Silicon Nanowires prepared by Metal-Assisted Chemical Etching have been nanopatterned into periodic and aperiodic array geometries displaying functionality at visible wavelengths using top-down planar processing techniques. Broadband photoluminescense enhancement up to approximately one order of magnitude is measured from golden-angle spiral arrays over a wide parameter space.
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.
Controllable and uniform doping of nanowires (NWs) is the ultimate challenge prior to their effective application. Si NWs amorphize and bend toward the impinging ions under ion irradiation as a result of viscous flow. We demonstrate that thermal annealing induces a full recovery of the crystalline phase corresponding to the unbending of the NWs. The competition between Solid Phase Epitaxy and Random Nucleation and Growth at the nanoscale is the key parameter controlling the recovery.
We demonstrated the heteroepitaxial growth of single-crystal faceted Ge nanowires (NWs) by electron-beam evaporation on top of Si(111) substrates. Despite the non-ultrahigh vacuum growth conditions, scanning electron microscope and transmission elec- tron microscope images show that NWs have specific crystallographic growth directions (111), (110), and (112) and that specific surface crystallographic planes (111) or (110) correspond to the (110) and (112) growth directions. Moreover, we studied in detail the Ge NWs structural properties. The temperature dependence of the NW length and of the frequency of each crystallographic orientation has been elucidated. Finally, the microscopic growth mechanisms have been investigated.
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.
We have elucidated the mechanism for B migration in the amorphous (a-) Si network. B diffusivity in a-Si is much higher than in crystalline Si; it is transient and increases with B concentration up to 2 x 1020 B/cm3. At higher density, B atoms in a-Si quickly precipitate. B diffusion is indirect, mediated by dangling bonds (DB) present in a-Si. The density of DB is enhanced by B accommodation in the a-Si network and decreases because of a-Si relaxation. Accurate data simulations allow one to extract the DB diffusivity, whose activation energy is 2.6 eV. Implications of these results are discussed.
B clustering in amorphous Si D. De Salvador, G. Bisognin, M. Di Marino, E. Napolitani, A. Carnera, S. Mirabella, E. Pecora, E. Bruno, F. Priolo, H. Graoui, M. A. Foad, F. Boscherini The Journal of Vacuum Science and Technology B 26, 382 (2008)
The authors have investigated the role of the Si excess on the photoluminescence properties of Er doped substoichiometric SiOx layers. They demonstrate that the Si excess has two competing roles: when agglomerated to form Si nanoclusters Si-nc’s it enhances the Er excitation efficiency but it also introduces new nonradiative decay channels. When Er is excited through an energy transfer from Si-nc’s, the beneficial effect on the enhanced excitation efficiency prevails and the Er emission increases with increasing Si content. However, when pumped resonantly, the Er luminescence intensity always decreases with increasing Si content. These data are presented and their implications are discussed.