Dislocation Half-loops Research Articles

Correlation of microscopic structures to the strain rate hardening of SPCC steel

The high strain rate properties of steel have been studied for accurate crashworthiness of vehicles. The mechanical properties at high strain rates show different characteristics from those at the quasi-static state. In order to seek for the physical phenomenon of the strain rate hardening, a microscopic investigation has been conducted for the texture evolution and dislocation behavior of steel. Tensile tests were performed at strain rates ranged from 0.001/s to 100/s, which is the common range in practical vehicle crashes to investigate the microstructural variation and dislocation behavior. According to experimental results of the EBSD and TEM experiment, the texture evolution does not develop remarkably with the variation in strain rates, while dislocation structure and behavior investigated by TEM experiments show significant difference with the variation in strain rates. The change in the dislocation structure, behavior, and density with the variation of strain rates clearly explains the mechanism of the strain rate hardening showing that round half-loop dislocations are replaced by straight and crossing dislocations and then by cell structured dislocations.

Heteroepitaxy of Ge x Si1 − x (x ∼ 0.4–0.5) films on Si(001) substrates misoriented to (111): Formation of short edge misfit dislocations alone in the misorientation direction

Relaxation of mechanical misfit stresses in GexSi1 − x (x ∼ 0.4–0.5) epitaxial films grown by molecular epitaxy on Si substrates misoriented from the exact orientation (001) by an angle of 6° has been studied. Possible cases of induced nucleation and interaction of 60° misfit dislocations (MDs) propagating in the misorientation direction with the formation of short edge MD segments are considered. Such configurations are classified and their various forms experimentally detected by TEM are presented. It is shown that short edge MDs are formed by two different mechanisms: (A) correlated or induced nucleation of a complementary 60° dislocation half-loop followed by the formation of an edge dislocation segment; and (B) the formation of a 90° MD segment upon intersection of the already existing complementary 60° MDs gliding in oppositely inclined {111} planes. The nonequivalency of the interaction of 60° MDs propagating in opposite directions along the substrate misorientation is shown.

Formation of misfit edge dislocations in Ge x Si1 − x films (x ∼ 0.4–0.5) grown on tilted Si(001) → (111) substrates

The dislocation structure of GexSi1 − x films (x ∼ 0.4–0.5) grown by molecular epitaxy on Si(001) substrates tilted by 6° about the 〈011〉 axis was studied. It is shown that, in the tilt direction, edge misfit dislocations (MDs) arise only in the form of short segments lying on the intersections of 60° MDs. As a result, the total length of edge MDs along the substrate tilt direction is smaller than that along the tilt axis. The deviation of the substrate surface from the singular plane made it possible to detect a dislocation configuration that consists of a short segment of an edge MD and two diverging 60° MDs propagating from it in the tilt direction. The formation of the segment is assumed to begin with simultaneous nucleation of complementary dislocation half-loops that form a short edge MD on the interface and then propagate on one side as two diverging 60° MD lines.

Defect-controlled growth of GaN nanorods on (0001)sapphire by molecular beam epitaxy

Transmission electron microscopy is used to reveal threading defects in single crystal c-oriented GaN nanorods grown on (0001)sapphire by molecular beam epitaxy. The defects are shown to be planar faults lying on {101¯0} planes and bounded by opposite partial screw dislocations with Burgers vectors of 1/2⟨0001⟩. The faults nucleate, as dislocation half-loops, from points close to the GaN/(0001)sapphire interface. It is proposed that the spiral growth of the partial atomic step joining the emerging dislocations controls nanorod growth and accounts for the growth surface morphology. The significance of these defects for nanorod growth and applications is discussed.

On the nucleation of partial dislocations in nanoparticles

Nanocrystalline face-centred-cubic (FCC) materials show unique mechanical properties. In this regard, an interesting but controversial discovery is that nanocrystalline FCC materials with high stacking-fault energy (SFE), such as aluminum (Al), exhibit a novel mechanism of deformation. In particular, both molecular dynamic simulations and high-resolution transmission electron microscopy observations show that for nanocrystalline Al mechanical twinning plays an important role in the deformation process. Yet these results are surprising, as the presence of partial dislocations in high SFE FCC metals are expected to be unstable. The purpose of this article is to offer a plausible explanation for the occurrence of stacking faults and deformation twins in nanocrystalline Al. A simple model is developed for the nucleation of both perfect and partial dislocation half-loops in individual single-crystal nanoparticles of Al and copper (Cu) with different sizes subjected to shear stress. The model shows that as the size of the nanoparticles decreases, partial dislocations are more likely to occur. Moreover, for a constant applied stress, there is a critical size for which the nucleation of partial dislocations occurs is more probable, compared to that of perfect dislocations.

Formation of edge misfit dislocations in Ge xSi 1−x ( x∼0.4–0.5) films grown on misoriented (0 0 1)→(1 1 1) Si substrates

We have studied the dislocation structure at the initial stage of relaxation of Ge x Si 1− x films ( x∼0.4–0.5) grown on Si substrates tilted 6° about the 〈0 1 1〉 axis. It is demonstrated that edge misfit dislocations (MDs) in the miscut direction arise in the form of short segments on intersections of 60° MDs. As a result, the total length of the edge MDs in the direction of substrate misorientation becomes substantially smaller than that in the direction of the miscut axis. Substrate misorientation from the singular plane made it possible to discover the MD configurations consisting of a short segment of an edge MD and only two 60° MDs diverging from this segment in the miscut direction. This configuration is assumed to start forming from simultaneous nucleation of two dislocation half-loops, which form a short edge MD in the interface and then propagate in only one direction in the form of two diverging branches of 60° MDs.

Influence of Growth Conditions and Substrate Properties on Formation of Interfacial Dislocations and Dislocation Half-loop Arrays in 4H-SiC(0001) and (000-1) Epitaxy

ABSTRACTFormation of interfacial dislocations (IDs) and dislocation half-loop arrays (HLAs) and their appearance in 4H-SiC epi-wafers are investigated by X-ray topography and KOH etching analysis. Synchrotron reflection X-ray topography demonstrates the ability to image IDs and HLAs simultaneously and reveal their densities as well as spatial distributions in the epi-wafers. The vertical location of IDs in the epi-wafer is also examined by this technique. The influence of wafer warp, in-situ H2 etching prior to epitaxial growth, substrate off-angle as well as the growth face (Si-face and C-face) on the densities and spatial distributions of IDs and HLAs are discussed.

Strain effects and microstructural evolution in Ge–Si system materials prepared by ion implantation and by ultrahigh vacuum chemical vapor deposition

Appropriately utilizing some microstructures may be very helpful to acquire desirable Ge–Si system materials. In this work, the Ge–Si system materials have been prepared either by ion implantation or ultrahigh vacuum chemical vapor deposition (UHVCVD). The interesting microstructures including half-loop dislocations, SiGe nanoclusters, and dislocation dipoles have been found in these two kinds of Ge–Si system materials. It is demonstrated that the evident surface strain state and adequate surface layer quality have been realized by employing these microstructures. Compared with the dipole dislocations in the Ge–Si systems deposited by UHVCVD on the compliant silicon on insulator, the half-loop dislocations and the SiGe nanoclusters induced by Ge ion implantation and subsequent annealing can relax the SiGe layer more effectively and lead to relatively large strain in the surface silicon. It may provide some new approaches to the control of misfit strains for fabricating desirable Ge–Si system materials.

Glide and multiplication of basal plane dislocations during 4H-SiC homoepitaxy

Basal plane dislocations (BPDs) are an important category of extended defects in SiC epilayers. They act as nucleation sites for single layer Shockley-type stacking faults which account for the degradation of the bipolar devices operating under forward bias. It is well documented that most of the BPDs in the SiC epilayers propagate from the substrates. However, two characteristic types of BPDs were suggested to be due to either nucleation or multiplication during epitaxy, including interfacial dislocations and short BPD arrays connected to the epilayer surface by threading segments. Combining molten KOH etching, plan-view transmission x-ray topography, and photoluminescence mapping, both types are determined to be two parts of one defect produced by the sideway glide of a BPD under the influence of shear stress. During the glide, the down-step end of the BPD frequently produces a series of short BPD segments at the moving growth front. These BPD segments will grow into an array of dislocation half loops. At the same time, the sideway glide of the BPD in the epilayer leaves an edge-type BPD segment at the epilayer∕substrate interface, which is the interfacial dislocation. The defect morphology provides the evidence of significant level of shear stresses present in SiC homoepitaxy of typical power device structures. The magnitude of such stresses is estimated.

Effects of temperature and surface step on the incipient plasticity in strained aluminium studied by atomistic simulations

Atomistic simulations using an EAM potential are carried out to investigate the first stages of plasticity in aluminium slabs, in particular the effect of both temperature and step geometry on the nucleation of dislocations from surface steps. Temperature is shown to significantly reduce the elastic limit, and to activate the nucleation of dislocation half-loops. Twinning occurs by successive nucleations in adjacent glide planes. The presence of a kinked step is shown to have no influence on the nucleation mechanisms.

Interlayer methods for reducing the dislocation density in gallium nitride

The ability of various interlayers to lower the density of threading dislocations (TDs) has been studied for the growth of c-plane (0 0 0 1) GaN epilayers on sapphire by metalorganic vapor-phase epitaxy (MOVPE). The TD density in the films may be reduced to 9×10 7 cm −2 using in-situ SiN x interlayers formed from silane and ammonia. This TD reduction method relies on the formation of facetted islands on the SiN x -treated GaN surface and the formation of dislocation half loops between bent-over TDs during the lateral overgrowth, and not by physically blocking the TDs. Thus, the TD density was reduced at the expense of greater film thickness by increasing the SiN x coverage and delaying intentionally the coalescence of the GaN islands. Scandium nitride interlayers have also proved effective for defect reduction; dislocation densities of 3×10 7 cm −2 have been reached by depositing thin (5–15 nm) Sc metal layers on 500 nm GaN-on-sapphire seed layers, then annealing in NH 3 to form ScN, followed by overgrowth of GaN. The low dislocation densities are attributed to a dislocation-blocking effect induced by the different (but compatible) crystal structures of GaN and ScN, whereby the dislocations threading from the seed layer cannot propagate through the ScN layer. The remaining dislocations appear to form on coalescence of the dislocation-free GaN islands which nucleate on the ScN interlayer.

Threading dislocation behavior in AlN nucleation layers for GaN growth on 4H-SiC

Threading dislocations in thin (<200nm) AlN nucleation layers (NLs) grown by metal-organic chemical vapor deposition on top of 4H-SiC on-axis mesas with atomic-scale steps were analyzed by transmission electron microscopy. The AlN NL controlled threading dislocations in an overlying ∼2μm GaN layer through two identified mechanisms: threading half-loop formation and dislocation bending at V-shaped pits. Threading dislocations in the AlN film could be traced directly to bilayer 4H-SiC steps at the substrate/film interface. These observations reveal several approaches to extended defect reduction in GaN films grown on 4H-SiC.

Out‐diffusion of nitrogen from float‐zone silicon measured by dislocation locking

AbstractA measurement of nitrogen out‐diffusion from nitrogen‐doped float‐zone silicon made using a dislocation locking technique is presented. Specimens containing a well‐defined array of dislocation half‐loops are subjected to identical anneals at 750 °C, during which nitrogen diffuses both to the surface and to the dislocations. The specimens are then chemically etched so as to remove different thicknesses of material from the surface. The stress required to move the dislocations away from the nitrogen is then measured. The variation in this unlocking stress with thickness of material removed allows some measure of nitrogen diffusivity to be deduced. The result obtained is consistent with an extrapolation of SIMS out‐diffusion measurements previously performed at higher temperatures, but indicates a different activation energy for out‐diffusion to that associated with dislocation locking by nitrogen. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characteristics of Dislocation Half-Loop Arrays in 4H-SiC Homo-Epilayer

Dislocation "half-loop arrays" (HLAs) in 4H-SiC homo-epilayers are studied by molten KOH etching and atomic force microscopy (AFM). It is found that the dislocation half-loops in an array exist at different depths in the epilayer, and they are aligned roughly but not exactly perpendicular to the off-cut direction. These results indicate that the dislocation half-loops in an array are not formed simultaneously, but the array extends by generation of new half-loops during growth. It is also demonstrated that the HLAs can be artificially induced by creating strain in the material, followed by annealing.

Basal-plane slip in InGaN∕GaN heterostructures in the presence of threading dislocations

Basal-plane slip has been observed in InGaN∕GaN heterostructures containing a high density of threading dislocations. The InGaN epilayers were deposited on a GaN mesa structure grown by metal-organic chemical vapor deposition on (0001) sapphire substrates using conventional methods. The glide process starts at the edge of the mesas, generating misfit dislocation half loops with a Burger vector of 1∕3⟨112¯0⟩. The misfit dislocation propagation is shown to depend on the crystalline orientation of the mesa edge and results in relaxation of over 80% of the misfit strain. Pinning of the misfit dislocation loops by threading dislocations does not stop them from propagating into the layer. No reaction is observed between the dislocation loops and the threading dislocations. The number of dislocation loops and their propagation distance increase with the indium content of the InGaN epilayers, indicating that the process is driven by the misfit strain. As a result of plastic relaxation, InGaN epilayers are observed to expand outside the edge of the GaN mesa, forming a ledge. These observations suggest possible ways to overcome lattice mismatch with GaN in order to grow high quality InGaN films with high indium concentration.

Two-Dimensional Growth Mode and Reduction of Dislocation Density in Nitride Layers

AbstractHeteroepitaxial two-dimensional (2D) and three-dimensional (3D) MOVPE growth of nitride layers on sapphire substrates using low temperature (LT) AlN nucleation layer are discussed. It is shown by Atomic Force Microscopy and Transmission Electron Microscopy that the 2D or 3D growth mode of successive AlGaN layers depends predominantly on the growth conditions of the underneath LT AlN nucleation layer.Modifications of growth conditions of LT AlN nucleation layer in respect of the growth in the conventional way, leads to a drastic change in properties of the successively grown at high temperature AlGaN layer. Main modification is connected with a drastic reduction of the growth rate of LT AlN nucleation layer at higher rector pressure. In this case the growth at high temperature starts evenly on the whole surface, retaining atomic flatness, and occurs in the 2D mode. Therefore, it is possible to grow even a very thin, continuous AlGaN layers directly on top of LT nucleation layer.The 2D growth mode leads to significant change in extended defect distribution. Edge dislocation density being close to 1010 cm−2, results in the effective relaxation of the internal stress. On the other hand mix dislocations density are close to 108 cm−2. All observed mix dislocations occur in pairs with dislocations with opposite sign. This suggests that all mix dislocations originate from dislocation half-loops. Both types of dislocations, edge and half loops can be effectively reduce in-situ, during the growth using annealing in the presence of silane. This process and precise control of the growth conditions leads to decreasing of mix dislocations density to 2·106 cm−2 and edge dislocations density to 5·108 cm−2.The newly developed two-dimensional growth mode of nitride layer opens up a new way to reduce strain, decrease density of dislocations and improvement of crystallographic properties.

Threading dislocation reduction in (0 0 01) GaN thin films using SiN x interlayers

The ability of in situ SiN x interlayers to lower the density of threading dislocations (TDs) has been studied for the growth of c-plane (0 0 0 1) GaN epilayers on sapphire by organometallic vapour-phase epitaxy (OMVPE). The TD density in the films may be reduced by up to a factor of 50 to 9×10 7 cm −2 and depends on the SiN x coverage and the conditions of the overgrowth. The TD reduction method relies on the formation of facetted islands on the SiN x -treated GaN surface and the formation of dislocation half-loops between bent-over TDs during the lateral overgrowth. Dislocations that are not annihilated at the interfacial region during the interlayer overgrowth may bend over at the islands’ inclined side facets and annihilate at their coalescence boundaries. Thus, the TD density was reduced at the expense of greater film thickness by increasing the SiN x coverage and delaying intentionally the coalescence of the GaN islands.

Enhanced oxygen diffusion in highly doped p-type Czochralski silicon

The locking of dislocations by oxygen has been investigated experimentally in Czochralski silicon (Cz-Si) with different concentrations of shallow dopants. Specimens containing well-defined arrays of dislocation half-loops were subjected to isothermal anneals in the 350–550°C temperature range, and the stress required to bring about dislocation motion at 550°C was then measured. This dislocation unlocking stress was found to increase with annealing time due to oxygen diffusion to the dislocation core. The dislocation unlocking stress was measured in n-type Cz-Si with a high antimony doping level (∼3.4×1018cm−3) and p-type Cz-Si with a low boron doping level (∼1.3×1015cm−3). An analysis of the data taking the different oxygen concentrations into account showed that the rate of increase in dislocation unlocking stress was unaffected by the high level of antimony doping. This indicates that a high antimony doping level has no significant effect on oxygen transport for the conditions used in this experiment. However, in p-type Cz-Si with a high boron doping level (∼5.4×1018cm−3), the dislocation unlocking stress was found to rise at a much faster rate than in Cz-Si with a low boron doping level or high antimony doping level. This enhancement in dislocation locking was by a factor of approximately 60 at 400°C. By performing a numerical simulation to solve the diffusion equation for oxygen transport to a dislocation, the effective diffusivity of oxygen was deduced from the dislocation unlocking data to be 2.7×10−6exp(−1.4eV∕kT)cm2s−1 in the highly boron doped Cz-Si. In the temperature range studied, the effective diffusion coefficient in the highly boron doped Cz-Si was found to be approximately 44 times higher than expected in low boron doped Cz-Si with an identical oxygen concentration.

Generation of misfit dislocations by basal-plane slip in InGaN∕GaN heterostructures

The authors have observed that for InxGa1−xN epitaxial layers grown on bulk GaN substrates exhibit slip on the basal plane, when in the presence of free surfaces that intercept the heterointerface and for indium compositions x⩾0.07. This leads to almost complete relaxation of the local misfit strain by generation of radial-shape dislocation half loops. For x⩾0.17, generation of straight misfit dislocations by glide on the secondary ⟨112¯3⟩ {112¯2} slip system is observed, in addition to the radial-shape half loops at surface pits. These two mechanisms act independently with no observed interaction between them, leading to the conclusion that slip on the basal plane occurs first during the growth process. The secondary slip system is activated later and involves a significantly higher critical stress energy.

Nitrogen in silicon: Transport and mechanical properties

A novel dislocation locking technique is applied to nitrogen-doped float-zone silicon. Specimens containing well-defined arrays of dislocation half-loops are subjected to isothermal anneals of controlled duration, during which nitrogen diffuses to the dislocations. The stress required to bring about dislocation motion is then measured, generally at 550°C. The segregation of nitrogen to dislocations is found to be stable to at least 1200°C, and the dislocation unlocking stress measured at 550°C is of similar magnitude to that for oxygen in Czochralski silicon. At all annealing temperatures studied, the measured unlocking stress as a function of annealing time initially rises linearly before taking a constant value. The rate of the initial rise is found to be strongly dependent on temperature, with an activation energy of 1.5eV. In a separate set of experiments, the magnitude of the dislocation unlocking stress measured is also found to depend upon the temperature at which the unlocking process takes place in the 500–700°C temperature range. The dislocation locking technique is also used to give some measure of nitrogen out-diffusion, by measuring the unlocking stress as a function of material etched away after annealing. For specimens annealed at 750°C for 15h, the dislocation unlocking stress is found to rise with increasing depth away from the specimen’s surface, before it takes an approximately constant value. By fitting an error function to these data, the diffusion coefficient of nitrogen in silicon is deduced to be approximately 4×10−11cm2s−1 at 750°C. This value is consistent with a SIMS investigation by Itoh and Abe.