In-plane Mismatch Research Articles

The Correlation between the Microstructure and the Characteristic Luminescence in the High Crystalline α-Ga2O3 Grown on the Thin-Wall Single Crystal Al2O3

A high-quality alpha-Ga2O3 thin film was grown on an Al2O3 single-crystal thin-wall channel structure. Alpha-Ga2O3 crystals are grown with a 3-fold symmetry template provided by the Al2O3 surface, which forms domain boundaries from the small rotation of the mosaic structure. The anti-phase domain (APD) was confirmed by convergent-beam electron diffraction (CBED), and it is believed to be developed at the stage of nucleation based on the unstable, or uneven Al-O bonding on the Al2O3 surface. The in-phase domain and the APD boundaries exhibit bright contrast in ADF, which originates from a local strain of crystallographic rotation and not from the local elemental fluctuations of gallium. As confirmed by CL measurement in TEM, the bandgap energy of alpha-Ga2O3 is inferred to be 5.56 eV with a near band edge transition of ~ 223 nm. Even though the heights and positions of peaks vary slightly from area to area, 4 distinct luminescence peaks and a long tail of wavelength are commonly observed throughout the crystal. 2-dimensional mapping with the specific wavelength window reveals a clear difference between the inside domain, in-phase domain boundary, and APD boundary. A luminescence peak of distinct 350 nm was observed at the APD boundary. Misorientation between domains was found, which leads to an in-plane mismatch in crystallographic lattices at the boundary when the two misoriented domains grow and the growth front merges. The imperfect lattice structure at the boundary is believed to create strain and intermediate states.

New insight into strain and composition of BaZrO3 nanorods in REBCO superconductor

We report on strain and composition effects associated with growth of self-assembled BaZrO3 (BZO) nanorods in REBa2Cu3O7−δ (REBCO) superconductors (RE = rare earth = Y and Gd), which have a profound effect on flux pinning and in-field critical current performance. The a-b plane mismatch between BZO and REBCO is never fully coherently accommodated. Instead, the nanorods always assume a size at least one unit cell smaller than the corresponding ‘hole’ in the REBCO matrix, thus providing deep minima of in-plane mismatch strain. Next, we show that the nominal BZO nanorods are in fact solid solution Ba2+(Zr4+ 1−z RE3+ z )O3−δ perovskite, thus strongly affecting the stoichiometry and relative amounts of REBCO and BZO. We demonstrate that by varying only the Ba content in the nominal composition of 15 mol.% BZO + REBCO, the unit cell density of BZO can be tuned from 5% to 23%, and the linear density of RE2O3 (REO) precipitates from 18 to 1 μm−1. The results explain the wide range of pinning performances observed for the same nominal amount of Zr addition and provide insight into the mechanisms behind the complex phenomenon of growth of nanorods by self-assembly in REBCO superconductors.

Formula omitted] Twin boundary structures in a Mg–Gd alloy

In this work, we report our direct observations of atomic structures of both end and broad interfaces of{101¯1} twin boundary (TB) in a deformed and annealed Mg–Gd solid solution single phase alloy using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The end interface is an asymmetric tilt boundary decorated by a periodic array of clusters comprising ordered Gd-rich columns. The broad interface consists of coherent {101¯1} twin boundaries that are disconnected by various steps and basal/pyramidal (BPy) facets, which all contain characteristic Gd segregation. The twin steps, including S1/1, S2/2, S3/2, S5/4, S7/6 and S7/7, are observed and classified in terms of their heights, orientations, and Burgers vectors. The S1/1 and S2/2 steps are not associated with any misfit dislocations, but S1/1 is always associated with an I1 stacking fault, which is free of Gd segregation, in the twin crystal. The S3/2, S5/4, S7/6 and S7/7 steps contain misfit dislocations but no stacking faults. The BPy facets, observed for the first time in the{101¯1} TB of Mg alloys, exhibit different widths and orientations, and they are associated with elastic strain arising from in-plane mismatch and plane disclination. A double-segregation-layer structure is also observed, with the coherent{101¯1}1} TB lying on one of these two layers. These observations are discussed in terms of existing crystallographic models.

Anisotropic thermal expansion of Ni3Sn4, Ag3Sn, Cu3Sn, Cu6Sn5 and βSn

The directional coefficient of thermal expansion (CTE) of intermetallics in electronic interconnections is a key thermophysical property that is required for microstructure-level modelling of solder joint reliability. Here, CTE ellipsoids are measured for key solder intermetallics using synchrotron x-ray diffraction (XRD). The role of the crystal structure used for refinement on the CTE shape and temperature dependence is investigated. The results are used to discuss the βSn-IMC orientation relationships (ORs) that minimise the in-plane CTE mismatch on IMC growth facets, which are measured with electron backscatter diffraction (EBSD) in solder joints on Cu and Ni substrates. The CTE mismatch in fully-intermetallic joints is discussed, and the relationship between the directional CTE of monoclinic and hexagonal polymorphs of Cu6Sn5 is explored.

Room-temperature ferromagnetic CuO thin film grown by plasma-assisted molecular beam epitaxy

CuO thin film was epitaxially grown on MgO (111) substrate by plasma-assisted molecular beam epitaxy method. Reflection high-energy electron diffraction monitoring and X-ray diffraction analysis indicates it is single crystal CuO (002) thin film. Without any detectable ferromagnetic dopant or second phase, the CuO thin film displays ferromagnetic characteristics at both 5K and 300K. With an in-plane mismatch and out of plane stretch, the existence of oxygen vacancies and the coexistence of Cu1+ and Cu2+ are inferred to be responsible for the origin of the room-temperature ferromagnetism in the CuO thin film.

Room-temperature selective epitaxial growth of CoO (1 1 1) and Co3O4 (1 1 1) thin films with atomic steps by pulsed laser deposition

Cobalt oxide epitaxial thin films of both rock-salt CoO and spinel Co3O4 were selectively synthesized on atomically stepped α-Al2O3 (0001) substrates at room temperature under well-controlled oxygen pressures by pulsed laser deposition. X-ray diffraction and reflection high-energy electron diffraction analyses demonstrated that the CoO and Co3O4 films were grown with phase control and good epitaxial quality at room temperature (20°C). The CoO (111) film was obtained in ultra-high vacuum of 1×10−8Torr, while the Co3O4 (111) film was grown in 1×10−2Torr of O2. X-ray reciprocal space mapping results indicated that the in-plane mismatches of the {1=10} planes of CoO (111) and Co3O4 (111) films with the substrate were 4.5% and 2.5%, respectively. The films were almost entirely relaxed with ratios of expansion less than ±2%; the films underwent slight elongation along the [111] axis and shrinkage in the (111) plane. The surfaces of the as-grown CoO and Co3O4 thin films revealed atomic steps reflective of those on the substrates. Their root-mean-square roughness values were about 0.1nm indicating suppressed grain growth on the substrates at room temperature. The optical bandgap of the epitaxial CoO (111) film was estimated to be 2.72eV accompanied with a broad absorption attributable to non-stoichiometry or d-d transition. The bandgap of the Co3O4 (111) film was evaluated as 1.42eV, and also absorption at 1.86eV was observed. The obtained room-temperature epitaxial growth of CoO and Co3O4 thin films contributes to enhance their catalytic ability and quality of layer-stacking devices in terms of improving surface/interface flatness and specific surface area.

Numerical Analysis of Constraint and Strength Mismatch Effects on Local Fracture Resistance of Bimetallic Joints

In this paper, the finite element method (FEM) based on GTN model was used to investigate the in-plane/out-of-plane constraint and strength mismatch effects on local fracture resistance of A508/Alloy52Mb bimetallic joint. TheJ-resistance curves, crack growth paths and local stress-stain distributions in front of crack tips were calculated for cracks with different constraints and strength mismatches. The results show that the local fracture resistance of the interface crack in this joint is sensitive to constraint and strength mismatch effects. With increasing in-plane constraint (crack deptha/W), out-of-plane constraint (specimen thicknessB) and strength mismatch degree, the plastic strain and stress triaxiality around crack tip increase, and the corresponding crack growth resistance decreases. The crack with strength mismatch factorM=1 displays a markedly higher crack growth resistance than the other cracks withM>1 andM<1. It also has been found that there is an interaction between in-plane/out-of-plane constraint and strength mismatch for the bimetallic joint. With increasing in-plane/out-of-plane constraint, strength mismatch effect on fracture toughness becomes weaken. For accurate and reliable safety design and failure assessment of the bimetallic joint structures, the effects of constraint and strength mismatch on local fracture resistance need to be considered.

柱坐标下推广的Stoney模型的应变能密度

材料的应力与应变关系是理解材料力学性能的重要方面，由此可以进一步得到材料的应变能函数，从而可以得到其它一系列力学特性。Stoney模型是从薄膜基底的应力应变关系出发，首先得到应变能函数，从而推得膜内应力与曲率的关系。这里，我们推广Stoney模型，考虑z方向的应力和应变，以及和z方向相关的剪切应变，得出柱坐标下的任意一点的应变能函数，并考虑膜内面间的均匀失配应变，得到体系的应变能的积分表达式。 The relation of strain-stress is an important aspect of understanding mechanical property of materials. According to it, we can further obtain the strain energy function of materials, and a series of other mechanical properties. The Stoney model is based on the relation of strain-stress of substrate, and the strain energy function is first obtained, and then the relation between stress in film and curvature is achieved. At the present case, we generalize the Stoney model, considering the strain and stress of z direction, and as well as shear strain related to z direction, and we obtain the strain energy function at any point with cylindrical coordinate. In addition, we obtain the integral expression of strain energy of system by considering of the in-plane uniform mismatch strain of the film.

Crystallographic tilting of AlN/GaN layers on miscut Si (111) substrates

We report on the crystallographic tilting of AlN/GaN layers grown on Si (111) substrates with different miscut angles toward [110] direction. The inclination angle of the Si (111), AlN (002) and GaN (002) planes to the sample surface was measured by high resolution X-ray diffraction. We found that the tilt angles of the AlN and GaN layers with respect to the Si (111) plane are significantly larger than that predicted by the classical Nagai model. We proposed that in-plane mismatch played an important role in the tilt epitaxy of AlN/GaN layers on miscut Si (111) substrate.

Ab initiostudy of the factors affecting the ground state of rare-earth nickelates

We have used first-principles methods to investigate the factors that control the ground state of rare-earth nickelates, studying in detail the case of NdNiO${}_{3}$. Our results suggest a complex phase diagram, with the bulk compounds standing on the edge of various instabilities that can be triggered by both electronic (e.g., changes in the Coulomb repulsion) and structural (e.g., epitaxial mismatch) means. In particular, we reveal that several phase transitions can be induced by epitaxial strain in thin films and predict that a continuous transformation between insulating spin-density-wave- and metallic spin-spiral-like solutions occurs at moderate values of the in-plane mismatch. Our results provide a coherent picture of structural and electronic effects in nickelates and have implications for current experimental and theoretical work on these compounds.

Tunneling Magnetoresistance Properties in Ballistic Spin Field-Effect Transistors

We investigate tunneling magnetoresistance (TMR) properties in ballistic spin field-effect transistors (SFETs) by taking into account the Rashba spin-orbit coupling (SOC), interface scattering, the presence of an in-plane magnetic field, band mismatch, and spin polarization in the ferromagnetic electrodes. It is shown that, for high potential barriers at the contact/channel interfaces, as the band mismatch is varied, the magnitude, amplitude, phase, and sign of the TMR are significantly modulated by the Rashba SOC, whereas the presence of the magnetic field makes the TMR oscillate between positive and negative values. Likewise, in an SFET with ohmic-contact interfaces, the Rashba SOC affects the band mismatch dependence of the TMR in a completely different fashion from the magnetic field. We also study the variation in the TMR when either the Rashba SOC strength or the magnitude or direction of the magnetic field is varied. Our theoretical results may be useful in analyzing experimental data of relevant devices.

Effects of mismatch strain and substrate surface corrugation on morphology of supported monolayer graphene

Graphene monolayers supported on oxide substrates have been demonstrated with superior charge mobility and thermal transport for potential device applications. Morphological corrugation can strongly influence the transport properties of the supported graphene. In this paper, we theoretically analyze the morphological stability of a graphene monolayer on an oxide substrate, subject to van der Waals interactions and in-plane mismatch strains. First, we define the equilibrium separation and the interfacial adhesion energy as the two key parameters that characterize the van der Waals interaction between a flat monolayer and a flat substrate surface. By a perturbation analysis, a critical compressive mismatch strain is predicted, beyond which the graphene monolayer undergoes strain-induced instability, forming corrugations with increasing amplitude and decreasing wavelength on a perfectly flat surface. When the substrate surface is not perfectly flat, the morphology of graphene depends on both the amplitude and the wavelength of surface corrugation. A transition from conformal (corrugated) to nonconformal (flat) morphology is predicted. The effects of substrate surface corrugation on the equilibrium mean thickness of the supported graphene and the interfacial adhesion energy are analyzed. Furthermore, by considering both the substrate surface corrugation and the mismatch strain, it is found that, while a tensile mismatch strain reduces the corrugation amplitude of graphene, a corrugated substrate surface promotes strain-induced instability under a compressive strain. These theoretical results suggest possible means to control the morphology of supported graphene monolayers by substrate surface patterning and strain engineering.

Ballistic transport properties in spin field-effect transistors

We investigate conductance properties in ballistic spin field-effect transistors (SFETs) by taking into account the Rashba spin-orbit coupling (SOC), interface scattering, the presence of an in-plane magnetic field, band mismatch, and spin polarization in the ferromagnetic electrodes. It is shown that the conductance of the SFET has high peaks for high potential barriers at the contact/channel interfaces. Therefore, switching on or off can be easily realized by tuning either the band mismatch, the Rashba SOC strength, or the magnitude or direction of the magnetic field. Moreover, in the SFET with Ohmic-contact interfaces, the conductance modulation results from a mixing between Fabry–Pérot-type spin channels interference and spin precession and becomes more and more pronounced as the spin polarization in the contacts increases.

Epitaxial Electrodeposition of Tin(II) Sulfide Nanodisks on Single-Crystal Au(100)

Epitaxial nanodisks of tin(II) sulfide (SnS) are deposited electrochemically on a [100]-oriented single-crystal Au substrate from an acidic solution at 70 °C. The SnS grows with two different out-of-plane orientations of [100] and [301], which each have four equivalent in-plane orientations. X-ray pole figures reveal the following epitaxial relationships: SnS(100)[010]//Au(100)[010], SnS(100)[010]//Au(100)[010], SnS(100)[010]//Au(100)[001], SnS(100)[010]//Au(100)[001], SnS(301)[010]//Au(100)[010], SnS(301)[010]//Au(100)[010], SnS(301)[010]//Au(100)[001], and SnS(301)[010]//Au(100)[001]. For the SnS[100] orientation, the in-plane mismatch is −2.4% in the [010] direction and 6.1% in the [001] direction. For the [301] orientation, the in-plane mismatch is −2.4% in the [010] direction and alternates between 3.4% and 6.7% in the [103] direction. The SnS deposits with a disklike morphology with a diameter of 300 nm and a thickness of 50 nm.

Initiation of geometric roughening in polycrystalline metal films

The mechanics of plasticity-induced roughening of initially planar polycrystalline metal films upon thermal cycling is presented. The emphasis is on films of FCC metals having grain size comparable to their thickness. In the model, the elastic substrate imposes a biaxial in-plane mismatch strain on the film. Calculations demonstrate how surface undulations develop with a length scale governed by the grain size, when plastic slip initiates above a critical strain. Under monotonic straining, the undulation amplitude grows in proportion to the imposed strain with extent dependent on the slip anisotropy. Upon cyclic plastic straining, three growth stages have been identified with the possibility of substantial roughness development. To cite this article: G. Parry et al., C. R. Mecanique 336 (2008).

High-resolution TEM observation of AlN grown on on-axis and off-cut SiC substrates

High-resolution TEM has been carried out on AlN epifilms grown on both on-axis and off-cut SiC substrates to study the state of strain relaxation at the interface and the defects formed in the AlN films. Prismatic stacking faults (PSF) are observed forming at I 1 type substrate steps. These PSFs expand into the further grown GaN film and form complicated intersecting configurations in the off-cut sample while they annihilate each other and form enclosed domains at the near-interface region in the on-axis sample. A set of 60° misfit dislocations (MDs) are observed along 〈 1 0 1 ¯ 0 〉 orientation, in contrast to the general observations of 60° complete MDs along 〈 1 1 2 ¯ 0 〉 orientation reported in literature. These MDs are suggested as geometric partial misfit dislocations which serve both to relax in-plane mismatch and accommodate stacking differences at substrate steps of I 2 type.

Synchrotron white beam X-ray topography, transmission electron microscopy and high-resolution X-ray diffraction studies of defects and strain relaxation processes in wide band gap semiconductor crystals and thin films

A review is presented of recent research into imaging on different length scales of defect structures in wide band gap semiconductors. This includes: synchrotron white beam X-ray topography (SWBXT) imaging of defects in SiC wafers and epilayers; high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) studies of strain relaxation in AlN and GaN epilayers on on-axis and vicinal SiC and sapphire substrates; TEM studies of fault structures in GaN/AlN films on SiC substrates; and TEM studies of threading dislocation density reduction driven by growth mode modification in AlN films grown on sapphire substrates. In detail, studies of closed-core and hollow-core screw dislocations, basal plane dislocations and stacking faults (SFs) in 4H- and 6H-SiC substrates and films are presented. The mechanism of improved mismatch strain relaxation associated with the growth of GaN (AlN) films on vicinal substrates is discussed. This is attributed to the combined effects of mutual tilt between the epilayer and the substrate, which helps to relax out-of-plane mismatch, and by the generation of geometric partial misfit dislocations (GPMDs), which serves both to relax in-plane mismatch and accommodate stacking differences between the epilayer and substrate at some proportion of the steps at the substrate/film interface. In addition, the formation mechanisms of intersecting SF structures observed in GaN/AlN/SiC epilayers are presented. These fault structures comprise SFs that fold back and forth from the basal plane (I 1 Basal Plane Faults; BSFs) to the prismatic plane (Prismatic Stacking Faults; PSFs). Finally, the threading dislocation density reduction in AlN/sapphire films is modeled based on overgrowth of the dislocation outcrops by macrosteps which appear during a transition from step flow to 2D layer-by-layer growth.

Step-Controlled Strain Relaxation in the Vicinal Surface Epitaxy of Nitrides

On-axis and vicinal GaN/AlN/6H-SiC structures grown under identical conditions have been studied by x-ray diffraction and transmission electron microscopy to demonstrate the distinctive features of vicinal surface epitaxy (VSE) of nitrides on SiC. In VSE, the epilayers are tilted from the substrate due to the out-of-plane lattice mismatch (Nagai tilts), and the in-plane mismatch strains are more relaxed. The majority of misfit dislocations (MDs) at the vicinal AlN/6H-SiC interface are found to be unpaired partial MDs that are geometrically necessary to correct the stacking sequences from 6H to 2H. This mechanism indicates that it is possible to develop "step-controlled-epitaxy" strategies to control strain relaxation by adjusting the substrate offcut angles.

A Lamination Theory Incorporating the Effect of Interlaminar Deformation

The effect of interlaminar deformation due to interlaminar shear stresses and delamination on the in-plane behavior of FRP laminates was investigated. The interlaminar shear stresses considered were induced by in-plane external load and stiffness mismatch. In order to develop a simple lamination theory which could take account of the effect of interlaminar shear deformation, a simplified laminate model and linear shear slip theory were employed. The lamination theory was applied to a series of two-layer angleply laminate tubes and the results show that interlaminar shear deformation affects significantly the mechanical behavior of the angle-ply laminate tubes with imperfect interfacial bonding.

Measurement of residual stresses in Al2O3/Ni laminated composites using an X-ray diffraction technique

Using an X-ray diffraction technique, macro-residual stresses were measured in laminated composites consisting of alternating layers of α-Al2O3 and nickel. The in-plane thermal mismatch stresses which develop during fabrication were found to be compressive and tensile in the α-Al2O3 and nickel layers, respectively. The magnitude of the in-plane stresses was found to be ≈ 110 MPa. Models of laminate structures predict the stress state to be biaxial in the plane of the layers. However, substantial stresses were observed perpendicular to the plane of the laminate; this stress might be due to the hot-pressing procedure used to fabricate the samples. The stress on the side surface of a laminate was measured using the indentation method and the results were consistent with those obtained by the X-ray method. Three samples were heated to 700, 900 and 1000 °C, respectively, and then cooled to test the effect of stress relaxation of the residual stresses due to the thermal expansion. The heat treatments (700–1000 °C) had no effect on the measured stress states of the laminates.