hydrogen concentration profiles and bonding configurations combined with the investigation of the atomic and nanoscale structures of the films indicate the solubility limit of hydrogen in a-Si to lie at 3-4 at. %. This limit is associated with the defect-related trap concentration in a-Si. If hydrogen is introduced in the matrix at a concentration well above its solubility, the alloy is intrinsically unstable to the formation of hydrogen complexes. Upon annealing at temperatures higher than 300 °C, the excess H (i.e., above the solubility) leaves the matrix, presumably forming H₂ molecules, which accumulate in nanoscale hydrogen complexes. Observations on nucleation and growth of these hydrogen complexes are discussed in light of H diffusion and solubility.