Large Misfit Research Articles

Adsorption of Bi on Si(001) surface: An atomic view

Coverage dependent adsorption of Bi on Si(001) has been studied using scanning tunneling microscope and low-energy electron diffraction. At low coverages, Bi atom adsorbs without breaking the Si(001)-(2×1) structure. At the coverage of ∼0.5 ML, Bi atoms begin to sit on the troughs between the Si dimer row and form a Bi (1×2) structure with broken Si dimers. Strain due to the large misfit is relieved by the formation of antiphase boundaries and Bi missing line defects.

Determination of interface coherency by x-ray double-crystal diffraction

The degree of coherency at the interface between an epitaxial layer and a substrate or between two different epilayers is defined as a coherency factor fCOH. By performing just three diffraction measurements on an x-ray double-crystal diffractometer and calculating the deformations of the corresponding epilayer according to the peak separations, this coherency factor can be determined together with the composition of the epilayer on any orientation substrate. The experimental results show that the coherency factor at the interface varies significantly with the composition and thickness of the epilayer for a system with large misfit, so the determination of the coherency factor is necessary for the calculation of the epilayer composition via the relaxed lattice constant.

On modeling the thermal evolution of the oceanic upper mantle: An assessment of the cooling plate model

The plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor. Several estimates of “best fitting” plate model parameters, derived from depth, heat flow, and age data, have been proposed, but the viability of the plate model itself has not been rigorously evaluated. We have used published mean depths and depths to basement at Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) drilling sites to test the plate cooling model based on two criteria: First, viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half‐space (or root t) subsidence of young seafloor is implicit in the plate model (i.e., the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model). Second, any viable physical model must fit the data with an acceptable degree of systematic misfit; large systematic misfits indicate that the model cannot explain the observations. Fits of half‐space (root t) models to depth versus age data for young (< ∼80 Ma) seafloor indicate basal temperatures in the range 1300 to 1370°C. Based on the age at which the depths deviate from the root t line, the minimum plate thickness that is compatible with the best fitting half‐space models for young seafloor is 120 km. In contrast, all best fitting plate models yield systematically higher temperatures (1450 to 1470°C) and thinner plates (102–118 km). The plate model can explain the depth to basement at DSDP/ODP drill sites with a satisfactory degree of systematic misfit, but we find that there is no plate cooling model that can explain the variation of the mean depths (derived from the DBDB5 database) with age over the entire range of ages (0 to 165 Ma). Models that “best fit” the entire data set have unacceptably large systematic misfits over the entire range of seafloor age, whereas models that minimize the systematic misfit for young seafloor (0 to 81 Ma) fail for older seafloor. The plate model clearly fails to explain the observations. The cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model. Previous studies have suggested, as an alternative to the plate model, that observed variations of basement depth with age are best explained by the combined effects of a “normal” half‐space cooling process and the dynamic and/or thermal effects of hot spots or mantle plumes. Our results are entirely consistent with the half‐space cooling model, and we find that the best reference model for “normal” subsidence is d(t) = (2600±20)m + (345±3)m (m.y.)−½ t½. We also find that the heat flow predicted by this half‐space model is consistent with the most reliable average heat flow values from the Pacific.

Pseudomorphic-to-close-packed transition. I. General formulation.

Monolayers (ML's) of Ni and Co grow at equilibrium on {110} W and Mo substrates, with which they bond strongly and have large negative misfits, pseudomorphically until completion of the pseudomorphic monolayer. When excess atoms are deposited atop the pseudomorphic monolayer, it transforms to a misfitting, more or less close-packed (cp) monolayer with misfit dislocations at the monolayer-substrate interface. In nonequilibrium growth, the pseudomorphic-to-close-packed transition occurs in monolayer islands before completion of the pseudomorphic ML. The main objectives of the paper are (i) to show that the pseudomorphic ML is more stable than the close-packed monolayer and (ii) to develop an understanding of the pseudomorphic-to-close-packed transition. In paper I we introduce force models and derive expressions for (a) equilibrium zero-order energies of complete and incomplete misfit dislocations having displacement vectors equal to a lattice and half a lattice vector, respectively, and (b) equilibrium first-order energy for incomplete misfit dislocations (assumed to yield the lowest energy configuration), these two being the candidates for minimum energy of the close-packed monolayer. The numerical computation of parameters (using embedded-atom methods) and derived energies (including anharmonic pseudomorphic strain energy), needed to answer the relevant questions, are left for paper II.

The Structure of Spinel/Oxide Reaction Fronts During Spinel-Forming Solid State Reactions

AbstractA series of spinels were grown by topotaxial solid state reaction on MgO(001) and sapphire(11.2) substrates. The structure of the various spinel/oxide reaction fronts was investigated by cross-sectional high resolution electron microscopy and other methods. While for extremely low misfit the reaction front is completely coherent, different interfacial defects form in other cases, depending on sign and amount of the spinel/oxide lattice misfit. For a large positive misfit, a network of misfit dislocations occured all running along <100<, with Burgers vectors of types a/2[101] and a/2[011] pointing out of the interface. The perpendicular Burgers vector component along [001] permits these dislocations to glide in order to cope with the advancing reaction front, avoiding kinetically unfavourable climb processes. The latter have, however, been observed in negative misfit, where the interfacial dislocations run along <110>, with their Burgers vectors lying in the interface plane. At the sapphire/MgAl2O4 front the structure is completely different. Here the h.c.p.-type oxygen sublattice of sapphire is reconstructed into the f.c.c-type oxygen sublattice of the spinel, which requires a tilt of the MgAl2O4 lattice and the formation of interfacial ledges.

Pulsed Laser Deposition and Characterization of Novel Cu/TiN/Si(100) Heterostructures Grown VIA Domain Epitaxy

Laser physical vapor deposition (LPVD) has been used to study the growth morphology of copper thin films on siliconi 100) substrates with epitaxially grown titanium nitride buffer layers. The epitaxial TiN film was grown on hydrogen-terminated Si substrate at 600°C. The Cu films were grown on TiN/Si at different substrate temperatures ranging from room temperature to 600°C in vacuum (∼10-7 torr) with a pulsed KrF excimer laser (τ = 248 nm, ι = 25 ns). The pulse repetition rates were 15 and 30 Hz. The X-ray diffraction (XRD) results indicate that below 200°C, the Cu film has a mosaic spread of (001) textures of about 2.3° and that a small fraction (0.05 ) is of (111) textures. But above 200°C there is only (001) textures Cu of about 0.42° to 0.40° depending on the substrate temperature (200° - 600°C). The Rutherford backscattering (RBS) spectra show that the TiN has a yield (xmin) of 13.5 % whereas that for Cu is 70 %. A detail high resolution transmission electron microscopy (HRTEM) was performed to reveal the growth morphology of Cu films with substrate temperature. It was observed that at 15 Hz, Cu grows three-dimensionally on TiN with the island sizes ranging from 0.3–1.5 μm depending upon the substrate temperature. All of these islands grow epitaxially on TiN giving rise to Cu ║ TiN ║ Si. At 30 Hz, there is a uniform growth of epitaxial cube-on-cube Cu film on TiN/Si. The Cu/TiN and TiN/Si interfaces are quite abrupt with absolutely no indication of interfacial reactions between them. Moire fringe spacings suggest that both TiN and Cu unit cells are completely relaxed. Although there exist a large lattice misfit between Cu and TiN (15.8%) and as well as between TiN and Si (24.6%), it has been possible to grow epitaxial Cu/TiN/Si(100) heterostructures by means of domain epitaxial growth. In this case, there are 4-to-3 match in unit cells for TiN/Si structure and 7-to-6 match for Cu/TiN structure giving rise to remaining lattice misfits of only 4.0% and 0.6% respectively.

X-ray diffraction analysis of lattice strain in metallic superlattice films

In-plane lattice spacings of [Au( xÅ)/Ni( xÅ)] and [Pd( xÅ)/Cu( xÅ)] superlattice films with [111] fcc textures and large lattice misfits have been measured by X-ray diffraction. With decrease of superlattice period ( Λ = 2 x), Au and Pd layers are compressed whereas Ni and Cu layers are expanded. The observed lattice spacings ( d 220) of the individual layers vary almost linearly depending on 1/Λ. The lattice spacings of two constituent layers become equal in a Au/Ni superlattice with Λ = 8 A ̊ (d 220 = 1.371 A ̊ ) and a Pd/Cu superlattice with Λ = 14 A ̊ (d 220 = 1.337 A ̊ ) .

In situ HVEM-investigations of the early stages of recrystallization in Cu-0.2 at.% Mn-single crystals

The influence of foreign atoms on the early stages of recrystallization in Cu-0.2 at.% Mn-single crystals were investigated in situ in a high voltage electron microscope (HVEM). The stacking fault energy (SFE) of copper does not change with Mn addition. Along the grain boundaries the orientations of the recrystallized grains and of the deformation microstructure were determined to establish the local orientation relationships. The new orientations can be described by multiple twinning of the deformation microstructure with additional small angle grain boundaries. Frequently Σ13b- (27.8°〈111〉) and a Σ29b- (46.39° 〈221〉) orientation relationships were found. As is known from growth selection experiments these orientation relationships produce the final recrystallization texture. Because of the relatively high SFE and the large misfit between Cu and Mn these orientation relationships were formed rapidly after nucleation. Thus, the orientation formation process is finished after several μm of growth. The experimental results are discussed in terms of nucleation, twinning and growth of the recrystallized grains. An explanation is given why particular grain boundaries are mobile in Cu Mn.

Structural Transformation During the Epitaxial Growth of a System with a Large Misfit: MBE Study of Ni on Au (100)

ABSTRACTThe growth of Ni on Au (100) has been investigated by MBE. It has been found that a Metastable BCT Ni (100) phase is coherent with the substrate up to a thickness of 5 ML. Then a structural transformation analogous to a raartensitic transition occurs, and the structure becomes FCC Ni (110). We show that the interfacial stress is progressively relieved by a high density of stacking faults, as was predicted by Bruinsma and Zangwill for epitaxial overlayers with a large Misfit.

Surface Stress Effects on Thin Film Growth

ABSTRACTA review of surface and interface stresses relevant to thin film growth is presented. It is shown that surface stress effects that are generally ignored in the analysis of epitaxy can have an influence on the critical thickness for epitaxy, especially at larger Misfits. A discussion of intrinsic stress generation from surface stress is also presented.

Crystallographic fatigue crack growth in incompatible aluminum bicrystals: Its dependence on secondary slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaamWaaeaaca% aIXaGaaGOmamaanaaabaGaaGymaaaaaiaawUfacaGLDbaaaaa!3A22! $$\left[ {12\overline 1 } \right]$$ tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystallographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems.

Accomodation of the misfit strain surrounding {III} precipitates (Ω) in AlCuMg(Ag)

The Ω phase in AlCuMg and AlCuMgAg was examined to determine (1) the influence of silver addition on the Ω precipitate structure and (2) the effect of the large misfit normal to the Ω plate habit plane on interfacial structure. Using electron diffraction and HREM, no difference in Ω structure could be discerned between the two alloys with and without Ag additions. Conventional TEM revealed a consistent vacancy-type strain field surrounding the Ω plates, independent of plate thickness. This strain field is due to the presence of many small 12 Ω unit cell high ledges on the plate face which are coherent with the matrix. Larger ledges which contain a misfit-compensating dislocation were also observed on the plate face, but were less common. Dislocations similar to those in these ledges were also observed at the plate edge. These dislocations have an average spacing of 212 or 3 Ω unit cells which, similar to the larger ledges, produce a minimum strain normal to the habit plane. This similarity indicates that the larger ledges may be the mechanism for plate thickening. A mechanism is proposed which describes the relationship between these two ledge types and their role in Ω plate thickening.

Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition

We report epitaxial growth of TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using x-ray diffraction, Rutherford backscattering, four-point-probe ac resistivity, high resolution transmission electron microscopy and scanning electron microscopy techniques and epitaxial relationship was found to be 〈100〉 TiN ∥ 〈100〉 Si. TiN films showed 10%–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Four-point-probe measurements show characteristic metallic behavior of these films as a function of temperature with a typical resistivity of about 15 μΩ cm at room temperature. Implications of low-resistivity epitaxial TiN in silicon device fabrication are discussed.

Structure of GaAs=GaSb incoherent interface after epitaxial growth

The structure of GaAs=GaSb(001) incoherent interface has been studied by grazing incidence x-ray scattering. The interface is periodic with a square array of edge dislocations. It is shown that the interface is nonplanar with 2–4 GaAs monolayers protruding in the GaSb epilayer at the dislocation core. This corrugated interface does not increase the strain energy by a large factor. Two possible mechanisms are suggested for the interface formation of large misfit epitactic growth: a stress induced mechanism or the presence of an ordered intermediate 2D layer.

Structural studies of Al/Si(111) surfaces by LEED

Al-adsorbed Si(111) surfaces were studied by observing patterns, line profiles and intensity versus energy (I–V) curves of low-energy electron diffraction (LEED). Four different surface structures, i.e., √3 × √3, √7 × √7, incommensurate and α-phase 7 × 7 structures observed as a function Al coverage and the substrate temperature. Dynamical LEED analysis of a Si(111)√3 × √3 R30° surface, appearing after the deposition of Al of about 13 monolayer at the substrate temperature of 650°C, shows adsorption of Al atoms above the second layer Si atoms (T4 site). The √7 × √7 structure was observed in the narrow range of coverage of about 0.5 monolayer at 650°C. Below 600°C, the structure with a √7 × √7 pattern (α-7 × 7) whose line profile and I–V curves were different from those of the clean Si(111) surface, was formed. Above the 0.65 monolayer, an incommensurate phase, which is often referred to as a “7 × 7” structure, was observed. The phase transition from the α-7 × 7 phase to this incommensurate phase is considered to accompany large reconstructions and misfits.

Heteroepitaxial growth of alkali halides on a GaAs(001) substrate

Heteroepitaxial growth of alkali halides on GaAs(001) substrates was studied by use of reflection high‐energy electron diffraction and electron energy loss spectroscopy (EELS). In the case of NaCl growth on a GaAs(001) substrate (misfit 0.40%), epitaxial growth occurred when the substrate temperature was between 100 and 300 °C. EELS revealed that the NaCl film thicker than 3 nm completely covers the substrate and that the grown film is stoichiometric. This contrasts with the case of KBr growth on a GaAs(001) substrate, where epitaxial growth was not observed. The latter result seems to be due to the large misfit (16.8%). However, KBr was found to grow epitaxially on a NaCl‐on‐GaAs(001) substrate. The NaCl epilayer enables heteroepitaxial growth of another foreign alkali halide.

Inversion of long-offset TEM soundings near the borehole Münsterland 1, Germany, and comparison with MT measurements

SUMMARY Data from a long-offset transient electromagnetic (LOTEM) survey in northwest Germany around the borehole Munsterland 1 are interpreted using 1-D inversion. Special emphasis is given to the fact that the best-fitting model is not necessarily the most realistic one. We therefore investigate different inversion algorithms in order to obtain a more geologically meaningful result. The 1-D inversion results using a second-order Marquardt algorithm show a three-layer structure, with a conductor embedded between two more resistive layers. This is in good agreement with the known lithology and the well logs. ‘Profile inversion’ uses the result from one site as the starting model for the next. The application of the technique yields a smoother section than the individual inversions, and corresponds more closely to the (presumably) layered structure in the area. If desired, using a parameter weighting scheme makes it possible to force consistency at the cost of a larger misfit, yielding a smooth section even in a part of the area where the data are influenced by a 3-D anomaly. Occam inversion is a smoothness-constrained technique which is independent of the starting model. The result of the Occam inversion shows the same features as the Marquardt inversion, including effects which are attributed to 3-D distortions in the data. Joint inversions of the LOTEM data were carried out with magnetotelluric data from the same area. In that case, the LOTEM electric field is recognized as crucial for the resolution of the resistive layer in the section.

Dislocation reduction in HgCdTe epilayers on GaAs by using CdTe/CdZnTe strained-layer superlattices in CdTe layers

The structure of CdTe/CdZnTe strained layer superlattices (SLSs) which block the threading dislocations has been studied. Since the residual strain in the heteroepitaxial CdTe offsets the SLS-induced strain, SLSs located near the interface of CdTe/GaAs should be constructed with large misfit force in place. The surface dislocation density of CdTe was found to be reduced by a factor of 2.5 by using the two SLSs, and by a factor of 4.3 for four SLSs. The etch pit density of a HgCdTe layer grown on this SLS buffer layer was reduced, a beneficial side effect of reducing the dislocations in the buffer layer. Thus, strained-layer superlattices effectively block dislocations if designed properly. The dislocation density can be further reduced by increasing the number of SLSs.

Solving Interface Structures by Combined Electron Microscopy and X-Ray Diffraction

AbstractBy a combination of high resolution imaging (HREM) and grazing incidence X-ray scattering (GIXS), periodic interfaces with large unit cell can be solved at an atomic scale. The advantage of recording information in the real space is that phases are directly encoded in the image. On the other hand the X-ray diffraction gives quantitative information at a resolution level better than with HREM. This combined analysis is illustrated on GaAs (001)-CdTe (111) and on GaAs(001)-GaSb(001) interfaces. In both cases the structure at the interface is obtained and some mechanisms for the strain relaxation at heterostructures with large misfit are proposed.

Atomic structure of heterophase interfaces

High-resolution electron microscopy (HREM) has been used to study internal interfaces between dissimilar materials, notably ceramic-metal interfaces. Structures observed for systems with small and large misfits are compared in metal-metal, metal-ceramic, and ceramic-ceramic boundaries. The interfaces were prepared by a variety of techniques, including internal reduduction, internal oxidation, and epitaxial growth by MOCVD and special thin-film techniques. While interfaces produced by internal oxidation and reduction in f.c.c. systems typically form boundaries on (111) planes, non-equilibrium boundaries have also been generated using special thin film techniques. All boundaries can be characterized by their tendency to form coherent structures. While it appears that the amount of misfit and the bond strength primarily determine the degree of coherency, kinetic factors and substrate defects also seem to play an important role in determining the local defect structure at the boundary and the type of misfit localization.