Interfacial Misfit Dislocations Research Articles

Surfactant-mediated epitaxy of thin germanium films on SiGe(001) virtual substrates

We report on the impact of a surfactant on the growth mode and strain relaxation of thin Ge films on Si0.21Ge0.79 virtual substrates grown by surfactant mediated epitaxy on Si(001) wafers. Ge epitaxy without surfactant results in island formation after deposition of only 5nm Ge. A certain part of the strain in the Ge islands is relaxed via interfacial misfit dislocations, which are located within the core part of the islands. We discuss the possibilities for the occurrence of three-dimensional growth at low Ge layer thickness. The use of Sb as a surfactant suppresses three-dimensional islanding and enables the growth of smooth pseudomorphically strained Ge films on Si0.21Ge0.79(001) virtual substrates up to a thickness of 10nm. At thicknesses higher than 20nm, the films relax via the formation of a misfit dislocation network at the Ge/ Si1−xGex interface. The surface roughness of up to 30nm thick layers is below 1.6nm. Our experimental results corroborate the calculated thickness for plastic relaxation of Ge on Si1−xGex. The effect of the surfactant on the growth of the virtual substrate and on the subsequent growth of Ge on Si0.21Ge0.79 is discussed.

Critical thickness for interface misfit dislocation formation in two-dimensional materials

Critical thickness for interface misfit dislocation formation in two-dimensional materials

Investigation of cross-hatch surface and study of anisotropic relaxation and dislocation on InGaAs on GaAs (001)

There exist discrepancies between reports on cross-hatch (CH) behaviour and its interaction with interfacial misfit dislocations in the literature. In this work, a thorough CH analysis has been presented by use of conventional and statistical analysis of AFM data. It has been shown that correlation between cross-hatch and misfit dislocation depends on the growth conditions and residual strain. Anisotropic relaxation and dislocations, composition and epitaxial tilt have been studied by HRXRD analysis. To illustrate these findings, molecular beam epitaxy (MBE) grown metamorphic InGaAs on GaAs (001) samples have been used. Reciprocal space mapping has been used to characterize the composition and relaxation while epilayer tilt and dislocation have been investigated by HRXRD rocking curve. A better understanding of CH pattern can enable us to minimize the surface roughness for metamorphic electronic devices and to fully utilize the quasi-periodic undulation in cross-hatch in applications, like ordered quantum dot growth.

In situ observation of low temperature growth of Ge on Si(1 1 1) by reflection high energy electron diffraction

In this paper we investigate the initial stages of epitaxial growth of Ge on Si(111) and the impact of growth temperature on strain evolution in situ by reflection high energy electron diffraction (RHEED) for temperatures between 200°C and 400°C. The change in surface morphology from a flat wetting layer to subsequent islanding that is characteristic for Stranski–Krastanov growth is monitored by spot intensity analysis. The corresponding critical layer thickness is determined to 3.1<dc<3.4ML. In situ monitoring of the strain relaxation process reveals a contribution of the Si(111) 7×7-surface reconstruction to the strain relaxation process. High resolution transmission electron microscopy confirms that the Ge islands exhibit a high degree of structural perfection and an ordered interfacial misfit dislocation network already at a growth temperature of 200°C is established. The temperature dependency of island shape, density and height is characterized by atomic force microscopy and compared to the RHEED investigations.

High-Resistivity Semi-insulating AlSb on GaAs Substrates Grown by Molecular Beam Epitaxy

Thin-film structures containing AlSb were grown using solid-source molecular beam epitaxy and characterized for material quality, carrier transport optimization, and room-temperature radiation detection response. Few surface defects were observed, including screw dislocations resulting from shear strain between lattice-mismatched layers. Strain was also indicated by broadening of the AlSb peak in x-ray diffraction measurements. Threading dislocations and interfacial misfit dislocations were seen with transmission electron microscopy imaging. Doping of the AlSb layer was introduced during growth using GaTe and Be to determine the effect on Hall transport properties. Hall mobility and resistivity were largest for undoped AlSb samples, at 3000 cm2/V s and 106 Ω cm, respectively, and increased doping levels progressively degraded these values. To test for radiation response, p-type/intrinsic/n-type (PIN) diode structures were grown using undoped AlSb on n-GaAs substrates, with p-GaSb cap layers to protect the AlSb from oxidation. Alpha-particle radiation detection was achieved and spectra were produced for 241Am, 252Cf, and 239Pu sources. Reducing the detector surface area increased the pulse height observed, as expected based on voltage–capacitance relationships for diodes.

Stabilization of coherent precipitates in nanoscale thin films

Coherent precipitates, on growth beyond a critical size (), become semi-coherent by the presence of interfacial misfit edge dislocations. In the case of precipitation in ‘small’ domains (matrix), the strain energy of the precipitate is altered with respect to its value in bulk domains, due to a lesser volume of strained material and domain deformations, resulting from the proximity of free surfaces. This results in a change in the value of critical size for the coherent to semi-coherent transition of the interface. In the current work, the Cu-2wt.%Fe system is used as model system to study the coherent to semi-coherent transition of precipitates. In this system, spherical Fe-2wt.%Cu precipitates form, which become cuboidal on growth beyond the critical size. Transmission electron microscopy is used to show experimentally for the first time that a coherent precipitate can be stable beyond , in nanoscale free-standing thin films (i.e. critical size for coherent to semi-coherent transition for thin films () > ). Electron energy loss spectroscopy has been used to determine the thickness of the sample. In parallel, finite element simulations are used to compute the critical size in representative domains and to comprehend the energetic basis for the observed phenomenon. Eigenstrains are used to simulate the precipitation process and the stress state due to an interfacial misfit dislocation in the finite element model.

Structural modifications due to interface chemistry at metal-nitride interfaces.

Based on accurate first principles density functional theory (DFT) calculations, an unusual phenomenon of interfacial structural modifications, due to the interface chemistry influence is identified at two metal-nitride interfaces with strong metal-nitrogen affinity, Al/TiN {111} and Al/VN {111} interfaces. It is shown that at such interfaces, a faulted stacking structure is energetically preferred on the Al side of the interface. And both intrinsic and extrinsic stacking fault energies in the vicinity Al layers are negligibly small. However, such phenomenon does not occur in Pt/TiN and Pt/VN interfaces because of the weak Pt-N affinity. Corresponding to structural energies of metal-nitride interfaces, the linear elasticity analysis predicts characteristics of interfacial misfit dislocations at metal-nitride interfaces.

Mesoscopic organization of cobalt thin films on clean and oxygen-saturated Fe(001) surfaces

The different morphologies of Co films grown on either the clean Fe(001) surface and the oxygen-saturated Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ substrate are investigated by means of scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory. The considered Co coverage range extends beyond the thickness at which layer-by-layer growth is destabilized by plastic deformations induced by the relaxation of the strain accumulated in the film. Our findings indicate that the oxygen overlayer of the Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ surface floats on top of the growing Co film and strongly influences both the Co nucleation process and the film structural evolution. The layer-dependent islands nucleation of Co films grown on clean Fe(001) substrates, recently associated with a thickness-dependent adatom mobility [A. Picone et al., Phys. Rev. Lett. 113, 046102 (2014)], is found to be suppressed by the oxygen overlayer. The latter also significantly delays the layer-by-layer instability with respect to the oxygen-free growth. Furthermore, the body-centered-tetragonal/hexagonal-close-packed transition is not observed in the case of Co/Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ sample, replaced by the development of highly ordered surface undulations. These form a mesoscopic square pattern with the sides aligned to the $\mathrm{Fe}\ensuremath{\langle}110\ensuremath{\rangle}$ directions, while the surface atomic structure retains the square $p(1\ifmmode\times\else\texttimes\fi{}1)$ symmetry in registry with the substrate. Such undulations are likely generated by a highly ordered array of interfacial misfit dislocations running along the $\mathrm{Fe}\ensuremath{\langle}110\ensuremath{\rangle}$ directions.

Anisotropy of strain relaxation in heterogeneous GaInNAs layers grown by AP-MOVPE

The structural investigations of thick undoped GaInNAs/GaAs heterostructures grown by atmospheric pressure vapor phase epitaxy (AP-MOVPE) have been performed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The cross-sectional view TEM images revealed plainly heterogeneous layer structure, consisting of a few sublayers with different indium and nitrogen contents. On the other hand, planar-view TEM images revealed 2D network of misfit dislocations oriented in two orthogonal 〈110〉 crystallographic directions at the (001) sublayer interface. Moreover, the XRD measurements provide information about gradient profiles of indium and nitrogen contents, as well as distinct anisotropy of strain relaxation in GaInNAs epitaxial layer, evoked by the asymmetry in formation of interfacial misfit dislocation. The obtained results were exploited to calculate the band structure diagram of the investigated GaInNAs/GaAs heterostructure and mean values of the lattice parameters of the distorted epitaxial layer unit cell, respectively.

Interfaces and defects in a successfully hot-rolled steel-based composite Fe–TiB2

Steel-based composites reinforced with titanium diborides (TiB2) and produced by eutectic solidification display a significant increase in specific stiffness (E/ρ) in comparison with usual steels. A significant lightening can be achieved in parts designed for stiffness with a high Young’s modulus and a low density. They can be produced through the use of industrial continuous casting devices and can be hot rolled without any damage. This production route guarantees a good cohesion of the steel/reinforcement interface and the control of the main parameters that determine the final properties of the product (size, shape, clustering of reinforcements). In order to get a better understanding of the composite properties, its microstructural features after hot rolling are investigated through transmission electron microscopy at microscopic and atomic levels. TiB2 reinforcements are homogeneously distributed in the ferritic iron matrix. Interfaces between Fe and TiB2 particles are mainly parallel to dense planes of the diboride, with a preferential growth of prismatic planes {101¯0}. Interfacial misfit dislocations occur at interfaces parallel to the prismatic planes. The TiB2 basal plane is covered by a TiC layer three atomic planes in width. The hot rolling operation induces a significant plastic deformation in the TiB2 particles, with the activation of basal and prism plane slip systems. All these characteristics may account for the good behaviour of the composite in terms of hot rolling capability.

Formation of interfacial misfit dislocation in GaSb/GaAs heteroepitaxy via anion exchange process

We report the formation of interfacial misfit dislocation found in GaSb layers grown on (001) GaAs substrates using anion exchange process at different growth temperatures. An in-situ reflection high-energy electron diffraction (RHEED) in conjunction with transmission electron microscopy (TEM), atomic force spectroscopy (AFM) and X-ray diffraction (XRD) analysis was applied to investigate the relations between substrate temperature, As to Sb anion exchange, the changes in threading dislocation density and the growth kinetics of the GaSb/GaAs heteroepitaxy system. A GaSb layer grown at 450°C exhibited the lowest threading dislocation density. Our results provide further understanding of the dominant substrate temperature-dependent growth mechanisms that can affect threading dislocation in the GaSb on a GaAs heteroepitaxy system, which uses anion exchange process to facilitate the formation of interfacial misfit dislocation.

The peculiarity of the metal-ceramic interface

Important properties of materials are strongly influenced or controlled by the presence of solid interfaces, i.e. from the atomic arrangement in a region which is a few atomic spacing wide. Using the quantitative analysis of atom column positions enabled by CS-corrected transmission electron microscopy and theoretical calculations, atom behaviors at and adjacent to the interface was carefully explored. A regular variation of Cu interplanar spacing at a representative metal-ceramic interface was experimentally revealed, i.e. Cu-MgO (001). We also found the periodic fluctuations of the Cu and Mg atomic positions triggered by the interfacial geometrical misfit dislocations, which are partially verified by theoretical calculations using empirical potential approach. Direct measurements of the bond length of Cu-O at the coherent regions of the interface showed close correspondence with theoretical results. By successively imaging of geometrical misfit dislocations at different crystallographic directions, the strain fields around the interfacial geometrical misfit dislocation are quantitatively demonstrated at a nearly three-dimensional view. A quantitative evaluation between the measured and calculated strain fields using simplified model around the geometrical misfit dislocation is shown.

Epitaxial growth of low threading dislocation density InSb on GaAs using self-assembled periodic interfacial misfit dislocations

We report a fully relaxed low threading dislocation density InSb layer grown on a GaAs substrate using self-assembled periodic interfacial misfit dislocations. The InSb layer was grown at 310°C by molecular beam epitaxy. The AFM measurement exhibited a root mean square (r.m.s.) roughness of 1.1nm. ω−2θ scan results from x-ray diffraction measurement indicated that the InSb layer is 98.9% relaxed. Images from the transmission electron microscope measurement showed a threading dislocation density of 1.38×108cm−2. The formation of highly uniform interfacial misfit dislocation array was also observed and the separation of dislocations is consistent with theoretical calculation. The InSb layer exhibited a 33,840cm2/Vs room temperature electron mobility.

Interface characteristics of cubic AlN film on MgO (100) substrate

Highly oriented cubic AlN film was deposited on MgO (100) substrate by laser molecular beam epitaxy (LMBE) technique, and the interface characteristics were investigated. Cubic AlN film and MgO substrate show the well-defined orientation relationship of AlN [001]//MgO [001] and AlN (100)//MgO (100). The AlN/MgO interface is characterized by coherent regions separated by periodically spaced misfit dislocations, forming a semi-coherent interface. The experimentally measured interface dislocation spacing is slightly smaller than that calculated from the lattice mismatch, implying that the lattice mismatch between cubic AlN film and MgO substrate is accommodated mainly by interface misfit dislocations.

Heterointegration of III–V on silicon using a crystalline oxide buffer layer

The integration of III–V compound semiconductors with Si can combine the cost advantage and maturity of Si technology with the superior performance of III–V materials. We have achieved the heteroepitaxial growth of III–V compound semiconductors on a crystalline SrTiO3 buffer layer grown on Si(001) substrates. A two-step growth process utilizing a high temperature nucleation layer of GaAs, followed by a low-temperature GaAs layer at a higher growth rate was employed to achieve highly crystalline thick GaAs layers on the SrTiO3/Si substrates with low surface roughness as seen by AFM. The effect of the GaAs nucleation layer on different surface terminations for the SrTiO3 layer was studied for both on axis and miscut wafers, which led to the conclusion that the Sr terminated surface on miscut substrates provides the best GaAs films. Using GaAs/STO/Si as virtual substrates, we have optimized the growth of high quality GaSb using the interfacial misfit (IMF) dislocation array technique. This work can lead to the possibility of realizing infrared detectors and next-generation high mobility III–V CMOS within the existing Si substrate infrastructure.

Temperature-dependent residual stresses in a hetero-epitaxial thin film system

This paper investigates the temperature dependence of residual stresses in a hetero-epitaxial thin film on a sapphire substrate. The X-ray diffraction technique was employed and a theoretical analysis was also carried out. It was found that the magnitude of compressive residual stresses decrease with increasing the temperature, and that the rate of the change can be well predicted theoretically. It was discovered, however, that the residual stresses vary with the film thickness. For a film of 0.3μm in thickness at all temperatures, the magnitudes of compressive stresses measured are greater than the theoretically predicted; but for that of 5μm in thickness, the magnitudes measured become smaller than the theoretical. This leads to the conclusion that the mitigation of lattice mismatch, essentially through interface misfit dislocations, could have varied with the change of the film thickness.

Interface structure and the inception of plasticity in Nb/NbC nanolayered composites

Molecular dynamics (MD) simulations were performed to explore the effect of interface structure on the inception of plastic deformation in Nb/NbC nanolayered composites. Using the atomistically informed Frank–Bilby method and disregistry analysis, we characterized the structure of the Nb/NbC interface, including misfit dislocations, dislocation nodes and three coherent interface structures. According to the crystallographic analysis of the interface, four possible coherent interface structures were identified. However, study of the interface energy showed that only three of these are energetically stable. After the relaxation of the interface, the unstable coherent region, which features Nb atoms in the Nb layer on the top of the Nb atoms in the NbC layer, evolves into a condensed interface dislocation node. Three stable coherent interface regions are retained in association with the formation, glide and reaction of interface misfit dislocation loops. Disregistry analysis of the Nb/NbC interface revealed that (i) all misfit dislocations are edge type, and (ii) misfit dislocations enclosing the coherent extended nodes have Burgers vectors along 〈110〉Nb. The role of the interface structure in the plastic deformation of Nb/NbC nanolayered composites was studied under two loading conditions, i.e. uniform compression and nanoindentation. Under uniform compression, lattice dislocations primarily nucleate from the condensed nodal regions where the local strains are the highest. Dislocations propagate in two {110} slip planes with the same Schmid factors. Under nanoindentation, by proper positioning of the indenter, lattice dislocations nucleate from the segments of the misfit dislocations and propagate in {112} slip planes. MD simulations also show cross-slip of lattice dislocations from {112} planes into {110} planes and the formation of vacancies as a result of climb of dislocation jogs.

OS0503-401 分子動力学法による多積層構造の界面特性評価

Advanced semiconductor products have structures with thickness and width in nanoscale. As the size of structures reduces to a nanometer level, a ratio of their surface to volume increase. In the present study, three-layered structure is analyzed using molecular dynamics (MD) method and the anisotropic elasticity theory. The analysis of stress and displacement near interface misfit dislocation in a two-phase anisotropic body is considering interface stress and interface elasticity. The stress and displacement distribution at the interface are compared the analysis considering the interface properties and the MD method.

Influence of initial indentation point on nanoindentation of Ni-based single crystal line alloys

Ni-based single crystal line alloy is constituted with γ phase and γ' phase in the form of coherency. Since an indenter for two-phase coherent structure is bigger than the usual nano-scale indenter, the press location of indenter may be unclear in nanoindentation simulation. Both γ phase and γ' phase may be pressed initially, and the mechanical properties shown are different because of the initial press locations. The nanoindentation of Ni-based single crystal line alloys is simulated by molecular dynamics method. Two models are used to study about the hardness in [001] crystal orientation, one is the model γ /γ' with the initial indentation on γ phase, and the other is the model γ'/γ with the initial indentation on γ' phase. The influence of misfit dislocation at (001) interface on nanoindentation of the two models is analyzed using a center-symmetry parameter. Results show that the misfit dislocation shape of the two models are different after relaxation. Lomer-Cottrell dislocation occurs on (001) interface in the γ'/γ model. Before 0.930 nm press depth is reached, there is little change in the (001) interface misfit dislocation of the two models. Relationship between press load and press depth is similar for the two models, and it is the same in the relationship between hardness and press depth. After press depth reaches 0.930 nm, the misfit dislocation at (001) interface for model γ'/γ grows big, which results in a smaller press load and a smaller hardness computation in the model γ'/γ than that in model γ /γ'. When the press depth reach 2.055 nm, we find only a small amount of dislocations in γ phase that can go into γ' phase since the misfit dislocation at (001) interface in model γ /γ' hinders the process. However, none of dislocations can go into γ phase because of the prevention caused by Lomer-Cottrell dislocation at the (001) interface in the model γ'/γ . That means the Lomer-Cottrell dislocation reinforces the material obviously. So the press load in model γ'/γ grows faster than that in model γ /γ'.

Graphene induced remote surface scattering in graphene/AlGaN/GaN heterostructures

The mobilities of single-layer graphene combined with AlGaN/GaN heterostructures on two-dimensional electron gases in graphene/AlGaN/GaN double heterojunction are calculated. The impact of electron density in single-layer graphene is also studied. Remote surface roughness (RSR) and remote interfacial charge (RIC) scatterings are introduced into this heterostructure. The mobilities limited by RSR and RIC are an order of magnitude higher than that of interface roughness and misfit dislocation. This study contributes to designing structures for generation of higher electron mobility in graphene/AlGaN/GaN double heterojunction.