Large Misfit Research Articles

Maximum aspect ratio of a coherent nanowire at equilibrium

Nanostructures elongated perpendicular to their supporting substrate have attracted attention owing to their ability to sustain the coherency at the interface with the misfitting substrate. Here simple qualitative scaling arguments are used to derive the equilibrium aspect ratio of an elongated coherent cylindrical nanowire depending on its volume V. It is found to increase as V1/4. Using the usual law for the critical radius of nanowires, the maximum aspect ratio of a coherent nanowire is found to increase linearly with the lattice misfit, demonstrating that coherent nanowires with large aspect ratios exist at equilibrium for large misfits. In addition, it is shown that when the height of the coherent crystal is limited, a possible equilibrium state consists of an assembly of non-interacting nanowires.

Atomic-scale characterization of the nucleation and growth of SnO2 particles in oxidized CuSn alloys

The internal oxidation of Sn was investigated to understand the oxidation kinetics of monophase CuSn alloys. SnO 2 particles were characterized by analytical transmission electron microscopy . The orientation relationship between SnO 2 and Cu was determined with a special emphasis on the atomic-scale structure of Cu/SnO 2 interfaces (misfit dislocations and chemical structure). Habit planes with a pure oxygen plane terminating the SnO 2 phase are greatly favored and large misfits promote the growth of plate-shaped precipitates.

Carrier localization in ZnO quantum wires

ZnO quantum wires are grown on patterned silicon substrates using plasma-assisted molecular beam epitaxy. A crystallographic etch process exposes the Si(111) planes from Si(001) substrates so that V-grooves are formed. The large misfit between the Si(111) and ZnO(0001) lattice constants leads to low ZnO film quality after growth. Subsequent thermal annealing improves the electronic and structural quality of the film substantially. In the center region of the wire we find a strong photoluminescence intensity enhancement, which arises from carrier localization in the ZnO quantum wire. Two-dimensional self-consistent calculations of the electronic structure show excellent agreement with the experimental results.

Spontaneous formation of filamentary Cd whiskers and degradation of CdMgYb icosahedral quasicrystal under ambient conditions

Abstract

Unusual precipitation of amorphous silicon nitride upon nitriding Fe–2at.%Si alloy

Silicon-nitride precipitation in ferritic Fe–2at.%Si alloy was investigated upon nitriding in NH3/H2 gas mixtures, using light microscopy, hardness measurements, scanning and transmission electron microscopy, X-ray diffraction and electron-probe microanalysis for microstructural characterisation. Surprisingly, ideally weak nitriding behaviour occurred upon nitriding thick (1 mm) recrystallised Fe–2at.%Si alloy specimens. This phenomenon can be attributed to the onset of silicon-nitride precipitation only after a certain degree of nitrogen supersaturation has been established at all depths in the specimen. Silicon-nitride precipitates formed inside the ferrite grains and also along the ferrite grain boundaries. The precipitates were amorphous and had a stoichiometry of Si3N4. The amorphous nature of the tiny precipitates has a thermodynamic origin. Nitride precipitation occurred very slowly due to the very large volume misfit of the nitride with the matrix. An anomalous non-monotonous hardness change occurred with increasing nitriding time, which was ascribed to the initially fully elastic accommodation of precipitate/matrix misfit. The nitrogen uptake rate increased upon continued nitriding as a result of “self-catalysis”. The possible, favourable application of amorphous silicon-nitride precipitates as grain-growth inhibitors in the production of grain-oriented electrical steel is discussed.

Effect of point defects on lattice constant in MgO thin film deposited on silicon(0 0 1) substrate

Epitaxial magnesium oxide (MgO) thin films prepared on Si(0 0 1) substrates revealed the contraction of its lattice constants along both out-of-plane and in-plane directions. X-ray Diffraction (XRD) verified the epitaxial growth with the relation of MgO(1 0 0) parallel to Si(1 0 0) [cubic on cubic growth] with large lattice misfit of ~22% instead of the relation of MgO(1 1 0) parallel to Si(1 0 0) [45° rotation growth] with lattice mismatch of ~9%. Although the domain epitaxy explaining the cubic on cubic growth is preferred in terms of crystallography, structural stability is not considered in the concept of the domain epitaxy. In order to explain the contraction of lattice constant from point of view of structural stability, ab initio method was used to evaluate all-electron total energy, and optimal lattice constant was estimated with point defects in the MgO structure.

Si and Ge nanostructures epitaxy on a crystalline insulating LaAlO3(001) substrate

AbstractWe present a comparative structural study of the growth of Si and Ge deposited by molecular beam epitaxy (MBE) on a c(2 × 2) reconstructed LaAlO3(001) substrate. Our findings are based on complementary experimental techniques such as in situ X‐ray photoelectron spectroscopy (XPS), reflection high‐energy electron diffraction (RHEED), low energy electron diffraction (LEED) and ex situ atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). While the layers are amorphous and wet uniformly the substrate in a low deposition temperature range, above 500 °C both Si and Ge growths proceed in a Volmer–Weber (VW) mode leading to the formation of nanocrystals (NCs). The islands are found to be composed of pure Si and Ge and to have abrupt interfaces with the substrate.Both semiconductors (SCs) crystallize in their own diamond structure leading to relaxed NCs. No facets could be observed on the crystalline islands. An epitaxial relationship is established for which the (001) planes of Si and Ge are parallel to the LaAlO3(001) surface but are rotated by 45° around the [001] growth axis. The Ge lattice undergoes a second rotation of 6° with respect to the (001) growth axis. This 6° tilt is an original mechanism to partially compensate the strain in the Ge islands induced by the large misfit. Whereas a unique epitaxial relationship is pointed out for Si NCs, many Ge NCs are randomly orientated on the surface. This is interpreted by the fact that the Ge islands are less anchored to the substrate due to a large misfit and to the fact that the GeO bonds are weaker than the SiO ones.

High Hole Mobility of GaSb Relaxed Epilayer Grown on GaAs Substrate by MOCVD through Interfacial Misfit Dislocations Array

The structural property of GaSb epilayers grown on semi-insulator GaAs (001) substrate by metalorganic chemical vapor deposition (MOCVD) using Triethylgallium (TEGa) and trimethylantimony (TMSb), was investigated by variation of the Sb:Ga (V/III) ratio. An optimum V/III ratio of 1.4 was determined in our growth conditions. Using transmission electron microscopy (TEM), we found that there was an interfacial misfit dislocations (IMF) growth mode in our experiment, in which the large misfit strain between epilayer and substrate is relaxed by periodic 90 deg. IMF array at the hetero-epitaxial interface. The rms roughness of a 300 nm-thick GaSb layer is only 2.7 nm in a 10 μm×10 μm scan from atomic force microscopy (AFM) result. The best hole density and mobility of 300 nm GaSb epilayer are 5.27×10 6 cm −3 (1.20× 10 6 ) and 553 cm 2 ·V −1 ·s −1 (2340) at RT (77 K) from Hall measurement, respectively. These results indicate that the IMF growth mode can be used in MOCVD epitaxial technology similar to molecular beam epitaxy (MBE) technology to produce the thinner GaSb layer with low density of dislocations and other defects on GaAs substrate for the application of devices.

Structure–property correlation in epitaxial (2 0 0) rutile films on sapphire substrates

We have investigated the influence of the deposition variables on photocatalytic properties of epitaxial rutile films. Despite a large lattice misfit of rutile with sapphire substrate, (200) epitaxial layers were grown on (0001)sapphire by domain matching epitaxy paradigm. Using φ-scan XRD and cross section TEM, the epitaxial relationship was determined to be rutile(100)||sapphire(0001), rutile(001)||sapphire(10−10), and rutile(010)||sapphire(1−210). Based on the XRD patterns, increasing the repetition rate introduced tensile stress along the film normal direction, which may arise as a result of trapped defects. Formation of such defects was studied by UV–VIS, PL, and XPS techniques. AFM studies showed that the film roughness increases with the repetition rate. Finally, photocatalytic performance of the layers was investigated through measuring decomposition rate of 4-chlorophenol on the films surface. The films grown at higher frequencies revealed higher photocatalytic efficiency. This behavior was mainly related to formation of point defects which enhance the charge separation.

Reconstruction of 2D Al3Ti on TiB2 in an aluminium melt

It has been widely considered that Al3Ti is involved in the aluminium nucleation on TiB2, although the mechanism has not been fully understood. In this paper molecular dynamics has been conducted to investigate this phenomenon at an atomistic scale. It was found that a two-dimensional Al3Ti layer may remain on TiB2 above the aluminium liquidus. In addition, the results showed that this 2D Al3Ti undergoes interface reconstruction by forming a triangular pattern. This triangular pattern consists of different alternative stacking sequences. The transition region between the triangles forms an area of strain concentration. By means of this mechanism, this interfacial Al3Ti layer stabilizes itself by localizing the large misfit strain between TiB2 and Al3Ti This reconstruction is similar to the hdp-fcc interface reconstruction in other systems which has been observed experimentally [1].

Effects of lattice mismatch on interfacial structures of liquid and solidified Al in contact with hetero-phase substrates: MD simulations

In this study, the effects of the misfit on in-plane structures of liquid Al and interfacial structure of solidified Al in contact with the heterophase substrates have been investigated, using molecular dynamics (MD) simulations. The MD simulations were conducted for Al/fcc (111) substrates with varied misfits. The order parameter and atomic arrangement indicated that the in-plane ordering of the liquid at the interface decreases significantly with an increase of the misfit, i.e., solid-like for small misfit and liquid-like for large misfit. Further, our MD simulation results revealed that a perfect orientation relationship forms at the interface between the substrate and the solidified Al for a misfit of less than -3% and the boundary is coherent. With an increase in the misfit, Shockley partial and extended dislocations form at the interface, and the boundary becomes a semi-coherent or low-angle twist boundary.

Simulation of morphological evolution and crystallographic tilt in heteroepitaxial growth using phase-field crystal method

The phase-field crystal (PFC) model is employed to study the morphological evolution and the crystallographic tilt of heteroepitaxial growth on vicinal substrates. The results are as follows: for heterostructures with large misfit (ε > 0.08) the crystallographic tilt of epitaxial layer is approximately proportional to the substrate miscut angle, while the elastic strain energy of the film will lead to the nucleation of dislocation, which contributes to step-flow growth mode. As for the heterostructures with small misfit (ε < 0.04) the elastic strain energy will be released in the form of surface energy, and the surface profile of epitaxial film is dislocation-free island. When exposed to high undercooling, the substrate with large misfit and miscut angle will result in small-angle grain boundary between the substrate and the epitaxial layer. The small-angle grain boundary is composed of arranged dislocations, and it significantly changes the growth orientation of epitaxial layer.

Line defects, planar defects and voids in SrTiO3 films grown on MgO by pulsed laser and pulsed laser interval deposition

Two and three dimensional growth of SrTiO3 films on (001) MgO substrate was achieved by pulsed laser interval and pulsed laser deposition respectively. The growth mode was monitored by in-situ reflection high energy electron diffraction. Interval deposition forces layer-by-layer growth of materials even with such a large lattice misfit (~7.9%). A titanium dioxide buffer monolayer was deposited to allow the film to wet the substrate to encourage two dimensional growth of the strontium titanate. A variety of defects was investigated using transmission electron microscopy and high resolution scanning transmission electron microscopy. Misfit dislocations, steps at the interface, Ti-rich defects and regularly shaped nano-holes connected by anti-phase boundaries were found to be the dominant defects in these films grown layer by layer. The edges of the nano-holes were mainly along [010] and [100] for a [001] growth direction. The large strain between the two crystal systems with large lattice mismatch leads to in-plane tensile stress during the layer-by-layer growth. The stress is relieved in part by the holes. The films with a three dimensional growth mode possess a uniform surface with dislocations as the dominant defects. The individual densities of the various defects, including a Ti-rich phase and misfit and threading dislocations, are determined by the kinetics of the deposition method.

Interfacial disconnections at Sb2Te3 precipitates in PbTe: Mechanisms of strain accommodation and phase transformation at a tetradymite/rocksalt telluride interface

Understanding the structure and formation mechanisms of interfaces between different telluride phases is important to the development of thermoelectric nanocomposites. Here, we investigate the interfacial structure of tetradymite precipitates in a rocksalt telluride matrix, focusing in particular on precipitates of Sb2Te3 in PbTe. Using high-resolution transmission electron microscopy, we investigate the structure and arrangement of interfacial disconnections—i.e. interfacial steps possessing dislocation character—observed in this system. Our analyses provide insight concerning the roles of these defects in accommodating the large interfacial misfit (6.7%) in this system and in mediating the transformation from the rocksalt to the tetradymite structure. Our observations also suggest how such interfacial disconnections could arise through the dissociation of crystal lattice dislocations that accommodate the misfit on initially flat segments of the interface.

Epitaxial strain and electric boundary condition effects on the structural and ferroelectric properties of BiFeO3films

The influence of both compressive and tensile epitaxial strain along with the electrical boundary conditions on the ferroelastic and ferroelectric domain patterns of bismuth ferrite films was studied. BiFeO${}_{3}$ films were grown on SrTiO${}_{3}$(001), DyScO${}_{3}$(110), GdScO${}_{3}$(110), and SmScO${}_{3}$(110) substrates to investigate the effect of room temperature in-plane strain ranging from $\ensuremath{-}1.4$% to $+0.75$%. Piezoresponse force microscopy, transmission electron microscopy, x-ray diffraction measurements, and ferroelectric polarization measurements were performed to study the properties of the films. We show that BiFeO${}_{3}$ films with and without SrRuO${}_{3}$ bottom electrode have different growth mechanisms and that in both cases reduction of the domain variants is possible. Without SrRuO${}_{3}$, stripe domains with reduced variants are formed on all rare earth scandate substrates because of their monoclinic symmetry. In addition, tensile strained films exhibit a rotation of the unit cell with increasing film thickness. On the other side, the presence of SrRuO${}_{3}$ promotes step flow growth of BiFeO${}_{3}$. In case of vicinal SrTiO${}_{3}$ and DyScO${}_{3}$ substrates with high quality SrRuO${}_{3}$ bottom electrode and a low miscut angle of $\ensuremath{\approx}$${0.15}^{\ensuremath{\circ}}$ we observed suppression of the formation of certain domain variants. The quite large in-plane misfit of SrRuO${}_{3}$ with GdScO${}_{3}$ and SmScO${}_{3}$ prevents the growth of high quality SrRuO${}_{3}$ films and subsequent domain variants reduction in BiFeO${}_{3}$ on these substrates, when SrRuO${}_{3}$ is used as a bottom electrode.

Dislocation Configurations in Single-Crystal Superalloys during High-Temperature Low-Stress Creep

Dislocation configurations in two single-crystal superalloys during high-temperature low-stress creep (1100°C, 137 MP) were illustrated schematically with the use of transmission electron microscope (TEM). For an alloy with a small lattice misfit, the dislocations move in the combination of climbing and gliding processes. In the primary stage, the dislocations first move by slip in the g-matrix channels. When they reach the g¢ cuboids, they move by climb along the g¢ cuboid surfaces. In the secondary creep stage, dislocation reorientation in the (001) interfacial planes happens slowly, deviating from the deposition orientation of <110> to the misfit orientation of <100>. For an alloy with a large lattice misfit, the dislocations are able to move smoothly by cross slip in the horizontal g channels. The dislocation reorientation from the deposition orientation of <110> to the misfit orientation of <100> in the (001) interfacial planes can be completed in the primary creep stage.

Graphene/graphane interface energy and implications for defects

Recent theoretical work has shown that electronic properties of graphene sheets can be systematically modified by the partial hydrogenation of the sheets. Two possible perfect and distinct graphene/graphane interfaces (called zigzag and armchair) have very different but potentially useful electronic properties, which are nevertheless likely to be affected by the presence of defects. In an effort to evaluate their relative energetics and their potential for defects, the structure and energies of the zigzag and armchair interfaces have been computed for infinite sheets of periodically alternating stripes of graphene and graphane ribbons of various widths. The presence of an interface causes significant strains in both the graphene and graphane regions, with both shear strains and area strains typically close to $1%$. The associated large strain energies may lead to defects that relieve the strain but disrupt the lattice. The energies per unit length associated with the interfaces alone are approximately 0.12 eV/\AA{} for the zigzag interface and 0.11 eV/\AA{} for the armchair. The large misfit strains and energies suggest that formation of strain-relieving defects at the interface should be highly favorable.

Preferred orientation relationships with large misfit interfaces between Ni3Sn4 and Ni in reactive wetting of eutectic SnPb on Ni

Ni3Sn4 grains were formed on Ni by reactive wetting between molten eutectic SnPb and thermally annealed Ni foil. Using synchrotron white beam micro x-ray diffraction analysis, two kinds of preferred orientation relationships between Ni3Sn4 and Ni were found. The existence of preferred orientation with large interfacial misfit is suggested as a general mechanism of intermetallic compound formation in reactive solder wetting on metals.

Strained and strain-relaxed epitaxial Ge1−xSnx alloys on Si(100) substrates

Epitaxial Ge1−xSnx alloys are grown separately on a Ge-buffer/Si(100) substrate and directly on a Si(100) substrate by molecular beam epitaxy (MBE) at low temperature. In the case of the Ge buffer/Si(100) substrate, a high crystalline quality strained Ge0.97Sn0.03 alloy is grown, with a χmin value of 6.7% measured by channeling and random Rutherford backscattering spectrometry (RBS), and a surface root-mean-square (RMS) roughness of 1.568 nm obtained by atomic force microscopy (AFM). In the case of the Si(100) substrate, strain-relaxed Ge0.97Sn0.03 alloys are epitaxially grown at 150 °C-300 °C, with the degree of strain relaxation being more than 96%. The X-ray diffraction (XRD) and AFM measurements demonstrate that the alloys each have a good crystalline quality and a relatively flat surface. The predominant defects accommodating the large misfit are Lomer edge dislocations at the interface, which are parallel to the interface plane and should not degrade electrical properties and device performance.

3C-SiC Film Growth on Si Substrates

The aim of this work is to give an overview on 3C-SiC growth on Si substrates. Starting from the reasons why SiC is considered such an interesting innovative material, with a survey of application already demonstrated, we will present data explaining the most important issues in this hetero-epitaxy system and how the chemical vapor deposition process influences the resulting 3C-SiC film properties. 3C-SiC crystal structure is strongly dependent on the process parameters within the reaction chamber during growth as well as the substrate surface properties. Part of this work is then focused on the main crystallographic defects characterizing the 3C-SiC/Si system and on the resulting wafer bow due to the large misfit between the materials. Defects and wafer bow, are a direct consequence of the large stress generated at the interface. The work closes discussing the encouraging improvements in 3C-SiC crystal quality obtained by the introduction of compliant Si substrates.