N. Maloufi

Sub-micron resolution selected area electron channeling patterns

Collection of selected area channeling patterns (SACPs) on a high resolution FEG-SEM is essential to carry out quantitative electron channeling contrast imaging (ECCI) studies, as it facilitates accurate determination of the crystal plane normal with respect to the incident beam direction and thus allows control the electron channeling conditions. Unfortunately commercial SACP modes developed in the past were limited in spatial resolution and are often no longer offered. In this contribution we present a novel approach for collecting high resolution SACPs (HR-SACPs) developed on a Gemini column. This HR-SACP technique combines the first demonstrated sub-micron spatial resolution with high angular accuracy of about 0.1°, at a convenient working distance of 10mm. This innovative approach integrates the use of aperture alignment coils to rock the beam with a digitally calibrated beam shift procedure to ensure the rocking beam is maintained on a point of interest. Moreover a new methodology to accurately measure SACP spatial resolution is proposed. While column considerations limit the rocking angle to 4°, this range is adequate to index the HR-SACP in conjunction with the pattern simulated from the approximate orientation deduced by EBSD. This new technique facilitates Accurate ECCI (A-ECCI) studies from very fine grained and/or highly strained materials. It offers also new insights for developing HR-SACP modes on new generation high-resolution electron columns.

Fundamental and experimental aspects of diffraction for characterizing dislocations by electron channeling contrast imaging in scanning electron microscope

Nowadays Field Emission Gun-Scanning Electron Microscopes provide detailed crystallographic information with high spatial and angular resolutions, and allow direct observation of crystalline defects, such as dislocations, through an attractive technique called Electron Channeling Contrast Imaging (ECCI). Dislocations play a crucial role in the properties of materials and ECCI has naturally emerged as an adapted tool for characterizing defects in bulk specimen. Nevertheless, fine control of the channeling conditions is absolutely required to get strong dislocation contrast for achieving comprehensive analysis. In this work, experiment-assisted fundamental aspects of the origin of dislocation contrast are studied. Experimentally, the potential of ECCI is explored in several dislocation configurations in Interstitial-Free steel (Fe − 1% Si) used as a model material. Full interpretations of dislocation contrast in (g, −g) and its evolution along the Kikuchi band are shown. Furthermore, a dislocation dipole is observed and fully characterized for the first time in an SEM.

Magnetic Melt Process and Texture Analysis of High Temperature Superconductor YBa2Cu3O7−δ for Electrical Applications

In most electrical applications of YBCO superconductors, the superconducting current should occur preferentially in a-b planes, so that conduction is strongly dependent on grain alignment. As a consequence, significant efforts have been made to control sample elaboration to optimize the texture and microstructure. In this work, melting texture method under a magnetic field of 12 T has been used under low oxygen partial pressure to induce and control the grain alignment during the crystallization of YBCO samples. The texture and microstructure of YBCO samples have been analyzed by optical microscopy with polarized light, X-ray diffraction and electron backscatter diffraction (EBSD).

Progress on new wide bandgap materials BGaN, BGaAlN and their potential applications

The development of wide band gap semiconductors extends their applications in optoelectronics devices to the UV domain. Compact lasers and high sensitivity APD detectors in UV range are currently needed for different applications such as, purification, covert communication and real time detection of airborne pathogens. Until now, the full exploitation of these potential materials has been limited by the lack of suitable GaN substrates. Recently, a novel class of materials has been reported based on BGaN and BAlN, potentially reducing the crystal defect densities by orders of magnitude compared to existing wide band gap heterostructures. Characteristics of these new alloys are similar to those of AlGaN materials with the advantage that these can be lattice matched to AlN and SiC substrates. In addition, these materials offer the possibility of using quaternary BAlGaN alloys at Ultra Violet (UV) wavelengths and hence lead to more degrees of freedom in designing sophisticated device structures. In this paper we describe the MOVPE growth conditions used to incorporate boron in GaN and AlGaN. Detailed characterization and analysis in terms of structural and electrical properties are discussed.

Textured YBaCuO films enhanced by cold rolling and melt growth process in low oxygen partial pressure

High critical current density Jc under high magnetic field is required in numerous applications for high temperature superconductor YBa2Cu3O7-δ (Y123). Significant efforts have been made to control sample preparation in view of optimized textures and microstructures. In YBa2Cu3O7-δ superconductor, Jc is limited by grain misalignment and by other defaults like secondary phase (Y211), microcracks... So YBaCuO films were prepared by electrophoretic deposition directly on polycrystalline Ag substrate. Melt textured growth (MTG) in low oxygen partial pressure and cold rolling were performed to YBaCuO samples respectively to improve texture growth of the films and to study deformation effect on the texture. The film obtained by melted growth after rolling has better texture orientation than the film without rolling, and presents higher critical current density.

DyFe2(110) nanostructures: Morphology and magnetic anisotropy

Single-crystalline DyFe2(110) nanosystems have been obtained by molecular-beam epitaxy. From reflection high-energy electron diffraction observations, the systems have been shown to grow in a Stranski–Krastanov mode. Depending on elaboration conditions (substrate temperature and nominal thickness), dots with anisotropic shape or continuous films with low surface roughness are obtained. Compared to the bulk compounds, the epitaxial systems are strained because of thermal differential contraction and exhibit modifications of easy-magnetization direction compared to bulk. The magnetization reversal process is correlated to the morphology of the layers.

A Dislocation-Scale Characterization of the Evolution of Deformation Microstructures around Nanoindentation Imprints in a TiAl alloy

In this work, plastic deformation was locally introduced at room temperature by nanoindentation on a γ-TiAl-based alloy. Comprehensive analyses of microstructures were performed before and after deformation. In particular, the Burgers vectors, the line directions, and the mechanical twinning systems were studied via accurate electron channeling contrast imaging. Accommodation of the deformation are reported and a scenario is proposed. All features help to explain the poor ductility of the TiAl-based alloys at room temperature.

Characterization of a Sub-Grain Boundary Using Accurate Electron Channeling Contrast Imaging

Electron Channeling Contrast Imaging (ECCI) is a powerful technique in scanning electron microscopy (SEM) for observing and characterizing crystallographic defect such as dislocations, stacking faults, and grain boundaries. In order to detect defects, ECCI uses the fact that backscattered electrons are very sensitive to the angle between the incident beam and the crystal lattice. To characterize defects, it is necessary to carry out ECCI under controlled two beam channeling conditions. In the past, these imaging conditions were established using electron channeling patterns (ECPs), selected area channeling patterns (SACPs), or electron backscatter diffraction patterns (EBSD), but all of these approaches have either spatial or angular resolution limitations.

From anisotropic dots to smooth RFe 2 (110) single crystal layers (R = rare earth)

Abstract:Single crystal RFe2(110) films were grown by molecular beam epitaxy to a total thickness of 1000 Å at different substrate temperatures ranging from 450 °C to 660 °C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe2(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound.