Dislocations In Thin Films Research Articles

Study of two hardening mechanism caused by geometrically necessary dislocations in thin films with passivation layer

At microscale, thin films with passivation layer exhibit strong size effect during loading and prominent Bauschinger effect during reverse loading. It is commonly recognized that the hardening, which is caused by geometrically necessary dislocations (GNDs) near the passivation layer, is responsible for these phenomena. However, GNDs may contribute to hardening either by slip resistance via a generalized Taylor equation or by back stress originating from elastic interaction between dislocations. Which one dominates the thin film plasticity has not been well understood yet. In this paper, a dislocation-based crystal plasticity model is proposed, which includes both slip resistance and back stress hardenings. The model is applied to simulate passivated thin films with different thickness and the results are compared with experimental data. In the case where slip resistance hardening is considered, the predicted size effect of flow stress is much smaller than that in experiments and there is no obvious Bauschinger effect. While in the case where back stress hardening is considered, the predicted size effect matches well with experiments and the Bauschinger effect is clearly revealed. Also, it is found that the reverse plastic flow during unloading comes from the heterogeneous distribution of stress caused by the pile up of dislocations near the passivation layer.

Anisotropic Elastic Field of 3D Prismatic Dislocation Loops in Bounded and Voided Single Crystal Films

Dislocations in a finite medium bring about image stresses. These image stresses play important roles in the dislocation behavior in finite sized systems such as thin films. Since the ratio of surface to volume is higher for thin films than for bulk materials, dislocation behaviors in thin films are greatly different from those in a corresponding infinite medium, which make it necessary to take into account the effects of free surfaces on the evolution of dislocations in thin films. In the investigations[4, 5], image stresses in an elastic cylinder and thin films are calculated by employing a Fourier transform (FFT) approach and isotropically elastic fields due to dislocations are adopted in their formulation. However, most crystals are anisotropic, and the anisotropic ratio changes with environment physical parameters, such as the temperature, moisture, electron field, magnetic field. A theorem based on anisotropic Stroh’s formula for calculating the image stress of infinite straight dislocations in anisotropic bicrystals has been developed by Barnett and Lothe[6]. Wu et al.[3] recently also make use of the FFT technique to investigate the general dislocation image stresses of cubic thin films, thus extending the formalism by Weinberger et al.[4,5] from isotropic to anisotropic thin films. It is clear that for the assumed in-plane elastic fields to be periodically defined within an unbounded region is an essential and indispensable prerequisite for the above FFT-based approach to be effectively implemented, thus ruling out the possibility of its being employed to analyse image stresses in bounded and/or voided thin cubic films. Our motivation here is then to make an further extension by first calculating the anisotropic elastic fields of dislocation loops in an unbounded thin film with cavities and then invoking FEM and the principle of superposition to seek the image stress solution.

Thermodynamics and Kinetics of Defect Motion and Annihilation in the Self-Assembly of Lamellar Diblock Copolymers

The thermodynamics of dislocations in thin films of lamella-forming diblock copolymers and their climb and glide motions are investigated using single-chain-in-mean-field (SCMF) simulations and self-consistent field theory (SCFT) in conjunction with the string method. The glide motion of a defect perpendicular to the stripe pattern is characterized by large free energy barriers. The barriers not only stem from altering the domain topology; an additional barrier arises from a small-amplitude but long-range domain displacement. In contrast, the climb motion along the stripes does not involve a free energy barrier in accord with the continuous translational invariance along the stripe. Thus, the perpendicular distance (“impact parameter”) between a pair of defects is approximately conserved. Dislocation pairs with opposite Burgers vectors attract each other and move toward each other (“collide”) via climb motion. We find that the forces between apposing defects significantly depend on system size, and the Pe...

Capture cross-section of threading dislocations in thin films

Abstract The capture cross section for annihilation of two threads with opposite Burgers vectors moving on orthogonal slip planes in a thin film is examined using a numerical model. The initial configurations of threads that lead to annihilation are mapped out for a range of applied film stresses. The area of the region of initial configurations that lead to annihilation at a given stress and thickness is the capture cross-section. The size of the capture cross-section is shown to be highly sensitive to the applied stress relative to the critical stress for dislocation motion imposed by the film thickness.

Threading dislocation interactions in an inhomogeneous stress field: A statistical model

A statistical model is presented to characterize the influence of stress field inhomogeneity on the probability of interactions between threading dislocations in thin films. Any degree of stress field inhomogeneity is shown to increase the likelihood of interactions. However, below a critical level, the effect of stress inhomogeneity is small, while above this level a dramatic effect is observed. The approach taken is applicable to any system of interacting particles with spatially dependent velocities.

A stress relaxation mechanism through buckling-induced dislocations in thin films

We report on molecular dynamics simulations of thin film buckling which show that during the buckling phenomena dislocations can be emitted from specific region of the film where the heterogeneous stress was found to be maximum and larger than in the planar adherent part. A scenario of formation of misfit dislocations in the planar interface which lead to stress relaxation is finally proposed.

Fabrication of electrically conductive nanowires using high-density dislocations in AlN thin films

We fabricated conductive nanowires in insulating AlN thin films by doping Mn along high-density unidirectional threading dislocations. Investigation of the dislocation microstructures by transmission electron microscopy (TEM) and high-resolution scanning TEM revealed that Mn segregates to the dislocation cores. Strain analysis around the dislocations suggests that the strong attractive interaction between the Mn ions and the strain field around the dislocations enhances the confinement of the Mn only in the vicinity of the dislocation cores. Atomic force microscopy measurements under contact-current mode detected the local electrical conduction along the Mn-doped dislocations. The present results open up the possibility for fabricating functional nanowires using dislocations in thin films.

A new method of strain determination in partially relaxed thin films

AbstractIt is well known that classical Stoney's formula for radius of thin film bowing fails when thin film is dislocated. Thereby, a method of determination lattice parameters for each of heterostructure epilayers is needed when inelastic relaxation takes place. In this article, we have developed a method of determination lattice parameters of a heterostructure from radius curvature measurement data. Description of deformation when dislocations are nucleated is carefully analyzed. It is shown that in order to take into account nucleated dislocations in thin film an additional term responsible for increased volume of layers should be included in the analysis. The proposed method is based on the finite deformation elasticity and uses the finite element method and bowing radius measurement by the laser beam reflection method. As an example the nitride alloy heterostructure with GaN as a substrate is analyzed. The verification is performed using X‐ray measurement of lattice parameters. A very good correspondence between numerically determined lattice parameters and XRD measurements data are observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the features of dislocation–obstacle interaction in thin films: large-scale atomistic simulation

Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in thin films have a number of remarkable features. A particular example is the interaction between a screw dislocation and a stacking fault tetrahedron (SFT) in Cu, which can be directly compared with in situ observations of quenched or irradiated fcc metals. If the specimen is thin, the dislocation velocity is slow, and the temperature is high enough, a segment of the original SFT can be transported towards the surface via a double cross-slip mechanism and fast glide of an edge dislocation segment formed during the interaction. The mechanisms observed in the simulations provide an explanation for the results of in situ straining experiments and the differences between bulk and thin film experiments.

Unstable mixing regions in CdMnHgTe and ZnMnHgTe solid solutions

The miscibility gaps and critical temperatures of the spinodal decompositions of the quaternary semiconducting epitaxial thin CdMnHgTe and ZnMnHgTe films have been studied. The model constant K of delta-lattice parameter method is determined by fitting the calculated spinodal isotherms to the experimental data. It was shown that the narrowing of the spinodal decomposition regions is caused by the deformation energy as well as by the effect of plastic relaxation due to mismatch dislocations in thin films.

Misfit dislocations in thin films on plastically deformed substrates

A theoretical model describing the nucleation of misfit dislocations (MD) in interfaces between films and plastically deformed substrates with disclinations is proposed. The ranges of the parameters (disclination strength, density of the disclination ensemble, film thickness, and degree of misfit) within which MD nucleation is energetically favorable are found. It is shown that at certain strengths of disclinations and densities of their ensemble the critical thickness of the film on a plastically deformed substrate with disclinations can exceed that on an undeformed defect-free substrate by a few times.

Some current topics in dislocation theory

Some topics of current interest in dislocation theory are discussed. Included are considerations of the elastic fields of dislocations with emphasis on nonlinear and anisotropic elastic effects and boundary conditions for computer simulations. Also discussed are core structures, extended dislocation arrays, boundaries and interfaces and dislocations in thin films.

Non-Local Interactions and Patterning of Misfit Dislocations in Thin Films

Non-Local Interactions and Patterning of Misfit Dislocations in Thin Films

Brownian Motion of Dislocations in Thin Films

The motion of edge dislocations in a single monolayer film of Cu on Ru(0001) was studied by time-resolved scanning tunneling microscopy. The dislocations were observed to make rapid 1D random walks in the film. This dislocation motion is attributed to the equilibrium thermal exchange of atoms between the solid film and the adatom gas covering the film. These results highlight a fundamental difference between the dynamics of dislocations in thin films and the bulk, which is in principle important in understanding the mechanical properties of thin films. {copyright} {ital 1997} {ital The American Physical Society}

Modeling the Development and Relaxation of Stresses in Films

The mechanics for the development and relaxation of stresses in films is briefly reviewed. The discussion initially focuses on how stresses are distributed within a film and substrate, and on the resultant deformations that these stresses induce. Although the particular case of an elastically homogeneous system with a uniform stress within the film is addressed in detail, the modifications that would result from a nonhomogeneous system, or a nonuniform stress, are also outlined. A brief discussion of stress relaxation in epitaxial systems and the glide of dislocations in thin films then follows. In polycrystalline films, diffusion of atoms as well as the motion of dislocations can contribute to the relaxation of stresses. It is possible to use constitutive models for the different relaxation processes to predict the development of stresses during thermal cycling. A comparison of these predictions with experimental observations allows general conclusions about the dominant relaxation mechanisms that operate in particular thin-film systems to be made. An additional mechanism of stress relief

Comment on "Measurement of the activation barrier to nucleation of dislocations in thin films"

Comment on "Measurement of the activation barrier to nucleation of dislocations in thin films"

{10}<110> edge dislocations in YSZ films grown on (100)MgO by PLD

Yttria-stabilized zirconia (YSZ) is currently used in a variety of applications including oxygen sensors, fuel cells, coatings for semiconductor lasers, and buffer layers for high-temperature superconducting films. Thin films of YSZ have been grown by metal-organic chemical vapor deposition, electrochemical vapor deposition, pulse-laser deposition (PLD), electron-beam evaporation, and sputtering. In this investigation, PLD was used to grow thin films of YSZ on (100) MgO substrates. This system proves to be an interesting example of relationships between interfaces and extrinsic dislocations in thin films of YSZ.In this experiment, a freshly cleaved (100) MgO substrate surface was prepared for deposition by cleaving a lmm-thick slice from a single-crystal MgO cube. The YSZ target material which contained 10mol% yttria was prepared from powders and sintered to 85% of theoretical density. The laser system used for the depositions was a Lambda Physik 210i excimer laser operating with KrF (λ=248nm, 1Hz repetition rate, average energy per pulse of 100mJ).

Measurement of the Activation Barrier to Nucleation of Dislocations in Thin Films

Measurement of the Activation Barrier to Nucleation of Dislocations in Thin Films

Measurement of the activation barrier to nucleation of dislocations in thin films.

We introduce a method to study the activation barrier to nucleation of dislocations during strain relaxation of thin films. By growing on substrates with a range of misorientations from (001), we change the applied shear stress on the different glide systems, leading to an imbalance between nucleation on the different glide systems. The resulting tilt between the relaxed layer and the substrate provides a direct measure of the stress derivative of the nucleation barrier. A phenomenological model, which makes no assumptions about the detailed nucleation mechanism, can then be used to infer from this the activation energy for nucleation. We obtain an activation energy of 5 eV for the observed nucleation mechanism.

On Behaviour of Dislocations in Thin Films

On Behaviour of Dislocations in Thin Films