Hydrogen induced optically-active defects in silicon photonic nanocavities

This work was supported by Era-NET NanoSci LECSIN project coordinated by F. Priolo, by the Italian Ministry of University and Research, FIRB contract No. RBAP06L4S5 and by the EPSRC UKSp project. Partial financial support by the Norwegian Research Council is also acknowledged.We demonstrate intense room temperature photoluminescence (PL) from optically active hydrogen- related defects incorporated into crystalline silicon. Hydrogen was incorporated into the device layer of a silicon on insulator (SOI) wafer by two methods: hydrogen plasma treatment and ion implantation. The room temperature PL spectra show two broad PL bands centered at 1300 and 1500 nm wavelengths: the first one relates to implanted defects while the other band mainly relates to the plasma treatment. Structural characterization reveals the presence of nanometric platelets and bubbles and we attribute different features of the emission spectrum to the presence of these different kind of defects. The emission is further enhanced by introducing defects into photonic crystal (PhC) nanocavities. Transmission electron microscopy analyses revealed that the isotropicity of plasma treatment causes the formation of a higher defects density around the whole cavity compared to the ion implantation technique, while ion implantation creates a lower density of defects embedded in the Si layer, resulting in a higher PL enhancement. These results further increase the understanding of the nature of optically active hydrogen defects and their relation with the observed photoluminescence, which will ultimately lead to the development of intense and tunable crystalline silicon light sources at room temperature.Publisher PDFPeer reviewe

Con Feuerstein, oltre Feuerstein /

La proposta di Reuven Feuerstein si basa sulla Teoria della Modificabilità Cogni- tiva Strutturale. Essa postula, per ogni individuo, anche se affetto da handicap, o fortemente deprivato sul piano psicosociale, la possibilità di modificare struttu- ralmente i suoi processi di pensiero e cambiare il modo con cui si accosta alla co- noscenza e alle esperienze di vita. Tale progresso dipende in larga misura, nella prospettiva di Feuerstein, dalla forza della Mediazione Educativa. Il concetto di mediazione è sicuramente la seconda colonna che costituisce l’impalcatura del- la proposta di Feuerstein (Feuerstein, Klein, Tannenbaum, 1999). Questi riprende in proposito le intuizioni di Vygotsky e le osservazioni sullo scaffolding di Bru- ner (1960) e le sviluppa ulteriormente facendo delle esperienze di apprendimen- to mediato la base metodologica del suo programma, ovvero la condizione per lo sviluppo a spirale della modificabilità cognitiva (Feuerstein, 1990). Ma la Zona di Sviluppo Prossimale proposta da Lev Vygotsky, l’Esperienza di Apprendimen- to Mediato proposta da Reuven Feuerstein, nonché le ricerche e il lavoro sugli anziani di Warner Schaie e Sherry Willis e i numerosi studi sugli effetti della sco- larizzazione sulle capacità intellettive, gli studi condotti da Gardner sulla plurali- tà delle intelligenze e le ricerche avviate dalla sua scuola (Fisher et alii) costitui- scono basi di conoscenza e di evidenze che non possiamo più ignorare. Possia- mo e dobbiamo pensare l’intelligenza in maniera diversa e più flessibile; ossia in maniera atta ad aiutare le persone a fronteggiare le sfide poste dall’ambiente in cui vivono grazie all’incremento dei livelli di metacognizione. Con tutto ciò il progresso continuo della ricerca scientifica sul funzionamen- to dei processi cognitivi, ed in particolare sui processi di metacognizione con- nessi alla prospettiva della embodied cognition, per un verso, e alla caratteristi- ca enattiva che intrama e mobilita le nostre scelte di comportamento (Rossi, 2011), per l’altro, ci impongono di rivisitare sia la teoria che la metodologia pro- poste da Reuven Feuerstein allo scopo di valorizzarla, e di superare, insieme, al- cuni nodi problematici che incontriamo nelle pratiche di counselling o di inter- vento riabilitativo e inclusivo

Fluorine effect on As diffusion in Ge

The enhanced diffusion of donor atoms, via a vacancy (V)-mechanism, severely affects the realization of ultrahigh doped regions in miniaturized germanium (Ge) based devices. In this work, we report a study about the effect of fluorine (F) on the diffusion of arsenic (As) in Ge and give insights on the physical mechanisms involved. With these aims we employed experiments in Ge co-implanted with F and As and density functional theory calculations. We demonstrate that the implantation of F enriches the Ge matrix in V, causing an enhanced diffusion of As within the layer amorphized by F and As implantation and subsequently regrown by solid phase epitaxy. Next to the end-of-range damaged region F forms complexes with Ge interstitials, that act as sinks for V and induce an abrupt suppression of As diffusion. The interaction of Ge interstitials with fluorine interstitials is confirmed by theoretical calculations. Finally, we prove that a possible F-As chemical interaction does not play any significant role on dopant diffusion. These results can be applied to realize abrupt ultra-shallow n-type doped regions in future generation of Ge-based devices

Role of interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2

Light emitting silicon nanocrystals embedded in SiO2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered TEM analysis and by ab-initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO2 is not sharp: an intermediate region of amorphous nature and of variable composition links the crystalline Si with the amorphous stoichiometric SiO2. This region plays an active role in the light emission process

Nanometric moiré stripes on the surface of Bi2Se3 topological insulator

Mismatch between adjacent atomic layers in low-dimensional materials, generating moiré patterns, has recently emerged as a suitable method to tune electronic properties by inducing strong electron correlations and generating novel phenomena. Beyond graphene, van der Waals structures such as three-dimensional (3D) topological insulators (TIs) appear as ideal candidates for the study of these phenomena due to the weak coupling between layers. Here we discover and investigate the origin of 1D moiré stripes on the surface of Bi2Se3TI thin films and nanobelts. Scanning tunneling microscopy and high-resolution transmission electron microscopy reveal a unidirectional strained top layer, in the range 14-25%, with respect to the relaxed bulk structure, which cannot be ascribed to the mismatch with the substrate lattice but rather to strain induced by a specific growth mechanism. The 1D stripes are characterized by a spatial modulation of the local density of states, which is strongly enhanced compared to the bulk system. Density functional theory calculations confirm the experimental findings, showing that the TI surface Dirac cone is preserved in the 1D moiré stripes, as expected from the topology, though with a heavily renormalized Fermi velocity that also changes between the top and valley of the stripes. The strongly enhanced density of surface states in the TI 1D moiré superstructure can be instrumental in promoting strong correlations in the topological surface states, which can be responsible for surface magnetism and topological superconductivity

Self-interstitials injection in crystalline Ge induced by GeO 2 nanoclusters

The effect of O implantation in crystalline Ge on the density of native point defects has been investigated through transmission electron microscopy and B diffusion experiments. Annealing at 650 • C following O implants produces a band of defects (∼5-10 nm), compatible with GeO 2 nanoclusters (NCs). A clear shape transformation from elongated to spherical forms occurs within 2 h, concomitant with a transient enhanced diffusion of B. A large injection of self-interstitials from GeO 2 NCs, giving a vacancy undersaturation, and a long-range migration of self-interstitials are evidenced and discussed

Atomic characterization of Si nanoclusters embedded in SiO2 by atom probe tomography

Silicon nanoclusters are of prime interest for new generation of optoelectronic and microelectronics components. Physical properties (light emission, carrier storage...) of systems using such nanoclusters are strongly dependent on nanostructural characteristics. These characteristics (size, composition, distribution, and interface nature) are until now obtained using conventional high-resolution analytic methods, such as high-resolution transmission electron microscopy, EFTEM, or EELS. In this article, a complementary technique, the atom probe tomography, was used for studying a multilayer (ML) system containing silicon clusters. Such a technique and its analysis give information on the structure at the atomic level and allow obtaining complementary information with respect to other techniques. A description of the different steps for such analysis: sample preparation, atom probe analysis, and data treatment are detailed. An atomic scale description of the Si nanoclusters/SiO2 ML will be fully described. This system is composed of 3.8-nm-thick SiO layers and 4-nm-thick SiO2 layers annealed 1 h at 900°C

Evidences of an intermediate rodlike defect during the transformation of {113} defects into dislocation loops

International audienceA detailed study of the transformation of the {113} defects into dislocation loops has been carried out in Ge preamorphized silicon (30keV, 1×1015Ge+/cm2) and annealed at 800°C for time ranging from 15to2700s. The presence of a stable defect, along the ⟨110⟩ directions, formed during the transformation from {113}’s into Frank dislocation loops (FDLs), has been revealed and studied. Performing a detailed transmission electron microscopy analysis in nonconventional zone axes, a 1∕3 [111]-type Burgers vector has been found. These defects are shown to be more stable than {113}’s but less than FDLs