Nanobeam Electron Diffraction Research Articles

Oxide-confined GaAs-based vertical-cavity surface-emitting laser: Measurement and modeling of the strain field

The strain field of the emitters of one unstressed and two electrically stressed oxide-confined GaAs-based vertical-cavity surface-emitting lasers has been analyzed. The components of the strain tensor in the optical aperture, in the quantum wells, and at the oxide edge of the lamellas have been measured by nano-beam precession electron diffraction and geometrical phase analysis. The measurements have been used to validate the simulations based on the finite element method (FEM) of the mechanical behavior of the emitters. The FEM model is used to quantify the strain caused by the lattice mismatch of the GaAs-based epitaxial layers and the volume variation of the Al 98Ga 2As layer due to selective oxidation. The first is calculated as a function of the misfit strain and corresponds very well to the measured values. The latter is calibrated assuming that the thickness of the oxide is expanded by 4 % in both the unstressed sample and the stressed sample with dislocations. Instead, it is contracted by 8 % for the sample where the stress test has caused further oxidation.

On the trigonal structure of the ζ-hydride in zirconium: A microstructural characterization approach by electron diffraction and energy-loss spectroscopy

The zirconium (Zr) ζ-hydride phase was first proposed in 2008. While electron diffraction experiments characterized a trigonal crystal structure for this phase, formation-energy calculations have predicted that trigonal is not a stable structure for that. In this work, we attempt to tackle this discrepancy between experimental and computational studies.For this purpose, nano-beam electron diffraction (NBED) and electron energy-loss spectroscopy (EELS) were utilized to characterize the crystal structure of nano-hydrides in annealed Zircaloy-2.Diffraction patterns (DPs) collected from multiple axes of precipitates (and identical to those reported for the ζ-hydride in previous works) are shown to be composed of typical α-Zr and δ-hydride reflections along with additional reflections that did not belong to either phase. It is shown that precipitates were δ-hydride and extra reflections did not originate from a trigonal structure. Instead, it is proposed that they originated from either δ-hydride and a thin surface (probably Zr-oxide) phase or double-diffraction scattering between the α-Zr and δ-hydride.Previous works reported a plasmon energy (PE) value of 17.4 ± 0.1 eV for the ζ-phase. PE maps in this work confirmed the δ-nature of the examined nano-hydrides and revealed that the measured 17.4 eV PE value belonged to interfacial ribbons as a normal interface effect, instead of a ζ-phase.Hence, as predicted by computations, it is concluded that the trigonal structure appears to not be the appropriate crystal structure for nano-hydrides and reported observations of the ζ-hydride in the past could, in fact, have been δ-phase that was misidentified.

Analysis of Mechanical Strain in AlGaN/GaN HFETs

Herein, the influence of mechanical strain induced by passivation layers on the electrical performance of AlGaN/GaN heterostructure field‐effect transistor is investigated. We studied the physical mechanism of a threshold voltage (Vth) shift for the monolithically fabricated on/off devices reported earlier by our group. For that, theoretical calculations, simulation‐based analysis, and nano‐beam electron diffraction (NBED) measurements based on STEM are used. Strain distribution in the gate vicinity of transistors is compared for a SiNx passivation layer with intrinsic stress from ≈0.5 to −1 GPa for normally on and normally off devices, respectively. The strain in epitaxial layers transferred by intrinsic stress of SiNx is quantitatively evaluated using NEBD method. Strain dissimilarity Δε = 0.23% is detected between normally on and normally off devices. Using this method, quantitative correlation between 1.13 V of Vth shift and microscopic strain difference in the epitaxial layers caused by 1.5 GPa intrinsic stress variation in passivation layer is provided. It is showed in this correlation that about half of the reported threshold voltage shift is induced by strain, i.e., by the piezoelectric effect. The rest of Vth shift is caused by the fabrication process. Therefore, various components/mechanisms contributing to the measured Vth shift are distinguished.

Phase formation in the Ni-enriched zone below the surface oxide on NiTi

Upon exposure to oxygen, NiTi forms a Ti-rich surface oxide layer, and mass balance requires that a Ni-enriched zone forms below the oxide. Both the surface oxide layer and the Ni-enriched zone are discussed to affect key properties of NiTi as material for minimally invasive medical application or for actuators, e.g., the release of Ni and the resistance to initiation and propagation of cracks when exploiting the shape memory effect/pseudoelasticity. However, owing to the small extension of the Ni-enriched zone of a few nanometers, little is known about its crystallinity, phase and evolution of composition.We study the formation of the surface layers during annealing at ∼500 °C for up to 10 min, as routinely applied for shape-setting. With an approach employing nano beam electron diffraction in a transmission electron microscope and an in-house software package for phase analysis it is shown for the first time that phase transformations in the Ni-enriched zone, specifically from NiTi to Ni4Ti3, occur already after 2 min of annealing, much earlier than documented in the literature. Furthermore, the phase transformation to the thermodynamically stable Ni3Ti in the Ni-enriched zone is shown to be mediated by metastable Ni4Ti3. In contrast to Ni4Ti3 precipitates in NiTi bulk, the morphology of the crystalline phases is granular. Considering the swift formation of Ni-rich phases below the surface oxide and the observation of a band-like diffraction contrast in the TEM images originating from crystal interfaces, a phase transformation mechanism in the Ni-enriched zone is suggested consisting of short range order rearrangements of atoms.

Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO2.

Nanoscale polycrystalline thin-film heterostructures are central to microelectronics, for example, metals used as interconnects and high-K oxides used in dynamic random-access memories (DRAMs). The polycrystalline microstructure and overall functional response therein are often dominated by the underlying substrate or layer, which, however, is poorly understood due to the difficulty of characterizing microstructural correlations at a statistically meaningful scale. Here, an automated, high-throughput method, based on the nanobeam electron diffraction technique, is introduced to investigate orientational relations and correlations between crystallinity of materials in polycrystalline heterostructures over a length scale of microns, containing several hundred individual grains. This technique is employed to perform an atomic-scale investigation of the prevalent near-coincident site epitaxy in nanocrystalline ZrO2 heterostructures, the workhorse system in DRAM technology. The power of this analysis is demonstrated by answering a puzzling question: why does polycrystalline ZrO2 transform dramatically from being antiferroelectric on polycrystalline TiN/Si to ferroelectric on amorphous SiO2/Si?

Nanobeam electron diffraction strain mapping in monocrystalline silicon of modern trench power MOSFETs

Despite a decisive influence of residual strains in Si on its electronic properties and mechanical stability, strain distributions in vertical power transistors are still not fully investigated and understood. In this work, transmission electron microscopy (TEM) nanobeam electron diffraction (NBED) and finite element (FE) modeling were applied to reveal cross-sectional residual strain distributions in monocrystalline Si of a modern trench power MOSFET. Scanning NBED was realized in a region of interest with lateral and vertical dimensions of 0.69 μm and 1.38 μm, respectively, with a spatial resolution of 5.4 nm and a high precision better than 0.05%. The acquired results are interpreted in terms of device structure as well as process flow and are compared with the FE simulation. Complex strain distributions for the vertical, lateral and shear strain components are shown, which are not only dependent on structure and geometry but also influenced by the doping profile in Si. The comparison with the FE strain simulation shows good agreement especially for the vertical and shear strain components. However, it also reveals a limitation of the FE strain simulation with regard to the influence of doping on the Si lattice strain.

Mapping Polar Distortions with Nanobeam Electron Diffraction with a Cepstral Approach

Mapping Polar Distortions with Nanobeam Electron Diffraction with a Cepstral Approach

Investigation of Graphite Electrode Degradation in Lithium-Ion Cells Using 4D-STEM

The development of longer lasting and faster charging lithium-ion batteries (LIBs) remains a critical engineering priority for the U.S. Department of Energy. To enable the next-generation technology, it is essential to understand the nature and extent of materials failure during battery operation. Under fast-charging conditions, it has been shown that graphite particles, which comprise the active material of the LIB anode, suffer irreversible material degradation that limits ultimate battery performance. This damage to the graphite manifests as turbostratic lattice disorder, severe morphological roughening, and anode swelling [1]. While these findings are essential to understand the nature of damage to the anode under fast-charge conditions, it remains to be seen whether such damage occurs at slower rates or is instead accelerated by extreme rate conditions and aging. In this regard, the recent advances in high-resolution electron microscopy (HREM) and four-dimensional scanning transmission electron microscopy (4D-STEM) methods provide the essential tools to enable nanoscopic quantification of battery degradation [2], [3].In this work, we use advanced analytical electron microscopy and diffraction strategies to analyze nanoscopic degradation of graphite anode material under different cycling rate and aging conditions. The results of this work will demonstrate novel strategies for identifying nanoscale structural disorder that arise in battery materials as well as provide key understanding of graphite material failure to aid in the development of high-performance anode materials. We study (in post-mortem) the graphite particle cross-sections harvested from LIBs at different aging conditions for both C/2 (i.e., 2 hours to full charge) and 6C (i.e., 10 minutes to full charge) rate. At the nano- to atomic-scale, we rely primarily on aberration-corrected STEM for imaging and diffraction acquisition for each sample (see Figure 1 for the results). High resolution imaging reveals the extent of lattice disorder and amorphous character localized at graphite pore edges. The 4D-STEM data helps to further quantify the extent and distribution of nanoscale structural disorder (including permanent strain and turbostratic disorder). The experimental methods employed to acquire the datasets and the strategies employed to interpret the data to understand the nature of the resulting damage, including recent progress into Cepstral STEM, will be discussed. Additional supporting evidences of graphite disorder from Raman spectroscopy studies and rate-dependent anode swelling from scanning electron microscopy will also be presented. References : [1] S. Pidaparthy, M.-T. F. Rodrigues, J.-M. Zuo, and D. P. Abraham, “Increased Disorder at Graphite Particle Edges Revealed by Multi-length Scale Characterization of Anodes from Fast-Charged Lithium-Ion Cells,” J. Electrochem. Soc., vol. 168, no. 10, p. 100509, 2021.[2] J.-M. Zuo et al., “Data-Driven Electron Microscopy: Electron Diffraction Imaging of Materials Structural Properties,” arXiv Prepr. arXiv2110.02070, 2021.[3] E. Padgett et al., “The exit-wave power-cepstrum transform for scanning nanobeam electron diffraction: robust strain mapping at subnanometer resolution and subpicometer precision,” Ultramicroscopy, vol. 214, p. 112994, 2020. Acknowledgements: SP acknowledges support from the U. S. Department of Energy Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the U. S. Department of Energy under contract number DE‐SC0014664. This work was carried out in part in the Materials Research Laboratory Central Research Facilities, University of Illinois. DA and MTFR acknowledge support from DOE’s Vehicle Technologies Office. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1

Nanoscale Stress Localization Effects on the Radiation Susceptibility of GaN High‐Mobility Transistors

Nanoscale localized mechanical stress fields develop unavoidably in microelectronic devices due to structural and processing aspects. Their global average is too small to influence bandgap or mobility, but it is proposed that stress localization can influence defect nucleation sites under radiation. This is investigated on gallium nitride high‐electron‐mobility transistors (GaN HEMTs). Using transmission electron microscopy, we spatially resolved the stress field in the AlGaN layer for both pristine and 10 Mrad gamma‐irradiated HEMTs. The quantitative nanobeam electron diffraction and geometric phase analysis indicate that tensile stressed localizations experience higher radiation‐induced strain. This finding is explained by the tensile stress dependence of the carrier concentration and mobility in the AlGaN layer. Since gamma radiation damage is inflicted by high‐energy electrons only, localized regions of higher tensile stress in the AlGaN layer are expected to be more susceptible to gamma rays.

Deformation-induced medium-range order changes in bulk metallic glasses

Bulk metallic glasses (BMGs) naturally have excellent strength and elasticity while structural rejuvenation into higher energy glassy states is often required to improve ductility. However, our understanding of the detailed atomic ordering changes that occur during rejuvenation processes, such as plastic deformation, remains limited. This study utilizes nanobeam electron diffraction in a transmission electron microscope as an effective method to reveal the structural changes that occur after deformation in two Zr-based BMGs. Our findings indicate that heavy deformation from indentation or fracture causes an increase in the size of fcc-like medium-range order (MRO) clusters in a harder icosahedral dominated matrix, which corresponds to local softening of the BMGs. By examining the structure evolution at different points in the fracture process, we reveal that the mechanism of growth of MRO clusters is likely driven by enhanced diffusion from local temperature rise and/or free volume generation rather than deformation-induced nucleation and growth of new MRO sites.

Quantitative Strain and Topography Mapping of 2D Materials Using Nanobeam Electron Diffraction.

It is known that 2D materials can exhibit a nonflat topography, which gives rise to an inherent strain. Since local curvature and strain influence mechanical, optical, and electrical properties, but are often difficult to distinguish from each other, a robust measurement technique is needed. In this study, a novel method is introduced, which is capable of obtaining quantitative strain and topography information of 2D materials with nanometer resolution. Relying on scanning nanobeam electron diffraction (NBED), it is possible to disentangle strain from the local sample slope. Using the positions of the diffraction spots of a specimen at two different tilts to reconstruct the locations and orientations of the reciprocal lattice rods, the local strain and slope can be simultaneously retrieved. We demonstrate the differences to strain measurements from a single NBED map in theory, simulation, and experiment. MoS2 monolayers with different shapes are used as simulation test structures. The slope and height information are recovered, as well as tensile and angular strain which have an absolute difference of less than 0.2% and 0.2° from the theoretical values. An experimental proof of concept using a freely suspended WSe2 monolayer together with a discussion of the accuracy of the method is provided.

Free, flexible and fast: Orientation mapping using the multi-core and GPU-accelerated template matching capabilities in the Python-based open source 4D-STEM analysis toolbox Pyxem

This work presents the new template matching capabilities implemented in Pyxem, an open source Python library for analyzing four-dimensional scanning transmission electron microscopy (4D-STEM) data. Template matching is a brute force approach for deriving local crystal orientations. It works by comparing a library of simulated diffraction patterns to experimental patterns collected with nano-beam and precession electron diffraction (NBED and PED). This is a computationally demanding task, therefore the implementation combines efficiency and scalability by utilizing multiple CPU cores or a graphical processing unit (GPU). The code is built on top of the scientific Python ecosystem, and is designed to support custom and reproducible workflows that combine the image processing, template library generation, indexation and visualization all in one environment. The tools are agnostic to file size and format, which is significant in light of the increased adoption of pixelated detectors from different manufacturers. This paper details the implementation and validation of the method. The method is illustrated by calculating orientation maps of nanocrystalline materials and precipitates embedded in a crystalline matrix. The combination of speed and flexibility opens the door for automated parameter studies and real-time on-line orientation mapping inside the TEM.

Passivation effect on the surface characteristics and corrosion properties of yttrium oxide films undergoing SF6 plasma treatment

This study investigates the structures, compositions and fluorocarbon-plasma etching behaviors of yttrium oxyfluoride (YOF) passivation films fabricated on sputter-deposited yttrium oxide (Y2O3) by high-density SF6 plasma irradiation. High-resolution transmission electron microscopy and nano-beam electron diffraction confirmed a YOF passivation film containing multiple phases of (104) and (006) crystal planes was formed on the fluorinated Y2O3 surface. X-ray photoelectron spectroscopy revealed few changes in the chemical compositions and surface roughness of the YOF passivation film after fluorocarbon plasma etching, confirming the chemical stability of the SF6 plasma-treated Y2O3 sample. The etching depth was ∼20% lower on the SF6 plasma-treated Y2O3 film than on the commercial Y2O3 coating. These results showed that the SF6 plasma-treated Y2O3 films have an excellent erosion resistance properties compared to the commercial Y2O3 coatings.

Towards the interpretation of a shift of the central beam in nano-beam electron diffraction as a change in mean inner potential

The measurement of electric fields in scanning transmission electron microscopy (STEM) is a highly investigated field of research. The constant improvement of spatial resolution in STEM and the development of new hardware for the fast acquisition of diffraction patterns even paved the way for the measurement of atomic electric fields. Although the basic principle that an electric field leads to a tilt of the focussed electron probe that can be detected as a shift of the diffraction pattern in the back focal plane of the objective lens seems quite simple, many challenges arose in the measurement of fields in a quantitative way. In the present study we investigate whether a shift of the diffraction pattern that occurs at an interface between two materials can be related to the electric field which is caused by the difference of the mean inner potentials of the two materials. To this end, experiments and simulations are compared. It is demonstrated that the difference in mean inner potential has an influence on the observed effect, but a quantitative interpretation is difficult. The influence of image recording effects such as shot noise and the modulation transfer function are investigated as well as further effects such as e.g. sample tilt. In addition, the influence of the observed effect on a strain measurement is shown.

Chirality distribution of single-walled carbon nanotubes grown from gold nanoparticles

Au nanoparticles with diameters about 1.4 nm were dispersed onto porous Si3N4 grids and applied for growing single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition. Particularly, SWCNTs with diameters in the range of 0.75–1.8 nm were produced at a reaction temperature of 700 °C using CO as the carbon precursor. Nanobeam electron diffraction characterizations revealed that most of the SWCNTs have large chiral angles and zigzag SWCNTs were hardly detected. Density functional theory calculations showed that the SWCNT-Au interface formation energy and the SWCNT growth kinetics account for the enrichment of large chiral angle SWCNTs in the product.

Direct Transition from Ultrathin Orthorhombic Dinickel Silicides to Epitaxial Nickel Disilicide Revealed by In Situ Synthesis and Analysis.

Understanding phase transitions of ultrathin metal silicides is crucial for the development of nanoscale silicon devices. Here, the phase transition of ultrathin (3.6nm) Ni silicides on Si(100) substrates is investigated using an in situ synthesis and characterization approach, supplemented with ex situ transmission electron microscopy and nano-beam electron diffraction. First, an ultrathin epitaxial layer and ordered structures at the interface are observed upon room-temperature deposition. At 290 °C, this structure is followed by formation of an orthorhombic δ-Ni2 Si phase exhibiting long-range order and extending to the whole film thickness. An unprecedented direct transition from this δ-Ni2 Si phase to the final NiSi2- x phase is observed at 290 °C, skipping the intermediate monosilicide phase. Additionally, the NiSi2- x phase is found epitaxial on the substrate. This transition process substantially differs from observations for thicker films. Furthermore, considering previous studies, the long-range ordered orthorhombic δ-Ni2 Si phase is suggested to occur regardless of the initial Ni thickness. The thickness of this ordered δ-Ni2 Si layer is, however, limited due to the competition of different orientations of the δ-Ni2 Si crystal. Whether the formed δ-Ni2 Si layer consumes all deposited nickel is expected to determine whether the monosilicide phase appears before the transition to the final NiSi2- x phase.

Precise strain mapping of nano-twinned axial ZnTe/CdTe hetero-nanowires by scanning nanobeam electron diffraction

The occurrence of strain is inevitable for the growth of lattice mismatched heterostructures. It affects greatly the mechanical, electrical and optical properties of nano-objects. It is also the case for nanowires which are characterized by a high surface to volume ratio. Thus, the knowledge of the strain distribution in nano-objects is critically important for their implementation into devices. This paper presents an experimental data for II-VI semiconductor system. Scanning nanobeam electron diffraction strain mapping technique for hetero-nanowires characterized by a large lattice mismatch (>6% in the case of CdTe/ZnTe) and containing segments with nano-twins has been described. The spatial resolution of about 2 nm is 10 times better than obtained in synchrotron nanobeam systems. The proposed approach allows us to overcome the difficulties related to nanowire thickness variations during the acquisition of the nano-beam electron diffraction data. In addition, the choice of optimal parameters used for the acquisition of nano-beam diffraction data for strain mapping has been discussed. The knowledge of the strain distribution enables, in our particular case, the improvement of the growth model of extremely strained axial nanowires synthetized by vapor-liquid solid growth mechanism. However, our method can be applied for the strain mapping in nanowire heterostructures grown by any other method.

Atomic-scale polar vortices in Na0.5Bi0.5TiO3 grains

Ferroelectric materials have great potential applications in non-volatile ferroelectric random access memories because of the singularity of one-dimensional topological defects. In this paper, we have found for the first time the atomic-level dipole rotations of vortex structures in crystalline materials. By the aberration-corrected high resolution scanning transmission electron microscopy (STEM), the atomic-scale multiple rotation domains and polar vortices in Na0.5Bi0.5TiO3 (NBT) grains have been clearly observed in lead-free perovskite NBT. Meanwhile, nanobeam electron diffraction experiments provided the strong evidences of the coexistence of tetragonal, orthorhombic and rhombohedra phases at room temperature. These polar vortices are constructed by the competition of these atomic-scale polar domains, which are originated from the multiphase coexistence of NBT and can be well confirmed by our theoretical calculations. These observations have put forward new implications for the creation of vortex topologies in crystalline materials without sophisticated thin-film technologies, and it can also help to understand the complicated polar mechanism in lead-free perovskite ferroelectric materials.

High Resolution Powder Electron Diffraction in Scanning Electron Microscopy.

A modern scanning electron microscope equipped with a pixelated detector of transmitted electrons can record a four-dimensional (4D) dataset containing a two-dimensional (2D) array of 2D nanobeam electron diffraction patterns; this is known as a four-dimensional scanning transmission electron microscopy (4D-STEM). In this work, we introduce a new version of our method called 4D-STEM/PNBD (powder nanobeam diffraction), which yields high-resolution powder diffractograms, whose quality is fully comparable to standard TEM/SAED (selected-area electron diffraction) patterns. Our method converts a complex 4D-STEM dataset measured on a nanocrystalline material to a single 2D powder electron diffractogram, which is easy to process with standard software. The original version of 4D-STEM/PNBD method, which suffered from low resolution, was improved in three important areas: (i) an optimized data collection protocol enables the experimental determination of the point spread function (PSF) of the primary electron beam, (ii) an improved data processing combines an entropy-based filtering of the whole dataset with a PSF-deconvolution of the individual 2D diffractograms and (iii) completely re-written software automates all calculations and requires just a minimal user input. The new method was applied to Au, TbF3 and TiO2 nanocrystals and the resolution of the 4D-STEM/PNBD diffractograms was even slightly better than that of TEM/SAED.

Identifying the true structure and origin of the water-quench induced hydride phase in Zr-2.5Nb alloy

The crystal structure of the hydride induced by water quenching in hydrided Zr-2.5Nb pressure tube was identified by a combined approach of synchrotron x-ray diffraction and TEM nano-beam electron diffraction, coupled with density functional theory. It has been found that the hydrides experimentally characterized have the same face-centered tetragonal lattice structure as that of the γ-phase, however the hydrogen atoms did not show evidence of ordering. The quenched hydrides subsequently transformed into both ordered face-centered tetragonal (i.e., γ-phase) as well as δ-phase during a one-year room temperature aging, implying a metastable nature; therefore, we argue it is appropriate to identify these initial quenched hydrides as γ′ instead of the γ hydride the majority of literature has used in the past. The amount of γ′ phase is found to increase with the temperature at which the sample is quenched from, while the amount of δ phase changed inversely. In-situ heating experiments with synchrotron diffraction on samples with various prior heat treatments were also conducted; the γ′ phase was found to be present at temperatures up to 278˚C. Transformation of γ′ into δ hydride was found to occur upon heating, confirming the relative stability of the two phases. The origin of the γ′ phase is compatible with the newly confirmed Zr-H phase diagram where γ-hydride exists as the low temperature stable phase.