Slow waves in locally resonant metamaterials line defect waveguides

In the past decades, many efforts have been devoted to the temporal manipulation of waves, especially focusing on slowing down their propagation. In electromagnetism, from microwave to optics, as well as in acoustics or for elastic waves, slow wave propagation indeed largely benefits both applied and fundamental physics. It is for instance essential in analog signal computing through the design of components such as delay lines and buffers, and it is one of the prerequisite for increased wave/matter interactions. Despite the interest of a broad community, researches have mostly been conducted in optics along with the development of wavelength scaled structured composite media, that appear promising candidates for compact slow light components. Yet their minimum structural scale prevents them from being transposed to lower frequencies where wavelengths range from sub-millimeter to meters. In this article, we propose to overcome this limitation thanks to the deep sub-wavelength scale of locally resonant metamaterials. In our approach, implemented here in the microwave regime, we show that introducing coupled resonant defects in such composite media allows the creation of deep sub-wavelength waveguides. We experimentally demonstrate that waves, while propagating in such waveguides, exhibit largely reduced group velocities. We qualitatively explain the mechanism underlying this slow wave propagation and first experimentally demonstrate, then numerically verify, how it can be taken advantage of to tune the velocity, achieving group indices ng as high as 227 over relatively large bandwidths. We conclude by highlighting the three beneficial consequences of our line defect slow wave waveguides in locally resonant metamaterials: the deep sub-wavelength scale, the very large group indices and the fact that slow wave propagation does not occur at the expense of drastic bandwidth reductions

Study of CuInS2, CuIn1-xGaxS2, and Cu2ZnSnS4 thin films, elaborated by sol-gel process, devoted to photovoltaic applications

Ce travail de recherche porte sur l’élaboration et la caractérisation de films minces photo-absorbants de CuInS2, CuIn1-xGaxS2, et de Cu2ZnSnS4 destinés aux applications photovoltaïques. Les films minces ont été préparés par voie sol-gel puis déposés par enduction centrifuge sur substrat de silicium ou de verre. Les sols, formés à partir d’acétates métalliques et d’alcanolamines, ont été étudiés par spectroscopie IR, viscosimétrie et ATD-ATG. Les paramètres de dépôts des sols, et les traitements de calcination, ont ensuite été optimisés. Des films d’oxydes multi-couches, sans fissuration, et de faibles rugosités ont ainsi été élaborés. Une dernière étape de sulfuration des films d’oxydes a été effectuée afin de former les composés souhaités. Les films sulfurés ont fait l’objet d’une étude approfondie par DRX, EDX, MEB, AFM, spectroscopie UV-VIS-nIR et mesures par effet Hall. Leurs structures, leurs morphologies, mais aussi leurs propriétés optiques et électriques ont ainsi pu être étudiées. L’interface des films de CuInS2 avec le film de Mo, utilisé comme contact ohmique arrière de la cellule solaire, a également été étudiée par micro-EDX à l’aide d’analyses MET. Les résultats obtenus montrent que le procédé sol-gel, bien que très peu développé dans le domaine des cellules photovoltaïques, est une voie de synthèse bien adaptée à l’élaboration de films minces à structure chalcopyrite et kësterite. Ces résultats sont très prometteurs pour la réalisation d’une cellule solaire par voie sol-gel.This research activity concerns the elaboration and characterization of photo-absorbing thin films of CuInS2, CuIn1-xGaxS2, and Cu2ZnSnS4 devoted to photovoltaic applications. The thin films were prepared by sol-gel process and deposited by spin-coating technique on silicon and glass substrates. The sols, synthesized from metallic acetates and alcanolamines, were studied by IR-spectroscopy, viscosimetry, and TDA-TGA. The deposition parameters of the sols, and the calcination treatments were then optimized. The multi-layers oxides films produced were obtained without cracks and with low roughness. The last step was to produce the desired compounds through the sulfurization of the oxides films. The sulfurized films were studied by XRD, EDX, SEM, AFM, UV-VIS-nIR spectroscopy, and Hall Effect measurements. Their structures, morphologies, as well as their optical and electrical properties have been investigated. The interface between CuInS2 films and Mo film, defined as a back-contact of the solar cell, was also studied by micro-EDX with TEM analysis. Despite the fact that sol-gel process is not well-developed in the photovoltaic field, the obtained results show that sol-gel process is a well-adapted technique for elaboration of thin films with chalcopyrite and kesterite structures. These results are very promising for the achievement of a sol-gel solar cell

Etude de films minces de CuInS2, CuIn1-xGaxS2, et Cu2ZnSnS4, élaborés par voie sol-gel, destinés aux applications photovoltaïques

Ce travail de recherche porte sur l élaboration et la caractérisation de films minces photo-absorbants de CuInS2, CuIn1-xGaxS2, et de Cu2ZnSnS4 destinés aux applications photovoltaïques. Les films minces ont été préparés par voie sol-gel puis déposés par enduction centrifuge sur substrat de silicium ou de verre. Les sols, formés à partir d acétates métalliques et d alcanolamines, ont été étudiés par spectroscopie IR, viscosimétrie et ATD-ATG. Les paramètres de dépôts des sols, et les traitements de calcination, ont ensuite été optimisés. Des films d oxydes multi-couches, sans fissuration, et de faibles rugosités ont ainsi été élaborés. Une dernière étape de sulfuration des films d oxydes a été effectuée afin de former les composés souhaités. Les films sulfurés ont fait l objet d une étude approfondie par DRX, EDX, MEB, AFM, spectroscopie UV-VIS-nIR et mesures par effet Hall. Leurs structures, leurs morphologies, mais aussi leurs propriétés optiques et électriques ont ainsi pu être étudiées. L interface des films de CuInS2 avec le film de Mo, utilisé comme contact ohmique arrière de la cellule solaire, a également été étudiée par micro-EDX à l aide d analyses MET. Les résultats obtenus montrent que le procédé sol-gel, bien que très peu développé dans le domaine des cellules photovoltaïques, est une voie de synthèse bien adaptée à l élaboration de films minces à structure chalcopyrite et kësterite. Ces résultats sont très prometteurs pour la réalisation d une cellule solaire par voie sol-gel.This research activity concerns the elaboration and characterization of photo-absorbing thin films of CuInS2, CuIn1-xGaxS2, and Cu2ZnSnS4 devoted to photovoltaic applications. The thin films were prepared by sol-gel process and deposited by spin-coating technique on silicon and glass substrates. The sols, synthesized from metallic acetates and alcanolamines, were studied by IR-spectroscopy, viscosimetry, and TDA-TGA. The deposition parameters of the sols, and the calcination treatments were then optimized. The multi-layers oxides films produced were obtained without cracks and with low roughness. The last step was to produce the desired compounds through the sulfurization of the oxides films. The sulfurized films were studied by XRD, EDX, SEM, AFM, UV-VIS-nIR spectroscopy, and Hall Effect measurements. Their structures, morphologies, as well as their optical and electrical properties have been investigated. The interface between CuInS2 films and Mo film, defined as a back-contact of the solar cell, was also studied by micro-EDX with TEM analysis. Despite the fact that sol-gel process is not well-developed in the photovoltaic field, the obtained results show that sol-gel process is a well-adapted technique for elaboration of thin films with chalcopyrite and kesterite structures. These results are very promising for the achievement of a sol-gel solar cell.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

XPS chemical state mapping in opto- and microelectronics

The strength of XPS imaging lies in its ability to (i) locate small patterns on sample surface, and (ii) inform, with micrometric lateral resolution, about the chemical environment of the elements detected at the surface. In this context, strontium-based perovskites appear to be well-adapted for such photoemission experiments thanks to their tunability and variability. These functional oxides have great potential for emerging optoand microelectronic applications, especially for transparent conductive oxide. Patterned heterostructure SrTiO3/SrVO3 was grown by pulsed laser deposition using a shadow mask. This stack was then analysed by XPS mapping in serial acquisition mode. Ti2p and V2p core level imaging clearly highlights the SrTiO3 and SrVO3 domains. The XPS mapping of the Sr3d core level will be extensively discussed: strontium being a common element to both oxides with a very similar chemical environment. Despite a lower contrast in Sr3d images, the two materials are discernible thanks to the topography. In addtion, the use of Sr3d FWHM image is a real asset to evidence the two phases. Finally, data processing by principal component analysis allows us to extract significant spectral information on the strontium atoms

XPS monitoring of SrVO3 thin films from demixing to air ageing: The asset of treatment in water

International audienceIn this work, 40 nm-thick films of SrVO3 (SVO) grown by Pulsed Laser Deposition (PLD) on SrTiO3 substrates are studied by X-Ray Photoelectron Spectroscopy (XPS). We develop here a systematic fitting procedure for both Sr3d, V2p3/2 and O1s spectral regions of interest associated to the different chemical environments. Joint angle resolved XPS and Ar ion depth profiling reveal that as-grown SVO thin films exhibits Sr-rich phases at the extreme surface and a near-stoichiometric SVO in the bulk. The removal of segregated Sr is proposed by a treatment in deionized ultrapure water. This step will become an essential technological step in order to obtain reproducible surfaces achieving a stoichiometric SVO oxide phase. Besides, these hydrolyzed SVO surfaces appears VO2 terminated and then more electrically active. Furthermore, an air-ageing comparative study between as-grown and H2O-treated samples reveals that hydrolyzed surfaces are more surface sensitive to air oxidation. Such observations will be crucial and must be carefully considered before performing any passivation process for the integration of SVO thin film into next generation electronics heterostructures

Surface Characterizations and Selective Etching of Sr‐Rich Segregation on Top of SrVO 3 Thin‐Films Grown by Pulsed Laser Deposition

International audienceSince SrVO3 (SVO) can be used as a highly conductive material for perovskite heterostructures, the control of surface morphology and chemistry of such thin-films are essential. Using pulsed laser deposition, two distinct topographies can be produced. Thus, by tuning oxygen pressure in the growth chamber during cooling, smooth or partially covered by self-oriented Sr3V2O8 nanorods surfaces can be grown. This study manages to correlate the two typical topographies, revealed by atomic force microscopy (AFM), with their chemical compositions obtained by X-ray photoelectron spectroscopy (XPS). At first, a model describes their initial surface chemistry through the Sr/V cationic ratio and the (Sr+V)/Oox ratio. Furthermore, using sputter-depth profiling, post-thermal treatments and wet chemical etching, SVO thin-film chemical compositions are extensively studied. We demonstrate that they are composed of stoichiometric SVO phase covered by Sr-rich layer on top. Finally, treatment in water for 180 seconds helps to remove Sr-rich phases. Sr3V2O8 nanorods are found selectively dissolved leaving a surface nano-imprint. Moreover, on smooth SVO surfaces, a balanced Sr/V cationic ratio of 1.0±0.1 is obtained. These results appear very promising for SVO thin-films surface preparation and further development as electrodes for electronic devices

Transfer of Epitaxial SrTiO 3 Nanothick Layers Using Water-Soluble Sacrificial Perovskite Oxides

International audienceThe integration of functional thin film materials with adapted properties is essential for the development of new paradigms in information technology. Among them, complex oxides with perovskite structures have huge potential based on the particularly large diversity of physical properties. Here we demonstrate the possibility of transferring perovskite oxide materials like SrTiO 3 onto silicon substrate using an environmentally friendly process at nanoscale, by means of a water-soluble perovskite sacrificial layer: SrVO 3. Based on in situ monitoring atomic force microscopy and photoemission studies, we reveal that dissolution is initiated from a strontium rich phase at the extreme surface of SrVO 3. The transferred nanothick SrTiO 3 layer on silicon presents an effective morphology and monocrystalline quality, providing a proof of concept for the integration and development of an all perovskite oxide electronics or "oxitronics" onto any Si-based substrates

Surface Characterizations and Selective Etching of Sr‐Rich Segregation on Top of SrVO 3 Thin‐Films Grown by Pulsed Laser Deposition

International audienceSince SrVO3 (SVO) can be used as a highly conductive material for perovskite heterostructures, the control of surface morphology and chemistry of such thin-films are essential. Using pulsed laser deposition, two distinct topographies can be produced. Thus, by tuning oxygen pressure in the growth chamber during cooling, smooth or partially covered by self-oriented Sr3V2O8 nanorods surfaces can be grown. This study manages to correlate the two typical topographies, revealed by atomic force microscopy (AFM), with their chemical compositions obtained by X-ray photoelectron spectroscopy (XPS). At first, a model describes their initial surface chemistry through the Sr/V cationic ratio and the (Sr+V)/Oox ratio. Furthermore, using sputter-depth profiling, post-thermal treatments and wet chemical etching, SVO thin-film chemical compositions are extensively studied. We demonstrate that they are composed of stoichiometric SVO phase covered by Sr-rich layer on top. Finally, treatment in water for 180 seconds helps to remove Sr-rich phases. Sr3V2O8 nanorods are found selectively dissolved leaving a surface nano-imprint. Moreover, on smooth SVO surfaces, a balanced Sr/V cationic ratio of 1.0±0.1 is obtained. These results appear very promising for SVO thin-films surface preparation and further development as electrodes for electronic devices

Transfer of Epitaxial SrTiO 3 Nanothick Layers Using Water-Soluble Sacrificial Perovskite Oxides

International audienceThe integration of functional thin film materials with adapted properties is essential for the development of new paradigms in information technology. Among them, complex oxides with perovskite structures have huge potential based on the particularly large diversity of physical properties. Here we demonstrate the possibility of transferring perovskite oxide materials like SrTiO 3 onto silicon substrate using an environmentally friendly process at nanoscale, by means of a water-soluble perovskite sacrificial layer: SrVO 3. Based on in situ monitoring atomic force microscopy and photoemission studies, we reveal that dissolution is initiated from a strontium rich phase at the extreme surface of SrVO 3. The transferred nanothick SrTiO 3 layer on silicon presents an effective morphology and monocrystalline quality, providing a proof of concept for the integration and development of an all perovskite oxide electronics or "oxitronics" onto any Si-based substrates

XPS monitoring of SrVO3 thin films from demixing to air ageing: The asset of treatment in water

International audienceIn this work, 40 nm-thick films of SrVO3 (SVO) grown by Pulsed Laser Deposition (PLD) on SrTiO3 substrates are studied by X-Ray Photoelectron Spectroscopy (XPS). We develop here a systematic fitting procedure for both Sr3d, V2p3/2 and O1s spectral regions of interest associated to the different chemical environments. Joint angle resolved XPS and Ar ion depth profiling reveal that as-grown SVO thin films exhibits Sr-rich phases at the extreme surface and a near-stoichiometric SVO in the bulk. The removal of segregated Sr is proposed by a treatment in deionized ultrapure water. This step will become an essential technological step in order to obtain reproducible surfaces achieving a stoichiometric SVO oxide phase. Besides, these hydrolyzed SVO surfaces appears VO2 terminated and then more electrically active. Furthermore, an air-ageing comparative study between as-grown and H2O-treated samples reveals that hydrolyzed surfaces are more surface sensitive to air oxidation. Such observations will be crucial and must be carefully considered before performing any passivation process for the integration of SVO thin film into next generation electronics heterostructures