Convergent-beam Electron Diffraction Method Research Articles

TEM characterization of InN films grown by RF‐MBE

Structural properties of InN films grown on (0001) sapphire by radio frequency molecular beam epitaxy were characterized using transmission electron microscopy. A high density of threading dislocations was observed in the InN films. The density of the dislocation was more than 2 × 1010 cm−2. Most threading dislocations were edge-type dislocations. A columnar structure of crystallites with a small angular distribution in the c-plane (twist) was clearly observed. X-ray rocking curve measurements showed that InN film had relatively large distribution of twist in contrast to that of tilt. Convergent beam electron diffraction method was used to determine the polarity of the InN film. It is found that the InN film grown on the sapphire (0001) substrate showed N-polarity. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A Combination Method of Charge Density Measurement in Hard Materials Using Accurate, Quantitative Electron and X-ray Diffraction: The α-Al2O3 Case

Current X-ray diffraction techniques intended for "ideally imperfect" specimens provide structure factors only on a relative scale and ever-present multiple scattering in strong low-angle Bragg reflections is difficult to correct. Multiple scattering is implicit in the quantitative convergent beam electron diffraction (QCBED) method, which provides absolutely scaled structure factors. Conventional single crystal X-ray diffraction has proved adequate in softer materials where crystal perfection is limited. In hard materials, the highly perfect nature of the crystals is often a difficulty, due to the inadequacy of the conventional corrections for multiple scattering (extinction corrections). The present study on alpha-Al2O3 exploits the complementarity of synchrotron X-ray measurements for weak and medium intensities and QCBED measurement of the strong low-angle reflections. Two-dimensional near zone axis QCBED data from different crystals at various accelerating voltages, thicknesses, and orientations have been matched using Bloch-wave and multislice methods. The reproducibility of QCBED data is better than 0.5%. The low-angle strong QCBED structure factors were combined with middle and high-angle extinction-free data from synchrotron X-ray diffraction measurements. Static deformation charge density maps for alpha-Al2O3 have been calculated from a multipole expansion model refined using the combined QCBED and X-ray data.

Antiphase boundaries in Ni3Al ordered intermetallic—application of the CBED method

The antiphase domain structure in Ni3Al ordered intermetallics with perfect L12 superstructure has been examined. Transmission electron microscopy (TEM) investigations were performed using Jeol JEM 3010 and Philips CM30 microscopes. Conventional TEM studies, convergent-beam electron diffraction (CBED) and high-resolution electron microscopy (HRTEM) were used to elucidate the structure of the alloys. For advanced studies of the nature of antiphase boundaries, the large-angle convergent-beam electron diffraction (LACBED) method was employed. The recognition between symmetry-related and displacive domains can be performed with high accuracy using LACBED and dark-field images, even for nanosized domains.

Defect structures in TaSi 2 thin films produced by co-sputtering

Phase transformation and defect structures in thin-film TaSi 2 produced by co-sputtering have been investigated as a function of annealing temperature by transmission electron microscopy. Crystallization of amorphous TaSi 2 thin films occurs at 400 °C without forming any metastable phases. Most of C40 TaSi 2 crystallites contain planar faults parallel to (0001) basal planes. Convergent-beam electron diffraction (CBED) indicates that these planar faults are not simple stacking faults but are twin boundaries bounded by two adjacent enantiomorphically-related twin domains; i.e., domains belonging to the space groups P6 222 (right-handed) and P6 422 (left-handed) having the identical crystal orientation arrange alternatively separated by twin boundaries parallel to (0001). The formation of these enantiomorphically-related domains in TaSi 2 thin films with the hexagonal C40 structures is discussed in comparison with the formation of twin-related domains in MoSi 2 (the tetragonal C11 b structure) and TiSi 2 (the orthorhombic C54 structure) thin films. A new CBED method is proposed for identification of enantiomorphically-related crystals, in which asymmetry in the intensity of Bijvoet pairs of FOLZ disks in an experimental symmetrical zone-axis CBED pattern is compared with that in a computer simulated CBED pattern.

Analysis of dislocation loops by means of large-angle convergent beam electrondiffraction

Diffusion-induced dislocation loops in GaP and GaAs were analysed by means oflarge-angle convergent beam electron diffraction (LACBED) and conventionalcontrast methods of transmission electron microscopy. It is demonstrated thatLACBED is perfectly suited for use in analysing dislocation loops. The methodcombines analyses of the dislocation-induced splitting of Bragg lines in aLACBED pattern for the determination of the Burgers vector with analyses of theloop contrast behaviour in transmission electron microscopy bright-field imagesduring tilt experiments, from which the habit plane of the dislocation loop isdetermined. Perfect dislocation loops formed by condensation of interstitial atomsor vacancies were found, depending on the diffusion conditions. The loops possess{110}-habit planes and Burgers vectors parallel to ⟨110⟩. TheLACBED method findings are compared with results of contrast analyses basedon the so-called ‘inside–outside’ contrast of dislocation loops. Advantages of theLACBED method consist in the possibility of determining the complete Burgersvector of the dislocation loops and of an unambiguous and fast loop type analysis.

Refinement of crystal structural parameters and charge density using convergent-beam electron diffraction--the rhombohedral phase of LaCrO3.

Atom positions and anisotropic Debye-Waller factors of the rhombohedral phase of LaCrO(3) have been refined simultaneously with the low-order structure factors, using a structure-analysis method of convergent-beam electron diffraction (CBED) proposed by Tsuda & Tanaka [Acta Cryst. (1999), A55, 939-954]. The method is based on the least-squares fitting between full dynamical calculations and energy-filtered intensities of two-dimensional higher-order Laue-zone (HOLZ) and zeroth-order Laue-zone (ZOLZ) CBED patterns. The positions of the oxygen atoms have been determined with a high precision. Clear anisotropy of the thermal vibrations of the oxygen atoms has been successfully determined by the CBED method for the first time. The charge transfer from the La and Cr atoms to the O atoms has been found from the deformation charge-density map.

Polarity Determination of Wurtzite and Zincblende Structures by TEM

The applicability of transmission electron microscopy (TEM) to determine polarity was examined for GaN, ZnO, AlN and GaAs. The adopted methods are the TEM-EDS (energy dispersive X-ray spectroscopy) and the conventional convergent beam electron diffraction (CBED) methods. TEM observations and dynamical electron diffraction calculations were carried out. The results show that the CBED method is useful for ZnO and GaN, while the TEM-EDS method must be used for AlN and GaAs and can also be used for GaN and ZnO. It is clearly indicated that the TEM-EDS method is of use for the case where the difference in atomic scattering factor between the two constituent elements is small, while the CBED method is useful for the case where the difference is large.

Transmissionselektronenmikroskopische Analysen zur Foliendickenbestimmung an kaltverfestigtem und thermisch behandeltem AlMg5Mn/ The Determination of the Thickness of Cold Rolled Thermally Treated AlMg5Mn Foils by Transmission Electron Microscopic Analysis

Dislocation density plays a central role in materials science as an important material characteristic, and Transmission Electron Microscopy (TEM) is the main tool used to measure this characteristic where it's average value is in the range 10 8 - 10 11 cm -2 . In order to calculate the dislocation density of a material two parameters need to be determined: The linear length of the dislocations visible in the section of the picture under observation and the foil thickness. This article concerns the determination of the foil thickness. In the literature a number of different methods are described for doing this, but in this article it is the Convergent Beam Electron Diffraction method (the CBD - method) which is discussed as this is the most accurate. In contrast with other published literature, in this article the technique is assessed critically, extrapolated for use with even thicker foils and an improved method suggested. The Aluminium alloy AlMg5Mn was chosen as the specimen material.

Measurement of mean free paths for inelastic electron scattering of Si and SiO2.

The effects of accelerating voltage and collection angle on the mean free path for all inelastic electron scattering (lambdap), which is an important parameter for determining specimen thickness by using electron energy-loss spectroscopy, were investigated with crystalline Si and amorphous SiO2. First, thickness of Si film was measured with the convergent-beam electron diffraction method, while thickness of SiO2 particles was estimated from their spherical shape. Then from electron energy-loss spectra, lambdap was evaluated for Si film and SiO2 particles by changing the accelerating voltage (100 to approximately 300 kV) and the collection angle for the scattered electrons. Under the condition of no objective aperture, lambdap for Si film and SiO2 particles was found to increase with the increase of accelerating voltage and to take values of 180+/-6 nm (Si) and 247+/-8 nm (SiO2) at 300 kV. Also, it was found that lambdap in both cases decreases drastically with the increase of collection angle in the range smaller than 25 mrad, while it tends to take a constant value at the collection angle larger than 25 mrad at 200 kV.

Polarity determination of III–V compound semiconductors by large-angle convergent beam electron diffraction

The large-angle convergent beam electron diffraction (LACBED) technique is used for determining the crystal polarity of GaP and GaAs single crystals from 〈1 1 0〉 cross-sectional samples. The method which is based on an earlier approach using convergent beam electron diffraction (CBED) evaluates the polarity-sensitive contrast of high odd-index Bragg-lines in {0 0 2} dark-field patterns. The polarity is determined by application of a simple contrast rule as well as by direct comparison with dynamical simulations. For the two materials the ranges of applicability are determined by a detailed analysis of the Bragg-line contrast as a function of the sample thickness. The coexistence of the Bragg-line pattern and the of shadow image of the defect in correct rotational relationship to each other makes the analysis straightforward and free from possible sources of errors. As an example, the crystal polarity of GaP is related to the morphology of facetted voids. The LACBED method is shown to be suitable for relating the analysis of extended crystal defects. The advantages and the disadvantages of the LACBED method are discussed in comparison with the corresponding CBED method and with a recent method based on the analysis of bend contours.

Determination of crystal polarity from bend contours in transmission electron microscope images

A new method for determining the polarity of crystals with sphalerite structure (GaAs, GaSb, InP, etc.) within the transmission electron microscope (TEM) is presented. The method is derived from an established convergent beam electron diffraction (CBED) method (J. Appl. Crystallogr. 15 (1982) 60) and exploits the effects of the dynamical scattering on the contrast of bend contour crossings in conventional TEM images. In contrast to the CBED method, the bend contour method is performed in the image mode of the TEM. The sample can, therefore, be viewed while performing the polarity analysis. Furthermore, in the presence of strong foil bending, the bend contour method has some advantages for practical work. A general contrast rule for the bend contour intersections is stated which allows to readily obtain the crystal polarity by comparing the contrast in experimental images with the prediction of the rule. Exemplarily, the polarity of GaAs in TEM samples prepared for investigation in the two frequently used projections 〈0 0 1〉 and 〈1 1 0〉 is determined. The validity of the rule for the cases studied is confirmed by simulations of the dynamical scattering process. Furthermore, an independent analysis of the crystal polarity by making use of a long-range-ordered (GaIn)P layer grown on top of the GaAs confirms the results obtained with the bend contour method. As an example, the usefulness of the method is demonstrated in an analysis of the α/β-character of misfit dislocations at the interface between the GaAs substrate and the (GaIn)P layer.

サファイア研磨表面の微小欠陥と格子ひずみの高精度測定

Mechanical properties of ceramics strongly depend on the machining condition. In the present study, surface defects and local strain, which was introduced by systematically machining the basal plane of sapphire, were characterized by cross-sectional transmission electron microscopy (XTEM) and convergent beam electron diffraction (CBED) technique. It was found that dislocations and strain were introduced from the surfaces for all specimens examined in this study. Depths of the introduced dislocation were measured by weak beam dark field (WBDF) method, and residual strain in the vicinity of the surfaces were precisely measured by CBED method using the probe size less than 10nm. The experimental CBED patterns were compared with the calculated patterns to quantitatively evaluate the lattice strain around the surfaces. Maximum penetration depths of the dislocations in the specimens ground with #500 diamond wheel, polished with 4-8μm and under 1μm diamond slurry were 1.7μm, 700nm and 250nm, respectively. In addition, twin defect was introduced in the specimen ground by #500 diamond to accommodated the large lattice strain during the machining. Residual local strain was found to remain at the depths of 1.0μm, 1.2μm and 300nm for the specimens ground with #500 diamond, polished with 4-8μm and under 1μm diamond slurry, respectively.

Distinction between space groups having principal rotation and screw axes, which are combined with twofold rotation axes, using the coherent convergent-beam electron diffraction method.

23 sets of space groups remain indistinguishable by the convergent-beam electron diffraction (CBED) method. Recently, Tsuda, Saitoh, Terauchi, Tanaka & Goodman [Acta Cryst. (2000), A56, 359-369] demonstrated that the coherent CBED method can distinguish two space-group pairs (I23, I2(1)3) and (I222, I2(1)2(1)2(1)) by observing the relative arrangements of 2-fold-rotation and 2(1)-screw axes. The other ten space-group sets, which are composed of principal rotation and screw axes and other 2-fold-rotation axes such as P321 and P3(1)21 (P3(2)21), are shown to be distinguishable using the coherent CBED method.

Space-group determination of cubic and its superlattice-ordered rhombohedral phases of Zn63Mg23Er14 using the convergent-beam electron diffraction method

Abstract The space group of a cubic phase of Zn63Mg23Er14 has been determined to be F4̅3m by convergent-beam electron diffraction (CBED). This space group is in agreement with that assumed by X-ray structure analysis (Sterzel et al., Philos. Mag. Lett. 80 (2000) 239-247). A rhombohedral phase, which is considered as a superlattice ordered phase of the cubic phase, was found. The space group of the rhombohedral phase has been determined to be R3m by the CBED method.

Evaluation of lattice distortion near c-ZrO2/α-Al2O3 interface by plasmon loss

Plasmon energies near a cubic-ZrO2 (c-ZrO2)/α-Al2O3 interface were studied by electron energy loss spectroscopy using a field-emission transmission electron microscope. The plasmon energies measured by scanning an electron probe of 1 nm in diameter across the interface showed that, approaching the interface, the plasmon energy of c-ZrO2 increases from 23.9 to 24.3 eV and that of α-Al2O3 decreases from 24.3 to 23.4 eV. Energy dispersive x-ray spectroscopy revealed that fluctuation of the chemical composition across the interface is small (⩽5%) and causes only a slight change in the plasmon energies (⩽0.16 eV). We conclude that the large energy changes (up to 0.9 eV) are mainly caused by lattice distortion. Using the Drude model, it was found that, 3 nm from the interface, the unit cell volume of c-ZrO2 decreases by 3% and that of α-Al2O3 increases by 6% compared with the volumes at 50 nm. The presence of lattice distortion near the interface was also confirmed by the convergent-beam electron diffraction method. The results show that plasmon loss is useful to evaluate lattice distortion.

Structural studies on decagonal quasicrystals using the HAADF-STEM and CBED methods

The applications of the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and convergent-beam electron diffraction (CBED) methods to decagonal quasicrystals and its approximant are presented by giving some examples. The HAADF method can show the atomic arrangement mainly composed of transition metal (TM) atoms in Al-TM systems. The CBED method can determine the super point- and space-groups of quasicrystals, and furthermore can be used to refine the structural parameters.

Distinction of space groups (I23 and I2(1)3) and (I222 and I2(1)2(1)2(1)) using coherent convergent-beam electron diffraction

Convergent-beam electron diffraction (CBED) is well known as a powerful tool to determine space groups of crystals. However, it cannot distinguish several sets of space groups. It is shown theoretically that the coherent CBED method can distinguish between space groups (I23 and I2(1)3) and between (I222 and I2(1)2(1)2(1)), which belong to indistinguishable sets. It is demonstrated by computer simulations that the relative arrangement of 2-fold-rotation and 2(1)-screw axes can be distinguished by examining the relative phases of specific reflections through the interference fringes of coherent CBED patterns. This fact implies that these space groups can be distinguished by the coherent CBED method.

Space-Group Determination of Monoclinic and Orthorhombic Al13Co4Approximants by the Convergent-Beam Electron Diffraction Method

Space groups of monoclinic and orthorhombic Al 13 Co 4 approximants have been determined to be C 2/ m and P n m n , respectively, by the convergent-beam electron diffraction method. These space groups agree with those assumed in an X-ray structure analysis [Freiburg et al .: Mater. Sci. Forum 228–231 (1996) 583] and a conventional electron diffraction method [Ma and Kuo: Metall. Trans. 23A (1992) 1121].

Structural features of tilt grain boundaries in GaAs thin films

(fa181-1) and low-angle tilt grain boundaries in GaAs thin films were investigated by high-resolution transmission electron microscopy (HRTEM). Models for the atomic structure of the boundaries are proposed in this study. The structural unit for the boundaries consists of 5- and 7-member rings. The polarity of the grain on either side of the boundaries was confirmed by the convergent beam electron diffraction method. (110) low-angle tilt grain boundaries were observed to consist of a/2 edge dislocations. Anti-site type bonds are present at the core of the dislocations.

Three-phase structure invariants and structure factors determined with the quantitative convergent-beam electron diffraction method

Quantitative convergent-beam electron diffraction is used to determine structure factors and three-phase structure invariants. The refinements are based on centre-disc intensities only. An algorithm for param-eter-sensitive pixel sampling of experimental intensities is implemented in the refinement procedure to increase sensitivity and computer speed. Typical three-beam effects are illustrated for the centrosymmetric case. The modified refinement method is applied to determine amplitudes and three-phase structure invariants in noncentrosymmetric InP. The accuracy of the results is shown to depend on the choice of the initial parameters in the refinement. Even unrealistic starting assumptions and incorrect temperature factor lead to stable results for the structure invariant. The examples show that the accuracy varies from 1 to 10 degrees in the electron three-phase invariants determined and from 0.5 to 5% for the amplitudes. Individual phases could not be determined in the present case owing to spatial intensity correlations between phase-sensitive pixels. However, for the three-phase structure invariant, stable solutions were found.