Theoretical analysis of electronic band structure of 2-to-3-nm Si nanocrystals

We introduce a general method which allows reconstruction of electronic band structure of nanocrystals from ordinary real-space electronic structure calculations. A comprehensive study of band structure of a realistic nanocrystal is given including full geometric and electronic relaxation with the surface passivating groups. In particular, we combine this method with large scale density functional theory calculations to obtain insight into the luminescence properties of silicon nanocrystals of up to 3 nm in size depending on the surface passivation and geometric distortion. We conclude that the band structure concept is applicable to silicon nanocrystals with diameter larger than $\approx$ 2 nm with certain limitations. We also show how perturbations due to polarized surface groups or geometric distortion can lead to considerable moderation of momentum space selection rules

Luminescence of free-standing versus matrix-embedded oxide-passivated silicon nanocrystals: The role of matrix-induced strain:

We collect a large number of experimental data from various sources to demonstrate that free-standing (FS) oxide-passivated silicon nanocrystals (SiNCs) exhibit considerably blueshifted emission, by 200 meV on average, compared to those prepared as matrix-embedded (ME) ones of the same size. This is suggested to arise from compressive strain, exerted on the nanocrystals by their matrix, which plays an important role in the light-emission process; this strain has been neglected up to now as opposed to the impact of quantum confinement or surface passivation. Our conclusion is also supported by the comparison of low-temperature behavior of photoluminescence of matrix-embedded and free-standing silicon nanocrystals

Towards a Germanium and Silicon Laser: The History and the Present

Various theoretical as well as empirical considerations about how to achieve lasing between the conduction and valence bands in indirect band gap semiconductors (germanium and silicon) are reviewed, starting from the dawn of the laser epoch in the beginning of the sixties. While in Ge the room-temperature lasing under electrical pumping has recently been achieved, in Si this objective remains still illusory. The necessity of applying a slightly different approach in Si as opposed to Ge is stressed. Recent advances in the field are discussed, based in particular on light-emitting Si quantum dots

Symmetry and symmetry-breaking in semiconductors: fine structure of exciton states

This book discusses group theory investigations of zincblende and wurtzite semiconductors under symmetry-breaking conditions. The text presents the group theory elements required to develop a multitude of symmetry-breaking problems, giving scientists a fast track to bypass the need for recalculating electronic states. The text is not only a valuable resource for speeding up calculations but also illustrates the construction of effective Hamiltonians for a chosen set of electronic states in crystalline semiconductors. Since Hamiltonians have to be invariant under the transformations of the point group, the crystal symmetry determines the multiplet structure of these states in the presence of spin-orbit, crystal-field, or exchange interactions. Symmetry-breaking leads to additional coupling of the states, resulting in shifts and/or splittings of the multiplets. Such interactions may be intrinsic, as in the case of the quasi-particle dispersion, or extrinsic, induced by magnetic, electric, or strain fields. Using a power expansion of the perturbations these interaction terms can be determined in their parameterized form in a unique way. The hierarchic structure of this invariant development allows to estimate the importance of particular symmetry-breaking effects in the Hamiltonian. A number of selected experimental curves are included to illustrate the symmetry-based discussions, which are especially important in optical spectroscopy. This text is written for graduate students and researchers who want to understand and simulate experimental findings reflecting the fine structure of electronic or excitonic states in crystalline semiconductors.

Luminescence spectroscopy of semiconductors

Semiconductor luminescence has been a rapidly expanding field over the last 50 years. This text reviews the whole subject of semiconductor luminescence in one volume

Etude de l'amplification optique dans des nanostructures à la base du silicium

Le but principal de ce travail fut de préparer un matériau photo-luminescent à base de nano-cristaux de Silicium dans une matrice de silice (SiO2) de qualité optique suffisante pour permettre l'observation d'un gain optique. Des nano-cristaux de silicium peu oxydés de tailles comprises entre 2 et 3 nm ont été obtenus par abrasion électrochimique de wafer de silicium. Les nano-cristaux avec une concentration variable permettant l'observation de leur émission stimulée sont dilués dans une matrice de silice obtenue par procédé sol-gel. Un dispositif optique dit "de zone à longueur variable" ("Variable Stripe Length" VSL) a été utilisé pour la mesure du gain optique des nano-cristaux. Cependant cette méthode seule reste peu fiable pour les matériaux à faible gain optiques tels que les nano-cristaux de silicium. Pour cette raison nous avons combiné la méthode VSL avec celle du "spot d'excitation déplacé " ("Shifting Excitation Spot" SES). Ceci nous permet d'observer des gains faibles qui n'auraient pas pu être atteint avec la méthode VSL seule. Nos résultats montrent clairement l'apparition d'un gain sous différentes conditions d'excitations. Pour préparer un laser il est nécessaire d'avoir un materiau, montrant du gain optique, mais il faut aussi appliquer une contra réaction optique suffisante. L'utilisation d'une cavité optique externe nécessite des échantillons de grande qualité optique. Ceci n'est pas compatible avec un gain élevé qui demande une concentration très forte en nano-cristaux de silicium. Pour cela nous avons construit un laser à "cavité à contra réaction distribuée" ("Distributed Feedback Laser" DFL). Dans ce type de cavité, la contra réaction est distribuée sur l'ensemble de l'échantillon. Le pas du réseau (166 nm) est inférieure aux variations moyennes de densité ( 0.5-1.0 m) et peut être facilement modifié. Nous espérons ainsi obtenir un gain faible mais suffisant pour être observable. La cavité DFL est tout d'abord calibrée à l'aide de différents colorants dilués dans une solution de méthanol où nous avons observé des modes laser biens définis. Des modes d'émissions laser similaires (des pics plus larges et moins intenses que dans le cas des colorants) ont été obtenus dans nos échantillons Si-ncs/SiO2. Ceci est principalement dû à la moindre qualitéoptique de nos échantillons. Pour comprendre les précédentes observations, nous avons developpé un modèle théorique simple nous permettant de retrouver et d'expliquer les modes experimentaux en jouant sur la variation de densité et les caractéristiques des Si-ncs. L'effet de la contra réaction de la cavité DFL sur nos échantillons est clairement identifié par ce modèle. Ceci nous permet d'entrevoir de nouvelles perspectives pour la caractérisation optique et l'amélioration de nos échantillons.The aim of this work was to prepare light-emitting structure on the basis of silicon nanocrystals (Si-ncs) embedded in a silicon dioxide (SiO2) based matrix of a sufficiently good optical quality and stable emission properties, which exhibits positive optical gain and can be used as an active material in a laser cavity. The technique of sample preparation is based on a combination of the modified electrochemical etching of silicon wafers and the SiO2 based sol-gel processing. This method enables us to achieve relatively small oxidized Si-ncs ( 2-3 nm), embedded at virtually arbitrary volume fraction in a SiO2 based matrix, which is believed to be advantageous for easier stimulated emission (StE) onset observation. The optical gain coefficient was measured using the standard "Variable Stripe Length" (VSL) method, the application of which, however, is limited for low gain. Therefore we implemented a supplemental "Shifting Excitation Spot" (SES) method, enabling us to determine the optical gain coefficient even of such a small magnitude that will not be recognized by the VSL method itself. We observed a positive net gain coecient originating from the StE in dierent Si-ncs/SiO2 samples under different excitation and detection conditions. To prepare a laser system, a positive net gain observation is essential as well as a positive optical feedback. Using an external cavity as a resonator requires a high optical quality sample. This is, however, hardly achievable under the high Si-ncs volume fraction requirements for the StE onset. Because of that we decided to build an optically induced "Distributed Feedback Laser" (DFL) system, where the cavity is distributed over the whole sample volume and the cavity grating constant ( 166 nm) is lower than expected mean homogeneity length in our sample ( 0.5-1.0 m). Therefore, a positive but low effect on the emission of Si-ncs is expected. Moreover, such type of DFL cavity is easily tuneable. The functionality of the DFL setup was tested using reference organic dye solutions in methanol, where a tuneable lasing action was successfully achieved. Similar tuneable cavity modes were also observed in different Si-ncs/SiO2 samples, however, of broader widths and less intense, compared to the organic dyes, which is mainly given by their lower optical quality. To understand and describe the mode selection in such a material, we developed a simple theoretical model, enabling us to determine the selected mode shape with respect to the sample homogeneity length and the character of the inhomogeneities. We proved the active feedback of the DFL cavity on the emission of our Si-ncs/SiO2 samples and proposed some further steps for future sample improvement.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Photoluminescence Studies of Li-Doped Si Nanocrystals

We investigate the optical properties of Lidoped Si nanocrystals (both freestanding and matrixembedded), which are potentially an important material for the new generation of lithium‐ion batteries. Our samples contain 10 – 100 Li atoms per one Si nanocrystal and their lattice is slightly expanded. The photoluminescence (PL) spectra of the S‐band of Lidoped Si nanocrystals are blue‐shifted by ~30nm compared to the undoped Si nanocrystals and their PL lifetime is correspondingly shorter. The F‐band emission is almost unaffected by Li doping. The observed changes in the PL performance are probably caused by Si lattice expansion induced by Li insertion. The reported spectral blue shift is favourable for certain photonic applications

Two-dimensional photonic crystals increasing vertical light emission from Si nanocrystal-rich thin layers

We have fabricated two-dimensional photonic crystals (PhCs) on the surface of Si nanocrystal-rich SiO2 layers with the goal to maximize the photoluminescence extraction efficiency in the normal direction. The fabricated periodic structures consist of columns ordered into square and hexagonal pattern with lattice constants computed such that the red photoluminescence of Si nanocrystals (SiNCs) could couple to leaky modes of the PhCs and could be efficiently extracted to surrounding air. Samples having different lattice constants and heights of columns were investigated in order to find the configuration with the best performance. Spectral overlap of the leaky modes with the luminescence spectrum of SiNCs was verified experimentally by measuring photonic band diagrams of the leaky modes employing angle-resolved spectroscopy and also theoretically by computing the reflectance spectra. The extraction enhancement within different spatial angles was evaluated by means of micro-photoluminescence spectroscopy. More than 18-fold extraction enhancement was achieved for light propagating in the normal direction and up to 22% increase in overall intensity was obtained at the spatial collection angle of 14°

Generation of Silicon Nanostructures by Atmospheric Microplasma Jet: The Role of Hydrogen Admixture

International audienceSilicon nanostructures are synthesized with a DC atmospheric pressure microplasma jet using an Ar/SiH4/H-2 gas mixture. The plasma is characterized by OES and imaged using an EMCCD camera. The effect of hydrogen admixture to the formed structures is studied by transmission electron microscopy. Under specific conditions, crystalline silicon nanoparticles grow in an amorphous matrix investigated by electron energy loss spectroscopy. As-grown silicon nanoparticles are collected in ethanol for dynamic light scattering and photoluminescence measurements. The size distribution peaks at 4nm. The silicon nanocrystals exhibit roomtemperature photoluminescence that peaks at approximate to 415 and approximate to 465 nm

Grains of porous silicon embedded in SiO(2): Studies of optical gain and electroluminescence

Recent reports on experimental observation of optical gain in silicon nanostructures in the visible region, performed at several laboratories all over the world, have triggered an extraordinary surge of interest in silicon lasing. However, attempts aimed at reproducing the red stimulated emission from "standard" silicon nanocrystals (sized 3-5 nm) at some other laboratories either failed, or. did not come to definite conclusions. Therefore, more detailed measurements of optical gain in a wider variety of samples containing Si nanocrystals are required in order to unravel whether or not the observation of optical gain is an intrinsic property of Si nanocrystals. We have performed a detailed study of optical gain in layers of densely packed Si nanocrystals in SiO(2), prepared on the basis of porous Si, using the variable-stripe-length (VSL) method in combination with the shifted-excitation-spot (SES) method. In selected samples we have observed a distinct difference in behaviour between VSL and SES curves, indicating the occurrence of positive optical gain of similar to24 cm(-1). Preliminary reports on transport and electroluminescence measurements in thin films of SiO(2) doped with porous silicon grains, prepared by spin-coating technique, are also discussed