Quantum confinement and electroluminescence in ultrathin silicon nanowires fabricated by a maskless etching technique.

We present a novel approach for the direct synthesis of ultrathin Si nanowires (NWs) exhibiting room temperature light emission. The synthesis is based on a wet etching process assisted by a metal thin film. The thickness-dependent morphology of the metal layer produces uncovered nanometer-size regions which act as precursor sites for NW formation. The process is cheap, fast, maskless and compatible with Si technology. Very dense arrays of long (several micrometers) and small (diameter of 5-9 nm) NWs have been synthesized. An efficient room temperature luminescence, visible with the naked eye, is observed when NWs are optically excited, exhibiting a blue-shift with decreasing NW size in agreement with quantum confinement effects. A prototype device based on Si NWs has been fabricated showing a strong and stable electroluminescence at low voltages. The relevance and the perspectives of the reported results are discussed, opening the route toward novel applications of Si NWs.

Control of growth mechanisms and orientation in epitaxial Si nanowires grown by electron beam evaporation.

The growth mechanisms of epitaxial Si nanowires (NWs) grown by electron beam evaporation (EBE) and catalyzed through gold droplets are identified. NWs are seen to grow both from adsorbed Si atoms diffusing from the substrate and forming a dip around them, and from directly impinging atoms. The growth of a 2D planar layer competing with the axial growth of the NWs is also observed and the experimental parameters determining which of the two processes prevails are identified. NWs with (111), (100) and (110) orientation have been found and the growth rate is observed to have a strong orientation dependence, suggesting a microscopic growth mechanism based on the atomic ordering along (110) ledges onto (111)-oriented terraces. By properly changing the range of experimental conditions we demonstrate how it is possible to favor the axial growth of the NWs, define their length and control their crystallographic orientation.

Localization of the dopant in Ge:Mn diluted magnetic semiconductors by x-ray absorption at the Mn K edge.

A unitary picture of the structural properties of Mn(x)Ge(1-x) diluted alloys fabricated by either ion implantation or molecular beam epitaxy (MBE), at various growth temperatures (from 80 to about 623 K) and few per cent concentrations, is proposed. Analysis is based on synchrotron radiation x-ray absorption spectroscopy at the Mn K edge. When the growth temperature exceeds 330 K, the MBE samples show a high number of precipitated ferromagnetic nanoparticles, mainly Mn(5)Ge(3), nucleated from the previous occupation of interstitial tetrahedral sites. Efficient substitution is observed in the case of MBE samples made by alternate layers of GeMn alloys grown at T ≤ 433 K and undoped Ge thick layers. Similar good dilution properties are obtained by implanting Mn ions at low temperatures (80 K). Possible precursors to preferential mechanisms in the alloy formation are discussed on the basis of the present comparative study.