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We report direct mapping of the grains and grain boundaries (GBs) of large-area monolayer polycrystalline graphene sheets, at large (several micrometer) and single-atom length scales. Global grain and GB mapping is performed using electron diffraction in scanning transmission electron microscopy (STEM) or using dark-field imaging in conventional TEM. Additionally, we employ aberration-corrected TEM to extract direct images of the local atomic arrangements of graphene GBs, which reveal the alternating pentagon−heptagon structure along high-angle GBs. Our findings provide a readily adaptable tool for graphene GB studies.

Time series of high-resolution lattice images are examined to probe for possible alterations of the indium distribution in GaN/InxGa1–xN/GaN quantum well structures during electron irradiation with energies of 150 kV and 800 kV. By comparison with theory it is reasoned that sample preparation, microscope stability, and chosen acceleration voltages are essential factors that determine the reliability of the results. If considered, it is shown that for relevant time scales of <2 minutes and current densities of 20–40 A/cm2 no measurable alteration of the initial element distribution occurs. A quantitative method is highlighted for the characterization of existing indium fluctuations that are concentration dependent. Consistency between measurements from lattice images, Z-contrast images, and local band gap measurements support our conclusion. It is argued that the formation of such indium clusters is driven by strain-dependent spinodal decomposition. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-resolution transmission electron microscopy (HR-TEM) has been used as the ultimate method of thickness measurement for thin films. The appearance of phase contrast interference patterns in HR-TEM images has long been confused as the appearance of a crystal lattice by nonspecialists. Relatively easy to interpret crystal lattice images are now directly observed with the introduction of annular dark-field detectors for scanning TEM (STEM). With the recent development of reliable lattice image processing software that creates crystal structure images from phase contrast data, HR-TEM can also provide crystal lattice images. The resolution of both methods has been steadily improved reaching now into the sub-Ångstrom region. Improvements in electron lens and image analysis software are increasing the spatial resolution of both methods. Optimum resolution for STEM requires that the probe beam be highly localized. In STEM, beam localization is enhanced by selection of the correct aperture. When STEM measurement is done using a highly localized probe beam, HR-TEM and STEM measurement of the thickness of silicon oxynitride films agree within experimental error. In this article, the optimum conditions for HR-TEM and STEM measurement are discussed along with a method for repeatable film thickness determination. The impact of sample thickness is also discussed. The key result in this article is the proposal of a reproducible method for film thickness determination.

High-resolution transmission electron microscopy (HR-TEM) has been used as the ultimate method of thickness measurement for thin films. The appearance of phase contrast interference patterns in HR-TEM images has long been confused as the appearance of a crystal lattice by nonspecialists. Relatively easy to interpret crystal lattice images are now directly observed with the introduction of annular dark-field detectors for scanning TEM (STEM). With the recent development of reliable lattice image processing software that creates crystal structure images from phase contrast data, HR-TEM can also provide crystal lattice images. The resolution of both methods has been steadily improved reaching now into the sub-Ångstrom region. Improvements in electron lens and image analysis software are increasing the spatial resolution of both methods. Optimum resolution for STEM requires that the probe beam be highly localized. In STEM, beam localization is enhanced by selection of the correct aperture. When STEM measurement is done using a highly localized probe beam, HR-TEM and STEM measurement of the thickness of silicon oxynitride films agree within experimental error. In this article, the optimum conditions for HR-TEM and STEM measurement are discussed along with a method for repeatable film thickness determination. The impact of sample thickness is also discussed. The key result in this article is the proposal of a reproducible method for film thickness determination.

Abstract: This paper describes TEM characterization of bulk GaN crystals grown at 1500–1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20–30 arc-sec. The plate thickness along thec axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to thec direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

Abstract: Thin films of AlN and GaN were grown by pulsed laser deposition on c-plane sapphire substrates. It is demonstrated that the structure and surface morphology of layers can actively be controlled by adjusting the nitrogen partial pressure during the growth. The observed trends in the structural quality of the thin films can be attributed to the changes in the surface diffusion of adatoms. It is argued that the surface diffusion of adatoms can be influenced by the collisions between the nitrogen gas molecules and the activated atoms which can reduce the kinetic energy of activated atoms and increase the rate of formation of immobile surface dimers. Through these nitrogen pressure related effects, thin films with microstructure ranging from crystalline to amorphous can be produced. The observed similar impact of nitrogen pressure on the growth of GaN and AlN thin films indicates that a pressure assisted growth procedure is generally applicable to design the surface morphology of group III-nitride thin films. A minimal surface root mean square roughness of 0.7 nm for amorphous AlN is obtained which compares well with the substrate roughness of 0.5 nm. Rutherford backscattering spectroscopy of thin films of GaN and AlN showed a large incorporation of oxygen which was found to reduce the lattice constants of GaN and AlN.

Recent progress in High Resolution Transmission Electron Microscopy makes it possible to investigate crystalline materials by phase contrast microscopy with a resolution close to the 80 pm information limit of a 300 kV field emission microscope'"". A reconstruction of the electron exit wave from a focal series of lattice images converts the recorded information into interpretable resolution. The present contribution illustrates some recent applications of this technique to interfaces.Fig. 1 shows a reconstructed electron exit wave of a heterophase interface between GaN and sapphire. The experiment takes advantage of three factors: First, we resolved the GaN lattice in projection, which requires at least 0.15 nm resolution. The projection eliminates the stacking fault contrast that usually obscures lattice images in the commonly recorded projection. Thus, image interpretation is drastically simplified. Second, all atom columns at the interface and in the sapphire are resolvable with a smallest projected aluminum - oxygen spacing of 85 pm in the sapphire.

The phenomenon of segregation is of long standing scientific interest and has been studied extensively, both theoretically as well as experimentally. For our investigations we have chosen the Σ5 symmetric tilt grain boundaries (STGB) in two face-centered cubic (FCC) metals, aluminum and copper. Both metals were doped with only 1 at% of the impurity species (Cu and Ag). One of our major goals in this study was to investigate the size effect on segregation of an impurity to distinct sites at the grain boundary. Therefore we have selected the Ag as an impurity in Cu and Cu as an impurity in Al. The latter one is of special interest for applications like interconnects in microcircuits, where one of the major controlling factors of electromigration is expected to be the diffusion or segregation of Cu atoms at Al grain boundaries.The model grain boundaries have been fabricated with ultra-high vacuum diffusion bonding of single crystals which bonds the bicrystals under highly controlled environmental conditions.

In 1999 NCEM's One Ångstrom Microscope (OAM) became fully operational. The OAM is a Philips CM300 FEG/UT field emission microscope with holographic capabilities that is equipped with a Gatan Image Filter (GIF) and operates at 300 kV. It was designed to reach a resolution close to the “magic barrier” around one Ångstrom (100 pm) by combining mid voltage technology with advanced computer processing [1,2]. Ahardware correction of the three-fold astigmatism allows for aberration free imaging down to sub Ångstrom values [3]. In this contribution it will be shown that the instrument's performance exceeded expectations because sub Ångstrom resolution can be achieved by reconstructing electron exit waves from focal series [4].Figure la depicts a simulated [110] lattice image of a 90° partial dislocation in silicon. Tersoff potentials were used to calculate the exact atomic positions around dislocations with different core structures [5].

Nowadays, High Resolution Electron Microscopes are capable to resolve structures on a scale below 100 pm. They can be equipped for Electron Holography in order to detect electric / magnetic fields and for chemical analyses (Electron Energy Loss Spectroscopy & Energy Dispersed X-ray's) that can be performed with a lateral resolution of 0.5 to 1 nm [1]. With the aid of computer sciences it became also possible to quantify local strain. We utilize Philips CM200 and CM300 field emission instruments with attached image filters, the JEOL Atomic Resolution Microscope and specialized software [2] to perform these tasks. On the other hand, a recent highlight in materials sciences is the development a GaN technology that is driven by a fast trial and error approach and aims to revolutionize lighting [3]. It was unavoidable that basic materials properties of the nano-structured thin films are barely understood because of the rapid progress [4].

Quantitative evaluation of coherency strains in Ni-12 at.% Al alloys has been performed by means of high resolution electron microscopy (HREM) and convergent beam electron diffraction (CBED) techniques. The alloy Ni-12 at.%Al is composed of two phases, the disordered fee Ni-Al solid solution and the coherent Ni3Al (γ') particles with an LI2structure. The presence of γ' particles can impart excellent mechanical properties to these alloys. However these particles coarsen if subject to high temperature with an important reduction of mechanical properties. Coarsening of coherent particles in solid alloys is driven by the decrease of surface and elastic energies. The effect of surface energy is well understood but that of the elastic energy still requires experimental investigation. The source of elastic interaction between particles is the lattice parameter mismatch between particles and matrix. Evaluation of the elastic strain fields around particles can give some insight into the elastic interaction between particles and thus into the coarsening mechanism from an experimental viewpoint.

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 [1]. However, lens aberrations such as the three-fold astigmatism often complicate an image interpretation in the 100 pm range or even make it impossible [2]. 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.

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.

The local element distribution in quantum wells largely affects physical properties of devices made from such materials. In the past, quantitative electron microscopy was developed to access the stoichiometry on an atomic scale as shown on the cover page of this issue's Editor's Choice [1] for the GaN/InxGa1–xN/GaN and the GaAs/AlxGa1–xAs/GaAs system by the application of QUANTITEM and Chemical Imaging, respectively. In case of GaN/InxGa1–xN/GaN local strain mapping allows for extracting similar data and an unusual large indium fluctuation can be observed if compared with the aluminum distribution in GaAs/AlxGa1–xAs/GaAs quantum well structures. However, radiation damage, sample preparation and microscope stability affect the data analyses and it is of essence to monitor and control such effects as outlined in the related paper.The first author Til Bartel is a PhD candidate in physics at the Technical University of Berlin, currently visiting LBNL in California to apply transmission electron microscopy to III-nitride semiconductors. Christian Kisielowski is Staff Scientist and Principle Investigator at the National Center for Electron Microscopy (NCEM) and is responsible for the development and application of high resolution electron microscopy.The present special issue of physica status solidi (a) is a compilation of presentations from the recent symposium on Indium Nitride and Indium Rich Related Alloys at the E-MRS 2005 Fall Meeting in Warsaw.