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

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

Porous silicon: a quantum sponge structure for silicon based optoelectronics

Nanochemistry: Synthesis in diminishing dimensions

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

Monolithic lasers on Si are ideal for high-volume and large-scale electronic-photonic integration. Ge is an interesting candidate owing to its pseudodirect gap properties and compatibility with Si complementary metal oxide semiconductor technology. Recently we have demonstrated room-temperature photoluminescence, electroluminescence, and optical gain from the direct gap transition of band-engineered Ge-on-Si using tensile strain and n-type doping. Here we report what we believe to be the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge-emitting waveguide device. The emission exhibited a gain spectrum of 1590-1610 nm, line narrowing and polarization evolution from a mixed TE/TM to predominantly TE with increasing gain, and a clear threshold behavior.

/pdf/ge-on-si-laser-operating-at-room-temperature-16bzfpxa0w.pdf

Ge-on-Si laser operating at room temperature.

It is demonstrated that photoluminescent porous Si (PS) layers exhibit definitely visible electroluminescence (EL). The PS layers were formed by anodization of single‐crystal nondegenerate p‐type Si wafers in an HF solution. The experimental EL cells are of the form semitransparent metal/PS layer/p‐type Si/Al electrode. These cells show a rectifying junction behavior. When the forward current density reaches a certain value, stable visible (orange) light is uniformly emitted through a semitransparent electrode. A possible explanation of this is the radiative transition due to electron and hole injection into quantized states in PS.

Visible electroluminescence from porous silicon

It is shown that UV-excited porous Si (PS) exhibits an efficient visible photoluminescence (PL) at room temperature. The PS layers were formed by anodization of p-type and n-type single-crystal Si wafers in aqueous HF solutions. The peak wavelength of PL spectra depends on the anodization parameters including the resistivity and the conduction type of Si substrates. The PL spectra can be tuned to a higher energy side by either adjustment of the anodizing conditions or chemical etching after anodization. These remarkable results can be interpreted as a result of quantum size effects in PS.

https://iopscience.iop.org/article/10.1143/JJAP.30.L1221/pdf

Efficient Visible Photoluminescence from Porous Silicon

It has been shown that the blue photoluminescence (PL) of porous silicon (PS) can be obtained simply from postanodization illumination by white light without any oxidation processes. The PS samples were formed on 5–6 Ω cm p‐type (100) Si wafers. The anodization was carried out in an ethanoic HF solution at a current density of 50 mA/cm2 for 5 min in the dark. After the anodization, the samples were illuminated under the open‐circuit condition by a 500 W tungsten lamp for several minutes. The PL measurements of prepared PS samples were carried out in vacuum. With increasing the postanodization illumination time, the PL band exhibited a continuous shift from red to blue. During this blue shift, fourier transform infrared spectra did not show any signs of the growth of the surface oxide. In addition, the emission energy dependence of the PL decay time for each PL band behaved in accordance with a universal curve. These results strongly suggest that there is an oxide‐free mechanism in the PL emission from PS throughout the visible range.

Oxide‐free blue photoluminescence from photochemically etched porous silicon

It is demonstrated that electroluminescent porous silicon (PS) diodes operate as surface-emitting cold cathodes The PS diode is composed of a semitransparent thin Au film, PS layer, n-type Si substrate and ohmic contact When a sufficient positive bias voltage is applied to the Au contact in vacuum, the diode uniformly emits electrons as well as visible light This cold emission is explained by the model that electrons are injected from the substrate, drifted by the field within the PS layer, and ejected as hot electrons through the thin Au film This mode gives important insight for understanding the electroluminescence mechanism of PS, and shows the potential of PS devices not only for optoelectronic applications, but also for vacuum microelectronic ones

Cold Electron Emission from Electroluminescent Porous Silicon Diodes

An electrochemical technique is proposed to form a solid‐state electrical contact of porous Si (PS) electroluminescence (EL) diodes. The PS layers were created by anodizing nondegenerate p‐type single‐crystal Si wafers in an HF solution, and then electrochemically polymerizing semitransparent conducting polypyrrole films into the PS layer. The current‐voltage characteristics and the voltage and current dependence of the EL intensity were significantly improved in comparison with our experimental PS‐EL diode with a thin Au film contact. Our result suggests that the electrode impregnation into PS is very useful for an efficient and stable EL operation.

Hideki Koyama

Papers

Visible electroluminescence from porous silicon

Efficient Visible Photoluminescence from Porous Silicon

Oxide‐free blue photoluminescence from photochemically etched porous silicon

Cold Electron Emission from Electroluminescent Porous Silicon Diodes

Visible electroluminescence from porous silicon diodes with an electropolymerized contact