We report on the production of ordered assemblies of silicon nanostructures
by means of irradiation of a Si(100) substrate with 1.2 keV Ar ions at normal
incidence. Atomic Force and High-Resolution Transmission Electron microscopies
show that the silicon structures are crystalline, display homogeneous height,
and spontaneously arrange into short-range hexagonal ordering. Under prolonged
irradiation (up to 16 hours) all dot characteristics remain largely unchanged
and a small corrugation develops at long wavelengths. We interpret the
formation of the dots as a result of an instability due to the sputtering yield
dependence on the local surface curvatureComment: 4 two-column pages (revtex4), 3 figures (higher quality copies in the
printed jrnl. version
A lack of universality with respect to ion species has been recently established in nanostructuring of semiconductor surfaces by low-energy ion-beam bombardment. This variability affects basic properties of the pattern formation process, like the critical incidence angle for pattern formation, and has remained unaccounted for. Here, we show that nonuniform generation of stress across the damaged amorphous layer induced by the irradiation is a key factor behind the range of experimental observations, as the form of the stress field is controlled by the ion/target combination. This effect acts in synergy with the nontrivial evolution of the amorphous-crystalline interface. We reach these conclusions by contrasting a multiscale theoretical approach, which combines molecular dynamics and a continuum viscous flow model, with experiments using Xe + and Ar + ions on a Si(100) target. Our general approach can apply to a variety of semiconductor systems and conditions.
We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa 2 Cu 3 O x superconducting ceramics on (001)-oriented LaAlO 3 substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa 2 -Cu 3 O 7 . Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba 1-x Y x F 2+x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa 2 Cu 3 O 7 nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y 2 O 3 which reacts with the CuO particles forming Y 2 Cu 2 O 5 at about 700°C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795°C, at a stage where the YBa 2 Cu 3
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