Burgers Vectors Of Misfit Dislocations Research Articles

Lattice mismatch and twist partitioning at commensurate dichromatic pattern of two-phase interfaces

Misfit dislocations are formed at a two-phase interface to reduce and even diminish coherency stress in the region far from the interface. Such semi-coherent interfaces are key structural features in a wide range of engineering materials. Burgers vectors of misfit dislocations are defined with the reference lattice named as commensurate dichromatic pattern (CDP) in the topological model. The CDP is not a geometrical average of boundary units as historically used in many examples. In this work, based on the Green’s function and theory of dislocations, both the mechanical effects of interfacial dislocation arrays and coherency strains due to geometry match at the interface are considered to get the CDP. We demonstrated that the mismatch and twist partitioning at a CDP are unequally partitioned in the two adjacent crystals, which only depends on elastic properties of the two crystals regardless of characters of misfit dislocations. Correspondingly, the method to determine the CDP of a two-phase interface in bi-crystal is developed.

Determining coherent reference states of general semicoherent interfaces

We present a new method for determining the unique reference state in which the Burgers vectors of misfit dislocations in semicoherent interfaces are defined. Similar to previous work, our method requires cancelation of coherency and dislocation stresses far from the interface as well as consistency of far-field rotations with a prescribed interface crystallographic character. We compute misfit dislocation elastic fields using Stroh’s formalism and the Sextic theory, accounting fully for elastic anisotropy. The method is applicable to all types of interfaces, including ones with three sets of dislocations, and does not require the definition of a transformation path between adjacent crystals. We examine the accuracy of our method by comparing it with previous results on interfaces with one or two sets of dislocations. We then use it to carry out the first calculations of the coherent reference state of an interface with three sets of dislocations.

Defects in nonpolar (134¯) ZnO epitaxial film grown on (114) LaAlO3 substrate

The defects in (134¯0)ZnO epitaxial film grown on (114)LaAlO3 (LAO) have been systematically investigated by using transmission electron microscopy. At the ZnO/LAO interface, the Burgers vectors of misfit dislocations are identified to be 1/3[1¯21¯0] and 1/2[0001]. Threading dislocations with the Burgers vectors of 1/3⟨112¯0⟩ and ⟨0001⟩ are distributed on the basal plane. In (134¯0)ZnO film, the predominant planar defects are basal stacking faults (BSFs) with 1/6⟨202¯3⟩ displacement vectors. The densities of dislocations and BSFs are about 3.8 × 1010 cm−2 and 3.1 × 105 cm−1, respectively.

Mechanism of formation of the misfit dislocations at the cubic materials interfaces

High-angle annular dark-field scanning transmission electron microscopy and molecular dynamic simulation are applied to study the misfit dislocations at the GaSb/GaAs interface. In the investigated samples, three types of misfit dislocations have been observed: shuffle and glide set Lomer dislocations and 60° dislocation pairs. The dislocation density tensor analysis is next used to quantify the Burgers vector of misfit dislocations and investigate the misfit dislocation formation mechanism. This work demonstrates that, in these hetero-structures, the dominant mechanism underlying the formation of misfit dislocations is the glide and reaction of 60° dislocations. It is shown that the final structure of each misfit dislocation depends on the Burgers vectors of the initial 60° dislocations. Finally, this analysis points out an approach to determine the local rotation at interface due to mixed type dislocations.

Atomistic study of misfit dislocation in metal/SiC(111) interfaces

The interatomic potentials across metal/SiC(111) interfaces are derived fromab initio adhesive energies by an inversion method. We use these potentials toinvestigate the structures, energies and Burgers vectors of misfit dislocations inmetal/SiC(111) interfaces. Two kinds of interfacial dislocations are found in M/SiC(111) (M = Au, Ag, Al, Pt) interfaces, where the M/SiC(111) (M = Au, Al) system has partial dislocations and the M/SiC(111) (M = Ag, Pt) system has perfect dislocations. The former has a coherent interface while the lattercorresponds to a semi-coherent interface.

Scanning moiré fringe imaging by scanning transmission electron microscopy

A type of artificial contrast found in annular dark-field imaging is generated by spatial interference between the scanning grating of the electron beam and the specimen atomic lattice. The contrast is analogous to moiré fringes observed in conventional transmission electron microscopy. We propose using this scanning interference for retrieving information about the atomic lattice structure at medium magnifications. Compared with the STEM atomic imaging at high magnifications, this approach might have several advantages including easy observation of lattice discontinuities and reduction of image degradation from carbon contamination and beam damage. Application of the technique to reveal the Burgers vector of misfit dislocations at the interface of epitaxial films is demonstrated and its potential for studying strain fields is discussed.

Novel TEM methods for large-area analysis of misfit dislocation networks in semiconductor heterostructures

Transmission electron microscopy (TEM) is a powerful tool for detailed investigations of individual misfit dislocations and misfit dislocation networks in semiconductor heterostructures. However, the characterization of large numbers of misfit dislocations distributed over large network areas has traditionally been a challenge. Such analyses are important for detailed understanding of phenomena, such as layer mosaicity, that originate from the misfit dislocations, but develop features on a length scale of several tens of microns. Moreover, quantitative interpretations of X-ray diffraction data, based on models that describe the arrangement of misfit dislocations in statistical terms, call for validation by a large-area analysis with microscopic details. In this paper, novel TEM sample preparation and analysis methods are presented, which allow for the efficient study of large numbers of misfit dislocations and provide new insight into the formation of misfit–dislocation networks. The methods discussed are: (i) a double-bevel-polish (DBP) sample preparation for systematic depth-sensitive large-area plan-view TEM imaging of three-dimensional misfit-dislocation networks, (ii) a bevel-polish and cleavage (BPC) sample preparation for efficient large-area Burgers vector analysis of misfit–dislocation networks, (iii) the use of bend contours for large-area plan-view analysis of the Burgers vectors of misfit dislocations in single networks and (iv) the post-growth retrieval of the sequence of dislocation formation by studying antiphase boundaries formed during dislocation glide in layers showing long-range atomic ordering. The feasibility of each method is demonstrated and their potential applications discussed.

HRTEM Observation and Atomic Modeling of α/β Interphase Boundary in a Ti-22V-4Al Alloy

In order to study interface boundary structures, a three dimensional Near-Coincidence Site lattice (Near-CSL) model based on Bollmann’s O-lattice method has been proposed. Previous studies have shown that intragranular α-phase precipitates in a Ti–22V–4Al alloy have a morphology of lath-like, with two well defined facet planes. A detailed TEM observation of the laths has been made in this study and the results correlated to the present analysis. Accommodation mechanism of misfit dislocations and the Burgers vector of the defects observed at the broad face and side facet plane of the intragranular α-phase precipitates have been determined. The structural ledges at the side facet, as predicted by the present analysis, is consistent with HRTEM observation. The Burgers vector of misfit dislocations at the broad face is in good agreement with that previously reported for a Zr–2.6Nb alloy, that has almost the same lattice parameter ratio as the Ti–22V–4Al alloy used in the present study. Although the Burgers vector of the defects on the broad face can be determined by HRTEM observation, the boundary plane is not well defined. However, it can be demonstrated that the interface boundary structure of the broad face can be rationalized from the results of both the Near-CSL analysis and the HRTEM observations.

Electron Microscopy Study of the Eutectic Structure in Ti<SUB>40</SUB>Ni<SUB>40</SUB>Nb<SUB>20</SUB> Alloy

The Ti40Ni40Nb20 eutectic is an interesting material with respect to the applications of shape memory alloys. The eutectic, which consists of TiNi(B2) α phase and Nb-rich(bcc) β phase, was studied by conventional and high resolution electron microscopy, and the following results were obtained. The morphology was invariably lamellar with the interface plane parallel to {110}α//{110}β. The orientation relationship was such that[100]α//[100]β[010]α//[010]β[001]α//[001]βThus, the equivalent directions and planes are completely parallel. Steps and facets on a larger scale were found along the interfaces, and the Burgers vectors of misfit dislocations were analyzed by the O-lattice theory.

Identification of the misfit dislocations at an FeAl/AlAs/GaAs interface using moiré fringe contrast in a transmission electron microscope

Moiré fringe contrast in plan-view transmission electron microscopy (TEM) is adapted to measure the Burgers vector of misfit dislocations at the interface between FeAl and AlAs. This technique had originally been used to determine the Burgers vector of dislocations in bulk materials. The aluminide was grown by molecular beam epitaxy on AlAs which was pseudomorphic on GaAs(001). The observed misfit dislocations are determined to have [100] and [010] Burgers vectors, as measured in the FeAl, with [010] and [100] line directions, respectively. These are pure edge dislocations which cannot glide on the {110} or {112} slip systems of FeAl. This requires that the misfit dislocations either form at the edges of islands, during three dimensional (3D) growth or by climb from the free surface during two-dimensional (2D) growth. The TEM results along with in situ reflection high-energy electron diffraction (RHEED) results show that the growth is indeed 2D which suggests the misfit dislocations must form by dislocation half-loops climbing from the free FeAl surface.

Misfit Dislocations in the Interface between the Metallic and Insulating Phases in Cr Doped V203

ABSTRACTThe formation of misfit dislocations in the interface between the metallic and insulating phases which occur in 1.2% Cr doped V203 near room temperature was confirmed by TEM. Orientations of Burgers vectors of misfit dislocations were determined by the weak beam technique. The Burger vectors were parallel to principal axes in the basal plane (<100>) and inclined somewhat to interfaces. The Burgers vectors of misfit dislocations are, however, close enough to the direction of the maximum misfit in the interface. Only one or two misfit dislocations appear in an interface regardless of the thickness of specimens, and these were always located near the surface.

Misfit dislocation characteristics in quarternary heterojunctions Ga 1- xAl xAs 1- yP y/GaAs analysed by synchrotron radiation white beam topography

A typical example of low misfit quaternary heterojunction Ga 0.7Al 0.3As 1- y P y /GaAs has been characterized by several X-ray techniques. The sources, propagation conditions and Burgers vectors of misfit dislocations have been identified by use of synchrotron radiation X-ray topography. Most of the dislocations are of 60° type, a few edge dislocations were found to result from reactions.