Effects of amorphous layers on ADF-STEM imaging.

A study of high-resolution ADF imaging in uncorrected and aberration-corrected STEMs was carried out by multislice simulation. The presence of amorphous layers at the surface of a crystalline specimen is shown to significantly alter the visibility of the atomic columns. After propagating through an amorphous layer a portion of the beam passes without any alteration while scattered electrons introduce a Gaussian background. The dependence of the image contrast on the crystal structure, orientation and the types of the atoms present in the crystal was studied. In the case of uncorrected probes an amorphous layer thicker than 200 A is necessary to achieve considerable reduction of the visibility of the atomic columns, but with aberration-corrected probes only 60 A is necessary. With changes in defocus, crystalline specimens with amorphous layers on the top can also be imaged and high-resolution ADF images can be obtained. An amorphous layer at the beam entry surface affects the ADF image more than that of an amorphous layer at the exit surface. Approximately linear reduction of the contrast (with a slop of 1) is expected with increased thickness of amorphous layer.

Effects of tilt on high-resolution ADF-STEM imaging.

A study of the effects of small-angle specimen tilt on high-resolution annular dark field images was carried out for scanning transmission electron microscopes with uncorrected and aberration-corrected probes using multislice simulations. The results indicate that even in the cases of specimen tilts of the order of 1 degree a factor of 2 reduction in the contrast of the high-resolution image should be expected. The effect holds for different orientations of the crystal. Calculations also indicate that as the tilted specimen gets thicker the contrast reduction increases. Images simulated with a low-angle annular dark field detector show that tilt effects are more pronounced in this case and suggest that these low-angle detectors can be used to correct specimen tilt during scanning transmission electron microscopes operation.

Separation of bulk and surface-losses in low-loss EELS measurements in STEM.

To identify major features in low electron energy loss spectra, the different excitations (bulk plasmons, interband transitions, surface plasmons, Cherenkov and surface guided modes) must be delineated from each other. In this paper, this process is achieved by noting the linear thickness dependence of bulk processes contrasted with the constant thickness behavior of surface excitations. An alternative approach of analyzing bulk plasmon-loss is also introduced. Using a new algorithm, the parameters of plasma generation-plasmon energy E(P,0), a damping parameter DeltaE(P) and the coefficient of the dispersion relation gamma were obtained from a single curve fitting on the example of Si. The ability to separate surface-losses from the rest of the data permitted identification of the fine structure of the surface-losses. The strong peak at 8.2 eV characteristic of non-radiative surface plasmon excitations was measured for Si. Analysis of surface excitations indicates that a 10ASiO2 surface coating layer is still present despite careful cleaning the specimen. Dielectric functions deduced from the EELS data prove to be considerably affected by the presence of the surface-losses for samples as thick as 800A.

Depth-dependent imaging of individual dopant atoms in silicon.

We have achieved atomic-resolution imaging of single dopant atoms buried inside a crystal, a key goal for microelectronic device characterization, in Sb-doped Si using annular dark-field scanning transmission electron microscopy. In an amorphous material, the dopant signal is largely independent of depth, but in a crystal, channeling of the electron probe causes the image intensity of the atomic columns to vary with the depths of the dopants in each column. We can determine the average dopant concentration in small volumes, and, at low concentrations, the depth in a column of a single dopant. Dopant atoms can also serve as tags for experimental measurements of probe spreading and channeling. Both effects remain crucial even with spherical aberration correction of the probe. Parameters are given for a corrected Bloch-wave model that qualitatively describes the channeling at thicknesses 20 nm, but does not account for probe spreading at larger thicknesses. In thick samples, column-to-column coupling of the probe can make a dopant atom appear in the image in a different atom column than its physical position.

Spatial distribution of competing ions around DNA in solution.

The competition of monovalent and divalent cations for proximity to negatively charged DNA is of biological importance and can provide strong constraints for theoretical treatments of polyelectrolytes. Resonant x-ray scattering experiments have allowed us to monitor the number and distribution of each cation in a mixed ion cloud around DNA. These measurements provide experimental evidence to support a general theoretical prediction: the normalized distribution of each ion around polyelectrolytes remains constant when ions are mixed at different ratios. In addition, the amplitudes of the scattering signals throughout the competition provide a measurement of the surface concentration parameter that predicts the competition behavior of these cations. The data suggest that ion size needs to be taken into account in applying Poisson-Boltzmann treatments to polyelectrolytes such as DNA.

Simulation of thermal diffuse scattering including a detailed phonon dispersion curve.

Thermal vibration of the atoms in a crystal give rise to a diffuse background in the diffraction pattern (in between the normal allowed Bragg reflections). The Einstein model for phonon vibrations in a crystal leads to Gaussian statistics for the phonons. However, the Einstein model ignores the possibility of correlation between the atoms. An accurate model of the phonon dispersion curves for silicon is used to generate a set of more accurate random atomic displacements. These displacements are used in a multislice-style simulation to gauge the validity of the Einstein approximation. The phonon dispersion curve yields a small additional oscillatory structure in the thermal diffuse scattering (TDS) pattern. This does not produce significant changes in the annular dark field scanning transmission electron microscope (ADF-STEM) image signal, but could have a large impact on convergent beam measurements of bond charges.

A high efficiency annular dark field detector for STEM.

A new high efficiency annular dark field (ADF) detector for an HB501 STEM (Scanning Transmission Electron Microscope) has been constructed and tested. This detector uses a single crystal YAP scintillator and a solid quartz light pipe extending from the scintillator (inside the vacuum) to the photomultiplier tube (outside the vacuum). A factor of approximately 100 improvement in signal relative to the original detector has been obtained. This has substantially improved the signal to noise ratio in the recorded high resolution ADF-STEM images.

Digital reconstruction of bright field phase contrast images from high resolution electron micrographs.

The theory of bright-field image formation of thin specimens in a conventional transmission electron microscope is presented. The recorded image contrast is shown to be predominantly linear in the electron atom scattering amplitudes which are in general complex (possessing phase and amplitude). A linearized image model describing multiple images of varying defocus (defocus series) is derived. Image degradation is characterized by an instrumental transfer function (including spherical aberration, defocus and partial coherence), a finite signal-to-noise ratio and a Debye--Waller temperature factor. Using the minimum mean square error criterion, a new method of image reconstruction to recover the real and imaginary parts of the ideal phase contrast image from a defocus series is derived. This new method of image reconstruction reduces to the well known Wiener filter in the appropriate limiting conditions. A defocus series of micrographs taken on the Kyoto 500 keV electron microscope using a radiation damage resistant specimen of chlorinated copper phthalocyanine is processed. The signal-to-noise ratio of this series is found to be approximately 10. A resolution of approximately 2 A is apparent in the unprocessed images. The complex image reconstructed from this defocus series shows increased resolution in the real part of the image (approximately 1.4 A) and increased heavy/light atom contrast in the imaginary part of the image.