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A study of screw dislocations in hydride-vapor-phase-epitaxy (HVPE)
template and molecular-beam-epitaxy (MBE) overlayers was performed
using transmission electron microscopy (TEM) in plan view and in cross
section. It was observed that screw dislocations in the HVPE layers
were decorated by small voids arranged along the screw axis. However,
no voids were observed along screw dislocations in MBE overlayers. This
was true both for MBE samples grown under Ga-lean and Ga-rich
conditions. Dislocation core structures have been studied in these
samples in the plan-view configuration. These experiments were
supported by image simulation using the most recent models. A direct
reconstruction of the phase and amplitude of the scattered electron
wave from a focal series of high-resolution images was applied. It was
shown that the core structures of screw dislocations in the studied
materials were filled. The filed dislocation cores in an MBE samples
were stoichiometric. However, in HVPE materials, single atomic columns
show substantial differences in intensities and might indicate the
possibility of higher Ga concentration in the core than in the matrix.
A much lower intensity of the atomic column at the tip of the void was
observed. This might suggest presence of lighter elements, such as
oxygen, responsible for their formation.
Sub-Ångstrom TEM to a resolution of 0.78Å has been demonstrated by the one-Ångstrom microscope (OÅM) project at the National Center for Electron Microscopy. The OÅM combines a modified CM300FEG-UT with computer software able to generate sub-Angstrom images from experimental image series.
Sub-Ångstrom HREM is gaining in importance as researchers design and build artificially-structured nanomaterials such as semiconductor devices, ceramic coatings, and nanomachines. Commonly, such nanostructures include atoms with bond lengths shorter in projection than the point resolution of a mid-voltage HREM. in addition, image simulations have shown that structure determinations of defects such as dislocation cores require sub-Angstrom resolution, as will hold true for grain boundaries and other interfaces.
Sub-Ångstrom microscopy with a transmission electron microscope requires meticulous attention to detail. As resolution is improved, resolution-limiting parameters need to be reduced. in particular, aberrations must be minimized, power supplies must be stabilized, and the microscope environment optimized to reduce acoustic and electromagnetic noise in addition to vibration.
High-resolution transmission electron microscopy (HRTEM) is used extensively in the semiconductor industry for device characterization, and has become one of the highly favored techniques for characterizing the latest generation of ultra-thin gate oxides in MOSFET devices. However, relatively little is understood (either quantitatively or experimentally) about the limitations of HRTEM in detecting structural defects in gate oxides that could affect device performance. To investigate model defects experimentally, it would be necessary to construct “perfect” gate oxides, introduce defects with size and morphology known perfectly a priori, successfully make thin specimens that capture the defects, and then perform imaging experiments in the HRTEM. Since that task is virtually impossible, we have performed HRTEM image simulations to assess the visibility of various structural defects in gate oxides. The gate oxide was modeled as an amorphous silicon oxide 16.3Å-thick, sandwiched between a gate and substrate. The substrate was (100) silicon viewed along the  direction.
High resolution transmission electron microscopy (HRTEM) has found extensive use in the semiconductor industry for performing device metrology and characterization. However, shrinking device dimensions (gate oxides are rapidly approaching 10Å) present challenges to the use of HRTEM for many applications, including gate oxide metrology. In this study, we performed HRTEM image simulations of a MOSFET device to examine the accuracy of HRTEM in measuring gate oxide thickness. Length measurements extracted from simulated images were compared to actual dimensions in the model structure to assess TEM accuracy. The effects of specimen tilt, specimen thickness, objective lens defocus and coefficient of spherical aberration (CS) on measurement accuracy were explored for nominal 10Å and 16Å gate oxide thicknesses.
The gate oxide was modeled as an amorphous silicon oxide situated between a gate electrode and substrate, both modeled as single crystal Si(100). Image simulations of the sandwich structure were performed in cross-section (with Si parallel to beam direction) using the multislice approximation for a 200 kV microscope with Cs=0.5mm.
High resolution electron microscopes with field emission sources opened the possibility to investigate solids on a 100 pm range. Either electron holograpy can be applied or an information limit that may even extend into a region below 100 pm can be exploited to reach this goal . However, lens aberrations such as the three-fold astigmatism often complicate an image interpretation in the 100 pm range or even make it impossible . On the other hand, there is growing need to understand physical processes at a mono-atomic level in order to further develop artificially structured materials such as nano-crystals, ceramic coatings or semiconductors. Commonly, such materials contain light elements like C, N, or O with bond lengths that are shorter than a typical 180 pm point resolution of a high resolution, electron microscope. The carbon-carbon distance of 150 pm is the shortest bond length value in crystalline solids. Moreover, any projection of a diamond lattice along a low index zone axis for lattice imaging leads to a reduced C-C distance.
GaN and the related AlN semiconducting materials have recently attracted considerable attention because of their versatile applications for optoelectronics. Large stresses are present in the GaN/AlN thin-film heterostructure and can exceed GPa's. They originate from a large lattice mismatch between the substrate(sapphire)/GaN (14%) and the AlN/GaN interface (-2.7%) in quantum well structures. The strain is expected to induce local piezoelectric fields in these polar materials. It is essential for a further development of GaN based thin films to fully understand the formation and the local strength of these fields. Previous studies show that there are unusual large fluctuations of the patterns in lattice images present across GaN/AlxGa1-xN and GaN/InyGa1-yN quantum wells. They are attributed to compositional fluctuations and have been studied by quantitative high resolution electron microscopy. Differences between the strain profiles and the electrostatic (scattering) potential profiles were observed.
In 1996 the DOE formed the Materials Microcharacterization Collaboratory (MMC) to bring together the four DOE-sponsored electron microscope user facilities in one collaboratory. The MMC will bring the microanalysis and microcharacterization tools that are available in national centers to geographically dispersed researchers working in industries, universities, and Government laboratories. It will enable these remote users to share on-line the instrumentation, knowledge and expertise available at the individual facilities making up the collaboratory.
LBNL first demonstrated on-line remote control of a high-voltage TEM from Kansas City in 1995 in a joint project by LBNL computer scientists and NCEM microscopists. The microscope chosen was a 1.5MeV Kratos EM-1500 that is used for in-situ electron microscopy. In the demonstration, a specimen of an Al/Pb alloy was heated and observed, with remote-operator control of heating rate, microscope focus, and stage movement (translation and tilt).
The latest transmission electron microscopes with field emission guns and imaging filters now provide much of the microanalysis and imaging necessary in applications such as ULSI device development. The installation and operating environment of the instruments are critical to their successful operation. Information from two such installations is presented here, one in a purpose built facility and the other in an existing building. Ground vibration, acoustic noise, stray electromagnetic fields, air flow and temperature variation are considered, and the measures implemented to achieve desirable levels of each parameter are discussed. The physical layout of an installation is also shown.
The National Center for Electron Microscopy has recently acquired a field-emission TEM to form thebasis of a project to achieve a resolution of one Ångstrom. To reach this resolution, both instrumental and environmental factors need to be considered. We have designed and constructed a new building to provide a suitable environment for this instrument, with emphasis on providing isolation from external influences detrimental to the achievement of ultra-high resolution. Such influences include mechanical vibration, temperature fluctuations, acoustic noise, and stray electromagnetic fields.
The microscope chosen for the one-Ångstrom project is a Philips CM300 Ultra-Twin equipped with a field-emission gun. Pre-installation specifications provided by Philips for this 1.7Å-resolution TEM specify maximum-allowable values for vibration levels in three mutually-perpendicular directions. In the most critical direction (console left to right), vibration is required to remain below 0.8)μm/sec in the frequency range from 1Hz to 5Hz, although allowed to rise to 6μm/sec above 10Hz (Region I in fig. 1). Even when resolution is not a critical requirement, vibration must be minimized at 2.5Hz (Region II in fig.1).
The transmission electron microscope (TEM) is one of the most useful tools available to the materials scientist. Yet both the complexity and expense of the equipment, and the huge investment in time necessary to become proficient in specimen preparation and image acquisition and analysis, mean that it is difficult for most industrial institutions to maintain a state-of-the-art TEM facility. How can industry overcome this problem? One solution is to set up a collaboration with a university, an industrial partner, or a government research laboratory. Such collaborations can be extremely valuable to the company, which gains access to microscopes, specimen-preparation equipment and the expertise of professional microscopists, and to the research laboratory, which benefits from the industrial perspective and the private sector's proficiency in materials preparation and processing.
Such collaborations exist, and they can produce excellent results. In this article, we present three case studies in which successful collaboration has occurred between industry and one of the Department of Energy's scientific user facilities, the National Center for Electron Microscopy (NCEM-see sidebar). Our aim is not only to describe results that we hope will be of scientific interest but also to encourage industrial researchers to consider collaborations with institutes such as NCEM.
Electron crystallography has now been used to investigate the structures of inorganic materials in three dimensions. As a test of the method, amplitudes and phases of structure factors were obtained experimentally from high resolution images of staurolite taken in a number of different projections. From images in five orientations, a three-dimensional Coulomb potential map was constructed with a resolution of better than 1.4Å. The map clearly resolves all the cations (Al,Si,Fe) in the structure, and all of the oxygen atoms. This method promises great potential for structure determinations of small domains in heterogeneous crystals which are inaccessible to x-ray analysis. Three-dimensional structure determinations should be possible on small domains only approximately 10 unit cells wide, and may resolve site occupancies in addition to atom positions. Given a microscope stage with a suitable range of tilt and enough mechanical stability, the method could also be applied to small crystalline particles larger than about 50Å to 100Å. In addition, it may be possible to apply the method to derive the two-dimensional structure of periodic defects.
High resolution electron micrographs have been obtained in various orientations for δ2-Y2Si2O7 using the Atomic Resolution Microscope (ARM) at the National Center for Electron Microscopy (NCEM). These ARM images were processed by masking the diffractogram of the digitized image in Fourier space and applying an inverse transform (using the SEMPER program at the NCEM Image Analysis Facility) in order to reveal details obscured by amorphous contrast originating in the glassy matrix. Processed experimental images from very thin regions of the crystalline phase were compared with images simulated from postulated models (SHRLI images); close agreement was obtained.
High resolution electron microscopy has been employed to elucidate fault defects and structural details of the δ1 and δ2-Y2Si2O7 crystalline phases. From this study δ1 and δ2-Y2Si2O7 have been found to be orthorhombic having the same cell parameters but different atomic arrangements due to a change in their space groups. Computer simulations were necessary for interpreting details from the high resolution electron micrographs.
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