A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Brownian motors: noisy transport far from equilibrium

Unidirectional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal/semiconductor interfaces with Schottky barriers. We report on a diode effect associated with the direction of bulk electric polarization in BiFeO3: a ferroelectric with a small optical gap edge of ∼2.2 electron volts. We found that bulk electric conduction in ferroelectric monodomain BiFeO3 crystals is highly nonlinear and unidirectional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. A substantial visible-light photovoltaic effect is observed in BiFeO3 diode structures. These results should improve understanding of charge conduction mechanisms in leaky ferroelectrics and advance the design of switchable devices combining ferroelectric, electronic, and optical functionalities.

Switchable ferroelectric diode and photovoltaic effect in BiFeO3.

http://nathan.instras.com/documentDB/paper-69.pdf

Porous silicon: a quantum sponge structure for silicon based optoelectronics

We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye–Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.

Parameter-free calculations of X-ray spectra with FEFF9

Light-emitting silicon nanocrystals embedded in ${\mathrm{SiO}}_{2}$ have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding ${\mathrm{SiO}}_{2}$ is not sharp: an intermediate region of amorphous nature and variable composition links the crystalline Si with the amorphous stoichiometric ${\mathrm{SiO}}_{2}.$ This region plays an active role in the light-emission process.

/pdf/role-of-the-interface-region-on-the-optoelectronic-3pyh6gtyhv.pdf

Role of the interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO 2

Extended x-ray-absorption fine-structure (EXAFS) of copper has been measured from 4 to 500 K and analyzed by the cumulant method, to check the effectiveness of EXAFS as a probe of local dynamics and thermal expansion. The comparison between parallel mean square relative displacements (MSRD) of the first four coordination shells has allowed detecting a significant deviation from a pure Debye behavior. The first-shell EXAFS thermal expansion is larger than the crystallographic one: the difference has allowed evaluating the perpendicular MSRD, whose Debye temperature is slightly larger than the one of the parallel MSRD, due to anisotropy effects. High-order first-shell cumulants are in good agreement with quantum perturbative models. The anharmonic contribution to the first-shell parallel MSRD amounts to less than 1.5 percent. The third cumulant cannot be neglected in the analysis, if accurate values of the first cumulant are sought; it cannot however be used to directly estimate the thermal expansion. The shape of the effective pair potential is independent of temperature; a rigid shift, partially due to the relative motion perpendicular to the bond direction, is however observed.

Extended x-ray-absorption fine-structure measurements of copper: Local dynamics, anharmonicity, and thermal expansion

The extended x-ray-absorption fine structure (EXAFS) at the K edge of silver has been measured in β-AgI at different temperatures in the range T=23-410 K. Amplitude and phase of the first-shell EXAFS have been separately analysed in terms of cumulant expansion within the photoelectron wave-vector range κ=2.5-16.5 A -1 taking the 23-K spectrum as reference. Tests on EXAFS simulated from excluded-volume-model distributions have been performed to check the convergence of the cumulant series and the correspondence between polynomial coefficients obtained from EXAFS analysis and exact cumulants

Cumulant analysis of the extended x-ray-absorption fine structure of β-AgI

Silicon-rich silicon oxide films deposited by plasma enhanced chemical vapor deposition with different total Si contents (from 39to46at.%) have been annealed at increasing temperature (up to 1250°C) in order to study the Si nanocrystal (Si-nc) nucleation as well as the structural changes induced in the amorphous embedding matrix. The comparison between x-ray absorption measurements in total electron yield mode, Raman spectroscopy, and photoluminescence spectra allowed us to gain insight about the Si nanocrystal formation, while the chemical composition and the nature of chemical bonds into the oxidized matrix was studied by Fourier transform infrared spectroscopy. A comprehensive picture of the nucleation process has been obtained, demonstrating the active role played by the hydrogen and nitrogen atoms in the formation of Si-nc and in the thermally induced evolution of the deposited films.

Silicon nanocrystal formation in annealed silicon-rich silicon oxide films prepared by plasma enhanced chemical vapor deposition

High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals ${\mathrm{Cu}}_{2}\mathrm{O}$ and ${\mathrm{Ag}}_{2}\mathrm{O}$ as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature intervals (from 9 to $240\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for ${\mathrm{Cu}}_{2}\mathrm{O}$, up to $470\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for ${\mathrm{Ag}}_{2}\mathrm{O}$) and anisotropic thermal displacements of M atoms ($\mathrm{M}=\mathrm{Cu}$,Ag). EXAFS measures a positive expansion of the nearest-neighbors $\mathrm{M}\text{\ensuremath{-}}\mathrm{O}$ pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The $\mathrm{M}\text{\ensuremath{-}}\mathrm{O}$ bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 $\mathrm{M}\text{\ensuremath{-}}\mathrm{M}$ next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid $\mathrm{M}\text{\ensuremath{-}}\mathrm{O}$ rods.

/pdf/negative-thermal-expansion-and-local-dynamics-in-cu2o-and-1pwmhdn98r.pdf

F. Rocca

Papers

Role of the interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO 2

Extended x-ray-absorption fine-structure measurements of copper: Local dynamics, anharmonicity, and thermal expansion

Cumulant analysis of the extended x-ray-absorption fine structure of β-AgI

Silicon nanocrystal formation in annealed silicon-rich silicon oxide films prepared by plasma enhanced chemical vapor deposition

Negative thermal expansion and local dynamics in Cu2O and Ag2O