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

Organometallic chemistry on silicon and germanium surfaces.

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

Porous silicon: a quantum sponge structure for silicon based optoelectronics

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

Introduction Sp3-hybridized semiconductors (including InP, GaAs, CdSe, and Si) are remarkable from the perspective of physical chemistry. A single electron created by HOMOLUMO promotion moves rapidly in response to an applied electric field, because there is little lattice distortion (i.e., small Franck-Condon factors) accompanying its creation. According to Marcus-Hush electron transfer theory, electron motion is resonant in the limit of vanishing reorganization energy. Franck-Condon factors are also small around an electron-hole pair, which is an electronically excited state. As a consequence, radiationless internal conversion (unimolecular decay converting electronic energy into heat) is extremely slow. Excited states decay radiatively in a defect-free, direct gap semiconductor such as CdSe. These simple spectroscopic facts have important practical consequences. Semiconductor lightemitting diodes have narrow emission bands and can show near 30% efficiency in converting electrical power into light. Semiconductor lasers and diodes also show excellent long-term stability against photochemical and current-induced degradation, when compared with many organic materials. All these properties reflect the strong chemical bonding, and the extremely delocalized nature of the electronic wave functions. Semiconductor nanocrystals lie between the traditional regimes of chemistry and solid-state physics.1 Nanocrystal research was initially motivated by an effort to understand the evolution of bulk structural and electronic properties from the molecular scale.2 Presently, technological interest in nanocrystals stems from the prospect of creating novel materials with distinct physical properties. Nanocrystals act like molecules as they interact with light via their electronic transition dipoles. Yet, their delocalized solid-state parentage causes them to display unusual photophysics relative to molecules. In many molecules vibronic interaction in the excited state is strong as the wave function is localized on just one or a few bonds. The molecular excited state has a different structure which promotes fast nonradiative deactivation into the ground state. Emission quantum yields can be low, and often sensitive to quenching by the local environment. The situation is different in nanocrystals. In a 23 A diameter nanocrystal, for example, the wave function is delocalized over ∼100 unit cells with little probability density at the surface. This suggests that, in the absence of defects, internal or surface, a nanocrystal should exhibit near unity fluorescence quantum yield, and partial protection from quenching. The emission spectrum should be sharp as the Franck-Condon factors are small. At room temperature nanocrystals can be better photoemitters than bulk semiconductors because in nanocrystals the electron and hole remain superimposed due to quantum confinement. Nanocrystals have the potential to serve as ideal chromophores if their surface chemistry can be understood and controlled. In this Account we describe nanocrystal photophysics and make comparisons between inorganic solid-state materials and organic dye molecules.

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Luminescence Photophysics in Semiconductor Nanocrystals

Porous silicon/silicon structures under anodic oxidation conditions give rise to an electroluminescence phenomenon in the visible range. Using an optical multichannel analyzer the spectral distribution of the emitted light was measured−in situ−during the anodic oxidation step. Recorded spectra show a maximum which shifts continuously from red‐orange at the beginning of the process towards the yellow range. The visible emission well above the band gap of bulk silicon is attributed to a quantum size effect in the very small size (5–20 A) silicon island which constitutes the porous silicon skeleton. The light emission is interrupted when the current flow stops due to the formation of a continuous oxide layer at the porous silicon/silicon interface.

Electroluminescence in the visible range during anodic oxidation of porous silicon films

A microstructural study of high‐porosity porous silicon layers formed on lightly P‐doped wafers has been performed by x‐ray small‐angle scattering (SAXS) using the powerful and parallel beam of the synchrotron radiation. When the porosity of the sample is increased from 55% to 85% there is a continuous modification in the shape of the scattering profiles. The silicon skeleton mass fractal dimension decreases continuously. For porosity around 85%, the value for which the sample starts to display a strong photoluminescence at room temperature, there is a large increase in the pore size. The scattering profiles are characteristic of an isotropic three‐dimensional structure.

Characterization of photoluminescent porous Si by small‐angle scattering of x rays

A small‐angle x‐ray scattering study of porous layers prepared on lightly doped and heavily doped silicon wafers reveals strong differences according to the dopant type. The p‐type silicon layers show a roughness between matter and voids below a space‐correlation length of about 10 nm and a fractal dimension of aggregates for a length scale higher than 25 nm. The lightly doped n‐type is different: the scattering curve obeys the Porod law according to the rod‐like structure of the pores. However, the structure of the heavily doped p+ sample is more complicated and not yet well understood.A small‐angle x‐ray scattering study of porous layers prepared on lightly doped and heavily doped silicon wafers reveals strong differences according to the dopant type. The p‐type silicon layers show a roughness between matter and voids below a space‐correlation length of about 10 nm and a fractal dimension of aggregates for a length scale higher than 25 nm. The lightly doped n‐type is different: the scattering curve obeys the Porod law according to the rod‐like structure of the pores. However, the structure of the heavily doped p+ sample is more complicated and not yet well understood.

X‐ray small‐angle scattering analysis of porous silicon layers

Small‐angle x‐ray scattering was used to investigate the microstructural change induced by electrochemical oxidation of porous silicon (PS) layers. It is shown that when the oxidation level increases, the size of the crystalline Si domains, which constitute the PS layer, decreases. This size reduction is correlated to the blue‐shift observed in the photoluminescence spectra when the oxidation level is increased. Moreover, we found that a ‘‘fuzzy’’ surface appears between pores (voids) and matter when the samples are electrochemically oxidized. This interface is found very sharp for the as‐prepared and nonoxidized PS layers.

Small‐angle x‐ray scattering study of anodically oxidized porous silicon layers

A new type of application of porous silicon formation is proposed, which does not deal with the material properties, but with the characteristics of the involved electrochemical silicon dissolution reaction. The anodization potential of p‐type silicon in concentrated hydrofluoric acid under galvanostatic conditions has been shown to be characteristic of the silicon doping concentration at the interface and independent of the porous silicon‐layer thickness. This close correlation between potential and doping level is used to determine concentration profiles of p‐type implanted dopants. During anodization of silicon presenting dopant concentration variations, the anodization potential varies according to silicon doping concentration. The potential values can be converted to dopant concentration, and the electrolysis time scale to a depth scale, leading then to the doping impurity profile. This method, which is used as well for deep and shallow diffused impurities, exhibits a very good agreement with spreadi...

G. Bomchil

Papers

Electroluminescence in the visible range during anodic oxidation of porous silicon films

Characterization of photoluminescent porous Si by small‐angle scattering of x rays

X‐ray small‐angle scattering analysis of porous silicon layers

Small‐angle x‐ray scattering study of anodically oxidized porous silicon layers

Application of porous silicon formation selectivity to impurity profiling in p-type silicon substrates