Visible Luminescence from Single Crystal‐Silicon Quantum Wells

Single crystal‐silicon quantum wells with SiO2 barriers have been grown from SIMOX silicon‐on‐insulator substrates. Photoluminescence in the red and near‐infrared is observed for average well width \u3c8 \u3enm, with peak signal for 2‐nm average width. The luminescence spectrum is independent of well width for SiO2 barriers, but shifts 0.3 eV to higher energy upon removal of the upper oxide layer with HF. Both results suggest the importance of radiation from surface states

Photoluminescence Properties of Silicon Quantum-Well Layers

Nanometer-scale crystal silicon films surrounded by SiO2 were prepared by oxidizing silicon-on-insulator substrates prepared from SIMOX (separation by implantation of oxygen) and crystallized hydrogenated amorphous silicon films. Average silicon layer thickness was determined from reflection spectra. When sufficiently thin (\u3c2 \u3enm), all layers emitted red photoluminescence under blue and UV cw excitation, with a spectrum that did not depend on the mean layer thickness. The spectrum was roughly Gaussian with a peak energy of 1.65 eV, which is lower than for most porous silicon spectra. The time scale for the luminescence decay was ~35 μs at room temperature and ~54 μs at 88 K; the decay was nonexponential and did not exhibit spectral diffusion. Atomic force microscope images of the silicon layers showed that luminescing layers were broken apart into regions ~50-100 μm in diameter, suggesting that luminescence comes only from regions small enough to have no nonradiative recombination centers in the band gap. These results are inconsistent with a simple quantum-confinement model for luminescence in two-dimensional silicon and suggest the importance of radiation from surface states

Distinguishing Surface and Bulk Contributions to Third-Harmonic Generation in Silicon

We report measurements of third-harmonic generation from ultrathin crystalline silicon layers of gradually varying thickness. Both the angular and thickness dependence of the third-harmonic light generated in transmission at normal incidence are consistent with negligible surface contribution to third-harmonic generation in silicon, even under tight focusing. This work illustrates a method for distinguishing surface and bulk contributions to harmonic generation

Plasmonics of supported nanoparticles reveals adhesion at the nanoscale: implications for metals on dielectrics

The morphology and adhesion energy of nanosized metal particles supported on dielectrics are a puzzling issue since, due to the increasing contribution of surfaces and interfaces in their energetics, their equilibrium shape escapes the rules established for large objects. The evolution of wetting during Volmer–Weber growth of nanoparticles is herein studied by in situ ultraviolet/visible surface differential reflectivity spectroscopy (SDRS). The integrated s-polarized SDR signal is shown to be proportional to the oscillator strength of the optically excited plasmon resonances parallel to the surface. Dielectric modelings show that this quantity, which is marginally affected by the size and density of the objects, depends mainly on the aspect ratio of the particles from which adhesion energy can be derived. Applied to noble (Ag, Au) or transition metals (Cr, Ni) and Zn on weakly interacting dielectric (Al2O3, SiO2, KBr) and semiconducting (TiO2, ZnO) substrates, this plasmonic approach evidences a robust U-shaped variation of the aspect ratio with film thickness and therefore size. In line with the thorough study of the Ag/Al2O3(0001) growth and linear elasticity predictions of the equilibrium shape of strained epitaxial particles, the first branch of the “U” is assigned to a size-dependent equilibrium shape related to surface/interface stress effects. A significant decrease in adhesion energy parallels a rounding of the particles. The second branch partly stems from flattening due to incomplete coalescence. The common behavior of poorly wetting supported metal nanoparticles that is revealed herein, with strong changes in shape and adhesion as a function of particle size, had not been evidenced so far. Both the proposed optical methodology and the final findings about adhesion at the nanoscale are of interest in the wide field of application of supported metal nanoparticles that involves heterogeneous catalysis and thin film growth.We thank all reviewers for their very constructive comments. R.C., E.C., and Q.H. thank ANRT (Agence Nationale de la Recherche et de la Technologie), Arcelor-Mittal Maizières Research, and Saint-Gobain Recherche for the CIFRE funding of their thesis (grants 2013/0521 and 2016/0650). M.M. and E.M. acknowledge the support of the French state fund managed by the ANR (Agence Nationale de la Recherche) within the Investissements d’Avenir program under reference ANR-11-IDEX-0004-02 and more specifically within the framework of the Cluster of Excellence MATISSE. R.L., I.G., and Q.H. acknowledge the support of ANR (Industrial chair FRAXOS, reference ANR-15-CHIN-0003). The SDRS setup was designed by S. Chenot (INSP, Paris).Peer ReviewedPostprint (published version

How Much can Guided Modes Enhance Absorption in Thin Solar Cells?

Absorption enhancement in thin metal-backed solar cells caused by dipole scatterers embedded in the absorbing layer is studied using a semi-analytical approach. The method accounts for changes in the radiation rate produced by layers above and below the dipole, and treats incoherently the subsequent scattering of light in guided modes from other dipoles. We find large absorption enhancements for strongly coupled dipoles, exceeding the ergodic limit in some configurations involving lossless dipoles. An antireflection-coated 100-nm layer of a-Si:H on Ag absorbs up to 87% of incident above-gap light. Thin layers of both strong and weak absorbers show similar strongly enhanced absorption

First-principle study of excitonic self-trapping in diamond

We present a first-principles study of excitonic self-trapping in diamond. Our calculation provides evidence for self-trapping of the 1s core exciton and gives a coherent interpretation of recent experimental X-ray absorption and emission data. Self-trapping does not occur in the case of a single valence exciton. We predict, however, that self-trapping should occur in the case of a valence biexciton. This process is accompanied by a large local relaxation of the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure

Γ to X Transport of Photoexcited Electrons in Type II GaAs/AlAs Multiple Quantum Well Structures

We report novel femtosecond time‐resolved measurements performed on staggered type II GaAs/AlAs multiple quantum well structures. Photoexcited electrons were determined to transfer from the Γ valley of the GaAs layers to the X valleys of the AlAs in 100 and 400 fs for 8‐ and 11‐monolayer‐thick GaAs samples, respectively

Harmonic Generation in Thin Films and Multilayers

A general method for computing harmonic generation in reflection and transmission from planar nonmagnetic multilayer structures is described. The method assumes plane waves and treats harmonic generation in the parametric approximation. The method is applied in studying the second- and third-harmonic generation properties of thin crystal silicon layers surrounded by thermal oxide. Most independent components of the nonlinear susceptibility tensor have unique signatures with silicon layer thickness d, allowing their strength to be determined in principle by measuring harmonic generation as a function of d. Surface and bulk contributions to third-harmonic generation are cleanly distinguished, with the bulk signal dominating. Four of six nonvanishing components of χ(2) are independent. An approximate value for the bulk susceptibility component δ\u27, which is accessible only in multibeam experiments and has not previously been measured, is obtained

Swift Heavy Ion Induced Modification Studies of C60 Thin Films

Modification induced by 110 MeV Ni ion irradiated thin film samples of C60 on Si and quartz substrates were studied at various fluences. The pristine and irradiated samples were investigated using Raman spectroscopy, electrical conductivity and optical absorption spectroscopy. The Raman data and band gap measurements indicate that swift ions at low fluences result in formations that involve multiple molecular units like dimer or polymer. High fluence irradiation resulted in sub-molecular formations and amorphous semiconducting carbon, indicating overall damage of the fullerene molecules. These sub-molecular units have been identified with nanocrystalline diamond and nanocrystalline graphite like formations.Comment: 7 pages, 29 references and 9 figures submitted to J. Appl. Phy