Développement du pompage de charges pour la caractérisation in-situ de nanocristaux de Si synthétisés localement dans SiO2 par implantation ionique basse énergie et lithographie stencil

Le regain d'attention des industriels pour les mémoires non volatiles intégrant des nanocristaux, illustré par l'introduction sur le marché de la Flexmemory de Freescale en technologie 90 nm, incite à poursuivre des études sur ce type de systèmes. Pour cela, nous avons mis au point des cellules mémoires élémentaires, à savoir des transistors MOS dont l'oxyde de grille contient une grille granulaire formée par un plan de nanocristaux de silicium (Si-ncx) stockant la charge électrique.Ce travail présente les principaux résultats issus de ces travaux, ceux-ci allant du procédé de fabrication à la caractérisation fine des dispositifs mémoires. Le parfait contrôle de l'élaboration de la grille granulaire de Si-ncx par implantation ionique à très basse énergie (ULE-IBS) est accompagné de caractéristiques mémoires répondant aux normes industrielles d'endurance et d'une discrimination des pièges responsables du chargement. Le stockage majoritaire par les Si-ncx est démontré, ce qui est essentiel pour la rétention de la charge. Nous avons développé une technique électrique permettant d'extraire à la fois la quantité de charge stockée par les Si-ncx mais également leurs principales caractéristiques structurales (taille, densité, position dans l'oxyde). Cette extension de la technique électrique de pompage de charges , non destructive et in-situ permet de suivre l'état du composant en fonctionnement et de caractériser des pièges (e.g. les Si-ncx) pour la première fois au-delà de 3 nm de profondeur dans l'oxyde. Ces résultats ont été validés par des observations TEM. La résolution du pompage de charge étant le piège unique, nous avons alors couplé l'ULE-IBS avec la lithographie Stencil pour réduire latéralement le nombre de Si-ncx synthétisés. Cette technique nous permet pour le moment de contrôler la synthèse locale à la position désirée dans l'oxyde de poches de Si-ncx de 400 nm. La synthèse de quelques Si-ncx est envisagée à très court terme. Nous serons alors en mesure de fabriquer des mémoires à nombre choisi de nanocristaux (par SM-ULE-IBS), dont les propriétés structurales (taille, densité, position) et électriques (quantité de charge stockée) seront vérifiées par pompage de charge, offrant ainsi des outils puissants pour la fabrication et la caractérisation de mémoires à nombre réduit de nanocristaux, notamment pour des longueurs de grilles inférieures à 90 nmThe aim of this thesis has been to fabricate and electrically characterize elementary memory cells containing silicon nanocrystals (Si-ncs), in other words MOSFET which insulating layer (SiO2) contains a Si-ncs array storing the electrical charge. We have shown that we perfectly control the synthesis of a 2D array of 3-4 nm Si-ncs embedded into the MOSFET oxide by low-energy ion implantation (1-3 keV) Reaching this goal implied two key steps: on the one hand develop a reliable MOSFET fabrication process incorporating the Si-ncs synthesis steps and on the other hand develop tools and methods for both memory window and Si-ncs array itself characterizations. We have developed an in-situ characterization technique based on the well-known charge pumping technique, allowing for the first time the extraction of traps depth (e.g. the Si-ncs array) further than 3 nm into the oxide layer leading to the characterization of both position of these Si-ncs into the SiO2 matrix and their structural properties (diameter, density). These results have been confirmed by EF-TEM measurements. Finally, we have worked on the improvement of controlled local synthesis of Si-ncs pockets by combining low-energy ion implantation and stencil lithography. We reduced the size of these pockets down to about 400 nm using this parallel, low cost and reliable technique and identified the limiting effect for the pockets size reduction. These results pave the way for memory cells containing a few Si-ncs with a well-defined position into the oxide and a well-controlled number of ncsTOULOUSE-INSA-Bib. electronique (315559905) / SudocSudocFranceF

Graphene in silicon photovoltaic cells

Graphene is an allotrope of carbon. Its structure is one-atom-thick planar sheets of carbon atoms that are densely packed in a honeycomb crystal lattice [1]. The richness of optical and electronic properties of graphene attracts enormous interest. Its true potential seems to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited. The optical absorption of graphene layers is proportional to the number of layers, each absorbing A=1-T=πα=2.3% over the visible spectrum [2].The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Current photovoltaic (PV) technology is dominated by Si cells, with an energy conversion coefficient up to 25% [3]. Such an inorganic PV consists in a current transparent conductor (TC) replacing one of the electrodes of a PIN photodiode. The standard material used so far for these electrodes is indium-tinoxide, or ITO. But indium is expensive and relatively rare, so the search has been on for a suitable replacement. A possible substitute made from inexpensive and ubiquitous carbon is graphene. Being only constituted of carbon, it will become cheap and easily recyclable. But at the moment, the major difficulty consists in its fabrication and/or transfer. Our project consists in synthetizing graphene by CVD (Chemical Vapor Deposition) on Cu and in transferring the obtained layer on silicon PV cells, and then in testing their energy conversion efficiency

Structured ZnO-based contacts deposited by non-reactive rf magnetron sputtering on ultra-thin SiO2/Si through a stencil mask

In this paper, we study the localized deposition of ZnO micro and nanostructures deposited by non-reactive rf-magnetron sputtering through a stencil mask on ultra-thin (10 nm) SiO2 layers containing a single plane of silicon nanocrystals (NCs), synthetized by ultra-low energy ion implantation followed by thermal annealing. The localized ZnO-deposited areas are reproducing the exact stencil mask patterns. A resistivity of around 5×10−3 Ω cm is measured on ZnO layer. The as-deposited ZnO material is 97% transparent above the wavelength at 400 nm. ZnO nanostructures can thus be used as transparent electrodes for Si NCs embedded in the gate-oxide of MOS devices

Transport in Magnetic Nanoparticles Super-Lattices : Coulomb Blockade, Hysteresis and Magnetic Field Induced Switching

We report on magnetotransport measurements on millimetric super-lattices of Co-Fe nanoparticles surrounded by an organic layer. At low temperature, the transition between the Coulomb blockade and the conductive regime becomes abrupt and hysteretic. The transition between both regime can be induced by a magnetic field, leading to a novel mechanism of magnetoresistance. Between 1.8 and 10 K, high-field magnetoresistance due to magnetic disorder at the surface of the particles is also observed. Below 1.8 K, this magnetoresistance abruptly collapses and a low-field magnetoresistance is observed.Comment: 9 pages (text, figures, figures legends and references). 3 figures reference 33 added : "arXiv:0710.1750v1

Practical Works on Nanotechnology: Middle School to Undergraduate Students

Since its emergence a few decades ago, nanotechnology has been shown to be a perfect example of a crossroad between different fundamentals sciences. In the last 10 years, the continuous progress of classical top-down lithography and the use of alternative bottom-up elaboration methods has allowed new and smaller components to be created. Their combination has led to very complex and innovative architectures. At the same time, flexible, low-cost, and low-ecological-footprint devices have emerged. Thus, the diversity and multidisciplinary features present challenges in addressing these issues in educational programs

Synthèse par implantation ionique, adressage, caractérisations électriques et optiques d'un nombre réduit de nanocristaux de Si dans SiO2

Ce travail est consacré à la synthèse localisée et contrôlée de nanocristaux de Si dans une couche de SiO2 (<10 nm) par deux techniques d implantation ionique. D une part, l implantation ionique à basse énergie (1keV) suivie d un recuit thermique (ULE-II) permet d élaborer un plan de nanocristaux dans une couche d oxyde, d autre part l ULE-II couplée à la lithographie stencil (méthode originale SMULE- II) permet de les synthétiser localement et de contrôler leur nombre. Les caractérisations par MEB, AFM, EFTEM, spectroscopie de photoluminescence permettent l étude structurale des nanocristaux (taille, forme, densité, position dans l oxyde, caractéristiques des zones implantées localement ). Puis, des capacités MOS de taille micro à nanométrique en adressent un certain nombre (grand 108 à réduit 50). Les études I-V et I-t réalisées à température ambiante mettent en évidence des effets de chargement collectif (nanocristaux adressés en grand nombre ou connectés) et discret (nanocristaux en faible nombre (<200) ou formés localement et oxydés). Un modèle électrique permet de corréler les caractéristiques électriques et structurales. De plus, les études I-V réalisées à basse température et les mesures KFM confirment que les charges sont certainement stockées préférentiellement dans les nanocristaux. Pour finir, les effets de stockage de charge des mesures C-V confirment l intérêt des nanocristaux de Si (élaborés par ULE-II ou SM-ULE-II) pour des dispositifs mémoires non volatiles, et les électrodes transparentes (ITO et ZnO) prouvent qu il sera possible de les exciter optiquement et de les adresser électriquement afin de réaliser des dispositifs électro-optiquesThis work is dedicated to the localized synthesis of a controlled number of Si nanocrystals into SiO2 layer, by two ion implantation methods. On the one hand, the ultra low energy ion implantation followed by thermal annealing (ULE-II) leads to create a two dimensional array of nanocrystals ; On the other hand, the original SM-ULE-II method where ULE-II is performed through a stencil mask leads to fabricate localized areas of Si nanocrystals while controlling their number. Characterizations by SEM, AFM, EFTEM, photoluminescence spectroscopy allow studying the structural properties of the nanocrystals (size, density, shape, localization into the oxide, implanted areas characteristics, ..). Then, a reduced number of nanocrystals elaborated by ULE-II (108 to 50) or SM-ULE-II is addressed by a micro to nanometer MOS capacitor. Room temperature I-V and I-t measurements exhibit collective charging effects (large number of nanocrystals addressed or connected nanocrystals) and discrete charging effects (a few number of nanocrystals or nanocrystals elaborated by SM-ULE-II and oxidized). An electrical model relates the electrical and structural properties. I-V characterizations realized at low temperature and KFM measurements confirm charge storage essentially into nanocrystals. C-V curves prove that nanocrystals are attractive to non volatile memory applications, and using transparent electrode (ZnO or ITO), nanocrystals can be optically excited and electrically addressed in order to create electro optical componentsTOULOUSE-INSA (315552106) / SudocSudocFranceF

Towards wireless highly sensitive capacitive strain sensors based on gold colloidal nanoparticles

International audienceWe designed, produced and characterized new capacitive strain sensors based on colloidal gold nanoparticles. The active area of these sensors, made up of a 1 mm2 close-packed assembly of gold nanoparticles between interdigitated electrodes, was designed to achieve measurable capacitance (>∼1 pF) and overcome parasitic capacitances. Electro-mechanical experiments revealed that the sensitivity of such capacitive sensors increases in relation to the size of the nanoparticles. In the case of 14 nm gold NPs, such sensors present a relative capacitance variation of −5.2% for a strain of 1.5%, which is more than 5 times higher than that observed for conventional capacitive strain gauges. The existence of two domains (pure capacitive domain and mixed capacitive–resistance domain) as a function of the frequency measurement allows for the adaptation of sensitivity of these capacitive sensors. A simple low-cost circuit based on a microcontroller board was finally developed to detect the capacitance variations of such NP based strain sensors. This low-cost equipment paves the way for the development of an entirely wireless application set-up

Towards wireless highly sensitive capacitive strain sensors based on gold colloidal nanoparticles

International audienceWe designed, produced and characterized new capacitive strain sensors based on colloidal gold nanoparticles. The active area of these sensors, made up of a 1 mm2 close-packed assembly of gold nanoparticles between interdigitated electrodes, was designed to achieve measurable capacitance (>∼1 pF) and overcome parasitic capacitances. Electro-mechanical experiments revealed that the sensitivity of such capacitive sensors increases in relation to the size of the nanoparticles. In the case of 14 nm gold NPs, such sensors present a relative capacitance variation of −5.2% for a strain of 1.5%, which is more than 5 times higher than that observed for conventional capacitive strain gauges. The existence of two domains (pure capacitive domain and mixed capacitive–resistance domain) as a function of the frequency measurement allows for the adaptation of sensitivity of these capacitive sensors. A simple low-cost circuit based on a microcontroller board was finally developed to detect the capacitance variations of such NP based strain sensors. This low-cost equipment paves the way for the development of an entirely wireless application set-up

Conducting AFM study of colloidal nanoparticle assemblies

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Electron transport within transparent assemblies of tin-doped indium oxide colloidal nanocrystals

International audienceStripe-like compact assemblies of tin-doped indium oxide (ITO) colloidal nanocrystals (NCs) are fabricated by stop-and-go convective self-assembly (CSA). Systematic evaluation of the electron transport mechanisms in these systems is carried out by varying the length of carboxylate ligands protecting the NCs: butanoate (C4), octanoate (C8) and oleate (C18). The interparticle edge-to-edge distance L0, along with a number of carbon atoms in the alkyl chain of the coating ligand, are deduced from small-angle x-ray scattering (SAXS) measurements and exhibit a linear relationship with a slope of 0.11 nm per carbon pair unit. Temperature-dependent resistance characteristics are analyzed using several electron transport models: Efros–Shklovskii variable range hopping (ES-VRH), inelastic cotunneling (IC), regular island array and percolation. The analysis indicated that the first two models (ES-VRH and IC) fail to explain the observed behavior, and that only simple activated transport takes place in these systems under the experimental conditions studied (T = 300 K to 77 K). Related transport parameters were then extracted using the regular island array and percolation models. The effective tunneling decay constant βeff of the ligands and the Coulomb charging energy EC are found to be around 5.5 nm−1 and 25 meV, respectively, irrespective of ligand lengths. The theoretical tunneling decay constant β calculated using the percolation model is in the range 9 nm−1. Electromechanical tests on the ITO nanoparticle assemblies indicate that their sensitivities are as high as ~30 and remain the same regardless of ligand lengths, which is in agreement with the constant effective βeff extracted from regular island array and percolation models