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Complex morphology is observed in the GaAs:Bi cap layer of Ga(As,Bi) films grown on GaAs(001) substrates by low-temperature (LT) molecular beam epitaxy (MBE). The microstructure is dominated by the presence of V-shaped domains in the LT-GaAs cap layer and by the (unintentional) inhomogeneous incorporation of Bi from the underlying Ga(As,Bi) layer growth. Aberration-corrected scanning transmission electron microscopy techniques enabled two types of domain to be identified: (i) twinned domains and (ii) Bi-rich atomically ordered domains with triple periodicity. The observed microstructure is discussed in the context of LT-MBE growth in combination with the presence of a small amount of Bi, which seemingly alters adatom diffusivity. Surface processes and (surface) kinetic factors play a key role in the resultant morphology and explain the appearance of both types of domains. [ABSTRACT FROM AUTHOR]

We report a study of the magnetic domain structure of nanocrystalline thin films of nickel-zinc ferrite. The ferrite films were synthesized using aqueous spin-spray coating at low temperature (∼90 °C) and showed high complex permeability in the GHz range. Electron microscopy and microanalysis revealed that the films consisted of columnar grains with uniform chemical composition. Off-axis electron holography combined with magnetic force microscopy indicated a multigrain domain structure with in-plane magnetization. The correlation between the magnetic domain morphology and crystal structure is briefly discussed. [ABSTRACT FROM AUTHOR]

We have used electron holography to investigate the local magnetic behavior of isolated ferromagnetic nanowires (NWs) in their remanent states. The NWs consisted of periodic magnetic layers of soft, high-saturation magnetization CoFeB alloys, and non-magnetic layers of Cu. All NWs were fabricated by pulsed-potential electrodeposition in nanoporous alumina membranes. The NW composition and layer thicknesses were measured using scanning transmission electron microscopy and energy dispersive spectroscopy. The magnetization of individual NWs depended upon the thicknesses of the layers and the direction of an external magnetic field, which had been applied in situ. When the CoFeB was thicker than the diameter (50 nm), magnetization was axial for all external field directions, while thinner layers could be randomized via a perpendicular field. In some cases, magnetization inside the wire was detected at an angle with respect to the axis of the wires. In thinner Cu/CoFeB (<10 nm each) multilayer, magnetic field vortices were detected, associated with opposing magnetization in neighbouring layers. The measured crystallinity, compositions, and layer thicknesses of individual NWs were found to be significantly different from those predicted from calibration growths based on uniform composition NWs. In particular, a significant fraction of Cu (up to 50 at. %) was present in the CoFeB layers such that the measured magnetic induction was lower than expected. These results will be used to better understand previously measured effective anisotropy fields of similar NW arrays. [ABSTRACT FROM AUTHOR]

Magnetic remanent states in a rectangular array of 75×280-nm2 NiFe/Cu/Co thin-film pseudo-spin-valve elements are studied using off-axis electron holography in the transmission electron microscope (TEM). An approach based on focused ion-beam milling is used to minimize damage to the magnetic properties of the elements during preparation for TEM examination in plan-view geometry. Experimental electron holographic phase images are used to measure the switching fields of the Co and NiFe layers in each of three adjacent elements separately, and comparisons with micromagnetic simulations are used to infer the true magnetic thicknesses and widths of the layers. Demagnetizing fields are included in the discussion of the results, and the possibility that the conclusions may be affected by the procedure used to analyze the holograms is discussed. [ABSTRACT FROM AUTHOR]

We report a study of the magnetic switching behavior of nanoscale Co rings using off-axis electron holography. Arrays of 10 nm thick polycrystalline Co rings with 400 nm outer diameter (OD) and different inner diameter (ID) were fabricated by electron-beam lithography. The switching behavior of the rings was studied for different OD/ID ratios, and two kinds of reversal mechanism were identified. For OD/ID of 400 nm/250 nm and 400 nm/50 nm, the reversal started from the so-called onion (bidomain) state, proceeding to a stable vortex state, and finally to the reversed onion state. For intermediate OD/ID of 400 nm/150 nm, the reversal was instead accomplished via rotation of head-to-head domain walls around the rings to the reversed onion state without formation of a vortex state. The OD/ID ratio of the rings thus played the most important role in determining the switching process. Irrespective of the reversal mechanism, the coercive field of the rings and the range of the field needed to reverse their magnetization, both increased as the inner ring diameter was increased (i.e., narrower ring). The significance of different contributions to the total energy in causing these differences in switching behavior is briefly discussed. [ABSTRACT FROM AUTHOR]

Provides information on a study wherein an asymmetrical triangular quantum well was grown by molecular-beam epitaxy using a digital alloy composition grading methods. Methods; Results; Conclusion.

The microstructure of Ba(Cd1/3Ta2/3)O3 ceramics with boron additive was investigated by high-resolution and analytical electron microscopy. Superlattice reflections were present at positions of (h ± 1/3, k ± 1/3, 1 ± 1/3) away from the fundamental reflections in the [110] zone diffraction pattern for the pseudocubic perovskite unit cell. Lattice images showed a well-ordered structure with hexagonal symmetry. No boron segregation and amorphous phase was observed along grain boundaries. An amorphous phase rich in boron-oxide was observed to form pockets partially penetrating along multiple grain junctions. [ABSTRACT FROM AUTHOR]

Stress concentration at the boundary of Ge/Si(100) islands drives strain relief mechanisms activated at higher growth temperature, T. Si interdiffusion for T ≥ 550°C forms a reduced misfit alloy allowing specific cluster morphologies to exist at sizes greater than those for pure Ge islands. This interdiffusion also affects the pathway for island shape changes. Trenches formed at the island base result from diffusion of the most highly strained material to regions of lower strain and precede dislocation formation for T ≥ 600°C. [ABSTRACT FROM AUTHOR]

We describe the magnetic properties of nanoscale Fe shells grown on GaAs nanowires (NWs) by molecular beam epitaxy. The ferromagnetic character of these tubular Fe shells has been confirmed by dc magnetization measurements, and is further studied by ferromagnetic resonance (FMR), small-angle neutron scattering (SANS), and off-axis electron holography (EH). [ABSTRACT FROM AUTHOR]

Quantitative comparisons have been made of the In concentration, strain, and internal electric field present in a pseudomorphic InGaN/GaN quantum well. Z-contrast scanning transmission electron microscopy was used for mapping In concentration with atomic resolution, variations of the c-lattice parameter of the InGaN layer were measured from (0001) lattice fringes in high-resolution transmission electron micrographs, and the internal electric fields were determined by differentiating phase images obtained by electron holography. Based on these measurements, it was concluded that local fluctuations of In concentration caused inhomogeneities in the internal electric field across the quantum well. The band structure of the quantum well would thus be altered not only by quantum dot effects but also by the additional modulation of the internal electric field, leading to further broadening of the light emission. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An approach based on the technique of convergent-beam electron diffraction (CBED) has been used to determine the elemental composition of a pseudomorphic GaN/In[sub x]Ga[sub 1-x]N/GaN quantum-well structure on the nanometer scale. Areas of triangles formed by intersecting higher order Laue-zone (HOLZ) lines are highly sensitive to lattice-parameter variations. By calculating the ratio of triangle areas, the local In concentration is estimated to within ±0.5% once lattice expansion in the growth direction has been taken into account. Moreover, shifts in the HOLZ-line positions as the CBED probe is moved along the quantum well provide a simple and rapid indication of local fluctuations in In content. The technique is applicable to other pseudomorphically strained epitaxial layers. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a theoretical and experimental investigation of Cr-doped A1N. Density functional calculations predict that the isolated Crt[sub 2] defect level in AlN is 1/3 full, falls approximately at midgap, and broadens into an impurity band for concentrations over 5%. Substitutional A1[sub 1 - x]Cr[sub x]N random alloys with 0.05≤x≤0.15 are predicted to have Curie temperatures over 600 K. Experimentally, we have characterized and optimized the molecular beam epitaxy thin film growth process, and observed room temperature ferromagnetism with a coercive field, H[sub c], of 120 Oe. The measured magnetic susceptibility indicates that over 33% of the Cr is magnetically active at room temperature and 40% at low temperature. [ABSTRACT FROM AUTHOR]

The one-dimensional dopant profile of a silicon p–n junction was characterized using off-axis electron holography in a transmission electron microscope (TEM). Quantitative comparisons were made with simulated voltage profiles based on data obtained from secondary-ion mass spectroscopy. Close agreement was obtained over a range of sample thicknesses, and a spatial resolution of 5 nm and sensitivity of 0.1 V were established. By using a sample that had been wedge polished and briefly ion milled, depleted surface layers did not need to be taken into account, and beam-induced charging was removed by carbon coating one exposed surface of the TEM specimen. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the development of a simple chemical route to growing Ge[sub 1-x]Sn[sub x] semiconductors using ultrahigh-vacuum chemical vapor deposition and the molecular precursor (Ph)SnD[sub 3] as the source of Sn atoms. Thin films were deposited on oxidized and oxide-free Si by reactions of (Ph)SnD[sub 3] with Ge[sub 2]H[sub 6] at 350 °C. The composition, microstructure, and bonding properties of the films were characterized by Rutherford backscattering, high-resolution analytical electron microscopy, and Raman spectroscopy. As-deposited Ge[sub 1-x]Sn[sub x] on oxidized Si displayed good crystallinity which improved significantly by annealing at 400 °C. High-resolution electron microscopy and diffraction indicated a diamond-cubic structure with lattice constants intermediate to those of Ge and α-Sn. As-deposited Ge[sub 1-x]Sn[sub x] on pure Si was monocrystalline and epitaxial. Nanoprobe analysis in plan view and cross section revealed that the as-deposited and annealed materials were homogeneous with good chemical purity. The Raman spectra showed bands corresponding to Ge-Ge and Sn-Ge vibrations with frequencies consistent with a random tetrahedral alloy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Off-axis electron holography and micromagnetic simulations have been used to investigate magnetization reversal mechanisms and remanent states in exchange-biased submicron Co[sub 84]Fe[sub 16]/Fe[sub 54]Mn[sub 46] patterned elements. Domain structures within the elements were characterized despite the narrow thickness (∼3 nm) of the ferromagnetic layer relative to the total element thickness (∼42 nm). Individual elements were able to support different remanent states and their magnetic microstructure was sensitive to their size. The simulations confirmed that the coercivities of the elements and their domain structures were highly sensitive to the strength and orientation of the pinning field. A good fit to the experimental data was provided by using an interface exchange field that had a fixed amplitude and direction in the simulations, and small disagreements were attributed to structural imperfections. These differences emphasize the value of an experimental technique such as electron holography for probing local micromagnetic structure in individual nanostructured elements. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

Electron holography has been used to image electrostatic potential variations across an AlGaN/InGaN/AlGaN heterojunction diode. Features in the energy profile have been interpreted in terms of the expected built-in voltage across the p-n junction of the diode as well as contributions from the spontaneous and piezoelectric polarization fields within the material. Additional profile features indicate the presence of two-dimensional electron gas at the upper interface of the InGaN well, and evidence for hole accumulation close to the lower interface. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The induction of 90° magnetic domains in a Nd[sub 2]Fe[sub 14]B hard magnet has been mapped at high spatial resolution using off-axis electron holography. Line profiles from the reconstructed phase image established an upper limit for domain wall width of 10 nm. Thermal annealing resulted in domain wall movement, magnetization rotation, and evidence for pinning at a structural defect. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The three-dimensional (3D) morphology of a nanometer-sized object can be obtained using electron tomography. Variations in composition or density of the object cause variations in the reconstructed intensity. When imaging homogeneous objects, variations in reconstructed intensity are caused by the imaging technique, imaging conditions, and reconstruction. In this paper, we describe data acquisition, image processing, and 3D reconstruction to obtain and compare tomograms of magnetite crystals from bright field (BF) transmission electron microscopy (TEM), annular dark-field (ADF) scanning transmission electron microscopy (STEM), and high-angle annular dark field (HAADF) STEM tilt series. We use histograms, which plot the number of volume elements (voxels) at a given intensity vs. the intensity, to measure and quantitatively compare intensity distributions among different tomograms. In combination with numerical simulations, we determine the influence of maximum tilt angle, tilt increment, contrast changes with tilt (diffraction contrast), and the signal-to-noise ratio (SNR) as well as the choice of the reconstruction approach (weighted backprojection (WB) and sequential iterative reconstruction technique (SIRT)) on the histogram. We conclude that because ADF and HAADF STEM techniques are less affected by diffraction, and because they have a higher SNR than BF TEM, they are better suited for tomography of nanometer-sized crystals.

A method for obtaining detailed two-dimensional strain maps in nanowires and related nanoscale structures has been developed. The approach relies on a combination of lattice imaging by high-resolution transmission electron microscopy and geometric phase analysis of the resulting micrographs using Fourier transform routines. We demonstrate the method for a germanium nanowire grown epitaxially on Si(111) by obtaining the strain components epsilon(xx), epsilon(yy), epsilon(xy), the mean dilatation, and the rotation of the lattice planes. The resulting strain maps are demonstrated to allow detailed evaluation of the strains and loading on nanowires.

The Gerchberg-Papoulis (GP) iterative algorithm is applied to electron holographic reconstruction to supplement the Fourier-transform method. The GP algorithm can recover the phase to high accuracy using simulated holograms. This method is useful when a region of known amplitude is present, such as a region of vacuum or uniform thickness. Modulations of the phase on the order of 0.1 rad can be corrected significantly using this algorithm.

Specimen charging may be one of the most significant factors that contribute to the high variability and generally low quality of images in cryo-electron microscopy. Understanding the nature of specimen charging can help in devising methods to reduce or even avoid its effects and thus improve the rate of data collection as well as the quality of the data. We describe a series of experiments that help to characterize the charging phenomenon, which has been termed the Berriman effect. The pattern of buildup and disappearance of the charge pattern has led to several suggestions for how to alleviate the effect. Experiments are described that demonstrate the feasibility of such charge mitigation.

During in situ transmission electron microscopy (TEM) field emission experiments, carbon nanotubes are observed to strongly diffract the imaging TEM electron beam. We demonstrate that this effect is identical to that of a standard electrostatic biprism. We also demonstrate that the nanotube biprism can be used to capture electron-holographic information.

Silicon wafers with a complex but known dopant profile were used to explore possible methods for improving the reliability of off-axis electron holography for quantitative determination of electrostatic potential profiles in doped semiconductor devices. The variability of results from nominally identical structures was attributed to local charging and associated external fields, forcing the development of a more robust approach to hologram analysis that incorporated an additional phase correction factor rather than rely on vacuum for phase flattening. Consistent results in close agreement with simulated profiles based on measured dopant distributions could then be obtained. Carbon coating was shown to be effective in reducing accumulation of charge caused by emission of secondary electrons. Overall, this work demonstrates that reliable potential profiles from unbiased samples should be obtainable on a routine basis provided that regions suitable for flattening of the phase profile can be identified.

In both human and rat tissues, complex patterns of transcripts are derived from the genes that encode the gamma-aminobutyric acid (GABA)(A) receptor epsilon subunit. An epsilon subunit transcript (approximately 3.6 kb) is expressed at relatively high levels in regions of the human brain and heart, but is not detected in most other major tissues. The encoded human epsilon subunit (epsilon (h)) confers distinctive properties to receptors into which it assembles. A distinct transcript of the gene (6.2 kb) is expressed abundantly in a variety of human tissues. This alternative transcript (ET2) appears to originate from within the epsilon subunit gene. It is possible that this transcript encodes a truncated subunit (epsilon (hS)), containing all of the transmembrane and intracellular domains. However, a combination of biochemical and electrophysiological analyses does not support this hypothesis. A distinct transcript of the epsilon subunit gene, encoding a large extracellular pro/glx domain, is expressed abundantly in rat and mouse brain. Functional analyses also failed to provide evidence for incorporation of this subunit (epsilon (rL)) into recombinant receptors. However, a shorter rat epsilon subunit (epsilon (r)), which lacks the pro/glx domain, conferred epsilon (h)-like properties to recombinant receptors, providing evidence for a functional rat epsilon subunit. In common with its human orthologue, incorporation of the epsilon (r) subunit into recombinant GABA(A) receptors confers several distinctive properties, including a reduced modulation by the anesthetic propofol and the appearance of spontaneous current.

Quantitative two-dimensional maps of electrostatic potential in device structures are obtained using off-axis electron holography with a spatial resolution of 6 nm and a sensitivity of 0.17 V. Estimates of junction depth and variation in electrostatic potential obtained by electron holography, process simulation, and secondary ion mass spectroscopy show close agreement. Measurement artifacts due to sample charging and surface "dead layers" do not need to be considered provided that proper care is taken with sample preparation. The results demonstrate that electron holography could become an effective method for quantitative 2D analysis of dopant diffusion in deep-submicron devices.

Electron holography performed in situ inside a high resolution transmission electron microscope has been used to determine the magnitude and spatial distribution of the electric field surrounding individual field-emitting carbon nanotubes. The electric field (and hence the associated field emission current) is concentrated precisely at the tips of the nanotubes and not at other nanotube defects such as sidewall imperfections. The electric field magnitude and distribution are stable in time, even in cases where the nanotube field emission current exhibits extensive temporal fluctuations.

Nanocrystals of magnetite (Fe(3)O(4)) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars.

Growth reactions based on a newly developed deuterium-stabilized Sn hydride [(Ph)SnD(3)] with Ge(2)H(6) produce a new family of Ge-Sn semiconductors with tunable band gaps and potential applications in high-speed, high-efficiency infrared optoelectronics. Metastable diamond-cubic films of Ge(1-x)Sn(x) alloys are created by chemical vapor deposition at 350 degrees C on Si(100). These exhibit unprecedented thermal stability and superior crystallinity despite the 17% lattice mismatch between the constituent materials. The composition, crystal structure, electronic structure, and optical properties of these materials are characterized by Rutherford backscattering, high-resolution electron microscopy, and X-ray diffraction, as well as Raman, IR, and spectroscopic ellipsometry. Electron diffraction reveals monocrystalline and perfectly epitaxial layers with lattice constants intermediate between those of Ge and alpha-Sn. X-ray diffraction in the theta-2theta mode shows well-defined peaks corresponding to random alloys, and in-plane rocking scans of the (004) reflection confirm a tightly aligned spread of the crystal mosaics. RBS ion-channeling including angular scans confirm that Sn occupies substitutional lattice sites and also provide evidence of local ordering of the elements with increasing Sn concentration. The Raman spectra show bands corresponding to Ge-Ge and Sn-Ge vibrations with frequencies consistent with random tetrahedral alloys. Resonance Raman and ellipsometry spectra indicate a band-gap reduction relative to Ge. The IR transmission spectra suggest that the band gap decreases monotonically with increasing Sn fraction. The synthesis, characterization, and gas-phase electron diffraction structure of (Ph)SnD(3) are also reported.