Large-angle Convergent-beam Electron Diffraction Technique Research Articles

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.

On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys – An application of the LACBED method to complex microstructures

Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000°C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <110> and the more complex <100> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <100> superdislocation, which shows pronounced residual contrast even under conditions of g·b=0 and g·b×u=0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed.

Convergent-beam electron diffraction

This paper reviews the convergent-beam electron diffraction (CBED) technique. Point- and space-group determination methods of ordinary crystals are described, along with an example of the determination method for Sr₃Ru₂O₇. The symmetry determination of one-dimensionally incommensurate crystals and quasicrystals is explained. The large-angle CBED technique, which is indispensable for lattice defect and lattice strain analysis, is also described. A real procedure for lattice strain analysis is provided, using an example of a multilayer Si₁-xGe(x)/Si material. A nanometer-scale crystal structure refinement method and charge density and crystal potential determination method by CBED are briefly described.

CBED and LACBED characterization of crystal defects

Convergent-beam electron diffraction (CBED) obtained with a focused incident beam is well known for the identification of the point and space groups but it can also be used for the analysis of stacking faults and antiphase boundaries. Large-angle convergent-beam electron diffraction (LACBED) is performed with a large defocused incident beam and is well adapted to the characterization of most types of crystal defects: point defects, perfect and partial dislocations, stacking faults, antiphase boundaries and grain boundaries. Among the advantages of these methods with respect to the conventional transmission electron microscopy methods, are that one or few patterns are required for a full analysis and the interpretations are easy and unambiguous. The LACBED technique is particularly useful for the analysis of dislocations present in anisotropic and beam-sensitive materials.

Lattice strain and static disorder in hydrogen-implanted and annealed single-crystal silicon as determined by large-angle convergent-beam electron diffraction

The large-angle convergent-beam electron-diffraction technique, available in a conventional transmission electron microscope, has been employed in order to characterize the defective layer present in heavily damaged high-dose hydrogen-implanted and low-temperature annealed $(T&lt;~500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ single-crystal silicon in terms of lattice strain and static disorder. These quantities have been measured to determine the mean relaxation volume and atomic concentration of defect clusters present in the implanted layers in order to investigate the structural features causing the reverse annealing phenomena observed in ion channeling measurements. In particular, the mean relaxation volume detected in the 300 \ifmmode^\circ\else\textdegree\fi{}C 2-h annealed sample (0.1 ${\mathrm{nm}}^{3}$) results in a factor three times higher than that measured in the as-implanted sample; on the contrary the mean atomic concentration of clusters does not vary appreciably after this thermal treatment. This experimental evidence suggests a nonconservative growth of clusters in the low-temperature annealing regime. After annealing at 500 \ifmmode^\circ\else\textdegree\fi{}C for 2h, an increase of the relaxation volume and a significant decrease of the mean concentration is found, thus suggesting that only after this thermal treatment, producing in the meantime intrinsic defects with extended internal surfaces, defects seem to follow a conservative ripening.

LACBED study of a Σ3 grain boundary in a Cu +6 at% Si alloy

A new application of the large-angle convergent-beam electron diffraction technique for the determination of the displacement vector R at a Σ3 grain boundary in Cu+6 at% Si alloy is presented here. It is shown that in large-angle convergent-beam diffraction many high-index reciprocal coincident site reflections, g c , can be utilized to obtain the phase factor of g c · R , allowing a high accuracy for the determination of R . In this study the value of R at an asymmetric boundary of (1 2 1)/(5 2 5) in a (1̄ 2 1̄) twin is accurately determined. This result indicates that there is no excess volume at this boundary.

Analytical electron microscopy of Si1−xGex/Si heterostructures and local isolation structures

AbstractThe convergent beam electron diffraction (CBED) technique has been applied to determine the lattice strain in Si1−xGex/Si heterostructures and in local isolation structures. Both plan and cross-sections have been investigated by transmission electron microscopy. In the heterostructures, the strain value obtained by CBED along the growth direction ɛCRED is affected by a relaxation induced by the thinning process in a direction normal to the cross-section plane, being generally smaller than the bulk tetragonal value ɛT This effect can be overcome using the large angle CBED technique on plan sections. In addition, in the heterostructures the Ge concentration has been determined by energy dispersive X-ray spectrometry, allowing the pseudomorphicity of the samples to be evaluated. All the values of strain and Ge concentration thus obtained are in good agreement with those deduced from Rutherford backscattering spectrometry. In isolation structures, the CBED technique has been applied to determine the d...

Analysis of partial and stair-rod dislocations by large angle convergent beam electron diffraction

It is shown how large angle convergent beam electron diffraction (LACBED) can be used to analyse the Burgers vectors of partial dislocations in Si, GaAs and CdTe. By selecting reflections in which adjoining stacking faults are out of contrast, it is demonstrated that the LACBED technique can be used to analyse Shockley and Frank partial dislocations, and stair-rod dislocations of 1 6 〈110〉 type present at stacking fault intersections. The potential of the technique for studying grain boundary and dissociated dislocations in general is discussed.

Effects of dislocation strain fields on Bragg lines in an Al70Co15Ni15 decagonal quasicrystal studied by an improved LACBED technique

Effects of dislocation strain fields on Bragg lines in an Al70Co15Ni15 decagonal quasicrystal have been studied by using an improved large-angle convergent-beam electron diffraction technique. It is found that the effects are the same as that in conventional crystals.

Burgers vector determination of dislocations in an Al70Co15Ni15decagonal quasicrystal

Abstract Not only the directions, but also the senses and magnitudes of the Burgers vectors b of dislocations in Al70Co15Ni15 decagonal quasicrystal have been determined for the first time by the large-angle convergent-beam electron diffraction technique. Right−hand screw dislocations with b = [3/400000] and [100000] have been determined.

{110} twinning in YBa2Cu3O7-x

Abstract Large-angle convergent-beam electron diffraction (LACBED) has been used in conjunction with dark-field imaging to determine the nature of {110} twinning in YBa2Cu3O7-x (YBCO). The orthorhombic ratio (b-a)/a has been shown to vary considerably across a number of twin domains in annealed material from 0.006 to 0.02. A number of samples were irradiated at different temperatures (by electrons and ions) and at room temperature this was seen to promote the degradation of the orthorhombic phase to its tetragonal counterpart. This was measured quantitatively using the LACBED technique. The coexistence of all four twin variants of the orthorhombic structure within the same region of crystal was examined in detail. The nature of the crystal strain associated with this phenomenon was investigated both by dark-field imaging and by LACBED. Kinematic simulations of trial displacement fields based on the misfit between intersecting twins are shown to agree closely with experimental patterns. The strain field al...

Convergent-beam electron diffraction from AlGaAs/GaAs single quantum wells

Abstract The large-angle convergent-beam electron diffraction technique has been used to obtain 200 rocking curves from a l500 Å Al,0·4 Ga0·6 As/30Å GaAs/3500Å Al0·4 Ga0·6 As sample observed in plan view. It is demonstrated that the rocking curves are sensitive to the presence of the GaAs layer and are in good agreement with simulations calculated from a simple kinematical approach. It is shown that both the layer thickness and depth in the foil may be examined and that GaAs layer thicknesses less than 10 Å in the 5000 Å foil may be detectable.