O. Bisi

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.

Transition metal silicides: aspects of the chemical bond and trends in the electronic structure

Presents a theoretical investigation of the electronic properties of bulk silicides of near-noble metals (NiSi2, NiSi, Ni2,Si, PdSi, Pd2Si, PtSi and Pt2Si). The theoretical approach is provided by the iterative extended Huckel method. The densities of states of various compounds, differing in silicon content and in the crystal structure, are presented and the main features of the chemical bond are discussed. A detailed comparison with the available experimental data and with other theoretical work is presented. The relevance of the present results for the interpretation of the silicide/silicon interfaces is also discussed.

Chemical Bonding at the Si-Metal Interface: Si-Ni and Si-Cr,

Chemical bonding at the interface of a near‐noble‐metal (Ni) and a transition metal (Cr) with Si is examined through synchrotron radiation photoelectron spectroscopy studies of in situ formed interfaces, of cleaved bulk silicides, and of disordered surfaces prepared by sputter etching of the silicides. Interpretation of these experimental results is guided by parallel linear combination of atomic orbitals (LCAO) (extended Huckel approximation) calculations of stoichiometric Ni and Cr silicides.

Induction-model analysis of SiH stretching mode in porous silicon

Abstract A detailed study of SiH stretching in porous silicon was performed based on the induction model. Good agreement with experimental infrared absorption spectra was achieved considering the oxygen nearest-neighbors and next nearest-neighbors. Carbon presence in the samples was detected and its influence on SiH stretching was determined.

Optical properties of isolated and interacting silicon quantum wires

Abstract We have studied the effect of hydrogen passivation and inter-wire interaction on the electronic structure and optical properties of nanoscale Si wires through two first-principle techniques: linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA) and norm-conserving pseudopotential. We have considered free, partially and totally H-passivated [001] Si quantum wires with various rectangular cross-sections; moreover we have investigated the inter-wire interaction, by varying the wire density. The optical properties have been computed by evaluating the imaginary part of the dielectric function and the absorption coefficient. We find that wires with diameters as small as 10–25 A are active in the visible range. Inter-wire interaction leads to the presence of localized interface states which lower the bandgap energy. These results are important for the discussion about the dimensionality of confined Si quantum particles in porous Si and for the debate on quantum confinement models.

Electronic structure of compounds at platinum - silicon (111) interface

Abstract The electronic structure of Platinum silicides produced by thin film reaction is studied using ultraviolet photoemission and Auger spectroscopy. Spectra have been taken during the various stages of Si-Pt intermixing, in order to monitor the changes in the valence band, which take place during the reaction. The experimental data are compared with semi-empirical LCAO calculations. The importance of the coupling between Silicon p and Platinum d-states in determining the basic features of the chemical bond is discussed.

Light emitting porous silicon diode based on a silicon/porous silicon heterojunction

A new structure is proposed to improve the external quantum efficiency of porous silicon (PS) light emitting diodes (LED). It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.

Perturbation theory and three body forces in hcp metals

An analysis is presented of the role of the non pair forces in the lattice dynamics of sp bonded hcp metals: Be, Mg and Li. By expanding the total energy up to third order in the electron-ion pseudopotential, a dynamical matrix is derived which contains terms arising from central pairwise interactions between the ions as well as contributions due to sums of three body forces. The phonon dispersion relations are calculated using a local form of the model potential, which allows for a realistic evaluation of the third order contributions. The results show that the three body forces are very important in Be and are responsible for the peculiarities of the spectrum, as is most clearly seen from the analysis of the frequencies at the K point of the Brillouin zone.

Effects of chemical environment in the lineshape of silicon L2,3VV Auger spectra of nickel silicides

The Si L2,3VV Auger lineshape in nickel silicides of different compositions has been studied both experimentally and theoretically with the purpose of understanding the behaviour of silicon states in compounds where the Si atom has different local chemical environments. The experimental spectra provide evidence of significant modifications in the lineshapes, which are associated with changes in the distribution of the valence electrons. These modifications can be explained by theoretical calculations based on a single-particle description of the bulk electronic structure. Some discrepancies between theory and experiments can be attributed to approximations in the treatment of the final state of the Auger electron and to the neglect of surface effects in the electronic structure calculation.

Surface bands in relaxed cleavance surface of GaP

We have studied the dependence of the surface states upon relaxation in GaP (110) surface. Calculations were performed using a tight binding model with an approximate treatment of the self‐consistency. Different relaxation models, involving both rotation and stretching of the bonds were considered. The location and orbital composition of the surface bands their dispersion and the local density of states at the surface are presented for the various models. Unlike the other III–V compounds, we find that the relaxation does not remove the empty surface states from the band gap. Such a conclusion agrees with the experimental information on the pinning of the Fermi level in n‐doped samples of GaP. A comparison with partial‐yield photoemission data and a theoretical estimate of the surface excitonic binding energy are also given.