Interaction Of Misfit Dislocations Research Articles

Dislocation dynamics in strain relaxation in GaAsSb∕GaAs heteroepitaxy

The real-time stress evolution has been investigated during molecular-beam epitaxial growth of GaAs1−xSbx∕GaAs metamorphic buffer. These real-time data were obtained using an in situ multibeam optical sensor measurement and has been combined with detailed analysis of data obtained from x-ray diffraction, transmission electron microscopy, and atomic force microscopy. We compare the strain relaxation of two different compositions of GaAs1−xSbx, and correlated the development of dislocation structure and morphology. Several distinct stages of the strain relaxation were observed during growth, which can be separated in three main regimes: pseudomorphic growth, fast strain relaxation, and saturation. Transmission electron microscopy data show that GaAs0.5Sb0.5 buffer layers have a larger fraction of pure-edge dislocations that arise during the earliest stages of growth. This could have a significant influence in the fabrication of buffer layers, since pure edges are favored over the threading dislocations. The strain relaxation profile for each film was modeled using a modified model of Dodson and Tsao [Phys. Rev. B 38, 12383 (1988)] that takes into account the elastic interactions of misfit dislocations. The model results agree with the experimental data and show that interaction of misfit dislocations is responsible for the large residual stress. In addition, following the description developed by Dodson and Tsao [Phys. Rev. B 38, 12383 (1988)] for the rate of dislocation multiplication, we were able to determine the line density of threading dislocations from the experimental data. This has a potential application in the design of metamorphic buffer layers because our observations are made in real time on individual growth, without the need of external characterization to measure the dislocation density.

Atomistic simulations of dislocation-interface interactions in the Cu-Ni multilayer system

Abstract Experimental results show that a nanolayered composite structure made of two kinds of metals strengthens dramatically as the layer thickness is reduced. In epitaxial systems, this strengthening has been attributed to the modulus, lattice parameter, gamma surface and slip-plane mismatches between adjacent layers. The modulus mismatch (the Koehler barrier) introduces a force between a dislocation and its image in the interface. The lattice parameter mismatch generates oscillating coherency stresses and van der Merwe misfit dislocations at or near the interfaces, which interact with mobile dislocations. The gamma surface (chemical) mismatch introduces a localized force on gliding dislocations due to core energy changes at or near the interfaces. Slip-plane misorientations across the interfaces require mobile screw dislocations to cross-slip for slip transmission and other dislocations to leave a difference dislocation at the interface. In this paper, atomistic simulations using the embedded-atom method are used to study the four components of dislocation–interface interactions in epitaxial Cu–Ni multilayers in a systematic fashion. The interaction of misfit dislocations with mobile dislocations is modelled using continuum theory. In thick Cu–Ni bilayers, the Koehler barrier is almost independent of interface orientation and dislocation character and is equal to 0.01μ–0.015μ but, when the layer thickness is comparable with the core width of a dislocation, the Koehler barrier falls rapidly (from 0.01μ at a wavelength of 10nm to 0.004μ at 1.75 nm). This behaviour is in accordance with available experimental observations in the literature on the yield of epitaxial Cu–Ni multilayered systems. The gamma surface mismatch or chemical strengthening component of the blocking strength of Cu–Ni interfaces to (a/2)(110) screw dislocations is 0.003μ, a factor of three lower than the Koehler stress. Coherency stresses, apart from exerting direct forces on dislocations, alter the barrier strengths by three mechanisms: firstly, they reduce the density of van der Merwe misfit dislocations, secondly, they enhance the Koehler barrier by altering the elastic constants of both Cu and Ni and, thirdly, non-glide stress components change the core structure of gliding dislocations, thereby altering the Koehler barrier. Overall, the barrier strength of (111) interfaces is independent of the wavelength of the multilayer and about 0.02μ up to the wavelength of Λc, the coherence wavelength limit. At Cu(001)–Ni(001) interfaces the total barrier strength decreases from a value of 0.02μ at long wavelengths (Λ ≈ ∞) to about 0.01μ at Λ = Λc, as considered by Rao et al. in 1995 in their yield stress model for Cu–Ni multilayered structures. Slip-plane misorientations provide powerful barriers to slip transmission. Even at a (111) twinned interface in a coherent Cu–Ni multilayer, screw dislocations cross-slip on to the interface rather than into Ni because the stacking-fault energy at the interface is lower than in Ni. The blocking strength of the same interface to 60° dislocations (which must leave a step and a residual dislocation in the boundary) is very large, 0.03–0.04μ.

Influence of strain relaxation of the AlxGa1−xN barrier on transport properties of the two-dimensional electron gas in modulation-doped AlxGa1−xN/GaN heterostructures

Influences of the thickness of the Si-doped n-type Al0.22Ga0.78N barrier and the thickness of the Al0.22Ga0.78N spacer on mobility and density of the two dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated. 2DEG mobilities of 1274 cm2/V s at 300 K and 4495 cm2/V s at 77 K were reached. Both 2DEG mobility and density decrease dramatically when the Al0.22Ga0.78N barrier becomes partially relaxed, indicating that transport properties of the 2DEG are influenced significantly by the piezoelectric polarization of the Al0.22Ga0.78N layer. From our results, the critical thickness of an Al0.22Ga0.78N layer on GaN is estimated to be between 65 and 75 nm, which is much higher than that predicted by theoretical calculation. This may be attributed to the interaction of misfit dislocations and the presence of a high density of extended defects in the Al0.22Ga0.78N layer.

Configuration and interaction of misfit dislocations in si1-xgex/si(001) epilayer heterostructures grown by gas-source mbe

Transmission electron microscopy has been employed to investigate dislocation configurations and interactions m Si1-xGex/Si(001) epilayer heterostructures grown by Gas-Source MBE Thick Si1-xGex/Si(001) epilayers had 3-dimensional configuration of misfit dislocations in the entire structure The main arrays of misfit dislocations at each single Si1-xGex/Si(001) interface were along the two directions, being 60° type in character Three-way dislocation configuration, resulting from the interaction between dislocations, was observed at the interface Observation of oriented dislocations having striation lines was new in low misfit (1<%) Si1-xGex/Si(001) epilayer heterostructures

Effect of In-content on the misfit dislocation interaction in InGaAs/GaAs layers

A previous published model (D. González, D. Araújo, G. Aragón, R. Garcia, Appl. Phys. Lett., 71 (1997) 2477) allows us to predict the plastic relaxation in the saturation regime of epitaxial layers. This model is in agreement with the experimental residual strain for In xGa 1− x As/ GaAs epilayers below 40% In-content. Above 40% In-content, the predicted values of residual strain become larger than the experimental one. This disagreement takes its origin in the change of growth mode that implies a transition from a 60 ° misfit dislocation (MD) network to a Lomer MD one. An almost complete relaxation can be reached by a Lomer MD network, which is not possible with 60 † MD networks. The difference between both kinds of networks comes from the repulsive interaction of 60 ° MD that inhibits the lattice relaxation.

The effects of misfit dislocation distribution and capping layer on excess stress

It is generally accepted that in the buried strained-layer structure, the strain is relaxed by paired misfit dislocations: one at the upper interface and the other at the lower interface. But, experimentally it is not so. In this letter, the effect of a mixture of single and paired misfit dislocations is incorporated in the formula of excess force. In this formula, the effects of the capping layer with arbitrary thickness and the interaction of misfit dislocations at different interfaces are also included. Based on the formula, the excess stresses are derived. These formulas can be used to predict the excess stress of strained layers with arbitrary heterostructure structures. They also can describe the transition process from the single-kink to the double-kink mechanism.

Effects of misfit dislocation interaction and 90°-type misfit dislocations on strain relaxation behavior in strained epilayer

The effects of misfit dislocation interaction and 90°-type misfit dislocations on strain relaxation behavior in a strained layer are considered theoretically. Our results show that the interaction of misfit dislocations is responsible for the large residual strain in the strained epilayer and that 90°-type misfit dislocations play a very important role in the final stage of strain relaxation. The strain is relaxed more completely by 90° misfit dislocations than by 60° dislocations. The theory is consistent with experimental results.

Characteristics of InAs epilayers for Hall effect devices grown on GaAs substrates by MBE

InAs thin films with good characteristics were grown on GaAs (0 0 1) substrates by molecular beam epitaxy. Cross-sectional transmission electron microscopy indicated that most of the threading dislocations formed by the interaction of misfit dislocations are annihilated above a small thickness. The high electron mobility and small temperature dependence of InAs epilayers are useful for magnetic sensors which is demonstrated by the properties of Hall effect devices.

In-situ X-ray imaging of III–V strained-layer relaxation processes

The important value of the X-ray topography (XRT) technique for the investigation of III–V strained-layer relaxation processes is described. In addition to post-growth ex-situ XRT studies, a unique combined XRT/MBE growth facility has been constructed which allows the generation, motion and interaction of misfit dislocations to be monitored in-situ during epilayer growth, for the first time. The in-situ data already obtained for (100) InGaAs strained-layer growth on both Czochralski- and vertical-gradient freeze-grown GaAs substrates indicates technologically important differences in the initial relaxation process, including, in the latter case, the observation of a previously unreported secondary relaxation phase. Initial results relating to the influence of both post-growth annealing and the subsequent cool-down process are also described.

In Situ Observations of Misfit Dislocations in Lattice-Mismatched Epitaxial Semiconductor Heterostructures

This article describes the application of transmission electron microscopy (TEM) to real-time, in situ dynamic observations of dislocations in strained epitaxial semiconductor heterostructures. Such experiments allow us to directly observe the formation, motion, and interaction of mis-fit dislocations. Preliminary extension of this work to the in situ measurement of the electrical properties of misfit dislocations will also be described.The Fundamental Scientific IssueIt is well established that it is possible to grow a thin, coherent epitaxial layer on a substrate with a slightly different lattice parameter, as illustrated in Figure la. This concept is known as strained layer epitaxy. In the fields of semiconductor physics and device design, strained layer epitaxy offers many exciting new opportunities (see Reference 1 for a review). A coherently strained structure, however, will store an enormous elastic strain energy density in the epitaxial layer, due to the distortion of interatomic bonds. Therefore, as the epitaxial layer increases in thickness during growth, it will become increasingly energetically favorable to relax this strain energy. A number of relaxation routes exist: (1) roughening of the epitaxial layer surface (see, for example, Reference 2); (2) interdiffusion of the layers (this will generally only be significant at temperatures which are a large fraction of the layer melting temperatures (e.g., Reference 3)); and (3) introduction of a dislocation network into the substrate/epilayer interface, which as shown schematically in Figure lb, will allow the epitaxial layer to relax toward its bulk lattice parameter. This dislocation mechanism is the most prevalent strain relaxation mechanism at typical crystal growth and processing temperatures, and we concentrate on this mechanism in our experimental studies.

Strain Relaxation Mechanisms in Lattice Mismatched Epitaxy

ABSTRACTThe relaxation of strained epitaxial layers by the introduction of misfit dislocations is reviewed. Current theoretical and experimental understanding of the nucleation, propagation and interaction of misfit dislocations are summarized. The ramifications for applicability of strained layer epitaxy to practical device structures are discussed.

A phenomenological description of strain relaxation in GexSi1−x/Si(100) heterostructures

We describe how in situ observation of strain relaxation of Gex Si1−x /Si(100) heterostructures in a transmission electron microscope enable us to obtain the fundamental parameters which describe the nucleation, propagation, and interaction of misfit dislocations. Activation energies and prefactors are obtained for the nucleation and propagation processes for x in the range 0.20–0.35, and a simple model to account for dislocation interactions is developed. These measured parameters are then incorporated into a predictive model of strain relaxation which successfully reproduces experimental data. This model relies only upon parameters which can be directly measured.