Weak-beam Dark-field Imaging Research Articles

On the true morphology of highly photoactive anatase TiO2 nanocrystals

Nanosized titania photocatalysts were prepared from a TiOSO4·2H2O–Na2CO3 mixture via high-energy milling for 5min, and subsequent annealing at a temperature range of 300–700°C/1h. Photoactivity of all samples monitored by electron paramagnetic resonance (EPR) spectroscopy was correlated with phase composition, crystallinity, crystal size and morphology of the annealed products. The anatase nanocrystals with an XRD average size of 24nm and well-faceted bipyramidal morphology exposed mainly by {101} type planes obtained on heating at 700°C, exhibited markedly higher photoactivity than the Aeroxide® P25 TiO2 standard and sample annealed at 600°C. The rapid increase in photoactivity was assigned to complete crystallization of the sample, optimal size, but mainly to the well-developed bipyramidal morphology of anatase nanocrystals and enlargement of the {101} type plane area due to final growth of bipyramids in the 〈001〉 direction.The weak beam dark field (WBDF) transmission electron microscopy (TEM) method was used to characterize morphology of the nanocrystals. Based on the thickness changes along the electron beam incidence inside the individual nanocrystals yielded by WBDF images, it was demonstrated that the mixture of polyhedral shaped anatase projections, such as square-like, hexagonal-like, rhomboid-like, rhombic-like, and rectangular-like prevailing in bright-field (BF) images, corresponds exclusively to tetragonal bipyramidal shape projected along different crystallographic directions. WBDF images of the most often occurring anatase nanocrystal projections are summarized and compared with both the schematic depictions of similarly oriented anatase bipyramids and that recorded in BF images. The difficulties of 3D morphology assessment from the projected nanocrystal shapes are highlighted. Attention is paid to the determination of facets limiting the nanocrystals, using the high resolution (HR)TEM method.

Systematic row and zone axis STEM defect image simulations

Transmission electron microscopy (TEM) has been instrumental in advancing the field of crystalline defect analysis. Conventional TEM imaging techniques, such as bright field (BF), dark field (DF), and weak beam dark field (WBDF or g–3g) imaging, have been well-documented in the scientific literature, with simulation methods readily available for each. The present contribution highlights the use of a field-emission TEM, operated in scanning transmission electron microscopy mode, as a viable tool for defect analysis. Common techniques such as two-beam diffraction contrast and zone axis imaging are applied to defect analysis; both experimental and computational results are presented. Effects of experimental parameters such as camera length, beam divergence angle, and diffraction aperture placement are also discussed and illustrated by both experimental and computed micrographs of stacking faults.

Three-dimensional visualization of dislocation-precipitate interactions in a Al–4Mg–0.3Sc alloy using weak-beam dark-field electron tomography

Abstract

Improvement in Junction Breakdown and GIDL using MFLA in DRAM Product

In this study, we successfully improved junction breakdown using millisecond flash anneal (MFLA) on a dynamic random access memory (DRAM) product. By replacing a rapid thermal annealing (RTA) process with MFLA after contact implantation, improvements in NMOS and PMOS junctions were observed and no degradation was found. Device simulation data show that the PMOS junction electric field (E-field) was reduced with MFLA and that MFLA has an advantage in junction profile tailoring. The behavior of gate-induced drain leakage (GIDL) at different process stages with different annealing temperatures was studied. The preheat and peak temperatures of MFLA were found to be correlated with GIDL improvement. The optimum condition in this study was 1200°C flash anneal with a 750°C preheat temperature. Bright field and weak-beam dark-field transmission electron microscopy images showed perfect dislocation loops and fault dislocation loops remaining in the {113} plane with a size of around 17 × 20 nm in the junction area.

On the Stacking Fault Energy of Fe-18 Pct Mn-0.6 Pct C-1.5 Pct Al Twinning-Induced Plasticity Steel

The stacking fault energy (SFE) of Fe-18 pct Mn-0.6 pct C-1.5 pct Al twinning-induced plasticity (TWIP) steel was measured using weak-beam dark-field imaging of dissociated dislocations observed in transmission electron microscopy. The SFE was found to be 30 mJ/m2. A relatively wide scatter was observed in the experimentally measured partial dislocation separation of screw dislocations. It is argued that the anomalously wide partial dislocation separation is due to the interaction of point-defect pairs involving interstitial C atoms with the strain field of the partial dislocations. Internal friction (IF) experiments were carried out to detect the presence of point-defect pairs that might affect the dislocation separation, and a clear Finkelshtein–Rosin (FR) peak related to point-defect pairs involving interstitial C atoms was observed in the IF spectrum.

Gate-Induced Drain Leakage (GIDL) Improvement for Millisecond Flash Anneal (MFLA) in DRAM Application

In this paper, we successfully demonstrated gate-induced drain leakage (GIDL) improvements by millisecond flash anneal (MFLA) on a DRAM product. Fundamental studies on blanket wafers and the device characteristics of product wafers showed positive results. These proved that MLFA has good potential for DRAM application. The periphery NMOS off current was reduced by 36%. This off-current improvement reduced the standby current by 5% in a DRAM functional test. The GIDL reductions for an NMOS array and periphery N- and PMOSs were 14.5%, 15%, and 39%, respectively. A model for GIDL improvement by MFLA application to DRAM production was proposed. It is believed that the main GIDL-impacted factor was the high electric field caused by defect-assisted tunneling and junction profile abruptness under the gate edge. Bright field and weak-beam dark-field TEM images showed perfect dislocation loops and fault dislocation loops staying in the {113} plane with a size of around 17 x 20 nm in the junction area. These defects could be one of the leakage sources for a defect-assisted trap charge, leading to trap-assisted tunneling.

Burgers vector determination in deformed perovskite and post-perovskite of CaIrO 3 using thickness fringes in weak-beam dark-field images

The thickness-fringe method [Ishida et al. , Philosophical Magazine 42 (1980) 453] for complete determination of the character of a dislocation Burgers vector has been performed in CaIrO 3 perovskite and post-perovskite deformed at high pressures and high temperatures. By selecting several main zone axes and determining the number of terminating thickness fringes at the extremity of a dislocation from a wedge-shaped thin-foil specimen in weak-beam dark-field transmission electron microscope (TEM) images, the Burgers vectors were unambiguously determined. The results demonstrate that [1 0 0] screw and edge dislocations on the (0 1 0) slip plane are dominant in the post-perovskite phase. Curved [1 0 0] and [0 1 0] dislocations and straight 〈1 1 0〉 screw dislocations on a potential (0 0 1) slip plane were identified in the perovskite phase as well as a high density of {1 1 0} twins. Low-angle tilt boundaries consisting of different groups of parallel edge dislocations on the {1 1 0} and (0 0 1) planes indicate diffusion-assisted climb in perovskite at high temperatures. The differences in dislocation microstructures could be due to activations of limited numbers of slip systems for post-perovskite and of a large number of multiple slip systems for perovskite, which may result in the strong crystallographic preferred orientation (CPO) in post-perovskite and the lack of CPO in deformed perovskite.

Dislocation tomography made easy: a reconstruction from ADF STEM images obtained using automated image shift correction

After previous work producing a successful 3D tomographic reconstruction of dislocations in GaN from conventional weak-beam dark-field (WBDF) images, we have reconstructed a cascade of dislocations in deformed and annealed silicon to a comparable standard using the more experimentally straightforward technique of STEM annular dark-field imaging (STEM ADF). In this mode, image contrast was much more consistent over the specimen tilt range than in conventional weak-beam dark-field imaging. Automatic acquisition software could thus restore the correct dislocation array to the field of view at each tilt angle, though manual focusing was still required. Reconstruction was carried out by sequential iterative reconstruction technique using FEI's Inspect3D software. Dislocations were distributed non-uniformly along cascades, with sparse areas between denser clumps in which individual dislocations of in-plane image width 24 nm could be distinguished in images and reconstruction. Denser areas showed more complicated stacking-fault contrast, hampering tomographic reconstruction. The general three-dimensional form of the denser areas was reproduced well, showing the dislocation array to be planar and not parallel to the foil surfaces.

Tilt generation in step-graded InxGa1−xAs metamorphic pseudosubstrate on a singular GaAs substrate using a low-temperature grown InGaP interlayer

Metamorphic pseudosubstrates of In0.42Ga0.58As were grown by molecular beam epitaxy using step-graded InxGa1−xAs buffer layers grown either directly on a (001) GaAs substrate or on a GaAs substrate overgrown with a layer of low-temperature grown In0.51Ga0.49P (LT-InGaP). The structures were examined using x-ray reciprocal space mapping to determine the characteristics of the pseudosubstrates and buffer layers. For the sample grown on the LT-InGaP layer, the pseudosubstrate exhibited an asymmetric tilt around [11¯0] toward the [110] direction. Weak-beam dark-field electron imaging shows an imbalance of misfit dislocations with opposite sign Burgers vector. An explanation for this tilt is given and it is suggested that it may be responsible for the improved quality of epitaxial layers grown on such pseudosubstrates.

The stability of screw dipoles in Ni 3Al studied by TEM

Transmission electron microscopy (TEM) methods were used to study the dislocation structures in L1 2 ordered binary Ni 3Al deformed at a temperature below and at a temperature above the onset of the anomalous increase of the yield stress (compression axis oriented for single slip). The corresponding change from a dislocation structure dominated by edge dipoles to a structure dominated by locked screw dipoles is caused by the transition from octahedral cross-slip to cube cross-slip. The measurements based on weak-beam dark-field TEM images show that stable configurations of screw dipoles occur above a minimum dipole height and within a certain range of dipole angles. In agreement with the calculations the stability region of the screw dipoles is limited since locked screws can become mobile in the vicinity of a screw of opposite sign. Remobilized screws can annihilate leading to the formation of antiphase boundary tubes or can pass each other generating glissile edge segments that contribute to plastic flow. The screw dipoles showing stable configurations are considered as strong locks and lead to an autocatalytic increase of their density with increasing deformation.

The Relationship between the Bending Direction and Burgers Vector of Threading Dislocations at a Coherent Heterointerface in AlGaN/AlN Layers

Transmission electron microscope analysis of microstructure in AlGaN/AlN thin layers was performed to investigate the behavior of threading dislocations (TDs). The absolute values of the Burgers vectors (B) of TDs were determined from the change in number of thickness fringes in the weak-beam dark-field images. In the AlGaN/AlN thin layers grown on α-Al 2 O 3 (0001) substrate, there were a lot of TDs propagating along [0001] with B=a, where a is the corresponding axis vector. These TDs were observed to bend at the hetero-interface of AlGaN/AlN. It was confirmed that the bending direction is normal to B and that the extra half plane associated with the TD decreases in area by the bending. Precise measurement of the in-plane lattice dimensions showed that there exists a compressive strain in AlGaN on the hetero-interface. These results revealed that control of internal stress is an important factor to govern the behavior of TDs.

Three-dimensional analysis of dislocation networks in GaN using weak-beam dark-field electron tomography

We have developed a new method of tomographically reconstructing extended three-dimensional dislocation networks using weak-beam dark-field (WBDF) imaging in a TEM. A series of WBDF images is recorded every few degrees over a large tilt range, while ensuring that the dark-field reflection used for imaging maintains a constant deviation parameter. With suitable filtering of the WBDF images prior to tomographic reconstruction, the three-dimensional distribution of dislocations is reproduced with high fidelity and high spatial resolution. The success of this approach is demonstrated for heteroepitaxial Mg-doped GaN films. The fidelity of the tomographic reconstruction varies with the dislocation line-vector and elastic anisotropy of the material.

Microstructure of FePt nanoparticles produced by nanocluster beam

FePt nanoparticles produced by condensation of sputtered atoms in a nanocluster beam source at different forming temperatures were investigated using transmission electron microscopy (TEM). At the temperature of 170 K, the particles were icosahedron and the mean particle size was 4 nm with a standard deviation of 16%. At 100 K polygonal shapes of particles with size 5–15 nm such as triangular, square, rhombic, pentagonal, hexagonal, octagonal, and irregular were observed. The three-dimensional (3-D) shapes of the 2-D TEM projections were derived from high resolution, bright field, dark field and weak beam dark field (WBDF) imaging and electron diffraction pattern. The polygonal shapes arose from octahedron, truncated octahedron, cubooctahedra, truncated tetrahedron, triangular platelet, Marks decahedron and icosahedron. The final growth structures did not necessarily have the lowest free energy, but was the one that corresponded to a local minimum governed by specific growth kinetics.

Weak-beam dark-field electron tomography of dislocations in GaN

By combining weak-beam dark-field imaging with tomography, we have been able to reconstruct the three-dimensional structure of dislocation arrays in GaN. With a mixture of both threading and in-plane dislocations, owing to plastic relaxation of the film, we look at how well each dislocation is reconstructed and what limits are imposed by way of dislocation density and material anisotropy.

Weak‐Beam Dark‐Field Microscopy of Complex Stress States in X7R‐Type BaTiO3 Dielectric Core–Shell Structures

X7R‐type BaTiO3 materials were analyzed using X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Powder XRD indicated that the materials had pseudocubic lattices, but core–shell grain structures predominated in bright‐field (BF) TEM images. Electron diffraction patterns across the core–shell boundaries and convergent beam electron diffraction patterns of cores and shells indicated that coherent grain boundaries existed between cores and shells. The flat dielectric constant–temperature curves obtained from these materials can be interpreted in terms of the internal stress states in individual grains. The stress states were observed using weak‐beam dark‐field (WBDF) microscopy, and strain contours formed by distorted crystal planes were visible in the WBDF images. The contours observed were dependent on the stress state of the crystal instead of crystal symmetry and the stress distribution in individual grains was determined by both the thickness ratio of shell and core, and the geometrical relationship of the core and the shell. Twins observed in this material were determined to be growth rather than mechanical twins, through observation of the strain contour distribution.

Electrical and Structural Characterization of a Low‐Angle Tilt Grain Boundary in Iron‐Doped Strontium Titanate

The atomistic structure and electrical properties of a symmetrical 5.4° [001] tilt grain boundary in Fe‐doped SrTiO3 have been investigated, respectively, by means of various transmission electron microscopy (TEM) techniques and impedance spectroscopy. In weak‐beam dark‐field images, the grain boundary is revealed to consist of a periodic array of dislocations; high‐resolution TEM images show that the dislocation cores are separated by regions of strained lattice. The impedance response of the bicrystal has been measured in the frequency range 20 Hz ≤f≤ 106 Hz as a function of temperature and oxygen partial pressure. The transport of charge across the array of dislocations that form the grain boundary is strongly hindered. Analysis of the impedance data in terms of a double‐Schottky‐barrier model yields a space‐charge potential that exhibits a weak dependence on temperature and oxygen partial pressure and is ∼0.55 V in the investigated regime.

The massive transformation in titanium aluminides: Initial stages of nucleation and growth

Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the α → γm massive transformation in titanium aluminides. Splat quenching Ti52Al48 and Ti50Al48Cr2 from the liquid at slightly below the melting point produces an equiaxed α solidification structure. Solid-state cooling rates that approach 106 K/s arrest the α → γm massive transformation with 1- to 5-µm-sized γm nuclei, especially in the Ti50Al48Cr2 alloy. Classical massive-transformation heterogenous nucleation occurs at α:α grain boundaries with an orientation relationship of [111]γ//[0001]α and \(\{ 110\} _\gamma //\{ 11\bar 20\} _\alpha \). The γm nucleus then grows into the adjacent α grain without the orientation relationship by forming an incoherent α:γ interface with {111}γ facets. Orthogonal variants of the tetragonal c-axis in the γm product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L10 phase. Nucleation of γm is not only observed at α:α grain boundaries and triple points, but also within the α grains. The intragranular γm nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular nuclei. However, the intragrain nuclei do not form {111}γ facets and retain a curved α:γm interface. Although analysis of the {111}γ faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations, lattice imaging of the {111}γ facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing (λ) to ledge height (h) for the {111}γ facets as λ/h=41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys.

Collinear partial dislocations in barium titanate perovskite

Dislocation substructures in CO 2-laser-sintered undoped BaTiO 3 have been investigated via transmission electron microscopy. Deformation of BaTiO 3 at 1390 °C occurs when the slip system of 〈100〉{001} is activated by the intrinsic thermodynamic driving force for sintering. Dissociation of dislocations with Burgers vector b 1=[001] into two collinear partials of b 2=1/2[001] and b 3=1/2[001] has been identified unambiguously using weak-beam dark-field imaging. Part of the b 3 partial further changes to a scallop-shape half-loop of b 3′=1/2[100] by a non-conservative loop formation mechanism of mixed climb and glide in the {111} plane. Results suggest that plastic flow by dislocation mechanisms contribute to the densification of BaTiO 3 at 1390 °C. It takes place by a climb-controlled glide mechanism, which is rate-determined kinetically by the lattice diffusion of Ba 2+.

TEM to Support Magnetic Media Development in YR2000

ABSTRACTTEM provides fast and statistical support for magnetic recording media development. Turn-around time including sample preparation, data collection and computer analysis is reduced to ∼3 hours. Rapid, systematic TEM analysis is shown to accurately measure crystallographic Orientation Ratio (OR) on mechanically textured media by diffraction analysis; elucidate grain separation by Weak Beam Dark Field Imaging; and determine the crystallographic relationship between cubic-NiAl seed layers and hexagonal cobalt-alloy (Co) layers. Novel evidence for reduction of the number of preferred orientations in sequentially grown epitaxial films is presented, which explains the “2-&-1/4-D isotropic” growth orientation and the high magnetic squareness of Co media grown on nearly randomly oriented “3D-isotropic” NiAl underlayers.

Nitrogen implantation in 4H and 6H–SiC

We have undertaken a physico-chemical and electrical characterisation of nitrogen ion implantations in SiC epitaxies using a large variety of experimental conditions. The samples were implanted either at room temperature (RT) or at 650°C (HT). SIMS measurements were done to compare the depth-distributions with results from TRIM92 simulations. Transmission electron microscope (TEM) analysis of these samples was carried out on cross-sectional samples using Weak Beam Dark Field imaging conditions. Van der Pauw test patterns were realised to extract the thermal dependencies of resistivity and mobility. A comparison with the conductivity and mobility obtained on films doped during epitaxy and implanted films is done. It is shown that, in the lower dose range, the ionisation and activation does not depend on the implantation temperature.