Stefano Ossicini

POROUS SILICON: A QUANTUM SPONGE STRUCTURE FOR SILICON BASED OPTOELECTRONICS

Abstract The striking photoluminescence properties of porous silicon have attracted considerable research interest since their discovery in 1990. Luminescence is due to excitonic recombination quantum confined in Si nanocrystals which remain after the partial electrochemical dissolution of silicon. Porous silicon is constituted by a nanocrystalline skeleton (quantum sponge) immersed in a network of pores. As a result, porous silicon is characterized by a very large internal surface area (of the order of 500 m 2 / cm 3 ). This internal surface is passivated but remains highly chemically reactive which is one of the essential features of this new and complex material. We present an overview of the experimental characterization and theoretical modeling of porous silicon, from the preparation up to various applications. Emphasis is devoted to the optical properties of porous silicon which are closely related to the quantum nature of the Si nanostructures. The characteristics of the various luminescence bands are analyzed and the underlying basic mechanisms are presented. In the quest of an efficient electroluminescent device, we survey the results for several porous silicon contacts, with particular attention to the interface properties, to the stability requirement and to the carrier injection mechanisms. Other device applications are discussed as well.

Light emitting silicon for microphotonics

Introduction: Fundamental Aspects.- Electron States and Optical Properties in Confined Silicon Structures.- Porous Silicon.- Silicon Nanostructures: Wells, Wires, and Dots.- Light Emission of Er3+ in Silicon.- Silicon Based Photonic Crystals.- Conclusions and Future Outlook.

Second-harmonic generation in silicon waveguides strained by silicon nitride

Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks. All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40 pm V(-1) at 2,300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light sources spanning the near- to mid-infrared spectrum from 1.2 to 10 μm.

Dynamics of stimulated emission in silicon nanocrystals

Time-resolved luminescence measurements on silicon nanocrystal waveguides obtained by thermal annealing of plasma-enhanced chemical-vapor-deposited thin layers of silicon-rich oxide have revealed fast recombination dynamics related to population inversion which leads to net optical gain. Variable stripe length measurements performed on the fast emission signal have shown an exponential growth of the amplified spontaneous emission with net gain values of about 10 cm−1. The fast emission component is strongly dependent on the pumping length for fixed excitation intensity. In addition, both the fast component intensity and its temporal decay revealed threshold behavior as a function of the incident pump intensity.

Self-energy and excitonic effects in the electronic and optical properties of TiO2 crystalline phases

We present a unified ab initio study of electronic and optical properties of TiO₂ rutile and anatase phases with a combination of density-functional theory and many-body perturbation-theory techniques. The consistent treatment of exchange and correlation, with the inclusion of many-body one-particle and two-particles effects in self-energy and electron-hole interaction, produces a high-quality description of electronic and optical properties, giving, for some quantities, the first available estimation for this compound. In particular, we give a quantitative estimate of the electronic and direct optical gaps, clarifying their role with respect to previous measurements obtained by various experimental techniques. We obtain a description for both electronic gap and optical spectra that is consistent with experiments by analyzing the role of different contributions to the experimental optical gap and relating them to the level of theory used in our calculations. We also show the spatial properties of excitons in the two crystalline phases, highlighting the localization character of different optical transitions. This paper aims at understanding and firmly establishing electro-optical bulk properties, yet to be clarified, of this material of fundamental and technological interest for green energy applications.

Carrier multiplication between interacting nanocrystals for fostering silicon-based photovoltaics

Carrier multiplication is a carrier-relaxation process that results in the generation of multiple electron–hole pairs after the absorption of a single photon. Researchers have now studied the role of nanoparticle interplay on the carrier-multiplication dynamics of two interacting silicon nanocrystals for photovoltaic applications.

AES ANALYSIS OF THE GROWTH-MECHANISM OF METAL LAYERS ON METAL-SURFACES

A discussion on the growth of thin metal films on metal substrates on the basis of the Auger technique is presented. Within a simple model the characteristics of the different growths are related to the adsorbate and substrate Auger peak‐to‐peak intensity ratios. The behavior of these ratios as a function of the number of deposited atoms is studied in the case of the Frank–van der Merwe, Volmer–Weber, and Stranski–Krastanov growth mechanism. A comparison between our calculations and reported measurements is discussed in detail for the Pd/W(110), Au/Pt(100), and Pt/Au(100) systems.

Silicon–Germanium Nanowires: Chemistry and Physics in Play, from Basic Principles to Advanced Applications

Basic Principles to Advanced Applications Michele Amato,*,† Maurizia Palummo,*,‡ Riccardo Rurali,* and Stefano Ossicini* †Institut d’Electronique Fondamentale, UMR8622, CNRS, Universite ́ Paris-Sud, 91405 Orsay, France ‡European Theoretical Spectroscopy Facility (ETSF), Dipartimento di Fisica, Universita ̀ di Roma, “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy Institut de Cieǹcia de Materials de Barcelona (ICMAB−CSIC), Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain “Centro S”, CNR-Istituto di Nanoscienze, Via Campi 213/A, 41125 Modena, Italy Dipartimento di Scienze e Metodi dell’Ingegneria, Centro Interdipartimentale En&Tech, Universita ̀ di Modena e Reggio Emilia, Via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia, Italy

Simultaneously B- and P-doped silicon nanoclusters: Formation energies and electronic properties

The effects of B and P codoping on the impurity formation energies and electronic properties of Si nanocrystals (Si-nc) are calculated by a first-principles method. We show that, if carriers in the Si-nc are perfectly compensated by simultaneous doping with n- and p-type impurities, the Si-nc undergo a minor structural distortion around the impurities and that the formation energies are always smaller than those for the corresponding single-doped cases. The band gap of the codoped Si-nc is strongly reduced with respect to the gap of the pure ones showing the possibility of an impurity based engineering of the photoluminescence properties of Si-nc.

Size, oxidation, and strain in small Si/SiO2 nanocrystals

The structural, electronic and optical properties of Si nanocrystals of different size and shape, passivated with hydrogens, OH groups, or embedded in a SiO2 matrix are studied. The comparison between the embedded and free, suspended nanocrystals shows that the silica matrix produces a strain on the embedded NCs, that contributes to determine the band gap value. By including the strain on the hydroxided nanocrystals we are able to reproduce the electronic and optical properties of the full Si/SiO2 systems. Moreover we found that, while the quantum confinement dominates in the hydrogenated nanocrystals of all sizes, the behaviour of hydroxided and embedded nanocrystals strongly depends on the interface oxidation degree, in particular for diameters below 2 nm. Here, the proportion of NC atoms at the Si/SiO2 interface becomes relevant, producing surface-related states that may affect the quantum confinement appearing as inner band gap states and then drastically changing the optical response of the system.