Nucleation Of Misfit Dislocations Research Articles

Elastic and inelastic strain in submicron-thick ZnO epilayers grown on r-sapphire substrates by metal-organic vapour phase deposition.

A significant part of the present and future of optoelectronic devices lies on thin multilayer heterostructures. Their optical properties depend strongly on strain, being essential to the knowledge of the stress level to optimize the growth process. Here the structural and microstructural characteristics of sub-micron a-ZnO epilayers (12 to 770 nm) grown on r-sapphire by metal-organic chemical vapour deposition are studied. Morphological and structural studies have been made using scanning electron microscopy and high-resolution X-ray diffraction. Plastic unit-cell distortion and corresponding strain have been determined as a function of film thickness. A critical thickness has been observed as separating the non-elastic/elastic states with an experimental value of 150-200 nm. This behaviour has been confirmed from ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy measurements. An equation that gives the balance of strains is proposed as an interesting method to experimentally determine this critical thickness. It is concluded that in the thinnest films an elongation of the Zn-O bond takes place and that the plastic strained ZnO films relax through nucleation of misfit dislocations, which is a consequence of three-dimensional surface morphology.

Molecular dynamics simulations of interface structure and deformation mechanisms in metal/ceramic composites under tension

The role of interfaces on plastic deformation of metal/ceramic composites is multifaceted, which is due to the fact that interfaces can emit, hinder, annihilate or store dislocations. In this work, molecular dynamics simulation was employed to investigate the interfacial structure and deformation mechanism of the coherent and semi-coherent Al/MgO(110) configurations under uniaxial tensile loading. In particular, rectangular mismatch dislocation networks were observed at the interface of a semi-coherent Al/MgO(110) configuration after relaxation, which were due to the difference in lattice constants between the constituent phases. Besides, the strain concentration at the dislocation nodes was found, being conducive to the emission of misfit dislocations into the Al matrix. Moreover, unlike the coherent configuration, the elastic zone in the stress-strain curve of the semi-coherent Al/MgO(110) configuration was relatively narrow and had two yield points. The first yield point was attributed to the nucleation of misfit dislocations from the interface dislocation nodes, whereas the second one was ascribed to the nucleation of lattice dislocations in the Al matrix. Finally, the failure stress of the semi-coherent configuration was less than that of the coherent system.

Atomic-Scale Insights on Large-Misfit Heterointerfaces in LSMO/MgO/c-Al2O3

Understanding the interfaces in heterostructures at an atomic scale is crucial in enabling the possibility to manipulate underlying functional properties in correlated materials. This work presents a detailed study on the atomic structures of heterogeneous interfaces in La0.7Sr0.3MnO3 (LSMO) film grown epitaxially on c-Al2O3 (0001) with a buffer layer of MgO. Using aberration-corrected scanning transmission electron microscopy, we detected nucleation of periodic misfit dislocations at the interfaces of the large misfit systems of LSMO/MgO and MgO/c-Al2O3 following the domain matching epitaxy paradigm. It was experimentally observed that the dislocations terminate with 4/5 lattice planes at the LSMO/MgO interface and with 12/13 lattice planes at the MgO/c-Al2O3 interface. This is consistent with theoretical predictions. Using the atomic-resolution image data analysis approach to generate atomic bond length maps, we investigated the atomic displacement in the LSMO/MgO and MgO/c-Al2O3 systems. Minimal presence of residual strain was shown at the respective interface due to strain relaxation following misfit dislocation formation. Further, based on electron energy-loss spectroscopy analysis, we confirmed an interfacial interdiffusion within two monolayers at both LSMO/MgO and MgO/c-Al2O3 interfaces. In essence, misfit dislocation configurations of the LSMO/MgO/c-Al2O3 system have been thoroughly investigated to understand atomic-scale insights on atomic structure and interfacial chemistry in these large misfit systems.

Релаксация напряжений несоответствия в гетероструктурах alpha-Ga-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-/alpha-Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- при образовании дислокаций несоответствия

We propose the analytical models describing misfit stress relaxation in α-Ga2O3/α-Al2O3 film/substrate heterostructures taking into account the crystal lattices anisotropy of the heterostructure materials. We consider the nucleation of misfit dislocations as a result of basal or prismatic slip in α-Ga2O3/α-Al2O3 heterostructures with various film orientations. We calculate and analyze the dependences of the critical thickness hc (film thickness above which the nucleation of misfit dislocations is favorable) on the angle ϑ between the polar c-axis and the normal to the film growth plane for α-Ga2O3/α-Al2O3 heterostructures. We demonstrate that accounting for elastic constant C14 is not critical in the considered relaxation models for the α Ga2O3/α Al2O3 heterostructures.

Experimental observation of nucleation of misfit dislocations induced by the stress field of primary dislocations

Lomer-type edge dislocations (L-dislocations) detected in strained films Ge-on-Si, GaAs-on-Si, etc., are formed from two 60° dislocations sliding along intersecting {111} planes inclined with respect to the (001) substrate.Kvam et al. [J. Mater. Res.5(1990) 1900] proposed a mechanism of induced generation of a secondary 60° dislocation as one of the variants of L-dislocation formation. The present paper describes the analysis of the dislocation structure arising at the initial stage of growth of strained Ge films on Si substrates with the surface tilted from the [001] orientation. Nucleation of the secondary misfit dislocation is found to be induced by the front of the expanding primary dislocation, i.e., by its segment reaching the film surface.

Fully strained epitaxial Ti1−xMgxN(001) layers

Ti1−xMgxN(001) layers with 0.00 ≤ x ≤ 0.49 are deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets in 5 mTorr pure N2 at 600 °C. X-ray diffraction ω-2θ scans, ω-rocking curves, φ-scans, and high resolution reciprocal space maps show that the Ti1−xMgxN layers are rock-salt structure single crystals with a cube-on-cube epitaxial relationship with the substrates: (001)TiMgN║(001)MgO and [100]TiMgN║[100]MgO. Layers with thickness d = 35–58 nm are fully strained, with an in-plane lattice parameter a|| = 4.212 ±0.001 Å matching that of the MgO substrate, while the out-of-plane lattice parameter a⊥ increases with x from 4.251 Å for TiN(001) to 4.289 Å for Ti0.51Mg0.49N(001). This yields a relaxed lattice parameter for Ti1−xMgxN(001) of ao = (1-x)aTiN + xaMgN – bx(1-x), where aTiN = 4.239 Å, aMgN = 4.345 Å, and the bowing parameter b = 0.113 Å. In contrast, thicker Ti1−xMgxN(001) layers with d = 110–275 nm are partially (pure TiN) or fully (x = 0.37 and 0.49) relaxed, indicating a critical thickness for relaxation of 50–100 nm. The in-plane x-ray coherence length is large (100–400 nm) for fully strained layers with 0.00 ≤ x ≤ 0.45 but drops by an order of magnitude for x = 0.49 as the composition approaches the phase stability limit. It is also an order of magnitude smaller for thicker (d ≥ 110 nm) layers, which is attributed to strain relaxation through the nucleation and growth of misfit dislocations facilitated by glide of threading dislocations.

Enhanced Sn incorporation in GeSn epitaxial semiconductors via strain relaxation

We investigate the effect of strain on the morphology and composition of GeSn layers grown on Ge/Si virtual substrates. By using buffer layers with controlled thickness and Sn content, we demonstrate that the lattice parameter can be tuned to reduce the strain in the growing top layer (TL) leading to the incorporation of Sn up to 18 at. %. For a 7 at. % bottom layer (BL) and a 11-13 at. % middle layer (ML), the optimal total thickness tGeSn = 250-400 nm provides a large degree of strain relaxation without apparent nucleation of dislocations in the TL, while incorporating Sn at concentrations of 15 at. % and higher. Besides facilitating the growth of Sn-rich GeSn, the engineering of the lattice parameter also suppresses the gradient in Sn content in the TL, yielding a uniform composition. We correlate the formation of the surface cross-hatch pattern with the critical thickness hG for the nucleation and gliding of misfit dislocations at the GeSn-Ge interface that originate from gliding of pre-existing threading dislocations in the substrate. When the GeSn layer thickness raises above a second critical thickness hN, multiple interactions between dislocations take place, leading to a more extended defective ML/BL, thus promoting additional strain relaxation and reduces the compositional gradient in the ML. From these studies, we infer that the growth rate and the Ge-hydride precursors seem to have a limited influence on the growth kinetics, while lowering temperature and enhancing strain relaxation are central in controlling the composition of GeSn. These results contribute to the fundamental understanding of the growth of metastable, Sn-containing group-IV semiconductors, which is crucial to improve the fabrication and design of silicon-compatible mid-infrared photonic devices.

New Relaxation Mechanism Enabling High-Quality, Laterally Grown Ge on Si

We demonstrate a novel growth technique, metal-catalyzed, lateral epitaxial growth, to grow Ge films laterally on Si with reduced threading dislocation densities (TDD). In contrast to traditional planar film growth on Si, this technique starts with a small crystal nucleation at a specific position on Si followed by Ge lateral film growth. In plan-view and cross-sectional transmission electron microscopy micrographs, high-density threading dislocations were found to be present in the initial growth areas, while the lateral overgrowth areas demonstrated substantially reduced TDD or even defect-free areas. Furthermore, the X-ray diffraction results showed that the films were fully relaxed and mostly pure Ge. Full relaxation and low TDD Ge films are two surprising results given that the growth occurred at a low temperature (375 °C), which cannot be understood with the current misfit dislocation nucleation and glide model of film relaxation. For these reasons, we suggest the presence of a new relaxation mechan...

A predictive model for plastic relaxation in (0001)-oriented wurtzite thin films and heterostructures

In this work, we present an experimental and theoretical study of the process of plastic strain relaxation of (0001)-oriented wurtzite heterostructures. By means of transmission electron microscopy and atomic force microscopy, we show that plastic relaxation of tensile strained AlxGa1-xN/GaN heterostructures proceeds predominantly by nucleation of a-type misfit dislocations in the 13⟨112¯0⟩|0001 slip-system driven by a three-dimensional surface morphology, either due to island growth or due to cracking of the layer. Based on our experimental results, we derive a quantitative model for the dislocation nucleation process. With the shear stress gradients at the nucleation sites of a-type misfit dislocations obtained by the finite element method, we calculate the critical thickness for plastic relaxation of strained wurtzite films and heterostructures as dependent on the surface morphology. The crucial role of the growth mode of the film on the strain relaxation process and the resulting consequences is discussed in the paper.

New strategies for producing defect free SiGe strained nanolayers

Strain engineering is seen as a cost-effective way to improve the properties of electronic devices. However, this technique is limited by the development of the Asarro Tiller Grinfeld growth instability and nucleation of dislocations. Two strain engineering processes have been developed, fabrication of stretchable nanomembranes by deposition of SiGe on a sacrificial compliant substrate and use of lateral stressors to strain SiGe on Silicon On Insulator. Here, we investigate the influence of substrate softness and pre-strain on growth instability and nucleation of dislocations. We show that while a soft pseudo-substrate could significantly enhance the growth rate of the instability in specific conditions, no effet is seen for SiGe heteroepitaxy, because of the normalized thickness of the layers. Such results were obtained for substrates up to 10 times softer than bulk silicon. The theoretical predictions are supported by experimental results obtained first on moderately soft Silicon On Insulator and second on highly soft porous silicon. On the contrary, the use of a tensily pre-strained substrate is far more efficient to inhibit both the development of the instability and the nucleation of misfit dislocations. Such inhibitions are nicely observed during the heteroepitaxy of SiGe on pre-strained porous silicon.

Anisotropic relaxation behavior of InGaAs/GaAs selectively grown in narrow trenches on (001) Si substrates

Selective area growth of InGaAs inside highly confined trenches on a pre-patterned (001) Si substrate has the potential of achieving a high III-V crystal quality due to high aspect ratio trapping for improved device functionalities in Si microelectronics. If the trench width is in the range of the hetero-layer thickness, the relaxation mechanism of the mismatched III-V layer is no longer isotropic, which has a strong impact on the device fabrication and performance if not controlled well. The hetero-epitaxial nucleation of InxGa1-xAs on Si can be simplified by using a binary nucleation buffer such as GaAs. A pronounced anisotropy in strain release was observed for the growth of InxGa1-xAs on a fully relaxed GaAs buffer with a (001) surface inside 20 and 100 nm wide trenches, exploring the full composition range from GaAs to InAs. Perpendicular to the trench orientation (direction of high confinement), the strain release in InxGa1-xAs is very efficiently caused by elastic relaxation without defect formation, although a small compressive force is still induced by the trench side walls. In contrast, the strain release along the trenches is governed by plastic relaxation once the vertical film thickness has clearly exceeded the critical layer thickness. On the other hand, the monolithic deposition of mismatched InxGa1-xAs directly into a V-shaped trench bottom with {111} Si planes leads instantly to a pronounced nucleation of misfit dislocations along the {111} Si/III-V interfaces. In this case, elastic relaxation no longer plays a role as the strain release is ensured by plastic relaxation in both directions. Hence, using a ternary seed layer facilitates the integration of InxGa1-xAs covering the full composition range.

Misfit dislocations induced by lithium-ion diffusion in a thin film anode

According to the diffusion kinetics and heteroepitaxial strained layer theory, this paper presents a theoretical model to investigate the generation and distribution of misfit dislocations in a thin film anode under galvanostatic and potentiostatic operations. The results show that the nucleation and density distribution of misfit dislocations largely depend on the thickness of the diffused layer and insertion time. When the thickness is less than a certain critical value, the total strain energy in the electrode is almost insusceptible and yet reduced by misfit dislocations for that of going beyond the critical value. Under potentiostatic operation, the rise range and response speed of the dislocation density are greater and faster. A certain region immediately possesses much lower dislocation density under the electrode surface compared with the region below it. These quantitative results can provide a new perspective into relieving diffusion-induced stress by misfit dislocations with the purpose of improving the mechanical durability of lithium-ion batteries.

Characterization of dislocations in germanium layers grown on (011)- and (111)-oriented silicon by coplanar and noncoplanar X-ray diffraction.

Strained germanium grown on silicon with nonstandard surface orientations like (011) or (111) is a promising material for various semiconductor applications, for example complementary metal-oxide semiconductor transistors. However, because of the large mismatch between the lattice constants of silicon and germanium, the growth of such systems is challenged by nucleation and propagation of threading and misfit dislocations that degrade the electrical properties. To analyze the dislocation microstructure of Ge films on Si(011) and Si(111), a set of reciprocal space maps and profiles measured in noncoplanar geometry was collected. To process the data, the approach proposed by Kaganer, Köhler, Schmidbauer, Opitz & Jenichen [Phys. Rev. B, (1997 ▶), 55, 1793-1810] has been generalized to an arbitrary surface orientation, arbitrary dislocation line direction and noncoplanar measurement scheme.

Role of edge dislocations in plastic relaxation of GeSi/Si(001) heterostructures: Dependence of introduction mechanisms on film thickness

It has been shown that, in the GeSi/Si(001) heterosystem at lattice parameter mismatches of ∼2% and more, a small critical thickness of the introduction of dislocations leads to the implementation of the mechanism of induced nucleation of misfit dislocations. This mechanism consists in that the stress field of an already existing 60° dislocation provokes introduction of a secondary 60° dislocation with an opposite-sign screw component. As a result of the interaction of such dislocation pairs, edge misfit dislocations are formed, which do control the plastic relaxation process. This mechanism is most efficient when dislocations are introduced at the GeSi film thickness only slightly exceeding the critical thickness of the introduction of 60° dislocations, and there are threading dislocations. The dominant type of misfit dislocations (60° or edge) in the Ge-on-Si(001) system can be controlled by varying the mismatch parameter in the heteropair.

The influence of atomic ordering on strain relaxation during the growth of metamorphic solar cells

The occurrence of single variant CuPtB ordering during growth of InGaP graded buffer layer structures on offcut (001) GaAs substrates for inverted metamorphic solar cells is found to have a strong influence on strain relaxation mechanisms. Since the surface-induced CuPtB ordering is metastable in the bulk of the material, a strong preference is observed for the nucleation and glide of 60° type misfit dislocations with Burgers vectors that introduce an antiphase boundary into the ordered structure. This results in an overall epitaxial layer tilt in the opposite sense to that normally observed for the direction of substrate offcut. Furthermore, in InGaP buffer layers graded to InP, a switch in the dislocation glide plane preference back to that normally observed for the direction of substrate offcut is observed as the degree of atomic ordering falls below a critical value. This results in the nucleation and glide of new misfit dislocations resulting in an increase in the threading dislocation density that is found to have a deleterious effect on device efficiency. Understanding the materials science behind this behavior will enable the engineering of more effective, lower threading dislocation density strain relief buffer layers resulting in improved performance of subsequently grown devices.

Mechanism of induced nucleation of misfit dislocations in the Ge-on-Si(0 0 1) system and its role in the formation of the core structure of edge misfit dislocations

Ge-on-Si(001) films are grown by molecular beam epitaxy via a three-step epitaxial growth method (Ge/Ge seed/GeSi buffer/Si(001)). The dislocation structure of the Ge/GeSi buffer interface is studied by high-resolution electron microscopy. Misfit dislocations on the interface are edge dislocations and are aligned regularly with a period of 9–10nm. A variety of atomic structures of the dislocation core is observed, known in the literature as dissociated or asymmetric Lomer edge dislocations. The assumption that atomic structures of various degrees of complexity are intermediate states in the formation of a perfect edge misfit dislocation in the course of plastic relaxation of a stressed film is justified. A model is proposed which explains the intermediate states in terms of statistical variation of the nucleation site of the complementary 60° dislocation which forms, together with the primary dislocation, a Lomer dislocation at the interface.

Nanostructured silicon for Ge nanoheteroepitaxy

A fabrication process for Si nanopillars (NPs) as template for the Ge nanoheteroepitaxy (NHE) was developed. The NHE concept suggests that by fabricating three-dimensional free standing NP, the NP can be elastically deformed by deposited heteroepitaxial material on the top due to lattice mismatch strain. To enable the elastic deformation, small diameter NPs are required with a specific height. A fabrication process was developed to create Si NPs with <100 nm width and >100 nm height. The top of Si NPs must be opened for selective Ge growth, while the sidewall and bottom surface must be covered by SiO2. Spectroscopic ellipsometry was used to control Si NPs before selective Ge epitaxial overgrowth. The process was optimized based on transmission electron microscopy (TEM) and X-ray diffraction investigations. Ge on top of NPs is fully relaxed. The relaxation occurs however not elastically but plastically by misfit dislocation nucleation at the Ge/Si interface due to SiO2 passivation of the sidewall. The possible approaches to enable elastic deformation of Si NPs are to reduce the SiO2 thickness of the sidewall and lateral dimension of NPs. The Ge of high crystallinity grown on top of the NPs only is indicating that the developed technology guarantees a clean top Si surfaces and a sufficient SiO2 protection of the rest of the structure. No threading dislocations are observed by TEM. This shows that the small lateral dimension of the seed area allows the TDs to glided out to the surfaces with high probability.

Growth and relaxation processes in Ge nanocrystals on free-standing Si(001) nanopillars

We study the growth and relaxation processes of Ge crystals selectively grown by chemical vapour deposition on free-standing 90 nm wide Si(001) nanopillars. Epi-Ge with thickness ranging from 4 to 80 nm was characterized by synchrotron based x-ray diffraction and transmission electron microscopy. We found that the strain in Ge nanostructures is plastically released by nucleation of misfit dislocations, leading to degrees of relaxation ranging from 50 to 100%. The growth of Ge nanocrystals follows the equilibrium crystal shape terminated by low surface energy (001) and {113} facets. Although the volumes of Ge nanocrystals are homogeneous, their shape is not uniform and the crystal quality is limited by volume defects on {111} planes. This is not the case for the Ge/Si nanostructures subjected to thermal treatment. Here, improved structure quality together with high levels of uniformity of the size and shape is observed.

Compliant substrate versus plastic relaxation effects in Ge nanoheteroepitaxy on free-standing Si(001) nanopillars

We report on the structural characterization of Ge clusters selectively grown by chemical vapor deposition on free-standing 50 nm wide Si(001) nanopillars. Synchrotron based x-ray diffraction studies and transmission electron microscopy were performed to experimentally verify the nanoheteroepitaxy theory as a technique to grow high quality Ge on Si(001). Although the structure dimensions are comparable to the theoretical values required for the strain partitioning phenomenon, the compliant character of Si is not unambiguously proven. In consequence, the strain is relieved by nucleation of misfit dislocations at the Ge/Si interface. By gliding out of threading arms, high quality Ge nanostructures are achieved.

Quasicontinuum study the influence of misfit dislocation interactions on nanoindentation

Multiscale simulations using the quasicontinuum (QC) method with the embedded-atom method (EAM) potential are performed to examine the mechanical response of Cu–Ag bilayer film during nanoindentation tests. An attempt is made, from the viewpoint of collective interaction among misfit dislocations on the interface, to account for the strengthening and weakening mechanisms of interface on Cu–Ag bilayer film. The details of misfit dislocation nucleation, motion and collective interaction on Cu/Ag and Ag/Cu interfaces are discussed systematically, respectively. The investigation shows that the property and performance of Cu–Ag bilayer film mainly depend on the mechanical property of upper film. Both the strengthening and weakening effects are closely related to the collective interaction among misfit dislocations on the interface. Due to the pinning effect of interface on misfit dislocation, both the local interface migration and the voids can be observed at the core region of misfit dislocations. For nanoindentation on Cu/Ag bilayer film, the plastic deformation is localized chiefly in the lower Ag substrate and the void will disappear with the redistribution of misfit dislocations, which indicate that there are distinct protective and strengthening effects of the upper Cu film on the lower Ag substrate. While, for nanoindentation on Ag/Cu bilayer film, both the upper Ag film and the lower Cu substrate experience plastic deformation and the voids will not disappear, which imply that there are an obvious weakening effect of the upper Ag film on the lower Cu substrate. In addition, the multiscale simulation results are consistent with the experimental results.