Mismatch Dislocations Research Articles

Structural features and electrical properties of si(al) thermal migration channels for high-voltage photovoltaic converters

The results of a study of the structural features and electrical properties of Si(Al) through thermomigration p-channels in a silicon wafer are presented. Structural studies were performed using X-ray methods of projection topography, diffraction reflection curves and scanning electron microscopy. It is shown that the channel-matrix interface is coherent without the formation of mismatch dislocations. The possibility of using an array of thermomigration p-channels of 15 elements to form a monolithic photovoltaic solar module in a Si(111) silicon wafer based on p-channels with a width of 100 microns with walls in the plane is shown. The monolithic solar module has a conversion efficiency of 13.1%, an idle voltage of 8.5 V and a short-circuit current density of 33 mA/cm².

Mechanical behavior analysis on SiC/Al composites via in-situ neutron diffraction combined with finite element methods

In this study, in-situ neutron diffraction and finite element (FE) analysis were employed to analyze the mechanical behavior of SiC/Al composites with 9 (M9) and 25 (M25) vol% SiC during tensile tests. The analysis of stress concentration factors in SiC/Al composites during the tensile process revealed that load-redistribution between the matrix and the reinforcement was accompanied by local and overall plastic deformation. The strength analysis shows that the load transfer and the dislocation strengthening were the main reasons for the strength increase. The dislocation strengthening and load transfer strengthening of M25 were 4 and 3 times higher than that of M9, respectively. In addition, during the tensile process, the load-redistribution occurred simultaneously with micro-yielding. The comparison between the external loading-macro elastic strain and stress-strain curves could reflect the micro-yielding beginning. It was found that the micro-yielding of M25 was more obvious, because more stress concentration around reinforcement and a large number of movable thermal mismatch dislocations were induced at high content reinforcement.

Temperature-dependent morphology and composition of ultra-thin GeSn epilayers prepared by remote plasma enhanced chemical vapor deposition

Two distinct ultra-thin Ge1−xSnx (x ≤ 0.1) epilayers were deposited on (001) Si substrates at 457 and 313 °C through remote plasma-enhanced chemical vapor deposition. These films are considered potential initiation layers for synthesizing thick epitaxial GeSn films. The GeSn film deposited at 313 °C has a thickness of 10 nm and exhibits a highly epitaxial continuous structure with its lattice being compressed along the interface plane to coherently match Si without mismatch dislocations. The GeSn film deposited at 457 °C exhibits a discrete epitaxial island-like morphology with a peak height of ∼30 nm and full-width half maximum (FWHM) varying from 20 to 100 nm. GeSn islands with an FWHM smaller than 20 nm are defect free, whereas those exceeding 25 nm encompass nanotwins and/or stacking faults. The GeSn islands form two-dimensional modulated superlattice structures at the interface with Si. The GeSn film deposited at 457 °C possesses a lower Sn content compared to the one deposited at lower temperature. The potential impact of using these two distinct ultra-thin layers as initiation layers for the direct growth of thicker GeSn epitaxial films on (001) Si substrates is discussed.

Low-Temperature Growth of InGaAs Quantum Wells Using Migration-Enhanced Epitaxy.

The growth of InGaAs quantum wells (QWs) epitaxially on InP substrates is of great interest due to their wide application in optoelectronic devices. However, conventional molecular beam epitaxy requires substrate temperatures between 400 and 500 °C, which can lead to disorder scattering, dopant diffusion, and interface roughening, adversely affecting device performance. Lower growth temperatures enable the fabrication of high-speed optoelectronic devices by increasing arsenic antisite defects and reducing carrier lifetimes. This work investigates the low-temperature epitaxial growth of InAs/GaAs short-period superlattices as an ordered replacement for InGaAs quantum wells, using migration-enhanced epitaxy (MEE) with low growth temperatures down to 200-250 °C. The InAs/GaAs multi-quantum wells with InAlAs barriers using MEE grown at 230 °C show good single crystals with sharp interfaces, without mismatch dislocations found. The Raman results reveal that the MEE mode enables the growth of (InAs)4(GaAs)3/InAlAs QWs with excellent periodicity, effectively reducing alloy scattering. The room temperature (RT) photoluminescence (PL) measurement shows the strong PL responses with narrow peaks, revealing the good quality of the MEE-grown QWs. The RT electron mobility of the sample grown in low-temperature MEE mode is as high as 2100 cm2/V∗s. In addition, the photoexcited band-edge carrier lifetime was about 3.3 ps at RT. The high-quality superlattices obtained confirm MEE's effectiveness for enabling advanced III-V device structures at reduced temperatures. This promises improved performance for applications in areas such as high-speed transistors, terahertz imaging, and optical communications.

Atomic-scale investigation of interface structures in polysynthetically twinned TiAl single crystals

Interface structures of γ-TiAl/α2-Ti3Al and differently oriented γ/γ lamellae in PST-TiAl crystals with 0° lamellar orientation have been systematically investigated on an atomistic scale using high-resolution scanning transmission electron microscopy. The γ/α2 lamellar interfaces are atomically abrupt and coherent except for steps with a height of even number (mainly two) of (111)γ planes. These steps are associated with Shockley partial dislocations that slip on alternate (111)γ or (0002)α2 planes to accomplish the γ ⇋ α2 transformation. Three kinds of γ/γ interface structures are all atomically flat and coherent. The γ/γ twin boundaries contain steps associated with Shockley and Frank partial dislocations, while there are usually no steps and mismatch dislocations on the γ/γ ordered boundaries and pseudo-twin boundaries. But there is a thin α2 lamella with a thickness of two or three lattice cells sandwiched between some γ/γ ordered boundaries. More intriguingly, the γ lamella terminated in the α2 lamella, or vice versa, forms the flat and coherent (1 1‾ 00)α2/(1 2‾ 1)γ end interfaces with the existence of steps associated with dislocations.

Improvement of crystallinity of CdZnTe epilayers on GaSb substrates by ZnTe buffer layer

Thick CdZnTe epitaxial film is a promising alternative for X-ray detectors, especially in the case of large-area imaging application. One of the key issues in growing thick heteroepitaxial film with high crystallinity is to reduce the mismatch dislocations that appear at the interface between CdZnTe epilayer and the substrate. In this paper, we prepared CdZnTe films without and with ZnTe buffer layers on GaSb (001) substrates by the closed space sublimation. All preparation conditions for ZnTe buffer layers, CdZnTe epilayers on ZnTe buffers and GaSb substrates, as well as the pretreatment of GaSb surface have been optimized through tremendous experiments. The results of electron backscatter diffraction and transmission electron microscope observation prove that ZnTe buffer layers suppress (1‾1‾1)M twins extending from GaSb. The deposition temperature for inserting ZnTe buffer about 495 K is optimized for getting smooth ZnTe surface, on which high crystalline quality CdZnTe epilayer with a narrower full-width at half-maximum (85 arcsec) of double crystal X-ray rocking curve for (004) reflection and lower intensity ratio IDcomplex/I(D0, h) of low-temperature photoluminescence spectroscopy were produced.

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.

The fabrication of flexible and large‐size inverted metamorphic five‐junction solar cells

AbstractThe inverted metamorphic AlGaInP/AlGaAs/GaAs/InGaAs/InGaAs five‐junction solar cells with the bandgap energies of 2.0/1.67/1.42/1.18/0.83 eV were grown by metal–organic chemical vapor deposition on GaAs substrates. The mismatch stress between the two bottom (In0.17Ga0.83As and In0.46Ga0.54As) cells and GaAs substrate was nearly fully relaxed owing to the growth of AlInGaAs compositionally step‐graded buffers. By employing the low‐temperature epilayer transfer technology with only one‐time temporary bonding, the flexible five‐junction solar cells with the size of 12 cm2 were successfully fabricated. The conversion efficiency was 35.1% with the open‐circuit voltage of 4.73 V under AM1.5G spectrum. The excellent performance of the large‐size five‐junction solar cells is attributed to the low amount of mismatch dislocations in the material as well as the simple fabrication process of the flexible device.

Dislocation-driven growth of WS2/WSe2 quantum well superlattices

The synthesis of two-dimensional lateral heterojunctions with nanoscale characteristic width and sharp interfaces remains challenging. The quantum confinement effects are still difficult to create on 2D materials since widths smaller than 5 nm are necessary for quantum confinement effects and quantum well applications. In this study, we demonstrated the growth of a sub-2-nm tungsten sulfide quantum well array in a monolayer of tungsten selenide, driven by the climb of mismatch dislocation in a heterointerface due to the lattice mismatch. Width-controllable 2D quantum well superlattices are theoretically formed by the mismatch dislocation-driven growth mechanism, according to our analysis. Thus, abundant photonic electronic properties can be obtained in 2D quantum well superlattices formed at varied lateral heterointerfaces, which will support the study of topological insulators and superconductors.

Dislocation-induced stop-and-go kinetics of interfacial transformations.

Most engineering materials are based on multiphase microstructures produced either through the control of phase equilibria or by the fabrication of different materials as in thin-film processing. In both processes, the microstructure relaxes towards equilibrium by mismatch dislocations (or geometric misfit dislocations) across the heterophase interfaces1-5. Despite their ubiquitous presence, directly probing the dynamic action of mismatch dislocations has been unachievable owing to their buried nature. Here, using the interfacial transformation of copper oxide to copper as an example, we demonstrate the role of mismatch dislocations in modulating oxide-to-metal interfacial transformations in an intermittent manner, by which the lateral flow of interfacial ledges is pinned at the core of mismatch dislocations until the dislocation climbs to the new oxide/metal interface location. Together with atomistic calculations, we identify that the pinning effect is associated with the non-local transport of metal atoms to fill vacancies at the dislocation core. These results provide mechanistic insight into solid-solid interfacial transformations and have substantial implications for utilizing structural defects at buried interfaces to modulate mass transport and transformation kinetics.

Effects of hybrid processing on microstructural and mechanical properties of thixoformed aluminum matrix composite

Abstract Hybrid processing is a promising technique that combines powder metallurgy and liquid casting to disperse reinforced particles in Al–5Si alloy matrix homogeneously. In this study, dual alumina-multiwalled carbon nanotubes (MWCNTs) and single MWCNT-reinforced particles were ball-milled and compacted into pellets. The pellets were then injected into molten Al–5Si at 650 °C, mechanically stirred for 10 min and poured into a mould via a cooling slope for semi-solid feedstock. All samples were subjected to thixoforming and standard T6 heat treatment. The effects of hybrid techniques on the distribution of reinforcements, microstructure and mechanical properties were investigated. Tensile fracture surface analyses showed that dual and single reinforcements were distributed homogeneously in the samples. However, severely damaged MWCNT structures were observed under dual reinforcement due to excessive shear stress during ball milling process. The highest yield strength, ultimate tensile strength and elongation to fracture were 262 MPa, 289 MPa and 7.2% and 316 MPa, 347 MPa and 13.3% for thixoformed-T6 alloy composites of dual and single reinforcements, respectively. The two strengthening mechanisms involved are as follows: by the matrix, microstructure evolution and precipitation; and by reinforcement, load transfer, thermal mismatch dislocation and Orowan looping. The more intact and the longer structure of MWCNTs in PB lead to better effective load transfer ability than the PA composite.

Epitaxial growth of InAs/GaAs quantum dots on {113}-faceted Ge/Si (001) hollow substrate

The direct epitaxial growth of GaAs on Si suffers from their nature of lattice mismatch, thermal mismatch and polarity difference induced anti-phase domains (APDs). Here, we report the high quality and thin GaAs film grown on {113}-faceted Ge/Si (001) hollow substrate by in-situ hybrid molecular beam epitaxy. By directly growth of Ge on U-shape patterned Si (001), a strain-relaxed high-quality Ge sawtooth hollow structure with {113} facets was obtained. With an additional 400 nm GaAs deposition, an APD-free surface with a root-mean-square roughness of merely 0.67 nm is obtained on such Ge {113} /Si (001) substrate. The lattice mismatch dislocation between Ge and Si is found to terminate mostly at the sidewalls of the hollow structures. The {113}-faceted Ge surface is acting as an equivalent to the miscut substrate, which annihilates the APDs at the GaAs/Ge interface. High-resolution X-ray diffraction characterization reveals that the hollow structures can effectively reduce the thermal strain, leading to a crack-free GaAs film up to 7 µm. Five-layer InAs/GaAs quantum dots (QDs) on such virtual GaAs/Ge {113} /Si (001) substrate without any dislocation filter layers exhibits almost the same photoluminescence (PL) intensity as that on the GaAs substrate, providing a promising method for integrating III-V QD lasers with silicon photonic platform.

Структурное совершенство и состав легированных галлием термомиграционных слоев кремния

We suggest a technique of fabrication of thick heavily Ga doped by thermomigration layers of Si for modern power electronics devices. Structure perfection and layers composition as a function of formation temperature were studied by techniques of X-ray Lang topography, X-ray rocking curves and secondary ion mass spectrometry. The fabricated layers are single crystalline, no mismatch dislocations were found on the interface with the Si substrate. The Ga concentration in the layers can be varied in the range (1.6-4.8)1019 cm-3, being higher than for Si doping with aluminum.

An investigation on particle weakening in T6-treated SiC/Al–Zn–Mg–Cu composites

This study found weakening effect of SiC particles on strength of a T6-treated SiC/Al–Zn–Mg–Cu composite. The mechanisms for the weakening were investigated in detail for the first time. The 15 vol% SiC/7085Al composite and 7085Al alloy were fabricated by powder metallurgy (PM) method and subjected to microstructural examination and mechanical properties evaluation. It was indicated that the as-extruded composite had 92 MPa enhancement in the ultimate tensile strength (UTS) compared to the as-extruded unreinforced alloy. However, After T6 treatment (solution treatment and artificial ageing), the UTS of the composite was 34 MPa lower than that of the unreinforced alloy. Microstructural examination disclosed that the content of η′-MgZn2 was reduced in the T6-treated composite due to interface reaction between Mg and oxide on the surface of SiC particles. Meanwhile, compared to the unreinforced alloy, the composite had coarser precipitates with relative lower density under the T6 condition. Furthermore, it was found that SiC particles accelerated ageing process, leading to formation of coarse precipitates and precipitate free zone in the vicinity of SiC–Al interface. According to the theoretical calculation, the weakened precipitation strengthening value was not compensated by mismatch dislocation strengthening and load transfer strengthening. By adding additional Mg (1.0 wt%) into the composite, the precipitation strengthening effect could be basically recovered, resulting in significantly enhanced strength compared to the 7085Al alloy.

Simultaneously improving elastic modulus and damping capacity of extruded Mg-Gd-Y-Zn-Mn alloy via alloying with Si

Developing magnesium (Mg) alloys with both high elastic modulus and damping capacity is a long-term challenge in the field of lightweight metals. Herein, it is shown that alloying with Si in Mg-Gd-Y-Zn-Mn simultaneously increased the elastic modulus and damping capacity. After Si alloying, the LPSO phase was greatly reduced and a large number of (RE + Si)-rich particles were formed. Due to the contribution of the high modulus second phases, the elastic modulus of the extruded Mg-Gd-Y-Zn-Mn alloy with a trace amount of Si was 49.3 GPa, 8 GPa greater than that of the extruded pure Mg. In addition, the extruded Mg-Gd-Y-Zn-Mn-Si alloy possessed good damping capacity at both room temperature (RT) and high temperature. At high strain amplitude, the extruded Mg-Gd-Y-Zn-Mn-Si alloy achieved a RT damping value of Q−1 > 0.01, significantly higher than the extruded pure Mg, which was related to the high density of thermal mismatch dislocations in the vicinity of the interfaces between the reinforcing phase and the α-Mg matrix. With the increasing temperature, the extruded Mg-Gd-Y-Zn-Mn-Si alloy showed obviously higher damping capacity than the extruded Si-free alloy, which was attributed to the incoherent phase interfaces, weakened texture and thermal mismatch dislocations.

Thermal mismatch stress relaxation and dislocation transformation of 45%SiCp/Al composites by continuous diode laser heating

The purpose of the study was to investigate the effect of laser beam scanning on residual stress relaxation and metallographic structure evolution of 45%SiCp/Al composites with an increase in the temperature. Based on the theory of elasticity and equivalent inclusions, the residual stresses distribution of individual component in 45%SiCp/Al composites with an increase in the temperature are obtained. The results demonstrated that the speed of stress relaxation for matrix and interface was faster with the temperature variation from 25 to 300 °C comparing with the temperature variation from 300 to 500 °C. The dislocation configurations and morphologies of second precipitated phases with laser treatment were characterized using the optical microscopy and transmission electron microscopy. It is found that the thermal residual stress relaxation mechanism with an increase in the temperature was caused due to the evolution of dislocation density and dislocation structure arrangements. A physical model of dislocation and grain evolutionary process with an increase in the temperature was proposed, and the effects of laser scanning on precipitated phase was analyzed in view of the interaction mechanism between the mobile dislocations and the precipitated phase. The nanoindentation test was also carried out to analyse the effect of residual stress and dislocations configuration on the mechanical response of individual component.

Influence of interface with mismatch dislocations on mechanical properties of Ti/Al nanolaminate

As a representative boundary, interphase-interface may affect the strength or ductility of multilayered composites dramatically. However, the effect of the interface with mismatch dislocations on the mechanical behavior of multilayered composites is still not clear. In the present work, we performed molecular dynamics simulations to investigate the effect of interface structures and layer spacing on the mechanical properties of the Ti/Al nanolaminate. The results indicate that there are two transitions of the plastic deformation mechanism in the Ti layer with the increase of layer spacing in the sample with a coherent interface. The plastic deformation mechanism evolves from one that is dominated by dislocation to the phase transformation from the hcp-Ti to the fcc-Ti mode, which transfers to the dislocation slip deformation again. For the samples with an incoherent interface, the plastic deformation is dominated by the transformation from hcp-Ti to fcc-Ti, regardless of the variation of layer spacing, while the plastic deformations in the Al layers are mainly dislocations confined in the layer slip in the samples with both coherent and incoherent interfaces. When the layer spacing is larger than 6.6 nm, an obvious second hardening is observed due to the superior dislocation storage ability of the Ti/Al laminate with the incoherent interface. Meanwhile, extraordinary ductility is obtained when optimal layer spacing is employed in the Ti/Al laminate. Moreover, the phase transformation mechanism of hcp-Ti to bcc-Ti has also been explicated in the present work. The general conclusions derived from this work may provide a guideline for the design of high-performance Ti/Al multilayer and alloy devices.

Microstructural assessment of 310S stainless steel during creep at 800 °C

A thorough investigation of the microstructural evolution of 310S stainless steel subjected to creep at 800 °C shed new light on the nucleation and growth mechanism of M23C6 precipitates and the σ-phase. During creep of 310S, the M23C6 precipitates form on defects such as dislocations, Ti(C,N) precipitates and twin boundaries, within the austenitic matrix. These precipitates have a cube to cube orientation relationship with the matrix and are initially semi-coherent; maintaining coherency by forming mismatch dislocations. As the M23C6 precipitates are consumed by growing σ-phase particles, they can no longer pin sub-grain boundaries and a decrease in dislocation density of the material occurs, as measured by TEM. Under low stress, formation of sufficient quantities of the σ-phase eventually causes sensitization of the stainless steel leading to oxidation and void formation at grain boundaries. Eventually the diffusion of nitrogen, causes formation of an extensive network of widmanstatten Cr2N throughout the bulk.

O-Band and C/L-Band III-V Quantum Dot Lasers Monolithically Grown on Ge and Si Substrate

Direct epitaxial growth of III-V heterostructure on CMOS-compatible silicon wafer offers substantial manufacturing cost and scalability advantages. Quantum dot (QD) devices are less sensitive to defect and temperature, which makes epitaxially grown III-V QD lasers on Si one of the most promising technologies for achieving low-cost, scalable integration with silicon photonics. The major challenges are that heteroepitaxial growth of III-V materials on Si normally encounters high densities of mismatch dislocations, antiphase boundaries and thermal cracks, which limit the device performance and lifetime. This paper reviews some of the recent developments on hybrid InAs/GaAs QD growth on Ge substrates and highly uniform (111)-faceted hollow Si (001) substrates by molecular beam epitaxy (MBE). By implementing step-graded epitaxial growth techniques, the emission wavelength can be tuned into either an O band or C/L band. Furthermore, microcavity QD laser devices are fabricated and characterized. The epitaxially grown III-V/IV hybrid platform paves the way to provide a promising approach for future on-chip silicon photonic integration.

Towards strength-ductility synergy with favorable strengthening effect through the formation of a quasi-continuous graphene nanosheets coated Ni structure in aluminum matrix composite

In this study, in situ synthesis of graphene nanosheets (GNS) coated nickel (Ni@GNS) as reinforcement for aluminum matrix composites (Ni@GNS/Al) was reported. Ni@GNS exhibits a quasi-continuous network-like structure. The addition of Ni could reduce the interfacial thermal expansion and improve the wettability between GNS and Al, resulting in a robust interfacial bonding among GNS, Ni and Al. Due to the unique structure, the composite exhibits a favorable strength-ductility synergy. The yield strength of the composite increases to 237 ± 2 MPa, ~ 80.9% improvement compared to that of the unreinforced Al, combined with an excellent elongation of 19.1 ± 2%. The main strengthening mechanisms are the interfacial thermal mismatch and Orowan dislocation strengthening, with some contributions of grain refinement, load transfer of GNS and precipitated-phase strengthening of Ni. Moreover, the interlocked effect between GNS and Ni could further prolong the deformation failure and improve the plasticity of the composites.