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.
The fluence effect in hydrogen-ion cleaving of silicon at the sub-100-nm scale
The implementation at the sub-100-nm scale of ion cleaving requires ion beams of ∼5 keV/amu or less. The blistering efficiency in 5-keV H-ion-implanted and annealed Si has been found to peak and vanish in a narrow range of ion fluence of (1.5–3.5)×1016 H/cm2. In order to understand this effect, the defect profiles in 5-keV H-irradiated Si were studied by Rutherford backscattering/channelling, while the Si-H bonding configurations during annealing were investigated by Raman scattering spectroscopy. Three types of defects play major roles: the ‘broad-band’ monohydride multivacancy complexes, the fully or partially passivated monovacancy VHn, and H-terminated internal surfaces Si(100):H. Blister absence at high fluence is characterised by the persistence up to 550 °C of the Si(100):H structures, which are blister embryos that failed to coalesce and grow. Radiation-induced stresses and fracture toughening may play roles in inhibiting cleavage at high fluence; however, widening towards the surface of the zone of high H and defect concentration is the likely major factor.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.