Electron emitters in vacuum microelectronic devices need sharp tips in order to permit electron emission at moderate voltages. A method has been found for preparing uniform silicon tips with a radius of curvature less than 1 nm. These tips are formed by oxidation of 5-μm-high silicon cones through exploitation of a known oxidation inhibition of silicon at regions of high curvature.
Sharp microtips of silicon have potential applications as field emitters and as electrical or mechanical microsensors. This study describes a single unified etching/oxidation treatment that results in uniform tips with controlled radii of atomic dimensions or larger. Variations in the etching/oxid~ti.o~ treat~ent for~ I?ultiple tips with two or four tips per etched pyramid, which offe~ t.h~ possIbil.lty of higher emlss~on current density for field emitter applications, and higher sensltivlty for rmcrosensor applicatlOns.
One problem with the growth of high quality c-axis oriented YBa2Cu3O7−x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c-axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c-axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a-axis films. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.
We evaluate defect concentrations and investigate the lifetime potential of p-type single-crystal kerfless silicon produced via epitaxy for photovoltaics. In gettered material, low interstitial iron concentrations (as low as (3.2 ± 2.2) × 109 cm−3) suggest that minority-carrier lifetime is not limited by dissolved iron. An increase in gettered lifetime from <20 to >300 μs is observed after increasing growth cleanliness. This improvement coincides with reductions in the concentration of Mo, V, Nb, and Cr impurities, but negligible change in the low area-fraction (<5%) of dislocated regions. Device simulations indicate that the high bulk lifetime of this material could support solar cell efficiencies >23%.
Various orientations of YBa2Cu3O7−x grains in polycrystalline films prepared on (001)MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in-plane orientations have been observed. These orientations agree well with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu3O7−x and MgO. The YBa2Cu3O7−x grains were found to have a high probability of forming low angle or low Σ boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu-O-Cu chains. Therefore, YBa2Cu3O7−x thin films on MgO can attain a Tc of ∼90 K and a Jc of ∼106 A/cm2 at 77 K.
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