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International audience; Silicon photonics is expected to allow the implementation of a wide range of applications including sensing, datacom and security with the potential to leverage the already existing CMOS fabrication facilities for cost-effective and large production. Among the building blocks to develop, optical modulator is considered as a key device for integrated circuits. The main physical effect to perform optical modulation in silicon is based on plasma dispersion effect. However the maximum modulation speed achievable with this approach is limited by the mobility and recombination time of the carriers. Furthermore, these types of modulators face some limitations in terms of power consumption, chirp and loss that may compromise their future integration in integrated circuits. The purpose of this work is to explore an alternative approach, based on the use of the Pockels effect. Such an effect, commonly used in Lithium-Niobate, allows high speed and low power consumption optical modulation. Unfortunately silicon is a centro-symmetric crystal leading to a vanishing of the second order nonlinear coefficient, i.e. no Pockels effect. To overcome this limitation, it is possible to break the crystal symmetry by straining the silicon lattice. The new approach developed here is to generate strain by the epitaxial growth of crystalline functional oxides, which exhibit more appropriate strain-induced characteristics in silicon than the use of silicon nitride with lower defect concentration at the Si/oxide interface. Indeed, these functional oxides are deposited by epitaxial growth at high temperature on silicon with pulsed-laser deposition. The induced strain due to lattice parameter mismatch and the difference in the thermal expansion coefficients between oxides and silicon are strong. Furthermore, these functional oxides exhibit very interesting multiferroicity and piezoelectricity properties that paves the way to a new route to implement silicon photonic circuits with unprecedented functionalities. Last results on the integration of high quality oxides on silicon waveguides will be presented as well as a full characterization of the strain-induced property changes including X-ray diffraction, AFM-SEM microscopies, and Raman spectroscopy. The first experiments of light propagation in hybrid oxides on silicon photonic structures will be also reported.

International audience; Silicon photonics is expected to allow the implementation of a wide range of applications including sensing, datacom and security with the potential to leverage the already existing CMOS fabrication facilities for cost-effective and large production. Among the building blocks to develop, optical modulator is considered as a key device for integrated circuits. The main physical effect to perform optical modulation in silicon is based on plasma dispersion effect. However the maximum modulation speed achievable with this approach is limited by the mobility and recombination time of the carriers. Furthermore, these types of modulators face some limitations in terms of power consumption, chirp and loss that may compromise their future integration in circuits. Our work focuses on the integration of Yttria-Stabilized Zirconia (YSZ) functional oxide on sapphire substrates and on silicon. The common aim for both integrations is to induce nonlinear optical effects at the wavelength around 1550nm, thanks to strain tuning in full oxide-or in hybrid oxide/Si-photonic structures. Indeed, we explore an alternative approach, based on the use of the Pockels effect for optical modulation [1]. Such an effect, commonly used in Lithium-Niobate, allows high speed and low power consumption optical modulation [2]. Unfortunately silicon is a centro-symmetric crystal leading to a vanishing of the second order nonlinear coefficient, i.e. no Pockels effect. To overcome this limitation, it is possible to break the crystal symmetry by straining the silicon lattice [3]. The new approach developed here is to generate strain by the epitaxial growth of crystalline functional oxides, which exhibit more appropriate strain-induced characteristics than the use of silicon nitride with lower defect concentration at the Si/oxide interface. Indeed, these functional oxides are deposited by epitaxial growth at high temperature with pulsed-laser deposition [4], [5]. The induced strain due to lattice parameter mismatch and the difference in the thermal expansion coefficients between oxides and silicon are strong. Furthermore, functional oxides exhibit very interesting multiferroicity and piezoelectricity properties that pave the way to a new route to implement silicon photonic circuits with unprecedented functionalities. Our study includes:-Epitaxial growth of YSZ functional oxide by pulsed-laser deposition and material characterizations with several complementary techniques such as X-ray diffraction, atomic-force (AFM) and electronic microscopies (SEM and TEM).-Fabrication processes with electronic lithography and different etching technics which allow us to fabricate photonic structures such as gratings coupler, strip or sub-wavelength waveguides on full oxide-and hybrid oxide/Si chips and their optical characterisation around 1550 nm.-Micro-Raman spectroscopy used to provide information about strain in silicon or YSZ structures throughout the processes [6]. Last results about full oxide-and hybrid oxide/Si-photonic structures will be presented and discussed.

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International audience; The hybrid integration of oxides on silicon or sapphire is promising to bring other properties (such as multiferroicity or piezoelectricity) that can be combined and tuned at the nanoscale to develop new devices. Moreover, their epitaxial growth and difference of thermal expansion induce strain that can be used to break the symmetry and reveal second-order nonlinear effect in silicon. This work is mainly devoted to optimization of the growth of functional oxides such as YSZ (Yttria-Stabilized Zirconia) on sapphire and silicon, in order to control the crystalline quality, the control of strain distribution and the interfaces YSZ/Si. Epitaxial growth of YSZ thin films Preliminary optical and strain measurements on YSZ on silicon Conclusions Perspectives POPSTAR In the case of YSZ, the yttrium content can be used to tune the lattice mismatch with silicon and thus the epitaxial strain. The growth of YSZ on sapphire substrate allows measuring optical properties of this oxide without silicon interface effects. Functional oxides are deposited by epitaxial growth (pulsed-laser deposition) on silicon and so naturally induce strong strain due to large lattice parameter mismatch. Furthermore the difference of thermal expansion coefficients between YSZ and silicon is 5 times higher than Si 3 N 4 Si YSZ S i O 2 SEM image of silicon strip waveguide strained with epitaxial YSZ thin film Pulsed-laser deposition (PLD) Red parameters have been tested :  Sapphire : oxygen pressure [3] and surface state of the substrate are the most important parameters which lead to the growth of (002) or (111)YSZ on sapphire C-cut. Even if we have seen only (002)YSZ on sapphire R-cut, the quality is lower than on C-cut.  Silicon : oxygen pressure is the key parameter to grow epitaxially YSZ on silicon and to reduce the silica thickness at the interface. Map of Si raman peak [2] Wolf, " Micro-Raman Spectroscopy to Study Local Mechanical Stress in Silicon Integrated Circuits. " Semicond. Sci. Technol. 1996 • Shift of Si Raman peak : Waveguide strained by YSZ about 500 MPa [2]. • Weak transmission of 30 dB/cm. Need to improve the quality of the growth and control YSZ/Si interface. Transmission measurement at 1,55 µm Sapphire structure • Efficiently control and understand the chemical and thermal treatment of the substrates • Fabricate optical structures of YSZ on sapphire including waveguides • Optimize the growth quality of YSZ on silicon to avoid formation of silica at interface and increase the strain generation • Study the strain on different structures of silicon with different process of YSZ growth in PLD • YSZ-strained Si waveguides :-From First measurements, transmission in YSZ-strained Si waveguides is quite low, showing that the quality of the waveguides has to be further improved.-Si waveguides have been elaborated and strained thanks to the difference of thermal expansion of YSZ thin films and silicon substrate (strain evidenced by Raman measurements). • Epitaxial growth on sapphire :-The growth of YSZ thin films on sapphire has been optimized by studying the effects of all the PLD parameters.-Oxygen pressure and state surface of the substrate are the key to control YSZ growth orientation and films quality.-Optimized films surface exhibits a very low roughness (below 1nm) with atomic steps (related to the ones of the substrate).-Films grown on C-cut substrates have significantly better crystalline quality than on R-cut substrates. Only one orientation of YSZ film (002) can be stabilized with substrate treatment.-2-1

International audience; Intensive researches are currently focused on the miniaturization of devices and on the combination of photonics and electronics in the same platform, in order to decrease the power consumption and to create novel functionalities. In this context, the hybrid integration of oxides on silicon is promising to bring other properties (such as multiferroicity or piezoelectricity) that can be combined and tuned at the nanoscale to develop new devices. However, such integration induces scientific and technological challenges to overcome. The aim of the project is to develop innovative silicon photonic devices by integrating complex epitaxial oxide thin films on silicon and to optimize their physical properties. This work is mainly focused on the optimization of the materials including the crystalline quality, the control of strain fields and the interfaces between oxide and silicon. The experimental study includes the growth of oxide layers on silicon and sapphire substrates by pulsed laser deposition and their characterization by several complementary techniques, such as X ray diffraction, AFM-SEM microscopies, and Raman spectroscopy. Furthermore, the first fabrication and characterization of optical waveguides will be presented and discussed.

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LGEP 2014 ID = 1522; International audience

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The scope of this study is the self-organization of metal nanostructures as small as 10 nm and with good edge quality. We developed a process that allow the elaboration of such nanostructures arrays, self-organized on Si(111) along a unidimensional step bunch network. The density (between 109 et 1011 islands/cm2) and size (between 10 and 50 nm) of thus obtained objects are controllable. Several crystalline structures were observed, some are gold silicides, metastable compounds stabilized by the small size of the objects. We used these small metal islands for two distinct applications, the elaboration of a network of magnetic nanodots and the growth of semiconductor nanowires made of Si and Ge. In parallel, an original process that allow to electrically contact nanostructures was developed. To be able to control the islands position according to a two-dimensional (2D) pattern, we developed a surface texturation process using electronic beam lithography, reactive ion etching followed by annealing under ultra-high vacuum. This process involves the formation of a 2D network of step bunches which would allow to control the exact position of the objects on these nucleation centers. Such textured silicon surfaces at the tenth nm scale were used for the AFM study of membrane proteins via a collaboration with the Institut Curie in Paris. In addition, as an alternative to silicon, we studied the structuration of silicon carbide (SiC) by reactive ion etching. An alumina membrane with a periodic network of pores is used as masks for the etch step. The etched SiC surface is then treated by an annealing at very high temperature under hydrogen.; L'objet de cette étude est l'auto-organisation de nanostructures métalliques de taille aussi petite que 10 nm et de bonne qualité de bord. Nous avons développé un procédé permettant d'élaborer un réseau de nanostructures, auto-organisées sur Si(111) selon un motif unidimensionnel de paquets de marches atomiques. La densité (entre 109 et 1011 îlots/cm2) et la taille (entre 10 et 50 nm) des objets ainsi obtenus sont contrôlables. Plusieurs structures cristallines de ces îlots ont été observées, dont certaines sont des siliciures d'or, composés métastables stabilisés par la petite taille des objets. Nous avons utilisé ces îlots métalliques pour deux applications distinctes, l'élaboration d'un réseau de nanoplots magnétiques et la croissance de nanofils semi-conducteurs de Si et Ge. En parallèle, un procédé technologique original permettant la prise de contact électrique avec ces nanostructures a été développé. Afin de contrôler la position des îlots selon un motif bidimensionnel (2D), nous avons développé un procédé de texturation par lithographie électronique, gravure ionique ré- active puis recuit sous ultravide. Ce procédé entraîne la formation d'un réseau 2D de paquets de marches qui devrait permettre de contrôler la position des objets sur ces centres de nucléation. Les surfaces de silicium texturées à l'échelle de la dizaine de nm ont été mises à profit pour l'étude par AFM de protéines membranaires dans le cadre d'une collaboration avec l'Institut Curie à Paris. Par ailleurs, comme alternative au silicium, nous avons étudié la structuration du carbure de silicium (SiC) par gravure ionique réactive. La gravure utilise comme masque une membrane d'alumine avec un réseau périodique de pores. La surface prégravée de SiC est ensuite traitée par un recuit très haute température sous hydrogène.