Extended Charge Layers in Metal-Oxide-Semiconductor Nanocapacitors Revealed by Operando Electron Holography.

The metal-oxide-semiconductor (MOS) capacitor is one of the fundamental electrical components used in integrated circuits. While much effort is currently being made to integrate new dielectric or ferroelectric materials, capacitors of silicon dioxide on silicon remain the most prevalent. It is perhaps surprising therefore that the electric field within such a capacitor has never been measured, or mapped out, at the nanoscale. Here we present results from operando electron holography experiments showing the electric potential across a working MOS nanocapacitor with unprecedented sensitivity and reveal unexpected charging of the dielectric material bordering the electrodes.

Optimization of off-axis electron holography performed with femtosecond electron pulses.

We report on electron holography experiments performed with femtosecond electron pulses in an ultrafast coherent Transmission Electron Microscope based on a laser-driven cold field emission gun. We first discuss the experimental requirements related to the long acquisition times imposed by the low emission/probe current available in these instruments. The experimental parameters are first optimized and electron holograms are then acquired in vacuum and on a nano-object showing that useful physical properties can nevertheless be extracted from the hologram phase in pulsed condition. Finally, we show that the acquisition of short exposure time holograms assembled in a stack, combined with a computer-assisted shift compensation of usual instabilities encountered in holography, such as beam and biprism wire instabilities, can yield electron holograms acquired with a much better contrast paving the way to ultrafast time-resolved electron holography.

Quantitative 3D electromagnetic field determination of 1D nanostructures from single projection.

One-dimensional (1D) nanostructures have been regarded as the most promising building blocks for nanoelectronics and nanocomposite material systems as well as for alternative energy applications. Although they result in confinement of a material, their properties and interactions with other nanostructures are still very much three-dimensional (3D) in nature. In this work, we present a novel method for quantitative determination of the 3D electromagnetic fields in and around 1D nanostructures using a single electron wave phase image, thereby eliminating the cumbersome acquisition of tomographic data. Using symmetry arguments, we have reconstructed the 3D magnetic field of a nickel nanowire as well as the 3D electric field around a carbon nanotube field emitter, from one single projection. The accuracy of quantitative values determined here is shown to be a better fit to the physics at play than the value obtained by conventional analysis. Moreover the 3D reconstructions can then directly be visualized and used in the design of functional 3D architectures built using 1D nanostructures.

Development of TEM and SEM high brightness electron guns using cold-field emission from a carbon nanotip.

A newly developed carbon cone nanotip (CCnT) has been used as field emission cathode both in low voltage SEM (30 kV) electron source and high voltage TEM (200 kV) electron source. The results clearly show, for both technologies, an unprecedented stability of the emission and the probe current with almost no decay during 1h, as well as a very small noise (rms less than 0.5%) compared to standard sources which use tungsten tips as emitting cathode. In addition, quantitative electric field mapping around the FE tip have been performed using in situ electron holography experiments during the emission of the new tip. These results show the advantage of the very high aspect ratio of the new CCnT which induces a strong enhancement of the electric field at the apex of the tip, leading to very small extraction voltage (some hundred of volts) for which the field emission will start. The combination of these experiments with emission current measurements has also allowed to extract an exit work function value of 4.8 eV.

Dynamical effects in strain measurements by dark-field electron holography.

Here, we study the effect of dynamic scattering on the projected geometric phase and strain maps reconstructed using dark-field electron holography (DFEH) for non-uniformly strained crystals. The investigated structure consists of a {SiGe/Si} superlattice grown on a (001)-Si substrate. The three-dimensional strain field within the thin TEM lamella is modelled by the finite element method. The observed projected strain is simulated in two ways by multiplying the strain at each depth in the crystal by a weighting function determined from a recently developed analytical two-beam dynamical theory, and by simply taking the average value. We demonstrate that the experimental results need to be understood in terms of the dynamical theory and good agreement is found between the experimental and simulated results. Discrepancies do remain for certain cases and are likely to be from an imprecision in the actual two-beam diffraction conditions, notably the deviation parameter, and points to limitations in the 2-beam approximation. Finally, a route towards a 3D reconstruction of strain fields is proposed.

Electromechanical coupling among edge dislocations, domain walls, and nanodomains in BiFeO3 revealed by unit-cell-wise strain and polarization maps.

The performance of ferroelectric devices, for example, the ferroelectric field effect transistor, is reduced by the presence of crystal defects such as edge dislocations. For example, it is well-known that edge dislocations play a crucial role in the formation of ferroelectric dead-layers at interfaces and hence finite size effects in ferroelectric thin films. The detailed lattice structure including the relevant electromechanical coupling mechanisms in close vicinity of the edge dislocations is, however, not well-understood, which hampers device optimization. Here, we investigate edge dislocations in ferroelectric BiFeO3 by means of spherical aberration-corrected scanning transmission electron microscopy, a dedicated model-based structure analysis, and phase field simulations. Unit-cell-wise resolved strain and polarization profiles around edge dislocation reveal a wealth of material states including polymorph nanodomains and multiple domain walls characteristically pinned to the dislocation. We locally determine the piezoelectric tensor and identify piezoelectric coupling as the driving force for the observed phenomena, explaining, for example, the orientation of the domain wall with respect to the edge dislocation. Furthermore, an atomic model for the dislocation core is derived.

Evidence of sharp and diffuse domain walls in BiFeO3 by means of unit-cell-wise strain and polarization maps obtained with high resolution scanning transmission electron microscopy.

Domain walls (DWs) substantially influence a large number of applications involving ferroelectric materials due to their limited mobility when shifted during polarization switching. The discovery of greatly enhanced conduction at BiFeO(3) DWs has highlighted yet another role of DWs as a local material state with unique properties. However, the lack of precise information on the local atomic structure is still hampering microscopical understanding of DW properties. Here, we examine the atomic structure of BiFeO(3) 109° DWs with pm precision by a combination of high-angle annular dark-field scanning transmission electron microscopy and a dedicated structural analysis. By measuring simultaneously local polarization and strain, we provide direct experimental proof for the straight DW structure predicted by ab initio calculations as well as the recently proposed theory of diffuse DWs, thus resolving a long-standing discrepancy between experimentally measured and theoretically predicted DW mobilities.

Dark-field electron holography for the measurement of geometric phase.

The genesis, theoretical basis and practical application of the new electron holographic dark-field technique for mapping strain in nanostructures are presented. The development places geometric phase within a unified theoretical framework for phase measurements by electron holography. The total phase of the transmitted and diffracted beams is described as a sum of four contributions: crystalline, electrostatic, magnetic and geometric. Each contribution is outlined briefly and leads to the proposal to measure geometric phase by dark-field electron holography (DFEH). The experimental conditions, phase reconstruction and analysis are detailed for off-axis electron holography using examples from the field of semiconductors. A method for correcting for thickness variations will be proposed and demonstrated using the phase from the corresponding bright-field electron hologram.

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

Interference 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 inelastic intensities in diffraction patterns of magnetic samples using the energy spectrum imaging technique.

We present the quantitative measurement of inelastic intensity distributions in diffraction patterns with the aim of studying magnetic materials. The relevant theory based on the mixed dynamic form factor (MDFF) is outlined. Experimentally, the challenge is to obtain sufficient signal for core losses of 3d magnetic materials (in the 700-900eV energy-loss range). We compare two experimental settings in diffraction mode, i.e. the parallel diffraction and the large-angle convergent-beam electron diffraction configurations, and demonstrate the interest of using a spherical aberration corrector. We show how the energy spectrum imaging (ESI) technique can be used to map the inelastic signal in a data cube of scattering angle and energy loss. The magnetic chiral dichroic signal is measured for a magnetite sample and compared with theory.

Calibration of projector lens distortions.

The distortions introduced into high-resolution transmission electron microscope (HRTEM) images by the projector lens system are an important source of systematic error for quantitative displacement and strain determination. Using geometric phase analysis of images of perfect crystals, we measured these errors for two different transmission electron microscopes. Local magnification varies by as much as 5%, and rotation can reach 2 degrees across a typical image. Our experimental results are compared with theory, and optical pincushion and spiral distortion coefficients are determined. A method for calibrating and removing these distortions is presented that enables quantification to 0.1% strain and 0.1 degrees rotation across the whole field of view. This calibration is also critical for the accurate measurement of local lattice parameters from HRTEM images.

Strain mapping in nanowires.

A method for obtaining detailed two-dimensional strain maps in nanowires and related nanoscale structures has been developed. The approach relies on a combination of lattice imaging by high-resolution transmission electron microscopy and geometric phase analysis of the resulting micrographs using Fourier transform routines. We demonstrate the method for a germanium nanowire grown epitaxially on Si(111) by obtaining the strain components epsilon(xx), epsilon(yy), epsilon(xy), the mean dilatation, and the rotation of the lattice planes. The resulting strain maps are demonstrated to allow detailed evaluation of the strains and loading on nanowires.

Imaging conditions for reliable measurement of displacement and strain in high-resolution electron microscopy.

We analyse the degree to which the lattice fringe displacements in an image correspond to displacements of the atomic planes in the specimen using lens transfer theory. Our basic assumption is that the exit wave function faithfully reproduces the displacements of the projected atomic structure. The way this information is imaged by the objective lens is then developed analytically. We observe an interchange of amplitude and phase information between the original and the reconstructed wave function. For symmetry-related reflections, we show that in the absence of beam amplitude variations, the displacements are imaged perfectly by the objective lens. The theoretical results are confirmed using one-dimensional simulations. For the more complicated case of non-centrosymmetric structures, beam tilts and crystal tilts, we study the implications for slowly varying displacement fields. Errors are found to be minimised in areas where the contrast of the lattice fringes is highest. Finally, we deduce from these theoretical results a number of practical rules.