Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source

We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The properties of the electron probe (brightness, angular current density, stability) are quantitatively determined. The measured brightness is the largest reported so far for UTEMs. Examples of imaging, diffraction and spectroscopy using ultrashort electron pulses are given. Finally, the potential of this instrument is illustrated by performing electron holography in the off-axis configuration using ultrashort electron pulses.Comment: 23 pages, 9 figure

Chapter Two - Cold field emission electron source: From higher brightness to ultrafast beam

This chapter presents the second strategy followed to optimize coherent electron microscopy methods, which consists in working directly on the source technology. Improving their intrinsic brightness or generating new properties while maximizing brightness are the two directions followed in our work. The chapter will describe the development of a new emitter based on a carbon nano-cone after introducing the technology of the brightest sources now encountered in electron optics, namely cold field emission guns (CFEG). We will then conclude with a description of the ultrafast CFEG currently used to develop new kind of coherent time-resolved microscopy methods

Contribution au développement du CBED et de l’holographie HREM pour l’analyse des déformations de couches épitaxiées

Epitaxial thin layers undergo an elastic strain, related to the difference in lattice parame- ters, accompanied or not by chemical segregation. These various effects influence directly the properties of the epitaxial thin layers (emission in quantum wells, transport and magnetic ani- sotropy...). They interest both fundamental research and applications aiming at adjusting the properties of epitaxial layers through the control of their state of strain. Most of the experimental techniques used to determine the lattice parameters in epitaxial layers only provide information averaged over the whole layer or its surface. On the contrary, Transmission electron microscopy (TEM) makes it possible to precisely select the studied zones, which is a considerable advantage in the presence of heterogeneities. In particular very accurate measurements can be obtained by convergent beam electron diffraction (CBED). This work aims at developing reliable methods to measure the strain in thin layers with a spatial resolution of the order of a nanometer. The selected systems for this study were SiGe/Si and GaInAs/GaAs epitaxial thin layers. CBED patterns obtained on cross-sectional specimens reveal a very heavy deformation in the layer and we observe, in the substrate, a strong evolution of the HOLZ line profile as a function of the distance between the studied zone and the interface. We showed that this evolution results from a free surface relaxation effect occurring in thin foils of strained specimens. This effect depends on many parameters, such as the thickness of the sample, the misfit and the distance between the studied zone and the interface. In order to retrieve the strain in the specimen, we developed a new method based on the combination of finite elements calculations and dynamical simulations obtained using an original formalism developed during this work and referred as TDDT (Time- Dependent Dynamical Theory). Elastic relaxation could also be observed in samples prepared in plan-view. The state of strain was thus determined in the various specimens through the com- parison of simulated and experimental line profiles : remarkable agreements have been reached. These various measurements were compared with studies carried out by electron holography in HREM configuration making it possible to combine structural and chemical characterizations at an atomic scale. This works benefited from the use of a TEM-FEG instrument fitted with both a spherical aberration corrector and an energy filter.Les couches minces épitaxiées sur substrat subissent une déformation élastique, liée à la différence de paramètres cristallins, accompagnée ou pas de ségrégation chimique. Ces différents effets influent directement sur les propriétés des couches minces épitaxiées (caractéristiques d’émission dans les puits quantiques, transport et anisotropie magnétique ...). Ils sont au coeur d’une recherche tant fondamentale qu’appliquée visant à ajuster les propriétés par la maitrise des états de déformation des systèmes épitaxiés. Les techniques expérimentales susceptibles de déterminer les paramètres cristallins des couches épitaxiées fournissent pour la plupart des informations partielles car moyennées sur l’ensemble de la couche ou sur sa surface. La microscopie électronique en transmission (MET) permet de sélectionner précisément les zones étudiées, ce qui est un avantage considérable en présence d’hétérogénéités. En particulier des mesures d’une très grande précision peuvent ainsi être obtenues par la diffraction électronique en faisceau convergent (CBED). Le but de ce travail est d’optimiser les techniques de mesure des déformations de couches minces avec une résolution spatiale de l’ordre du nanomètre. Nous avons choisi d’étudier l’état de déformation de couches minces épitaxiées dans les systèmes SiGe/Si et GaInAs/GaAs. Les diagrammes CBED sur des sections transverses révèlent une très forte déformation dans la couche et nous observons dans le substrat des modifications du profil des lignes de HOLZ en fonction de la distance de la zone diffractante à l’interface couche/substrat. Nous interprétons cette évolution comme caractéristique d’un phénomène de relaxation de la contrainte épitaxiale de la couche du à la faible épaisseur de l’échantillon. Cet effet dépend de nombreux paramètres, tels que l’épaisseur de l’échantillon, le misfit et la distance de la zone diffractante a` l’interface couche/substrat. Afin de remonter a` la déformation, nous avons développé une nouvelle méthode de mesure basée sur la combinaison de calculs par éléments finis et de simulations dynamiques obtenues grâce à un formalisme original développé au cours de ce travail appelé formalisme TDDT (Théorie dynamique dépendante du temps). Les effets de relaxation élastique ont pu aussi être mis en évidence dans les échantillons préparés en vue plane. Leur état de déformation a pu être déterminé à partir du CBED, et les simulations TDDT rendent bien compte de tous les contrastes observés. Ces différentes mesures de déformation ont été comparées à des études réalisées par holographie électronique en configuration HREM permettant de combiner les caractérisations structurales (déformations) et chimique à l’échelle atomique. Ce travail a grandement bénéficié de l’utilisation d’un microscope TEM-FEG doté d’un correcteur d’aberration sphérique et d’un filtre en énergie

Chapter One - Characterization of nanomaterials properties using FE-TEM

This chapter reviews the fundamental concepts describing coherent transmission electron microscopy methods. It begins by discussing the notions of charged particle optics that will be necessary in the implementation of these new methods. Then we discuss the natural link between the property of electron beam coherence and the brightness of the electron source. The second section briefly summarizes the advancements of two strain measurement techniques for materials and highlights the value of doing these measurements using a coherent source, such as one with high brightness. Finally, it discusses a first approach aimed at further improving the sensitivity of these measurements by trying to optimize the optical configurations accessible in a transmission electron microscope. This development was carried out within the framework of a first collaboration with the Japanese company Hitachi High-technologies and gave rise to the instrument called I2TEM dedicated to this kind of development. This chapter presents for the first time in detail the optical properties of this unique instrument, which was installed at CEMES in 2012

Chapter Three - Every electron counts: Toward the development of aberration-optimized and aberration-corrected electron sources

The project is addressed in the last chapter, which follows logically from the developments described in the first two chapters. After an introduction to the strategy, the description is separated into two sections. The first section outlines the computational methods we are implementing and which will be required to build and create the new ultra-bright and ultra-fast sources. The concepts and preliminary results connected to the design of these new sources are ultimately covered in the second section of this final chapter

Diffracted phase and amplitude measurements by energy-filtered convergent-beam holography (CHEF)

International audienceInterference between transmitted and diffracted disks in convergent-beam electron diffraction (CBED) patterns using the CBED+EBI method proposed by Herring et al. is explored using different optical configurations on a spherical aberration corrected transmission electron microscope equipped with a biprism and imaging energy filter: the SACTEM-Toulouse. We will relate the amplitude and phase of these interference patterns, which we call convergent-beam holography (CHEF), to microscope transfer theory and the complex amplitudes of the diffracted beams. Experimental CHEF patterns recorded in the absence of aberration correction will be compared with simulations to validate the theory concerning the effect of microscope aberrations and current instabilities. Then, using aberration correction, we propose a scheme for eliminating the effect of the microscope, so that the diffracted amplitudes and phase due to dynamical scattering within the specimen can be studied. Experimental results are compared with simulations performed using the full dynamical theory. The potential for studying diffracted amplitudes and phases using CHEF analysis is discussed

Mapping stress and strain in nanostructures by high-resolution transmission electron microscopy

International audienceWe present the current state-of-the-art of geometric phase analysis (GPA), a technique for measuring stress and strain at the nanoscale by high-resolution transmission electron microscopy (HRTEM). The method will be illustrated with an experimental study of SiGe strained layers using the SACTEM-Toulouse, an aberration-corrected transmission electron microscope. This latest generation machine improves signal-to-noise allowing deformations to be measured to an accuracy of 0.1% at nanometre scale resolution. The relation between strain and deformation will be discussed in the light of thin film relaxation and chemical interdiffusion