J.K. Weiss

Accurate measurements of mean inner potential of crystal wedges using digital electron holograms

Abstract The mean inner potential of a solid is a fundamental property of the material and depends on both composition and structure. By using cleaved crystal wedges of known angle, combined with dogital recording of off-axis electron holograms and with theoretical calculations of dynamical effects, the mean inner potential of Si (9.26±0.08 V), MgO (13.01±0.08 V), GaAs (14.53±0.17 V) and PbS (17.19±0.12 V) is measured with high accuracy of about 1%. Dynamical contributions to the phase of the transmitted beam are found by Bloch wave calculations to be less than 5% when the crystal wedges are titled away from zone-axis orientations and from major Kikuchi bands. The accuracy of the present method is a factor of 3 better than previously achieved by reflection high-energy electron diffraction and electron interferometry. The major causes of uncertainty were specimen imperfections and errors in phase measurement and magnification calibration.

Detection limits in quantitative off-axis electron holography

Abstract The phase of an electron wave is altered by electric and magnetic fields as it passes through a specimen. This phase change can be accurately quantified from off-axis electron holograms acquired using a slow-scan CCD camera, and small changes can be observed over small dimensions. Expressions for the precision of the phase estimate, which is limited by shot noise, have been developed. These include most of the real experimental parameters. It is found that the typical precision of practical phase measurements is better than π/100 for spatial resolutions of 1–3 nm, in good agreement with the theoretical optimal phase precision. In order to attain such small errors the effects of geometric distortion, which can introduce phase differences of up to π, must be carefully corrected.

Methods to measure properties of slow‐scan CCD cameras for electron detection

New ways to measure detection properties of slow‐scan charge‐coupled device (CCD) cameras suitable for electron microscopy, mainly based on the statistics of single‐electron events, are discussed. The experiments concentrate on the newly introduced Gatan 679 slow‐scan CCD camera. It has been established that for this instrument (if equipped with a thin YAG scintillator) 20% of the true intensity is recorded at the highest detectable spatial frequency (Nyquist frequency), limited mostly by the modulation transfer function of the YAG scintillator. The detection quantum efficiency is 0.45 for single 100‐kV electrons and 0.15 for single 400‐kV electrons, and is approaching unity for intensities higher than ten electrons. Furthermore, nonlinearity of the response is smaller than deviations in the image intensity due to shot noise. Examples are presented illustrating the detection properties of slow‐scan CCD cameras for electron imaging, which also include high dynamic range and negligible geometric distortion.

Electron-beam-induced reactions at transition-metal oxide surfaces

Abstract Electron irradiation of maximally-valent transition-metal oxides within the transmission electron microscope leads to a variety of surface reactions which depend primarily upon the energy and local current density of the incident electron beam. When the oxides Nb 2 O 5 , V 2 O 5 , TiO 2 and WO 3 are irradiated at current densities of 5–50 A cm −2 then formation of the respective metallic monoxide is observed. The surface profile imaging technique reveals that the surface monoxide invariably has a well-defined epitaxial relationship with the original oxide. When a small focused probe of extreme current density (⋍10 3 -10 4 A cm −2 ) is used, reduction beyond the monoxide phase for WO 3 , V 2 O 5 and TiO 2 is observed. Extensive pitting, amorphization and some evidence for the based metal is also found. Measurements have also been performed under uhv conditions and show that sputtering from the exit surface is an important damage mechanism.

Applications of electron holography to the study of interfaces

Abstract Electron holography has been applied to a variety of layered structures to assess its usefulness for supplying information about composition profiles across heterogeneous interfaces. The phase of the exit-surface electron wave, which to a first approximation is dependent upon the mean inner potential and the specimen thickness, was extracted from electron holograms acquired from suitable cross-sectional multilayer specimens. Line profiles from the reconstructed phase images were analyzed to obtain information about interface diffuseness and layer width with a spatial resolution of about 5 A. Using spatial averaging parallel to the interface, increased measurement precision was obtainable in some special cases. Differences in interdiffusion widths between Mo-Si and Si-Mo interfaces in an Mo/Si multilayer structure were confirmed, and the width of the amorphous layer at Si 3 N 4 grain boundaries was measured to be about 12 A. It was concluded that off-axis electron holography represented a useful complementary technique for characterizing interfaces.

A computer system for imaging and spectroscopy in analytical electron microscopy

Abstract The modern analytical transmission electron microscope gives signals from both elastically and inellastically scattered electrons at rates up to a few magabytes per second. A flexible multiasking system is needed to effectively integrate the information from a microscope with multiple detectors while synchronously controlling selected optical parameters of the instrument. We describe a system based on a Digital Equipment LSI-11 microcomputer equipped with a frame buffer and show how it has been used to develop new experimental capabilities such as time-resolved spectroscopy and multi-spectroscopy elemental mapping.

Progress towards quantitative high-resolution electron microscopy

Abstract In recent years there has been a great upsurge of applications involving quantitative high-resolution electron microscopy, in particular comparing experimental micrographs with image simulations for determination of defect structures. Emphasis has been given to the determination of experimental parameters, the utilization of slow-scan CCD cameras for digital recording and extraction of quantitative structural and chemical information. More attention to surface cleanliness is needed to improve signal quality and the possibility of electron irradiation damage should not be overlooked. Issues related to adoption of a reliability of R -factor are briefly discussed.

A study of small electron probe formation in a field emission gun TEM/STEM

Abstract The optics of the illuminating system of a TEM/STEM equipped with a high-brightness field emission source has been studied in order to determine the current density distributions of small electron probes used for microanalysis. The experimental distributions were measured from high-magnification TEM images of the small probes, and compared to theoretical distributions calculated using both coherent and incoherent optics. The microscope was shown to be capable of producing an electron probe with a FWHM diameter of 1.7 nm and a current of 1 nA. The size of the probe is limited by electrical and mechanical instabilities.

Computer-assisted extrapolation method for absorption correction in quantitative X-ray microanalysis

Abstract This letter presents results of rapid processing for X-ray absorption correction in quantitative X-ray microanalysis using an analytical electron microscope. A computer-based system developed for analytical electron microscopy is used to assist in the extrapolation method, which does not require thickness measurement but only the recording of characteristic X-rays for absorption correction.

Factors limiting the spatial resolution and sensitivity of EELS microanalysis in a STEM

Abstract The experimental parameters which limit the spatial resolution and sensitivity of parallel electron-energy-loss microanalysis have been reviewed in order to determine the limiting factors over a range of experimental conditions. The effects of the signal-detector characteristics and electron optics on the performance of the microanalytical technique have been quantified. The importance of statistical considerations (signal-to-noise ratios) and the specimen itself are considered in order to develop a non-arbitrary definition of microanalytical performance based not only on the electron beam dimensions, but also incorporating other experimental limitations.