Biomedical subjects

D A Muller

Publications and source records attributed to D A Muller.

7 recordsLinked to original sources

PubMed2003

Imaging individual atoms inside crystals with ADF-STEM.

The quantitative imaging of individual impurity atoms in annular dark-field scanning transmission electron microscopy (ADF-STEM) requires a clear theoretical understanding of ADF-STEM lattice imaging, nearly ideal thin samples, and careful attention to image processing. We explore the theory using plane-wave multislice simulations that show the image intensity of substitutional impurities is depth-dependent due to probe channeling, but the intensity of interstitial impurities need not be. The images are only directly interpretable in thin samples. For this reason, we describe a wedge mechanical polishing technique to produce samples less than <50 A thick, with low surface roughness and no amorphous surface oxide. This allows us to image individual dopants as they exist within a bulk-like silicon environment. We also discuss the image analysis techniques used to extract maximum quantitative information from the images. Based on this information, we conclude that the primary nanocluster defect responsible for the electrical inactivity of Sb in Si at high concentration consists of only two atoms.

Journal Article↗

PubMed2002

Artificial charge-modulationin atomic-scale perovskite titanate superlattices.

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

Journal Article↗

PubMed2002

Atomic-scale imaging of individual dopant atoms and clusters in highly n-type bulk Si.

As silicon-based transistors in integrated circuits grow smaller, the concentration of charge carriers generated by the introduction of impurity dopant atoms must steadily increase. Current technology, however, is rapidly approaching the limit at which introducing additional dopant atoms ceases to generate additional charge carriers because the dopants form electrically inactive clusters. Using annular dark-field scanning transmission electron microscopy, we report the direct, atomic-resolution observation of individual antimony (Sb) dopant atoms in crystalline Si, and identify the Sb clusters responsible for the saturation of charge carriers. The size, structure, and distribution of these clusters are determined with a Sb-atom detection efficiency of almost 100%. Although single heavy atoms on surfaces or supporting films have been visualized previously, our technique permits the imaging of individual dopants and clusters as they exist within actual devices.

Journal Article↗

PubMed2002

Fluctuation microscopy in the STEM.

Fluctuation electron microscopy is a technique for studying medium-range order in disordered materials. We present an implementation of fluctuation microscopy using nanodiffraction in a scanning transmission electron microscope (STEM) at a spatial resolution varying from 0.8 to 5.0 nm. Compared to conventional TEM (CTEM), the STEM-based technique offers a denser scattering vector sampling at a reduced sample dose and easier access to variable resolution information. We have reproduced results on amorphous silicon previously obtained by CTEM-based fluctuation microscopy, and report initial variable-resolution measurements on amorphous germanium.

Journal Article↗

PubMed2002

Direct observation of defect-mediated cluster nucleation.

Ion implantation is widely used to introduce electrically or optically active dopant atoms into semiconductor devices. At high concentrations, the dopants can cluster and ultimately form deactivating precipitates, but deliberate nanocrystal formation offers an approach to self-assembled device fabrication. However, there is very little understanding of the early stages of how these precipitates nucleate and grow, in no small part because it requires imaging an inhomogenous distribution of defects and dopant atoms buried inside the host material. Here we demonstrate this, and address the long-standing question of whether the cluster nucleation is defect-mediated or spontaneous. Atomic-resolution illustrations are given for the chemically dissimilar cases of erbium and germanium implanted into silicon carbide. Whereas interstitial loops act as nucleation sites in both cases, the evolution of nanocrystals is strikingly different: Erbium is found to gather in lines, planes and finally three-dimensional precipitates, whereas germanium favours compact, three-dimensional structures.

Carbon Compounds, Inorganic↗

PubMed2001

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.

Journal Article↗

PubMed2001

Optimizing the environment for sub-0.2 nm scanning transmission electron microscopy.

Sub-0.2 nm probes can now be readily obtained on Schottky field-emission microscopes. However, environmental instabilities are proving to be the limiting factors for atomic resolution spectroscopy and distortion-free annular-dark field imaging. This is a result of the long acquisition times and the serial nature of the scanning system where instabilities result in image distortions rather than reductions in contrast. Troubleshooting the most common environmental problems is discussed here. In addition to the expected sensitivity to mechanical vibration, electromagnetic interference and temperature variations, air-pressure fluctuations are found to have a significant impact on microscopes with side-entry goiniometers.

Journal Article↗