Convergent Beam Electron Diffraction Experiments Research Articles

Electron diffraction study of crystal structures of (Sr1−xBax)2Nb2O7

Selected-area electron diffraction and convergent-beam electron diffraction (CBED) techniques are used to reveal the Ba doping effect on the crystal structure of (${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{)}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ [SBN(x)] in the temperature range from 293 to 693 K. Ba doping of ${\mathrm{Sr}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ (SN) causes weakening of the incommensurate modulation and deviation from the C-centered lattice. On the other hand, symmetries of fundamental reflections in CBED patterns of SBN(x) are seen to be the same as those of SN, showing a small structural deviation from Cmc${2}_{1}$. Crystal symmetries above and below the temperature of a relaxorlike dielectric anomaly in SBN ($x=0.32$) at 465 K remain unchanged. CBED experiments with a nanometer-sized electron probe do not show any indication of the presence of a nanodomain around this temperature. However, the presence of a few-micron-sized polar-inversion domains is found at around 573 K in SBN ($x=0.32$). The origin of the reported relaxorlike dielectric anomaly in SBN ($x=0.32$) is discussed on the basis of the experimental results of CBED.

Characterization of Crystalline Zinc Oxide in the Form of Hexagonal Bipods

We present a solution-phase preparation method for the synthesis of ZnO in an open reactor (with constant stirring) or in a closed reactor (autoclave). The method is based on a combination of the polyol method and the homogeneous precipitation of zinc nitrate with urea. ZnO particles with a rod-like shape and with a regular bipod structure were prepared in an open reactor, while needle-like shapes were prepared in the autoclave. A detailed characterization of the planar base-plane and prismatic defects obtained in the hexagonal ZnO bipod particles with electron microscopy revealed that in the middle of the bipod the planar defects contain a small amount of silicon. A rough estimation indicated that there was a single atomic layer of silicon at the defect. From convergent-beam electron-diffraction experiments, we found that these planar defects were inversion domain boundaries (IDBs) with a head-to-head orientation of the polar axis. In the needle-like particles, besides IDBs on the basal planes, prismatic IDBs with a small amount of silicon were also found. The formation mechanism for the prismatic IDB was thought to be the overgrowth of one part of the bipod over another. In this case, not just the head-to-head arrangement of the polar axis, but also the tail-to-tail arrangement, must be able to occur.

Recent developments in understanding the mechanism of five-power-law creep

This work describes recent advances on the understanding of elevated temperature plasticity within five-power-law creep. Particular attention is devoted to recent developments regarding dislocation network hardening, internal back-stresses and Harper–Dorn creep. This will include discussions of recent convergent beam electron diffraction experiments on metals to evaluate internal stresses in association with dislocation heterogeneities and Taylor hardening analyses in both copper and aluminum.

On the site occupancy of dopants in 4H-SiC

Silicon carbide (SiC) is a semiconductor material which is widely used for devices operating under extreme conditions. The fabrication of planar electronic devices requires a high concentration of pand ndopants. This can be archieved preferentally by ion implantation because of the small diffusion coefficients of dopants in SiC. After implantation, the implan-ted atoms predominantly occupy interstitial lattice sites [1], where they are usually not electrically active. Therefore, a thermal postimplantation annealing process is necessary to electrically activate the implanted dopants and to reduce the lattice damage, however, for the case of high foreign atom concentration, the SiC matrix remains defective always [2]. Another possibility to avoid lattice defects is to incorporate the doping element in-situ during the epitaxial growth of the crystal which can be performed by chemical vapour deposition (CVD) Concerning the site occupancy of p and n dopants a number of questions remains [3,4] from Hall mobility measurements. In this paper this question will be addressed using a combined analysis of convergent beam electron diffraction (CBED) and ALCHEMI (atomic location by channelling enhanced microanalysis) for the case of Al implanted 4H-SiC. The shift of Higher Order Laue Zone (HOLZ) lines appearing in CBED patterns can be used for detection of very small lattice parameter changes ([5,6,7]). However from this shift the origin of the strain can not be distinguished e.g. whether the foreign atoms occupy lattice or interstitial sites. By means of ALCHEMI the site occupancy of atoms within a matrix lattice can be determined [8]. This method has been applied for the case of Ge implanted in SiC already [9]. Aluminium implanted hexagonal 4H-SiC (concentrations: 5*10 cm for ion implantation at 160keV and 7*10 cm 2*10 cm for CVD implanation) have been investigated using CTEM, CBED and ALCHEMI. CBED patterns were obtained on [120 80 1] at an operating high voltage of 300kV. The EDX analysis was made on [01-10] zone axis incidence for tilts of ±1/2g to ±5/2g with g=0004. CBED patterns revealed different changes in HOLZ line shapes for different dopant concentrations, which can be attributed to different lattice distortion. Models of lattice distortion were suggested and calculated by Molecular Dynamics. Their influence on the HOLZ lines appearance was simulated by the multislice method. To determine the positions of the implanted atoms inside the unit cell, the CBED simulations have been carried out for probable locations of dopants in the SiC lattice (on lattice sites, on interstitials sites or both lattice and interstitial sites) by using the results of the ALCHEMI experiments. The comparison of ALCHEMI simulations and experiments (Fig. 1 and 2) showed that in the CVD growth case the implanted Al is mostly substituting Si in the matrix and in the ionimplanted sample Al is occupying Si-lattice positions and interstitials in equal parts. The CBED experiments (Fig. 3) show a line split of a line (1-1-22) in the CBED pattern in the ionimplanted samples and no split of that line in the CVD growth samples which can be understood from the substitution of Si by Al.

Control of the polarity of GaN films using an Mg adsorption layer

The polarity of GaN epilayers grown by molecular beam epitaxy is controlled using Mg. This is achieved by simultaneously exposing the surface to Mg and NH 3 fluxes during growth interruption. Reflection high-energy electron diffraction (RHEED) indicates the formation of a Mg 3N 2 layer. Overgrowing GaN on this surface leads to a polarity inversion either from Ga to N or N to Ga. The change of the polarity is followed in situ by RHEED via surface reconstructions of the GaN surface. The polarity inversion is further confirmed by convergent beam electron diffraction experiments. Finally, high-resolution transmission electron microscopy images show different interface morphologies between Ga/N and N/Ga polarity boundaries. The control of the GaN polarity opens the way for novel periodic polarity structures dedicated to non-linear optics.

Crystal polarity of sphalerite semiconductor compounds, as determined by convergent-beam electron diffraction experiments on plan-view (001) samples: application to ZnSe crystals

Abstract A novel convergent-beam-electron-diffraction method is discussed, which permits us to determine unambiguously the polarity of {001}-oriented semiconductor compounds with sphalerite structure. It is based on a comparison between experimental patterns and simulated patterns. At variance with {110}-oriented crystals, the polarity cannot be uniquely determined from observed intensity asymmetry in the ±{200} discs. Additional features have to be considered, such as the geometry and intensity of Kikuchi lines in the transmitted disc. Examples from ZnSe crystals are given.

Crystal polarity of sphalerite semiconductor compounds, as determined by convergent-beam electron diffraction experiments on plan-view (001) samples: Application to ZnSe crystals

Crystal polarity of sphalerite semiconductor compounds, as determined by convergent-beam electron diffraction experiments on plan-view (001) samples: Application to ZnSe crystals

Investigation of Defects and Polarity in GaN Using Hot Wet Etching, Atomic Force and Transmission Electron Microscopy and Convergent Beam Electron Diffraction

Availability of reliable and quick methods to investigate defects and polarity in GaN films is of great interest. We have used photo-electrochemical (PEC) and hot wet etching to determine the defect density. We found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM). Hot wet etching was used also to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic force microscopy (AFM). We have found that hot H3PO4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology. On the contrary, the acid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the morphology unchanged. The polarity assignments, confirmed by CBED experiments, were related to the as-grown surface morphology and to the growth conditions.

Investigation of Defects and Polarity in GaN Using Hot Wet Etching, Atomic Force and Transmission Electron Microscopy and Convergent Beam Electron Diffraction

Availability of reliable and quick methods to investigate defects and polarity in GaN films is of great interest. We have used photo-electrochemical (PEC) and hot wet etching to determine the defect density. We found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM). Hot wet etching was used also to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic force microscopy (AFM). We have found that hot H 3 PO 4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology. On the contrary, the acid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the morphology unchanged. The polarity assignments, confirmed by CBED experiments, were related to the as-grown surface morphology and to the growth conditions.

Stress relaxation in c⊥–c// YBaCuO thin films on MgO substrate studied by LACBED

Thin films of YBaCuO grown by in situ pulsed laser deposition on MgO substrate are studied by means of high resolution transmission electron microscopy imaging and large angle convergent beam electron diffraction experiments. Both c// and c⊥ YBaCuO crystals coexist in the film. The diffraction experiments are performed on the MgO substrate along the interface with the YBaCuO film. Some of the Bragg lines exhibit a dramatic broadening when the electron probe is just below the c// oriented crystal. This result is interpreted in terms of lattice strain relaxation due to the thinning of the TEM sample. This relaxation phenomenon indicates that the c// oriented grains embedded in a c⊥ oriented host matrix of YBaCuO, are under stress.

Quantitative zone-axis convergent-beam electron diffraction (CBED) studies of metals. II. Debye-Waller-factor measurements.

Quantitative CBED techniques, such as the ZAPMATCH zone-axis pattern-matching method [Bird & Saunders (1992). Ultramicroscopy, 45, 241-251], have been applied with great success to the accurate refinement of low-order structure factors. The major limitation on the accuracy of the structure-factor measurements is uncertainty in the Debye-Waller factors describing the temperature-dependent atomic vibrations. While X-ray and neutron diffraction tech-niques are both capable of accurate measurements of Debye-Waller factors, the frequent use of liquid-nitrogen-cooled samples in CBED experiments means that previous measurements are rarely available at the temperatures required. This has prompted attempts to determine Debye-Waller factors from electron diffraction data obtained under experimental conditions that match those used for the quantitative CBED work. In this paper, the possibility of extracting accurate Debye-Waller factors from the low-order reflections of a zone-axis CBED pattern is investigated. In this way, the Debye-Waller factors and structure factors could be obtained from the same data set. With this new approach, it is shown that errors lower than +/-0.02 Å(2) can be obtained for the measurement of Debye-Waller factors from room- and liquid-nitrogen-temperature nickel and copper <110> zone-axis data. The results obtained are compared with previous measurements and theoretical predictions.

Elastic Strain Relaxation in Si1-xGex Layers Epitaxially Grown on Si Substrates

We have investigated the elastic strain relaxation in Si 1-x Ge x layers grown by the molecular beam epitaxy (MBE) technique and in situ controlled with RHEED. Up to ≈0.8% critical lattice mismatch (about 20% Ge) uniform strained and flat layers were grown both on (111) and on (001) Si substrates. Calculations of the elastic constants evidenced a tetragonal distortion about 50% higher on (001) than on (111) in the same experimental conditions. At higher misfits (and/or thicknesses) a growth instability was evidenced only on (001) Si substrates. Si 1-x Ge x layers there displayed a surface layer undulation. On the contrary, Si 1-x Ge x layers grown on (111) Si substrates remained smooth throughout the growth up to the plastic relaxation of the layers. To determine stress fields in the Si 1-x Ge x layers, a high spatial resolution convergent beam electron diffraction (CBED) experiment was performed with a field effect analytical microscope. The CBED technique was applied to two typical cases: totally strained layer and undulated dislocation-free layer. In the latter case, CBED patterns recorded on nanometer scale areas of an undulation crest (cross-section sample) showed a gradual elastic relaxation mainly directed along the growth axis (z). Moreover a triclinic distortion of the unit cell was pointed out. These results were confirmed on a plane view sample. In conclusion, our results show that the driving force for the undulation is not the in-plane elastic relaxation since CBED experiments proved an important elastic relaxation of the (001) Si 1-x Ge x layers along the z axis. This was in agreement with the calculations of the elastic constants. We think that this could be at the origin of the undulation.

The key role of polarity in the growth process of (0001) nitrides

A direct method for the determination of the polarity of (0001) wurtzite GaN films is presented, using a combination of ion channeling and convergent beam electron diffraction experiments (CBDE). The samples were grown by metal organic chemical vapor deposition (MOCVD) on (0001) Al 2O 3. Depending on the nitridation conditions of the buffer, the samples are flat or rough. It is found that flat samples are Ga-terminated. The rough samples exhibit numerous pyramidal defects. In that case, a mixture of N- or Ga-terminated domains is put in evidence. These results unambiguously demonstrate that the nitride layer quality is closely related to the control of the polarity from the very first stages of the growth.

Observation and analysis of extended dislocations in an Al–Pd–Mn icosahedral quasicrystal by transmission electron microscopy

Extended dislocations including partial dislocations and a stacking fault in Al70Pd20Mn10 icosahedral quasicrystal have been observed and identified for the first time. The diffraction contrast and defocus convergent-beam electron diffraction experiments show that the dissociation of the extended dislocations is of the form 1/2&lt;1 −2 0 0 −2 1&gt; → 1/4&lt;1 −3 1 −1 −1 1&gt;+ 1/4&lt;1 −1 −1 1 −xs3 1&gt; with a stacking fault between these two partial dislocations. For the partial dislocations, the Burgers vector components in physical space b¶part are along different fivefold axes with a magnitude of 0.17 nm, which is about one seventh of that in complementary space. For the perfect dislocation, the Burgers vector component in physical subspace b¶perf is along a twofold axis with a magnitude of 0.183 nm, which is about an eleventh of that in complementary space.

Structure factor measurement in TiAl and silicon

The accurate measurement of low order structure factors is required for the determination of the electron charge density distribution in crystals. In this work the energy-filtered convergent beam electron diffraction (CBED) rocking curve method has been used for accurate structure factor measurements. This CBED method for structure factor refinement involves matching of the experimental CBED intensities to those calculated using dynamical electron diffraction theory. The CBED experiments were conducted with a Philips EM420 Transmission Electron Microscope coupled with a custom built energy-filtering attachment enabling single electron counting. The theoretical pattern matching was performed using FORTRAN programs which were developed by Swaminathan. Initially the experimental plan involved an attempt to refine structure factors of TiAl by two dimensional Bloch wave calculations. The results of this project have been reported elsewhere. Subsequently it proved impossible to obtain results with sufficient precision for TiAl reproducibly, i.e. less than 0.1%, from samples of different thicknesses.

Friedel's law and non-centrosymmetric space groups

It is shown that, in projections of non-centrosymmetric space groups having symmetry p3ml or p31m, loci exist along which Friedel's law is obeyed even in the presence of n-beam interactions and phenomenological absorption. This theoretical prediction is verified by means of convergent-beam electron diffraction experiments on Li2ZnTi3O8.

Simultaneous Identification of the Bravais Lattice and Glide Planes From Microdiffraction Patterns: Deduction of the Possible Space Groups

Microdiffraction experiments obtained with a small spot size and a nearly parallel electron beam are well-adapted to most specimens and especially to small particles. They are particularly useful when Convergent Beam Electron Diffraction (CBED) patterns do not give the 2D and 3D information required to identify the point and the space groups. They can also be used to start the identification or the determination of a crystal structure. Specific and simplest CBED experiments are later realized to remove left ambiguities.The microdiffraction Zone Axis Patterns (ZAP) are composed of sharp reflections whose high angular resolution helps to appreciate the shifts and the periodicity differences between the Zero (ZOLZ) and First (FOLZ) Order Laue Zones reflection nets. Observations of thin areas at low temperature are recommended in order to produce many well visible FOLZ reflections.It was already indicated that the crystal system and the Laue class are deduced from observation of the whole pattern symmetry of the reflections.

Transmission electron microscopy of Ru 7B 3 borides

Thin foils of Ru 7B 3 boride have been examined by convergent beam electron diffraction (CBED), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The P6 3 mc space group for the structure of Ru 7B 3 was confirmed by CBED experiments. HREM images of inversion boundaries in this boride reveal a rigid body translation between the domains either side of these boundaries in addition to the inversion operation. A model of inversion boundaries in Ru 7B 3 which is consistent with these experimental observations is proposed.