Perfect Misfit Dislocations Research Articles

Generation of misfit dislocations in a core-shell nanowire near the edge of prismatic core

The misfit strain relaxation via generation of partial and perfect misfit dislocations and their dipoles at the interface in core-shell nanowires with faceted cores is considered. The core has the shape of a long parallelepiped of a square cross-section and is placed symmetrically with respect to the cylindrical shell surface. The energy change caused by dislocation generation in such nanowires is calculated. Critical conditions for the onset of such dislocations are calculated and analyzed. The energy barriers for the dislocation generation are defined with respect to different values of the nanowire parameters. The preference of dislocation configuration and generation sites in the nanowire are determined.

Generation of Misfit Dislocations in a Core-Shell Nanowire Near the Edge of Prismatic Core

The misfit strain relaxation via generation of partial and perfect misfit dislocations and their di-poles at the interface in core-shell nanowires with faceted cores is considered. The core has the shape of a long parallelepiped of a square cross-section and placed symmetrically with respect to the cylindrical shell surface. The energy change caused by dislocation generation in such nanowires is calculated. Critical conditions for the onset of such dislocations are calculated and analyzed. The energy barriers for the dislocation generation are defined with respect to different values of the nanowire parameters. The preference of dislocation configuration and generation sites in the nanowire are determined.

Molecular dynamics studies on the interface evolution characteristics and deformation mechanisms of Cu/Al multilayers during compression process

The interface evolution characteristics and deformation mechanisms of Cu/Al multilayers are investigated via systematic molecular dynamics simulations. It is found that both the yield strength and ductility increase slightly with increasing strain rate, and the stress-strain curves exhibit two main yield points for all strain rate loadings. The first yield point correlates with the decomposition of perfect misfit dislocations on the interface and the propagation of partial dislocations inside the Al layer, and the second yield point relates with the dislocation transmission from the Al layer into the Cu layer. The lower the loading strain rate, the more severe the fluctuations on the stress-strain curve. However, the strain rates do not change the evolution way of dislocation networks. The calculated evolution curves of dislocation numbers indicate that the dislocation density inside the Cu layer is lower than that inside the Al layer. The interface region displays a serrated structure without voids or cracks, and the higher the loading strain rate, the more serious the interface roughening deformation. The main deformation mechanisms, respectively, are the formation of a lamellar twin structure in the Cu layer and dislocation slip in the Al layer, and the interface roughening is mainly dominated by the formation of a lamellar twin structure. Furthermore, the deformation mechanisms do not depend on the strain rate applied in this paper. In addition, we also discuss the growth curve of interface thickness which is divided into three stages.

Effect of substrate miscut on the microstructure in epitaxial Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films

Pb(Mg1/3Nb2/3)O3-PbTiO3 is known for its excellent ferroelectric and electrocaloric properties. Epitaxial films, which allow applying high voltages, can be grown in a much broader parameter range when depositing them on miscut substrates than on those without miscut. To understand the influence of substrate miscut, here the structure and microstructure of two films, with and without miscut, is compared in detail. In both cases the epitaxial strain is relaxed, but while in films without miscut perfect misfit dislocations are observed, the film with miscut exhibits many partial dislocations and stacking faults. The stacking faults may play an important role for the stabilization of the pure perovskite Pb(Mg1/3Nb2/3)O3-PbTiO3 phase in thin films on miscut substrates.

Nature of interfacial defects and their roles in strain relaxation at highly lattice mismatched 3C-SiC/Si (001) interface

Misfit defects in a 3C-SiC/Si (001) interface were investigated using a 200 kV high-resolution electron microscope with a point resolution of 0.194 nm. The [110] high-resolution electron microscopic images that do not directly reflect the crystal structure were transformed into the structure map through image deconvolution. Based on this analysis, four types of misfit dislocations at the 3C-SiC/Si (001) interface were determined. In turn, the strain relaxation mechanism was clarified through the generation of grow-in perfect misfit dislocations (including 90° Lomer dislocations and 60° shuffle dislocations) and 90° partial dislocations associated with stacking faults.

Delocalized dislocations in quantum dots

A theoretical model describing nucleation of the misfit dislocations with delocalized cores in island films is proposed. The parameters of nanoislands (quantum dots) with such delocalized misfit dislocations in the Ge/Si system are estimated. Within the framework of the proposed model, nucleation of the delocalized misfit dislocations at the quantum dots in a broad range of parameters is energetically more favorable than nucleation of the perfect misfit dislocations usually considered in the models of dislocated quantum dots.

Perfect, partial, and split dislocations in quantum dots

A mechanism for relaxation of misfit stresses in nanoislands (quantum dots) is theoretically examined which is the formation of partial and split misfit dislocations. The parameters of nanoislands with perfect, partial, and split misfit dislocations are estimated and compared, with emphasis on the case of Ge/Si nanoislands. Different dislocation structures are shown to be energetically preferred in different regions of the nanoisland/substrate interface. It is theoretically revealed that perfect misfit dislocations (conventionally considered in models of dislocated nanoislands) are not energetically favorable in pyramidlike nanoislands in Ge/Si system. Also, it is shown that misfit dislocation formation is capable of causing nanoisland shape transformation as an energetically favorable process.

⟨310⟩misfit dislocations in ZnSe/GaAs(001) heterostructure

Strain relaxation in ZnSe/GaAs(001) heterostructure grown by molecular beamepitaxy is studied by transmission electron microscopy. In as-grown samples, anarray of perfect misfit dislocations, lying along ⟨310⟩directions, with Burgers vector (1/2)⟨011⟩inclined to the interface is observed. The corresponding threading segmentspropagate by glide in {331} planes, leaving misfit segments in the interface.From a mechanical equilibrium analysis, it is concluded that, in the caseof low misfit (0.27%), the critical thickness for {331} planes is lessthan for {111} glide. Dislocations with the (1/2)⟨011⟩Burgers vector lying along ⟨310⟩ directionsare more efficient at relaxing the misfit strain than dislocations lying along ⟨110⟩directions.

Generation and evolution of partial misfit dislocations and stacking faults in thin-film heterostructures

An analysis is made of the specific features in the generation and evolution of partial misfit dislo- cations at the vertices of V-shaped configurations of stacking fault bands, which terminate in the bulk of the growing film at 90 ° partial Shockley dislocations. The critical thicknesses h c of an epitaxial film, at which gen- eration of such defect configurations becomes energetically favorable, are calculated. It is shown that at small misfits, the first to be generated are perfect misfit dislocations and at large misfits, partial ones, which are located at the vertices of V-shaped stacking-fault band configurations emerging onto the film surface. Possible further evolution of stacking-fault band configurations with increasing film thickness are studied. © 2001 MAIK "Nauka/Interperiodica".

Transmissionelectron microscopy study of crystal defects in ZnSe/GaAs(001)epilayers

ZnSe thin films grown on GaAs(001) substrate by molecular beamepitaxy to a thickness of 2500 Å have been studied bytransmission electron microscopy (TEM). Three types of structuraldefect have been observed: (i) Triangle-shaped stacking faults,with the apex close to the interface, either isolated or paired.They are bounded by two different Shockley dislocations. (ii)Stacking faults generated from the surface of the ZnSe epilayer bymovement of a Shockley half-loop. (iii) An array of perfect misfitdislocations. Their Burgers vectors are inclined to the interface.Most of them lie along ⟨310⟩ directions; only a feware parallel to ⟨110⟩.

Transmission electron microscopy study of triple-ribbon contrast features in a ZnTe epitaxial layer

A transmission electron microscopy study of triple-ribbon contrast features in a ZnTe layer grown epitaxially on a vicinal GaAs (001) substrate is reported. The ribbons go through the layer as threading dislocations near the [112] (111) or [112] (111) directions. Each of these (with a 40 nm width) has two narrow parts enclosed by three partial dislocations (with a 20 nm spacing). By contrast analysis and contrast simulation, the ribbons have been shown to be composed of two partially overlapping stacking faults. Their origin is attributed to a forced reaction between two crossing perfect misfit dislocations.

Strain relaxation in lattice-mismatched epitaxy

Growth of lattice mismatched layers epitaxially on top of each other is a challenge for today's semiconductor industry. For strained layer devices, it requires that the strained layers are not relaxed through the formation of misfit dislocations. For other device applications, such as GaAs on Si, it may be desirable that the overlayers are completely relaxed and all the misfit defects are confined to the lower interfaces. This article investigates the theories on strain relaxation mechanisms in face-centered-cubic (f.c.c.) systems. The tensile layers are found to initially relax through the formation of partial dislocations, at a critical thickness about half the thickness predicted by the conventional model based on the formation of perfect misfit dislocations. This poses a stringent condition on the maximum thickness useful for tensile layer devices. We also explain the difficulties involved in preparing thick strained layers with few threading dislocations and microtwins.

High-resolution electron microscopy of epitaxial YBCO/Y2O3/YSZ on Si(001)

The microstructures of YBa2Cu3O7−δ (YBCO) thin films grown on Si with Y-stabilized ZrO2 (YSZ) and Y2O3 buffer layers were characterized by means of high-resolution electron microscopy. At the Si–YSZ interface, a 2.5 nm thick layer of regrown amorphous SiOx is present. The layer is interrupted by crystalline regions, typically 5 to 10 nm wide and 10 to 50 nm apart. Close to the crystalline regions, {111} defects are present in the Si substrate. The typical defect observed is an extrinsic stacking fault plus a perfect dislocation close to the stacking fault which terminates extra {111} planes in the upper part of the Si. These defects are probably formed by condensation of Si self-interstitials created during oxide regrowth. Precipitates are present in the Si close to the Si–YSZ interface and indicate that in-diffusion of Zr has occurred. The YSZ–Y2O3 interface is atomically sharp and essentially planar and contains no second phases. Perfect misfit dislocations with Burgers vector 1/2〈110〉 are present at this interface along with unrelaxed elastic misfit stresses. The Y2O3–YBCO interface is atomically sharp and planar, but contains steps. (001) stacking faults are present in the YBCO above these steps; the faults are, however, healed a few unit cells away from the interface. By HREM analysis of ultrathin specimen areas, the atomic layer of the YBCO closest to the Y2O3 was found to be a barium-oxygen layer.

YBa2Cu3O7−δ films on Si with Y-stabilized ZrO2 and Y2O3 buffer layers: High-resolution electron microscopy of the interfaces

The interfaces of YBa2Cu3O7−δ (YBCO) thin films grown on Si substrates with Y-stabilized ZrO2 (YSZ) and Y2O3 buffer layers have been studied by high-resolution electron microscopy. At the Si-YSZ interfaces a 4-nm-thick amorphous silica layer is found, bridged by 10-nm-wide crystalline YSZ regions. Close to these regions the Si substrate contains planar {111} faults. At the YSZ-Y2O3 interfaces perfect misfit dislocations are present, with Burgers vector 1/2〈110〉. They occur either as separated single dislocations or as separated groups of closely spaced dislocations. The atomic coordination at the planar and atomically sharp Y2O3-YBCO interface could be revealed. The first atomic layer of the YBCO is found to be a Ba layer.

Metastable and equilibrium defect structure of II–VI/GaAs interfaces

Defect characterization by high-resolution electron microscopy is presented for (001) epitaxy of the CdTe/GaAs and ZnTe/GaAs heterojunctions, before and after a long vacuum anneal. The annealed interface structure consists of a periodic array of perfect edge Lomer misfit dislocations, with spacing corresponding to a strain-free thin film. Since this is the most efficient manner to obtain complete relaxation, it represents the equilibrium microstructure. The as-deposited films are very thick, three to four orders of magnitude greater than the critical thicknesses, which are both less than a monolayer for these large lattice mismatch systems. Their microstructure corresponds to a metastable distribution of defects in that the thin film residual strain is nearly zero and neighboring defects can react to form Lomer misfits. A variety of defects exist both in the form of perfect misfit dislocations at the interface and extended defects into the thin film. The extended defects result from formation of stacking faults bounded by either Shockley or Frank partials, and more complicated defect structures due to interacting perfect and partial dislocations on intersecting slip planes. The purpose of this paper is to investigate the means by which complete stress relaxation can occur through dislocation reactions during annealing of very thick as-deposited films.

Misfit Dislocations at II-VI/GaAs Interfaces

AbstractHigh-resolution electron microscopy -is used to characterize the defect structure of CdTe/GaAs and ZnTe/GaAs heterojunctions before and after annealing. For as-deposited films, a variety of defects exist both in the form of perfect misfit dislocations at the interface and extended defects into the thin film. The extended defects result from dissociation of 60° dislocations and reactions between perfect and partial dislocations lying on intersecting slip planes. The annealed interfaces consist of a periodic array of perfect edge Lomer dislocations, the most efficient type of misfit dislocation for accomodating the lattice mismatch, 14.6 % for CdTe/GaAs and 8 % for ZnTe/GaAs. In both cases, the spacing between dislocations corresponds to the value predicted for completely strain-free thin fims, 31 and 54 Å for CdTe and ZnTe respectively. This paper concentrates on the different dislocation reactions which transform the interfacial structure from the as-deposited case to the annealed case.

Effect of the sign of misfit strain on the dislocation structure at interfaces of heteroepitaxial GaAsxP1−x films

It was shown, with the use of transmission electron microscopy, that the dislocation structure at interfaces of heteroepitaxial GaAsxP1−x films is dependent on the sign of the misfit strain. Under tensile strain the interface contains a square grid network of both perfect and partial misfit dislocations. The dislocations are straight and exist predominantly along 〈110〉 directions. This configuration of misfit dislocations is consistent with the nucleation of partial dislocations in tensile strained films. The compressive interface also contains a network of misfit dislocations many of which have curved segments. This indicates that a cross slip mechanism has occurred that is only possible with a perfect dislocation movement. This result supports the nucleation of perfect dislocations in films under a compressive strain. The different structural characteristics of the tensile and compressive systems can play a significant role in subsequent processing or devices made from them.

Structural properties of the ZnSe/GaAs system grown by molecular-beam epitaxy

We report the results of an investigation of the structural properties and relaxation of misfit stress with transmission electron microscopy and x-ray diffraction techniques. Epitaxial films of ZnSe were grown on GaAs by molecular-beam epitaxy of thicknesses ranging from 0.05 to 4.9 μm. The films contain stacking fault defects up to thicknesses of about 150 nm. Above 150 nm perfect misfit dislocations are generated from surface sources and the stacking fault defects to accommodate the lattice mismatch. The majority of dislocations observed are of the 60° type with Lomer edge type dislocations observed in a much lower concentration. The density of misfit dislocations increases with increasing epilayer thickness. Above about 1 μm the films exhibit biaxial tension which we believe is due to thermal expansion differences of ZnSe and GaAs. Good agreement is observed between microscopic and diffraction measurements of the relaxation phenomena.

Electron diffraction from periodic arrays of misfit dislocations formed during epitaxial growth

Changes in transmission electron diffraction patterns were examined during the epitaxial growth of copper and palladium on (111) gold substrates. Bicrystal patterns contain discrete reflections which can be explained by diffraction from uniformly strained deposit and substrate layers or by double diffraction between these two layers. Extra reflections are also observed, which vary markedly in intensity with increasing deposit thickness. It is shown that the extra reflections may be ascribed to periodic arrays of perfect edge misfit dislocations which are observed directly in both Cu/(111)Au and Pd/(111)Au films. The interpretation of these results is discussed and their significance for more general studies of epitaxial growth is briefly considered.

Misfit dislocation sources in epitaxial films part I: Growth of Pd on (001) Au substrates

The epitaxial growth of Pd films on (001) Au substrates has been studied in situ in the transmission electron microscope and new misfit dislocation sources have been found. The growth of the Pd film was pseudomorphic up to a mean deposit thickness of 10 Å when Shockley edge-type misfit dislocations nucleated as half-loops at the Pd surface and expanded into the interface. In deposits of 10–20 Å thickness, secondary surface nucleation and growth of Frank misfit dislocations caused most Schockley misfit dislocations to convert to perfect misfit dislocations with Burgers vectors lying in the film plane. Anomalous dislocation images are explained by theoretical images computed to allow for surface relaxation. Expressions for the displacement field are given for a general edge dislocation lying parallel to a free surface.