Convergent-beam Electron Diffraction Method Research Articles

Visualizing optically-induced strains by five-dimensional ultrafast electron microscopy

Ultrafast optical control of strain is crucial for the future development of nanometric acoustic devices. Although ultrafast electron microscopy has played an important role in the visualization of strain dynamics in the GHz frequency region, quantitative strain evaluation withnm × ps spatio-temporal resolution is still challenging. Five-dimensional scanning transmission electron microscopy (5D-STEM) is a powerful technique that measures time-dependent diffraction or deflection of the electron beam at the respective two-dimensional sample positions in real space. In this paper, we demonstrate that convergent beam electron diffraction (CBED) measurements using 5D-STEM are capable of quantitative time-dependent strain mapping in the nm × ps scale. We observe the generation and propagation of acoustic waves in a nanofabricated silicon thin plate of 100 nm thickness. The polarization and amplitude of the acoustic waves propagating in the silicon plate are quantitatively determined from the CBED analysis. Further Fourier-transformation analysis reveals the strain distribution in the momentum-frequency space, which gives the dispersion relation in arbitrary directions along the plate. Versatility of 5D-STEM-CBED analysis enables quantitative strain mapping even in complex nanofabricated samples, as demonstrated in this study.

Comparative study on the specimen thickness measurement using EELS and CBED methods

Two thickness measurement methods using an electron energy loss spectroscopy (EELS) and 10a convergent beam electron diffraction (CBED) were compared in an Fe-18Mn-0.7C alloy. The thin foil specimen was firstly tilted to satisfy 10a two-beam condition. Low loss spectra of EELS and CBED patterns were acquired in scanning transmission electron microscopy (STEM) and TEM-CBED modes under the two-beam condition. The log-ratio method was used for measuring the thin foil thickness. Kossel-Möllenstedt (K-M) fringe of the mathbf{13}overline{mathbf{1}} diffracted disk of austenite was analyzed to evaluate the thickness. The results prove the good coherency between both methods in the thickness range of 72 ~ 113 nm with a difference of less than 5%.

Observation of reaction between a-type dislocations in GaN layer grown on 4-in. Si(111) substrate with AlGaN/AlN strained layer superlattice after dislocation propagation

Dislocation reaction in a GaN layer grown on 4-in. Si(111) with AlGaN/AlN strained layer superlattice was observed by transmission electron microscopy. The reaction between a dislocation (b=1/3[12¯10]) and another dislocation (b =1/3[112¯0]) to form a dislocation segment (b =1/3[21¯1¯0]) was demonstrated by plan-view observation using weak-beam dark-field and large-angle convergent-beam electron diffraction methods. Observed reaction occurred with dislocation motion after dislocation propagation with epitaxial growth.

Direct observation of the symmetry breaking of the nanometer-scale local structure in the paraelectric cubic phase of BaTiO3 using convergent-beam electron diffraction

Nanometer-scale local structures of the cubic phase of barium titanate (BaTiO3) are investigated by convergent-beam electron diffraction (CBED) using a nanometer-size electron probe. Breaking of the cubic symmetry has been discovered in the nanometer-scale areas of the cubic phase. This indicates the existence of local polarization clusters as a precursor of the phase transition. Symmetry-breaking index maps for the fourfold rotation symmetry are given at different temperatures with the combined use of scanning transmission electron microscopy and CBED methods (STEM-CBED). A spatially hierarchical structure of the cubic phase is proposed, which may explain different local symmetries reported using different experimental probes.

Liquid-target reactive magnetron sputter epitaxy of High quality GaN(0001̄) nanorods on Si(111)

Direct current magnetron sputter epitaxy with a liquid Ga sputtering target has been used to grow single-crystal GaN(0001̄) nanorods directly on Si(111) substrates at different working pressures ranging from 5 to 20mTorr of pure N2. The as-grown GaN nanorods exhibit very good crystal quality from bottom to top without stacking faults, as determined by transmission electron microscopy. The crystal quality is found to increase with increasing working pressure. X-ray diffraction results show that the rods are highly (0001)-oriented. The nanorods exhibit an N-polarity, as determined by convergent beam electron diffraction methods. Sharp and well-resolved 4K photoluminescence peaks at ~3.474eV with a FWHM ranging from 1.7meV to 35meV are attributed to the intrinsic GaN band edge emission and corroborate the superior structural properties of the material. Texture measurements reveal that the rods have random in-plane orientation when grown on Si(111) with native oxide, while they have an in-plane epitaxial relationship of GaN[112̅0]//Si[11̄0] when grown on substrates without surface oxide.

Appropriate zone-axis orientations for the determination of crystal polarity by convergent-beam electron diffraction

A convergent-beam electron diffraction (CBED) method is proposed for polarity determination, in which polarity is determined from the intensity asymmetry of any of the hkl–\overline h\overline k\overline l Friedel pairs appearing in a zone-axis CBED pattern with a symmetric arrangement of Bijvoet pairs of reflections. The intensity asymmetry occurs as a result of multiple scattering among Bijvoet pairs of reflections in the CBED pattern. The appropriate zone-axis orientations for polarity determination are deduced for 19 of the 25 polar point groups from symmetry considerations so as to observe Bijvoet pairs of reflections symmetrically in a single CBED pattern. These appropriate zone-axis orientations deduced for the 19 polar point groups coincide with nonpolar directions. This is because the nonpolar directions for these point groups are perpendicular to an even-fold rotation axis, which guarantees the symmetric arrangement of Bijvoet pairs of reflections with respect to the symmetry (m–m′) line in a CBED pattern taken along any of the appropriate zone-axis orientations. The m–m′ line in the CBED pattern is proved to be perpendicular to the trace of the even-fold rotation axis. On the other hand, if the nonpolar direction is either perpendicular to a mirror plane or parallel to a roto-inversion axis as in the four point groups m, 3m1, 31m, \overline 6, the nonpolar direction cannot be used as the appropriate zone-axis orientation for polarity determination because the Bijvoet pairs of reflections are not arranged symmetrically in the CBED pattern. The validity of the CBED method is confirmed both by experiment and by calculation of CBED patterns.

Study of nanoscale local structures of ferroelectric barium titanate using CBED

It is well known that BaTiO3 undergoes successive phase transformations from the cubic paraelectric phase to three ferroelectric phases: tetragonal, orthorhombic and rhombohedral ones. Coexistence of the displacive and order-disorder characters in the phase transformations of BaTiO3 was pointed out from many experiments and theories. However, local structures related to the order-disorder character were discovered neither in crystal structure analyses using neutron and X-ray diffraction nor by TEM observations. In the present study, the convergent-beam electron diffraction (CBED) method was applied to examine nanometer-scale local structures of BaTiO3. Rhombohedral nanostructures were observed in the orthorhombic and tetragonal phases of BaTiO3 using CBED [1]. It was found that the symmetry of the orthorhombic phase is formed as the average of two rhombohedral variants with different polarizations, and that of the tetragonal phase is formed as the average of four rhombohedral variants. These results indicate an order-disorder character in their phase transformations. Similar results were obtained in the ferroelectric orthorhombic phase of KNbO3 [2], while it was found that the ferroelectric tetragonal phase of PbTiO3 does not have such rhombohedral nanostructures. We also proposed a combined use of STEM and CBED methods (STEM-CBED method) to observe the nanostructures of polarizations [3]. Using the method, two-dimensional distribution of the rhombohedral nanostructures, or nanoscale fluctuations of the polarization clusters, were successfully visualized in the tetragonal phase of BaTiO3.

収束電子回折法によるBaTiO<sub>3</sub>強誘電相の局所構造揺らぎの研究

Local structures of the rhombohedral, orthorhombic, and tetragonal phases of barium titanate （BaTiO3）have been investigated using convergent-beam electron diffraction （CBED）. Nanometer sized rhombohedral structures have been found in both the orthorhombic and tetragonal phases. This indicates the existence of an order-disorder character in their phase transformations. The nanostructures in these phases are discussed in terms of an order-disorder model with off-centered Ti in the ［111］ directions. Spatial distributions of the nanostructures were examined by the combined use of the scanning transmission electron microscopy and convergent-beam electron diffraction methods （STEM-CBED method）. Two-dimensional structural fluctuations were successfully visualized in the tetragonal phase of BaTiO3.

Applicability study of the structure-factor phase method for determining the polarity of binary semiconductors

The structure-factor phase method of convergent-beam electron diffraction (CBED) has been widely applied as an effective tool in determining the polarity of binary compound materials, for example, the typical sphalerite material, GaAs. However, its validity on other polar materials is still unknown. In this paper we extensively investigated its potential applicability onto 11 AB-type semiconductors by dynamical simulations of CBED. Two key factors during the simulation, the difference between A and B atomic numbers and the sample thickness, are discussed in detail. It was found that this method is efficient to determine the polarity for a sphalerite structure under certain conditions, and, reversely, limited to determine the polarity for a wurtzite structure even though it is very similar to the sphalerite structure.

Two-dimensional mapping of polarizations of rhombohedral nanostructures in the tetragonal phase of BaTiO3 by the combined use of the scanning transmission electron microscopy and convergent-beam electron diffraction methods

Nanometer-sized structures with rhombohedral symmetry were discovered in the tetragonal and orthorhombic phases of perovskite-type ferroelectric BaTiO3, using convergent-beam electron diffraction [K. Tsuda, R. Sano, and M. Tanaka, Phys. Rev. B 86, 214106 (2012)]. The existence of rhombohedral nanostructures indicates an order-disorder character in their phase transformations. In the present study, spatial distributions of the nanostructures have been examined by the combined use of the scanning transmission electron microscopy and convergent-beam electron diffraction methods. Two-dimensional distributions of polarizations of the rhombohedral nanostructures have been visualized in the tetragonal phase of BaTiO3.

Comparison of convergent beam electron diffraction methods for simultaneous structure and Debye Waller factor determination

The measurement of accurate and precise structure factors and Debye Waller (DW) factors by quantitative convergent beam electron diffraction (QCBED) permits experimental determination of the electron density distribution and probing of interatomic bonding in crystals. The three QCBED methods used successfully for high precision measurements of low order structure factors to date, namely the zone axis pattern (ZAP) method, the excited row ER method and the multi-beam off-zone axis (MBOZA) technique, differ from each other regarding the crystal orientation relative to the incident electron beam. Consequently, the details of their respective dispersion surface representations differ regarding the number, relative amplitudes and phases of excited Bloch wave branches. Under the same experimental setup conditions, the factors most important to the degree of accuracy and precision achievable in electron density determination for crystals with QCBED methods ultimately depend on the sensitivity of the excited Bloch wave branches and the resultant contrast in the respective CBED patterns to changes in both structure and DW factors. In general, a QCBED pattern will be more sensitive to changes in both structure and DW factor, if it contains more and stronger excited Bloch wave branches, as dynamic interactions of the Bloch waves increase the sensitivity of the pattern. In this work we analyzed Bloch wave excitation and dispersion surfaces for the three most popular QCBED methods. The analysis indicates, that the QCBED patterns obtained using the MBOZA orientation generally contain more and stronger excited Bloch wave branches. Hence, MBOZA diffraction patterns are more sensitive than the ZAP and the ER patterns to changes in both DW and structure factors and therefore allow in differences to the other two methods simultaneous refinements effectively and robustly.

Polarity determination of rough and smooth surface grains in AlN crystals

AbstractThe polarity of rough and smooth grains in textured aluminum nitride boules were analyzed by transmission electron microscopy. Specifically, convergent beam electron diffraction method was applied to determine polarity. The grains corresponding to smooth and rough surfaces were identified as having Al and N polarities, respectively. Aluminum oxide (Al2O3) was observed to form at the grain boundaries. The oxide may precipitate due to the low mutual solubility between Al2O3 and AlN at the high crystal growth temperature (∼2000°C). Oxygen may be the cause of polarity inversion that leads to the formation of Al and N polar grains.

Study of strain fields caused by crystallization of boron doped amorphous silicon using scanning transmission electron microscopy convergent beam electron diffraction method

Study of strain fields around boron doped silicon electrodes in silicon devices was carried out with scanning transmission electron microscopy convergent beam electron diffraction. Leak current, which was one of the crucial failures of this kind of structure, depends on boron concentration in the electrodes. Transmission electron microscopy and diffraction analysis showed that the electrodes consist of epitaxial phase and poly-Si phase, and the proportion of these phases depends on the boron concentration in the electrode. Strain distribution obtained with scanning transmission electron microscopy convergent beam electron diffraction revealed that the origin of the strain is volume shrinkage of the epitaxial phase in the electrode, and the poly-Si phase acts as buffer against this strain. Electron energy-loss spectroscopy analyses revealed that boron segregation occurred in samples having a higher boron concentration, and prevented epitaxial growth in the electrodes. As a result, the core of the electrode remains as poly-Si or amorphous Si and acts as strain buffer. Our analysis concluded that boron concentration in the electrodes is one of the most important factors enabling high production yield for the structure.

The Art and Application of Large Angle Convergent Beam Electron Diffraction

The antiphase domain structure in Ni3Al and Al3Ti+Cu intermetallic alloys was recognized by conventional transmission electron microscopy and large angle convergent beam electron diffraction methods. In the case of antiphase boundary the superlattice excess line is split into two lines with equal intensity on bright and dark field LACBED pattern. This splitting can be considered as typical and used to identify APBs. The recognition between perfect structure of the defect-free matrix and the screw deviation around the nanopipes in GaN epilayers was performed with high accuracy using Zone Axis LACBED images.

Measurement of strain and strain relaxation in free-standing Si membranes by convergent beam electron diffraction and finite element method

Bridge-shaped free-standing Si membranes (FSSM), strained by low-pressure (LP) Si x N y , plasma-enhanced (PE) Si x N y and Si x Ge 1− x stressors, were measured by convergent beam electron diffraction (CBED) and the finite element method (FEM). The results of CBED show that, while the strain along the length of the FSSM is compressive in an LPSi x N y /Si sample, those along the length of the FSSM are tensile in PESi x N y /Si and Si x Ge 1− x /Si samples. The average absolute values of strains are different in FSSM with LPSi x N y , PESi x N y and Si x Ge 1− x as stressors. The FEM was used to compensate the results of CBED taking into account the strain relaxation in transmission electron microscopy (TEM) sample preparation. The FEM results give the strain properties in three dimensions, and are in good agreement with the results of CBED. There is approximately no strain relaxation along the length of FSSM, and the elastic strains along the other two axes in FSSM are partially relaxed by thinning down for the preparation of TEM samples.

Experimental Studies of Bonding Related Properties in Binary Intermetallics by Convergent Beam Electron Diffraction

ABSTRACTAccurate Debye-Waller (DW) factors of chemically ordered β-NiAl (B2, cP2, ${\rm{Pm}}\bar 3 {\rm{m}}$) have been measured at different temperatures using an off-zone axis multi-beam convergent beam electron diffraction (CBED) method. We determined a cross over temperature below which the DW factor of Ni becomes smaller than that of Al of ~90K. Additionally, we measured for the first time DW factors and structure factors of chemically ordered γ1-FePd (L10, tP2, P4/mmm) at 120K. We were able to simultaneously determine all four anisotropic DW factors and several low order structure factors using different special off-zone axis multi-beam convergent beam electron diffraction patterns with high precision and accuracy. An electron charge density deformation map was constructed from measured X-ray diffraction structure factors for γ1-FePd.

Determination of Debye–Waller factor and structure factors for Si by quantitative convergent-beam electron diffraction using off-axis multi-beam orientations

Debye-Waller (DW) factors and structure factors have been measured for Si using convergent-beam electron diffraction (CBED) experiments with a transmission electron microscope equipped with a field-emission gun and a post-column energy-filtering device. Si has been used here to evaluate the accuracy of multi-beam near-zone-axis orientations for the simultaneous refinement of DW factors and multiple structure factors. Strong dynamic interactions among different beams are obtained by tilting the crystal to specific four- or six-beam orientations near major zone axes, which provide sufficient sensitivity to determine accurate DW factors and structure factors. The DW factors of Si were measured using four-beam conditions near the [001] zone axis for temperatures ranging from 96 to 300 K. A comparison of the multi-beam near-zone-axis orientations with other CBED methods for DW and structure factor F(g) refinement is presented.

Simultaneous determination of highly precise Debye–Waller factors and structure factors for chemically ordered NiAl

Accurate Debye-Waller (DW) factors and several low-index structure factors of chemically ordered β-NiAl at different temperatures have been measured using an off-zone-axis multi-beam convergent-beam electron diffraction method. The temperature dependences of DW factors of Ni and Al atoms are compared with previous experimental measurements and theoretical calculations. The temperature below which the DW factor of Ni becomes smaller than that of Al was found to be lower than previously reported. Structure factors are determined with an accuracy of 0.05% and compared with prior reports.

Measurement of Strain in Freestanding Si/SixNy Membrane by Convergent Beam Electron Diffraction and Finite Element Method

Strain in a bridge-shaped freestanding Si membrane (FSSM) induced by depositing an amorphous SixNy layer was measured by convergent-beam electron diffraction (CBED) and the finite element method (FEM). CBED results show that the strain magnitude depends negatively on the FSSM thickness, and compressive strain along the length of the FSSM is increased by approximately 0.1% at the end of the FSSM. FEM is used as a supplementary method to CBED for calculating the strain relaxation in three-dimension in the FSSM.

Polarity determination of ferroelectric LiNbO 3 crystals and BiFeO 3 thin films by the convergent beam electron diffraction technique

The applicability of various convergent beam electron diffraction (CBED) methods to the polarity determination of LiNbO 3 crystals in the space group R3c was examined. We show that the method of zone-axis CBED combined with dynamical simulation is applicable to the polarity determination of LiNbO 3. The effects of the zone axis, the specimen thickness and the displacement of the homopolar atom on the polarity determination were studied. This method was also successfully applied to the polarity determination of a multiferroic BiFeO 3 film deposited on a (111)Pt/TiO 2/SiO 2/Si substrate by pulsed laser deposition. It shows that about 30% of the grains in the BiFeO 3 film are grown along the positive polar direction.