Hydrogen above saturation at silicon vacancies: H-pair reservoirs and metastability sites

Vacancies, microstructure and the moments of nuclear magnetic resonance: the case of hydrogenated amorphous silicon

Recent experiments on hydrogenated amorphous silicon using infrared absorption spectroscopy have indicated the presence of mono- and divacancies in samples for concentrations of up to 14% hydrogen. Motivated by this observation, we study the microstructure of hydrogen in two model networks of hydrogen-rich amorphous silicon with particular emphasis on the nature of the distribution (of hydrogen), the presence of defects and the characteristic features of the nuclear magnetic resonance spectra at low and high concentrations of hydrogen. Our study reveals the presence of vacancies, which are the built-in features of the model networks. The study also confirms the presence of various hydride configurations in the networks, from silicon monohydrides and dihydrides to open chain-like structures, that have been observed in the infrared and nuclear magnetic resonance experiments. The broad and the narrow line widths of the nuclear magnetic resonance spectra are calculated from a knowledge of the distribution of spins (hydrogen) in the networks.

Theory of hydrogen-related metastability in disordered silicon

Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of a weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability are discussed for amorphous and polycrystalline silicon.

Raman-scattering elucidation of the giant isotope effect in hydrogen-ion blistering of silicon

In this work, we investigate the origin of a giant isotope effect discovered in the blistering of hydrogen-ion-implanted and annealed silicon. Si(001) samples were implanted or co-implanted with 5 keV of H and/or D ions to total fluences of 2 x 10(16) and 6 x 10(16) ion/cm(2). The lower fluence is sufficient for blistering by pure H, but the higher one is required for the maximum blister coverage whenever D is involved. On these samples, we carried out Raman-scattering investigations of the evolution of Si-H/D complexes upon a stepwise thermal annealing from 200 to 550 degrees C. We have identified the critical chemical transformations characterizing the hydrogen-deuterium-induced blistering of silicon. The puzzling dependence on ion mass appears to be mainly connected with the nature of the radiation damage. We have found that H is more efficient in "preparing the ground" for blistering by nucleating platelets parallel to the surface, essentially due to its ability to agglomerate in the multihydride monovacancy complexes that evolve into hydrogenated extended internal surfaces. By contrast, D is preferentially trapped in the surprisingly stable monodeuteride multivacancies.

Direct role of hydrogen in the Staebler-Wronski effect in hydrogenated amorphous silicon

We report a hydrogen-related defect that establishes the direct role of hydrogen in stabilizing the silicon dangling bonds created in the Staebler-Wronski effect in hydrogenated amorphous silicon. A specific NMR signal due to paired hydrogen atoms occurs only after optical excitation, exists at an intensity that is consistent with the density of optically induced silicon dangling bonds, and anneals at temperatures that are consistent with the annealing of the optically induced silicon dangling bonds. At this defect the hydrogen atoms are 2.3+/-0.2 A apart.