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

The EXAFS Debye-Waller factor depends on the correlation of atomic motion and can yield original information on the vibrational dynamics of crystalline solids In this paper an introductory treatment of thermal disorder in EXAFS, based on thc cumulant approach, is given Thc general relation between mean-square relative displacement (MSRD) measured by EXAFS and atomic thermal vibrations in harmonic approximation is explored Strengths and limitations of the phenomenological Einstein- and Debye-correlated models are discussed Some of the most significant results so far obtained are reviewed A relatively simple method for estimating anharmonic corrections to the MSRD is proposed

EXAFS Debye-Waller factor and thermal vibrations of crystals.

The sensitivity of extended x-ray-absorption fine structure (EXAFS) to thermal expansion has been studied by temperature-dependent measurements on germanium. The first cumulant does not reproduce the thermal expansion owing to vibrations normal to the bond. The perpendicular relative displacement $〈\ensuremath{\Delta}{u}_{\ensuremath{\perp}}^{2}〉$ has been for the first time experimentally obtained; the ratio $〈\ensuremath{\Delta}{u}_{\ensuremath{\perp}}^{2}〉/〈\ensuremath{\Delta}{u}_{\ensuremath{\Vert}}^{2}〉$ is in agreement with vibrational model calculations. Low-temperature quantum effects on the 3rd cumulant have been for the first time observed. The possibility of measuring thermal expansion from the 3rd cumulant is demonstrated, provided that quantum effects are taken into account.

Sensitivity of Extended X-Ray-Absorption Fine Structure to Thermal Expansion

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

Giuseppe Dalba

Papers

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

EXAFS Debye-Waller factor and thermal vibrations of crystals.

Sensitivity of Extended X-Ray-Absorption Fine Structure to Thermal Expansion

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