Convergent Beam Electron Diffraction Technique Research Articles

Development and applications ofultrafast transmission electron microscopy.

We present a review on the development and applications of the ultrafast transmission electron microscopy (UTEM) at RIKEN. We introduce the UTEM system for the pump-probe TEM observation in a wide temporal range. By combining the UTEM and pixelated detector, we further develop five-dimansional scanning TEM (5D STEM) which provides the ultrafast nanoscale movie of physical quantities in nano-materials, such as crystal lattice information and electromagnetic field, by convergent-beam electron diffraction (CBED) and differential phase contrast (DPC) imaging technique. We show our recent results on the nanosecond-to-microsecond magnetic skyrmion dynamics observed by Lorentz TEM (LTEM) and photo-induced acoustic wave generation in picosecond regime by bright-field TEM and electron diffraction measurements by UTEM. We also show the demonstartion of the 5D STEM on the quantitative time (t)-dependent strain mapping by CBED with an accuracy of 4 ps and 8 nm, and the ultrafast demagnetization under zero magnetic field observed by DPC with 10 ns and 400 nm resolution.

Sensitivity of quantitative symmetry measurement algorithms for convergent beam electron diffraction technique

We investigate the sensitivity of symmetry quantification algorithms based on the profile R-factor (Rp) and the normalized cross-correlation (NCC) coefficient (γ). A DM (Digital Micrograph©) script embedded in the Gatan digital microscopy software is used to develop the symmetry quantification program. Using the Bloch method, a variety of CBED patterns are simulated and used to investigate the sensitivity of symmetry quantification algorithms. The quantification results show that two symmetry quantification coefficients are significantly sensitive to structural changes even for small strain values of < 1%.

Propagation of threading dislocations and effects of Burgers vectors in HVPE-grown GaN bulk crystals on Na-flux-grown GaN substrates

The propagation behavior of threading dislocations (TDs) and the effects of Burgers vectors in hydride vapor phase epitaxy (HVPE) GaN bulk crystals generated on Na-flux-grown GaN and in a commercially available HVPE-grown GaN bulk crystal were investigated. Analyses based on chemical etching and transmission electron microscopy (TEM) revealed a close correlation between the etch pit sizes and the Burgers vector of these TDs. The existence of TDs with the unique Burgers vector b = 1m + 1c was observed for the first time ever using a large-angle convergent-beam electron diffraction technique and plan-view bright-field scanning TEM. Multi-photon excitation photoluminescence microscopy observations showed that TDs with b = 1c had a meandering morphology in contrast to the linear morphology of TDs with b = 1a, 1a + 1c, or 1m + 1c in both types of HVPE-grown GaN crystals. The inclinations of TDs with b = 1a and 1a + 1c in HVPE-grown GaN on Na-flux-grown GaN were greatly affected by large symmetrical hexagonal hillocks. The TDs with b = 1a were inclined in the slope directions of the hillock planes, while those with b = 1a + 1c were inclined in the a directions parallel or antiparallel to the a component in their Burgers vector. These inclinations were readily explained by the isotropic elastic theory of an individual TD. The hillocks were produced around pairs of parallel mixed TDs for which the a components were opposite to one another. This phenomenon was attributed to inclusions at the interface between the Na-flux-grown GaN substrate and the HVPE-grown layer. The origins of other TDs including unusual ones having b = 1m + 1c are also discussed herein.

A new electron diffraction approach for structure refinement applied to Ca3Mn2O7.

The digital large-angle convergent-beam electron diffraction (D-LACBED) technique is applied to Ca3Mn2O7 for a range of temperatures. Bloch-wave simulations are used to examine the effects that changes in different parameters have on the intensity in D-LACBED patterns, and atomic coordinates, thermal atomic displacement parameters and apparent occupancy are refined to achieve a good fit between simulation and experiment. The sensitivity of the technique to subtle changes in structure is demonstrated. Refined structures are in good agreement with previous determinations of Ca3Mn2O7 and show the decay of anti-phase oxygen octahedral tilts perpendicular to the c axis of the A21am unit cell with increasing temperature, as well as the robustness of oxygen octahedral tilts about the c axis up to ∼400°C. The technique samples only the zero-order Laue zone and is therefore insensitive to atom displacements along the electron-beam direction. For this reason it is not possible to distinguish between in-phase and anti-phase oxygen octahedral tilting about the c axis using the [110] data collected in this study.

Structural and Li-ion diffusion properties of lithium tantalum phosphate LiTa2PO8

We synthesized a polycrystalline LiTa2PO8 sample at 1323 K by a solid-state reaction. The crystal structure of LiTa2PO8 was examined by selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) techniques. Electrochemical impedance measurements showed that the total conductivity at room temperature was 4.85 × 10−4 S cm−1 and the activation energy was 0.52 eV. The high Li-ion conductive properties were confirmed by 7Li pulse-field gradient (PFG) or pulse-gradient spin-echo (PGSE) NMR spectroscopy. The activation energy of the Li diffusion (0.51 eV) measured by PFG NMR was well consistent with the result of impedance measurement. We believe that the LiTa2PO8 would be as one of the important candidates for the solid electrolyte used in all solid-state lithium batteries.

Evolution of domain structure in PbZr0.52Ti0.48O3 thin film by adding dysprosium

In this study, we improve the thermal stability of PbZr0.52Ti0.48O3 thin film by Dy doping (Dy = 0–4 at.%). The PZT solution is prepared from lead acetate trihydrate, zirconium n-propoxide, and titanium (IV) isopropoxide to have the composition in the morphotropic phase boundary of PbZr(1-x)TixO3 (x = 0.48 at.%); using a spin coater, the solution is then deposited onto a Pt coated Si substrate. The polarization-electric field hysteresis loop shows that the polarization density (μC/cm2) of Dy-doped PZT thin films are enhanced as the amount of Dy increases. Local symmetry study using convergent beam electron diffraction technique then reveals that the Dy doping induces phase transition from P4mm-like to Pm-like symmetry, which has not been found in PZT.

Two-dimensional Gaussian fitting for precise measurement of lattice constant deviation from a selected-area diffraction map.

Unlike X-ray diffraction or Raman techniques, which suffer from low spatial resolution, transmission electron microscopy can be used to obtain strain maps of nanoscaled materials and devices. Convergent-beam electron diffraction (CBED) and nanobeam electron diffraction (NBED) techniques detect the deviation of a lattice constant (i.e. an indicator of strain) within 0.01%; however, their use is restricted to beam-insensitive samples. Selected-area electron diffraction (SAED) does not have such limitations but has low spatial resolution and precision. The use of a spherical aberration corrector and a nanosized selected-area aperture improves the spatial resolution, but the precision is still low. In this study, a two-dimensional stage-scanning system is used to acquire arrays of diffraction patterns at different positions of the sample under fixed beam conditions. Data processing with iterative nonlinear least-squares fitting enabled the spot displacement for each point of the scan area to be measured with precision comparable to that of the CBED or NBED technique. The precise strain determination, in combination with the simplicity of the measurement process, makes the nanosized SAED technique competitive with other methods for strain mapping at nanoscale dimensions.

Nanoscale symmetry fluctuations in ferroelectric barium titanate, BaTiO3.

Crystal charge density is a ground-state electronic property. In ferroelectrics, charge is strongly influenced by lattice and vice versa, leading to a range of interesting temperature-dependent physical properties. However, experimental determination of charge in ferroelectrics is challenging because of the formation of ferroelectric domains. Demonstrated here is the scanning convergent-beam electron diffraction (SCBED) technique that can be simultaneously used for imaging ferroelectric domains and identifying crystal symmetry and its fluctuations. Results from SCBED confirm the acentric tetragonal, orthorhombic and rhombohedral symmetry for the ferroelectric phases of BaTiO3. However, the symmetry is not homogeneous; regions of a few tens of nanometres retaining almost perfect symmetry are interspersed in regions of lower symmetry. While the observed highest symmetry is consistent with the displacive model of ferroelectric phase transitions in BaTiO3, the observed nanoscale symmetry fluctuations are consistent with the predictions of the order-disorder phase-transition mechanism.

Quantitative convergent-beam electron diffraction and quantum crystallography—the metallic bond in aluminium

The technique of quantitative convergent-beam electron diffraction (QCBED) is reviewed as a method for making very accurate and precise measurements of low-order bonding-sensitive structure factors. As such, it is a technique with the potential to make significant contributions to the field of quantum crystallography. To demonstrate this, the application of QCBED in determining the nature of the metallic bond in aluminium is examined in detail. The importance of precision in structure factor measurement when it comes to determining bonding electron distributions, especially in metals, becomes obvious from this examination. Uncertainties as low as ±0.1% are routinely attainable by QCBED and are shown to be important in reliably locating bonding charge. In the case of aluminium, a nearly free electron gas metal where the bonding electron distribution is subtler than in most other materials, the bonds are entirely tetrahedrally centred. This is a conclusion that could not have been reached experimentally without the levels of precision offered by QCBED.

The low-loss EELS spectra from radiation damaged gold nanoparticles

The advantage of the EELS spectra in transmission electron microscopy is the ability to obtain plasmon information of the structure of individual nanoparticles. The EELS analyses of the plasmonic information of gold nanoparticles were obtained before and after irradiation by the convergent beam electron diffraction (CBED) technique. The results indicate that before irradiating the gold nanoparticles (35 nm in diameter) by the CBED technique, only the EELS spectrum of carbon is obtained. Nevertheless, the EELS spectra of gold were obtained after CBED irradiation of the nanoparticles. The spectrum of gold shows five plasmonic modes at 15.9, 24.8, 32.8, 45.4, and 63.2 eV, but theoretical surface and bulk plasmons of gold are at 6.378 and 9.020, respectively. When compared with the experimental plasmon peaks, we determined that peaks at energies above 9.026 eV are multiples of the surface and bulk (valence) plasmons of the gold nanoparticles.

Microstructural and Compositional Characterisation of Electronic Materials

&lt;p class="p1"&gt; &lt;/p&gt;&lt;p class="p2"&gt;&lt;span class="s1"&gt; &lt;/span&gt;Microstructural and compositional characterisation of electronic materials in support of the development of GaAs, GaN, and GaSb based multilayer device structures is described. Electron microscopy techniques employing nanometer and sub-nanometer scale imaging capability of structure and chemistry have been widely used to characterise various aspects of electronic and optoelectronic device structures such as InGaAs quantum dots, InGaAs pseudomorphic (pHEMT), and metamorphic (mHEMT) layers and the ohmic metallisation of GaAs and GaN high electron mobility transistors, nichrome thin film resistors, GaN heteroepitaxy on sapphire and silicon substrates, as well as InAs and GaN nanowires. They also established convergent beam electron diffraction techniques for determination of lattice distortions in III-V compound semiconductors, EBSD for crystalline misorientation studies of GaN epilayers and high-angle annular dark field techniques coupled with digital image analysis for the mapping of composition and strain in the nanometric layered structures. Also, &lt;em&gt;in-situ &lt;/em&gt;SEM experiments were performed on ohmic metallisation of pHEMT device structures. The established electron microscopy expertise for electronic materials with demonstrated examples is presented.&lt;/p&gt;

An Analysis of Germanium–Silicon/Silicon Strained Superlattice Structure Using Convergent Beam Electron Diffraction

AbstractStrained superlattices (SSLs) are typically found inside the p‐n junction area of semiconductor devices and consist of very thin alternating layers of different material. There exists a small lattice mismatch between these materials which results in localised strain, as in the case of germanium‐silicon/silicon SSLs. Strain measurements using a convergent beam electron diffraction (CBED) technique inside a transmission electron microscope (TEM) have indicated that the strain measured normal to these germanium–silicon/silicon SSLs varies almost sinusoidally, in spite of theoretical predictions which indicate a much sharper change in strain between these layers. A theoretical formulation involving an elasticity solution has been developed to predict the strain inside these SSL structures. The comparison of theoretical and experimental results clearly quantifies the effect of beam size on the spatial resolution of CBED measurements. Given that beam size is critically dependent on the spot size of the beam, the convergence angle, the specimen thickness and the position of the focused plane, these parameters are all clearly accounted for in the theoretical predictions.

Post-thinning using Ar ion-milling system for transmission electron microscopy specimens prepared by focused ion beam system.

We investigate Ar ion-milling rates and Ga-ion induced damage on sample surfaces of Si and GaAs single crystals prepared by focused ion beam (FIB) method for transmission electron microscopy observation. The convergent beam electron diffraction technique with Bloch simulation is used to measure the thickness of the Ar-ion milled samples to calculate the milling rates of Si and GaAs single crystals. The measurement shows that an amorphous layer is formed on the sample surface and can be removed by further Ar-ion milling. In addition, the local symmetry breaking induced by FIB is investigated using quantitative symmetry measurement. The FIBed-GaAs sample shows local symmetry breaking after FIB milling, although the FIBed-Si sample has no considerable symmetry breaking.

Quantitative symmetry determination and symmetry mapping using convergent beam electron diffraction technique

The symmetry recorded in convergent beam electron diffraction (CBED) patterns is in general determined by direct visual inspection [1], which does not provide uniform measurement. Furthermore, experimental CBED patterns are often noisy and deviate from the ideal symmetry because of the sample geometry and defects. Thus, the imperfection in experimental CBED patterns can lead to uncertainty in the symmetry determination [2]. Here, we propose a symmetry quantification method for CBED patterns using the profile R-factor (Rp) [3, 4] and the normalized cross-correlation coefficient (γ) [5]. We have also developed computer algorithms to automate these procedures. We demonstrate that the method proposed here is highly effective and provides a more precise way to determine the symmetry in CBED patterns. The symmetry quantification method can be also combined with a scanning electron diffraction technique for symmetry mapping [6]. Figure 1 shows the image processing procedures for mirror symmetry quantification. First, the symmetry related two diffraction discs (A, A’) are selected about the mirror plane (yellow line) as shown in Fig. 1(a). The template A is used as the reference motif so that the symmetry element is calculated by comparing with template A’. The template A and A’ are first aligned (Fig. 1(c)), and A’ is flipped horizontally to obtain a mirror image (A’m, Fig. 1(h)). For the rotational operation, the template A’ is simply rotated by 360/no with respect to n-fold rotation. The circular mask (Figs. 1(d) and (i)) is finally used to remove areas affected by CBED disk edge and to obtain the final templates (Figs. 1(e) and (j)). Then, we applied the profile R-factor (Rp) and the normalized cross-correlation coefficient (γ) to quantify the similarity between A (=IA(x, y)) and A’m (=IB(x, y)) as given in Eqs. (1) and (2).

The microstructure and ferroelectric property of Nd-doped multiferroic ceramics Bi0.85Nd0.15FeO3

The microstructures of Bi0.85Nd0.15FeO3 ceramics were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), convergent-beam electron diffraction (CBED) and X-ray energy dispersive spectrometry (EDS). The superstructure phase related to 1/4(hh0)p and 1/4(00l)p diffraction spots were observed in the samples. It is found that the superstructure phase can co-exist with R3c phase in a single grain. The bright and dark field images accompanied with SAED patterns provided evidences of the superstructure phase dispersed into the matrix with R3c symmetry and evolution of the domain. The ferroelectric domain wall was observed according to the displacement of Fe3+ ions respect to Bi3+ sub-lattice in the ferroelectric R3c phase based on HRTEM observations. The space group Pnam of the superstructure was identified by combining SAED and CBED techniques. EDS measurement revealed that the concentration of Nd in the Pnam phase is higher than that in R3c phase. This might mean that the transition from R3c to Pnam structure is due to the inhomogeneous distribution of the Nd concentration and the weakened stereochemical activity of Bi3+ lone electron pair, arisen from the increase in Nd content.

The large angle technique and lattice defect identifications

The history of the Convergent Beam Electron Diffraction (CBED) is shortly introduced. Symmetry determinations[1] of crystals or the point groups and space groups of 3, 4, 5 and 6 dimensional crystals, and crystal structure analysis including the determination of charge density distribution and potential distribution of a crystal are briefly reviewed.[2] Then, the large angle CBED (LACBED) technique is described.[3] Applications of the LACBED technique to the determinations of the Burgers vector of a dislocation, the shift vector at a stacking fault, the precise orientation difference of a twin domain and the strain of an advanced multi-layer material are reviewed.

Crystal structure and formation mechanism of (Cr2/3Ti1/3)3AlC2 MAX phase

The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)3AlC2 was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)3AlC2 is determined to be P63/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)3AlC2, Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M3AX2 lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)3AlC2 becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)3AlC2 were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)3AlC2 with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms.

Transmission electron microscopy characterization of the dislocations and slip systems of the dense hydrous magnesium silicate superhydrous B

Superhydrous B (shy B) is a dense hydrous magnesium silicate (DHMS) phase, for which experimental studies have indicated a large stability field along cold subduction geotherms in the Earth’s transition zone and top region of the lower mantle. The rheological properties of shy B may therefore be relevant to the rheology and possibly the deep seismic characteristics of subduction zones. Samples of shy B have been synthesized and deformed in a multianvil apparatus at 20 GPa and at temperatures 1000–1100 °C. The resulting dislocations have been characterized by transmission electron microscopy (TEM) with the weak-beam dark-field (WBDF) and the large-angle convergent-beam electron diffraction (LACBED) techniques. We emphasize the role of (010) plane in the plasticity of phase shy B. We report that [100] and [001] glide in (010), and also climb of [100]. We also observe 〈101〉 glide in (010) and {111} planes under high stress conditions.

Symmetry of the charge-ordered phases in Pr0.5Ca0.5MnO3

The crystallographic symmetry of the charge-ordered Pr0.5Ca0.5MnO3 manganite is studied at ∼98 K using convergent beam electron diffraction technique. These studies have revealed appearance of two different type of charge-ordered phases in different domains of the same grain. These correspond to be monoclinic and triclinic phases. These studies reveal that the symmetry of the charge-ordered phase is highly sensitive to the local strain conditions.

Crystal chirality and skyrmion helicity in MnSi and (Fe, Co)Si as determined by transmission electron microscopy

The magnetic skyrmions as derived from the proper screw spin state in chiral-lattice magnets (e.g., MnSi) show the intrinsic spin swirling direction or the magnetic helicity. The crystal chirality (${\ensuremath{\Gamma}}_{\mathrm{c}}$) and skyrmion helicity (${\ensuremath{\gamma}}_{\mathrm{s}}$) were simultaneously determined at the identical specimen areas of MnSi and Fe${}_{1\ensuremath{-}x}$Co${}_{x}$Si ($x=0.2,0.5$) by using convergent-beam electron diffraction and Lorentz transmission electron microscopy techniques, respectively. The reversal of the ${\ensuremath{\Gamma}}_{\mathrm{c}}$-${\ensuremath{\gamma}}_{\mathrm{s}}$ correlation is observed between MnSi and Fe${}_{1\ensuremath{-}x}$Co${}_{x}$Si and ascribed to the different sign of the Dzyaloshinskii-Moriya interaction between both compounds.