Electron scattering

Résumé

Electron scattering occurs when electrons are displaced from their original trajectory. This is due to the electrostatic forces within matter interaction or, if an external magnetic field is present, the electron may be deflected by the Lorentz force. This scattering typically happens with solids such as metals, semiconductors and insulators; and is a limiting factor in integrated circuits and transistors. The application of electron scattering is such that it can be used as a high resolution microscope for hadronic systems, that allows the measurement of the distribution of charges for nucleons and nuclear structure. The scattering of electrons has allowed us to understand that protons and neutrons are made up of the smaller elementary subatomic particles called quarks. Electrons may be scattered through a solid in several ways: *Not at all: no electron scattering occurs at all and the beam passes straight through. *Single scattering: wh

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https://en.wikipedia.org/wiki/Electron_scattering

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Introduction to the path integral formulation of quantum mechanics. Derivation of the perturbation expansion of Green's functions in terms of Feynman diagrams. Several applications will be presented, including non-perturbative effects, such as tunneling and instantons.

Presentation of particle properties, their symmetries and interactions. Introduction to quantum electrodynamics and to the Feynman rules.

This course will present the fundamentals of electronâmatter interactions, as occuring in the energy range available in modern transmission electron microscopes, namely 60-300 keV electrons. Diffraction and high-resolution image formation as well as electron energy-loss spectrometry will be covere

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Angular Resolved low loss EELS for Materials Characterization

Simon Schneider

Electron Energy Loss Spectrometry (EELS) in Transmission Electron Microscopy (TEM) is a powerful tool for the investigation of the electronic structure of materials. In the low loss regime, one can access the optical properties which are governed by the dielectric function ε(ω), which may be retrieved thanks to the so-called "loss function". The energy range covered by EELS goes from the infrared (less than 1 eV) to the hard X-rays (about 3 keV), each region having its own interest. The low loss region (0–100 eV) serves to investigate the valence electron structure and the inter-band transition behavior. Using angular resolved EELS gives further information about the dispersion behavior of the investigated material. This work is a part of a large project aiming a closer agreement between theory and experiment in angular resolved EELS by improving the state of the art in both fields. The purpose of this work is to set up a reliable experimental technique to retrieve the momentum transfer resolved single scattering distribution at very low energy with good accuracy and resolutions. One of the main challenges was to use the newly installed JEOL 2200 FS transmission electron microscope that is equipped with an in-column Ω filter for retrieval of the loss function in diffraction energy filtered TEM. Issues such as the angular resolution, contamination, image-coupling in diffraction mode and plural scattering have been solved. Several optical alignments and operational modes of the JEOL 2200 FS have been tested, and finally nano-beam diffraction was found to be the optimal running mode. This work provides for the first time a full 3D energy loss data cube in diffraction mode with energy and angular resolutions of 1 eV and 0.5 nm−1 respectively. After processing, the energy and angular relevant data span an energy loss range from 2.75 eV to 40.25 eV and the angular momentum transfer up to 12.5 nm−1. Although theoretically, without CCD camera binning and the maximum available camera length of the JEOL (250 cm), a momentum transfer resolution of 0.04 nm−1 would have been possible, the effective angular resolution was limited by the smallest obtainable beam convergence as well as by the remaining aberrations of the microscope. The presented technique can be reproduced on any transmission electron microscope equipped with an in-column energy filter. Furthermore, it is demonstrated that the proposed method provides working ranges in energy, spatial and angular resolutions limited only by the perfor- mance of the microscope. Previous techniques did not provide experimental results showing an energy resolution independent of the spatial and angular resolutions.

EPFL2013

Chargement

Sondheimer oscillations as a probe of non-ohmic flow in WP2 crystals

Jonas De Jesus Diaz Gomez, Chunyu Guo, Philip Johannes Walter Moll, Jacopo Oswald, Matthias Carsten Putzke, Yi-Chiang Sun, Maarten Ruud van Delft

As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving (l(MC)) and relaxing (l(MR)) electron scattering, and the device size (d), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain l(MR) even when l(MR) >> d. We extract l(MR) from the Sondheimer amplitude in WP2, at temperatures up to T similar to 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on mu m-sized devices are in excellent agreement with experimental reports of the bulk l(MR) and confirm that WP2 can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of l(MR) in micro-devices.

NATURE PORTFOLIO2021

Chargement

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. Due to their good optoelectronic properties and the production scalability, Sn-doped indium oxide (In2O3:Sn) is the preferred TCO in industrial applications. Nonetheless indium is scarce in the Earth's crust and its availability might be compromised over the next decades. To address this issue, this doctoral project aims to (i) improve the optoelectronic properties of In-free TCOs seeking to match those of In2O3:Sn, and (ii) refine the optoelectronic properties of In2O3-based films to decrease the In consumption in applications. To accomplish this, we investigated the links between the defects, the microstructure and the optoelectronic properties of two families of TCOs, indium- and tin-based oxides. First, we studied the evolution in the optoelectronic properties and microstructure of amorphous zinc tin oxide (a-ZTO) when annealed up to 500C in oxidizing, neutral, and reducing atmospheres. We show that annealing in atmospheric pressure at temperatures > 300C decreases the detrimental subgap absorptance while increasing the electron mobility (mu). Thermal treatments in reducing atmospheres increase the free-carrier density (Ne) and the subgap absorptance. None of the thermal treatments resulted in important changes in the amorphous microstructure. Combining these results and density functional theory (DFT) calculations, oxygen deficiencies (VO) were identified as the source of detrimental subgap absorption. VO can act as donors but also as electron scattering centres. Based on these results, a-ZTO with ÎŒ up to 35 cm2/Vs, is demonstrated by high-temperature defect passivation. Profiting from the microstructure stability of a-ZTO, this material was used as a recombination junction in a tandem solar cell which require a high-temperature step in its processing. The passivation scheme might be problematic for temperature-sensitive technologies. Therefore, we demonstrated an alternative low-temperature passivation method, which relies on co-sputtering a-ZTO with SiO2 . Using two Sn-based oxides with different composition and microstructure: a-ZTO and SnO2 , and results of DFT calculations, we demonstrate that SiO2 contribution is twofold. (i) The oxygen from SiO2 passivate the VO in SnO2 and a-ZTO. (ii) The formation energy of the ionized VO is lowered by the silicon-atoms, enabling defects that do not contribute to the subgap absorptance. This passivation scheme improves the optical properties without affecting the electrical conductivity, overcoming the optoelectronic trade-off in Sn-based TCOs. Finally, we study an In-based high-ÎŒ TCO: Zr-doped In2O3. Films of In2O3:Zr with thickness from 15 nm-100 nm were sputtered in the amorphous state and annealed in different atmospheres. Annealing in air yields fully crystalline films with high transparency and a high-mu limited by phonon and ionized impurity scattering. 15 nm-thick films exhibit an average absorptance of < 0.5 % (between 390 nm-2000 nm) and an mu of 50 cm2/Vs increasing to 105 cm2/Vs for 100 nm-thick films. Alternatively, annealing in a neutral or reducing atmospheres result in higher mu for films thinner than 50 nm as a high Ne is maintained. The demonstration of thickness reduction while keeping high lateral mu makes In2O3:Zr is an alternative to reduce In in applications such as flexible displays, solar cells and light emitting diodes.

EPFL2019

Chargement

Angular Resolved low loss EELS for Materials Characterization

Simon Schneider

EPFL2013

EPFL2019