Appearance Of Misfit Dislocations Research Articles

A new complex-regular eutectic in the Fe7(CoNiMn)93-xBx high entropy alloys

Phase constituents and solidification microstructures of as-cast Fe7(CoNiMn)93-xBx (x = 9, 13) HEAs were investigated via electron backscattered diffraction and transmission electron microscopy. It was observed that the phase constituents contain an FCC1 solid solution and an M23B6 compound. The microstructure of hypoeutectic Fe7(CoNiMn)84B9 high-entropy alloy consists of primary face-centered-cubic (FCC1) dendrite and trigonal-shaped complex-regular eutectic, and that of Fe7(CoNiMn)80B13 eutectic high-entropy alloy is fully consisted of trigonal-shaped complex-regular eutectic. The formation of this rare complex-regular eutectic is attributed to the preferential growth of M23B6 in a trigonal-shaped trunk due to its crystallography anisotropy. In addition, the FCC1 eutectic phase has a cube-on-cube orientation relationship with the M23B6 phase where the same orientation but different lattice constants for them leads to the appearance of misfit dislocations at the interface, which enhances the strength of materials. The compression strength for Fe7(CoNiMn)80B13 eutectic high-entropy alloy is higher than that of hypoeutectic Fe7(CoNiMn)84B9 high-entropy alloy.

Grain boundary relaxation behavior and phase stability of AlCrTiVx (x = 0, 0.5 and 1) high-entropy alloys

The defect relaxation behavior of AlCrTiVx (x = 0, 0.5, and 1) high entropy alloys was reported in this paper. A pronounced internal friction (IF) peak superimposing on a monotonously increasing high-temperature background (HTBG) was observed in all samples, which was ascribed to grain boundary peak. The activation energy of grain boundary relaxation increases with the increasing V content, implying an increasing complexity and difficulty of atomic diffusion with the increasing mixing entropy. After annealing at 1100 °C for 5 h, the HTBG of AlCrTiV alloy abnormally increases, which was attributed to the appearance of misfit dislocations induced by precipitation of nano-sized Ti-rich phases as confirmed by microstructure analysis. These findings may provide a new degree of freedom for evaluating and designing high entropy alloys with high thermal stability.

1502 電気めっき法で作製したナノ多層膜の微視的構造と強度(OS15-1 ナノ・マイクロの視点からの変形と破壊の力学)

Mechanical properties and microstructure of Ni/Cu multilayered films have been investigated. The Ni/Cu multilayers were fabricated by electrodeposition technique. Thickness of individual Ni and Cu layers was set at h=20nm and 50nm, and total thickness of the multilayers was about 4μm. From electron diffraction patterns obtained by transmission electron microscope (TEM) observation, both the Ni and Cu layers were epitaxially grown during the electrodeposition. After being removed from a substrate, the Ni/Cu multilayered films were monotomcally strained to examine tensile strength. Prior to the tensile test, some multilayers were annealed. Ultimate tensile strength of the as-deposited multilayer of h=20nm was about 900MPa. After the annealing at 573K for 1 hour, the ultimate tensile strength of the 20nm multilayer increased up to 1GPa. However, the annealing at 773K significantly lowered tensile strength because disappearance of the multilayered microstructure. From the X-ray diffraction analysis of the as-deposited 20nm multilayer, the X-ray peaks from the Ni and Cu layers were unclear: both the layers had almost the same lattice constant. On the other hand, clear Ni and Cu peaks were detected at the annealed 20nm multilayer. It is thus probable that the increased strength in the annealed 20nm multilayer was caused by appearance of misfit dislocations which normally exist at an interface between the crystals having different lattice constants.

Thick-Strained-Si/Relaxed-SiGe Structure of High-Performance RF Power LDMOSFETs for Cellular Handsets

A strained-Si/relaxed-SiGe structure was applied to laterally diffused MOSFETs (LDMOSFETs) in order to improve the PAE of cellular handset RF power-amplifier applications. The LDMOSFETs were fabricated in a 70-nm-thick strained-Si/relaxed-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> structure. Despite the appearance of misfit dislocations, the thick strained-Si was essential for high efficiency and low leakage. The self-heating effects on the power performance were estimated to be negligible by using dynamic thermal simulation. The devices exhibited 46.6% PAE at a P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> of 27.5 dBm for wideband code-division multiple-access handset applications, which was a 3.8-point improvement over Si controls

Kinetic Monte Carlo simulations of heteroepitaxial growth

We introduce an algorithm for off-lattice kinetic Monte Carlo simulations of heteroepitaxial crystal growth. In heteroepitaxy a mismatch of the lattice constants in adsorbate and substrate can lead to a variety of phenomena already within the first monolayers of growth. This includes the appearance of misfit dislocations or the formation of self-assembled islands. In order to gain general insight into the relevant mechanisms we study, as a first example, a Lennard–Jones system in 1+1 dimensions. The critical layer thickness for the occurrence of dislocations is determined as a function of the misfit. Furthermore, we investigate the 2D–3D transition which marks the emergence of mounds from initially flat islands on the surface.

A Kinetic Monte Carlo method for the simulation of heteroepitaxial growth

We introduce a simulation algorithm which allows the off-lattice simulation of various phenomena observed in heteroepitaxial growth (see e.g. [Politi et al., Phys. Rep. 324 (2000) 271–404]) like a critical layer thickness for the appearance of misfit dislocations, or self-assembled island formation in 1+1 dimensions. The only parameters of the model are deposition flux, simulation temperature and an interaction potential between the particles of the system.

Atomistic simulation study of misfit strain relaxation mechanisms in heteroepitaxial islands

ABSTRACTThe mechanisms of the misfit strain relaxation in heteroepitaxial islands are investigated in two-dimensional molecular dynamics simulations. Stress distributions are analyzed for coherent and dislocated islands. Thermally-activated nucleation of misfit dislocations upon annealing at an elevated temperature and their motion from the edges of the islands towards the positions corresponding to the maximum strain relief is observed and related to the corresponding decrease of the total strain energy of the system. Differences between the predictions of the energy balance and force balance criteria for the appearance of misfit dislocations is discussed. Simulations of an island located at different distances form the edge of a mesa indicate that the energy of the system decreases sharply as the island position shifts toward the edge. These results suggest that there may be a region near the edge of a mesa where nucleation and growth of ordered arrays of islands is favored.

Misfit dislocations and other defects in thin films

The methods and the results of calculations of the elastic properties of misfit and lattice dislocations and disclinations in thin films are considered. For one-phase films the following problems are solved: a straight edge dislocation perpendicular to the film surfaces; wedge disclinations parallel or perpendicular to the surfaces. In the case of a film on a substrate, the critical thickness for the appearance of misfit dislocations is determined in the continuum model. For multilayer films (superlattices) the effect of a reduction in the grown-in dislocation density is analysed. The behaviour of dislocations in two-phase films is also briefly reviewed.

Evolution of Surface Roughness of a Strained Epitaxial Film Due to Interface Misfit Dislocations

ABSTRACTThe appearance of misfit dislocations at the interface between a strained layer and its substrate induces a nonuniform chemical potential along the surface of the layer. Under the assumption that the resulting chemical potential gradient will drive coherent mass rearrangement by surface diffusion, a model of evolution of surface shape is considered which leads to estimates of the surface roughness induced by this mechanism.

Anisotropic electrical conduction in GaAs/In0.2Ga0.8As/Al0.3Ga0.7As strained heterostructures beyond the critical layer thickness

The anisotropic conduction of GaAs/In0.2Ga0.8As/Al0.3Ga0.7As inverted high-electron-mobility transistor (HEMT) structures has been investigated. The heterostructures were grown by molecular-beam epitaxy on (100) GaAs substrates. The thickness of the pseudomorphic layer was increased stepwise (150–300 Å) beyond the critical layer thickness as determined by the appearance of misfit dislocations. These mixed 60° dislocations surrounded by depletion regions were observed as straight dark lines in cathodoluminescence. The measured resistance Rs was higher in the [01̄1] direction than in the perpendicular [011] direction. At T=30 K the conduction ratio of these two directions exceeded 105 in the 300-Å-thick layer. The magnitude and anisotropy of Rs was correlated with the anisotropic dislocation patterns resulting from the preferential generation of the α dislocations ∥ [011] as compared to the orthogonal β dislocations ∥ [01̄1]. In both directions Rs depended exponentially on the number of dark lines perpendicular to the probing current. Simultaneously, the functional form of the temperature-dependent Rs(T) strongly varied with layer thickness. The thin, still elastically strained layers showed the usual behavior of HEMT structures. For the thicker layers a completely different temperature dependence was gradually developing, eventually leading to an exponential increase of Rs with inverse temperature between 300 and 100 K. Below this range Rs(1/T) changed more slowly and leveled off at 30 K. All these features are convincingly explained by a model assuming that the electrons can surmount the insulating depletion barriers in the conducting channel by a thermally induced tunneling mechanism.

Use of Valved, Solid Group V Sources for the Growth of GaAs/GaInP Heterostructures by Molecular Beam Epitaxy

ABSTRACTWe report on the first growth of GaAs/Ga0.5In0.5P heterostructures by conventional molecular beam epitaxy using solid-source valved crackers to supply both the arsenic and the phosphorus fluxes. By regulating the group V fluxes with the cracker needle valves, arsenide-phosphide heterostructures were successfully grown with virtually no group V intermixing between layers. For comparison, similar heterostructure samples were grown using only the mechanical shutters to switch between group V fluxes, and the resulting layers were severely intermixed. The amount of group V intermixing was shown to be independent of whether As2 or As4 fluxes were used to grow the layers. A GaAs/Ga0.5In0.5P multiple quantum well sample was also grown using the valved crackers. Photoluminescence peaks were clearly observed from 40 Å, 80 Å, and 300 Å GaAs quantum wells, but no luminescence was detected from a 20 Å well. An 80Å GaAs/ 80Å Ga0.5In0.5P superlattice was grown, and superlattice satellite peaks were observed in the X-ray rocking curves. The appearance of misfit dislocations suggests localized intermixing at the interfaces.

Growth interfaces of Si1−xGex/Si heterostructures studied by i n s i t u laser light scattering

Elastic scattering of laser light has been used to monitor surface topography in real time during growth of Si1−xGex heterostructures. Large increases in scattered intensity were caused by surface height fluctuations in the alloy layers which, after growth, could be imaged in cross section by transmission electron microscopy and in plan view by scanning optical microscopy. For pseudomorphic alloy layers (x=0.19±0.01) grown at 750 °C the surface height fluctuations were periodic, the wavelength of approximately 270 nm giving strong laser scattering. Plan view transmission electron micrographs on such a layer showed periodic strain fluctuations with the same wavelength, whereas an equivalent layer grown at 610 °C showed strain fluctuations which were irregular in shape and spatial distribution. The intensity of light scattered from the surface of this layer was much lower. Thick alloy layers grown beyond the critical thickness for appearance of misfit dislocations showed strong scatter which increased more quickly in intensity at higher growth temperature.

Misfit dislocations and critical thickness of heteroepitaxy

An expression is derived for the critical thickness of heteroepitaxial films by taking the energy minimization approach. It is exactly identical to the critical thickness obtained by Matthew and Blakeslee [J. Cryst. Growth 27, 118 (1974)] using a force balance approach, after allowing for a discrepancy judged to be an unmentioned approximation in the latter. The equivalence of the two formulations is explained conceptually. The reason is pointed out for the very different critical thickness obtained by People and Bean [Appl. Phys. Lett. 47, 322 (1985); 49, 229 (1986)] using the energy minimization approach. It is further shown that misfit dislocations do not appear en masse catastrophically when the critical thickness is reached. Rather, their density increases gradually with the epitaxial thickness, approaching only asymptotically a value required for a complete relief of film stress as the thickness tends to infinity. Any observed sudden mass appearance of misfit dislocations must be attributed to a substantial supercritical thickness actually existing, prior to nucleation of dislocations.