Density Of Misfit Dislocations Research Articles

Investigation of the anisotropic strain relaxation in GaSb islands on GaP

The strain relaxation at the initial stages of highly mismatched (11.8%) GaSb grown on a GaP substrate following a Ga-rich surface treatment by molecular beam epitaxy has been investigated. High resolution transmission electron microscopy and moiré fringe analysis were used to determine the relaxation state in these GaSb islands in the [110] and [1–10] directions. The measurements revealed an anisotropic strain relaxation in these two directions; there is a higher misfit strain relaxation along the [110] direction where the islands are elongated, which is in agreement with a higher density of misfit dislocations. By combining molecular dynamics simulations and TEM results, the anisotropy in the strain relaxation is shown to be related to the asymmetry in the formation of interface misfit dislocations. The P-core glide set 60° dislocations (α type) and the Ga-core shuffle set Lomer dislocations serve as the primary misfit dislocation which contributes to the strain relaxation in the (1–10) interface, and the Ga-core glide set 60° dislocations (β type) and the P-core shuffle set Lomer dislocations for the (110) interface, respectively. The lower formation energy and higher glide velocity of the P-core glide set 60° dislocations (α type) result in a higher line density and more uniform periodical distribution of the misfit dislocation in the (1–10) interface. The higher fraction of Lomer dislocations, which is related to the dislocation configuration stability and surface treatment, promotes a better strain relief in the (1–10) interface, with a corresponding elongation of islands in the [110] direction.

Concentration and relaxation depth profiles of InxGa1−xAs/GaAs and GaAs1−xPx/GaAs graded epitaxial films studied by x-ray diffraction

A method is proposed to determine the concentration and relaxation depth profiles in graded epitaxial films from x-ray reciprocal space maps (RSMs). Various approximations in the kinematical x-ray diffraction from epitaxial films with the misfit dislocation density depth profile are developed. We show that a symmetric and an asymmetric RSM, or two asymmetric RSMs, contain enough information to obtain the concentration, relaxation, and lattice tilt depth profiles without any additional assumptions. The proposed approach is applied to In${}_{x}$Ga${}_{1\ensuremath{-}x}$As/GaAs and GaAs${}_{1\ensuremath{-}x}$P${}_{x}$/GaAs epitaxial graded films. The reconstructed concentration and dislocation density depth profiles are found to be in an agreement with the ones expected from the growth conditions.

Growth Mode and Defect Evaluation of GaSb on GaAs Substrate: A Transmission Electron Microscopy Study

We use transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques to confirm and analyze the interfacial misfit (IMF) and non-IMF growth modes for GaSb epilayers on GaAs substrates. Under optimized IMF growth conditions, only pure 90 degrees dislocations are generated along both [110] and [1-10] directions and located exactly at the epi-substrate interface, which leads to very low density of misfit dislocations propagating from the GaSb/GaAs interface along the growth direction, compared to the non-IMF growth condition. The mechanism of defect annihilation indicates that this IMF mergence process happens without formation of threading dislocations into the GaSb epilayer, which is a completely relaxed growth mode with extremely low defect density. Based on scanning several sets of wafer surfaces, plan-view TEM confirms that the misfit defect densities are estimated to be approximately 5 x 10(5) cm(-2) for IMF growth mode and approximately 10(9) cm(-2) for non-IMF growth mode.

Study of the TiO to Anatase Transformation by Thermal Oxidation of Ti Film in Air

The TiO to anatase phase transformation has been studied by transmission electron microscopy in this Article. It is shown that prior formation of TiO from Ti film can induce the formation of anatase by thermal oxidation in air; otherwise only rutile is formed. Ti film deposited on the NaCl (001) surface is induced to form epitaxial TiO film by thermal oxidation in air. Further thermal oxidation in air partially transformed TiO into anatase (A) with a parallel orientation relationship of {200}A//{200}TiO. Detailed analysis of the lattice fringes image of the specimen reveals the presence of very high density of misfit dislocations. The TiO to anatase transformation is reversible as further annealing in a vacuum can turn the anatase back into TiO and eliminates the misfit dislocations. The transformation is analyzed in terms of the crystal structure, orientation relationship, and the dislocation distribution, which show that the TiO to anatase transformation is due to the close similarity between their structures.

Crystallinity and Transport Properties in Fe/MgAl2O4/Fe (001) Epitaxial Magnetic Tunnel Junctions

We investigated in detail the crystallinity and transport properties of fully epitaxial Fe(001)/spinel MgAl2O4(001)/Fe(001) magnetic tunnel junctions (MTJs). We found that high (001) orientation (< 0.20°) and an extremely flat surface (averaged roughness ∼ 0.09 nm) were achieved in the optimized Fe/MgAl2O4/Fe MTJs exhibiting tunnel magnetoresistance (TMR) of more than 100% as well as excellent bias-voltage dependence. The misfit dislocation density of the MgAl2O4/Fe interface was determined to be precisely 5 × 10-2 nm-1. In addition, post-annealing in the preparation process has a significant influence on TMR and its bias-voltage dependence in Fe/MgAl2O4/Fe MTJs, suggesting that the crystallinity of the MgAl2O4 barrier interfaces was effectively improved by postannealing of each layer. The results provided useful information for understanding and developing epitaxial MTJs with coherent tunneling.

Increased critical thickness for high Ge-content strained SiGe-on-Si using selective epitaxial growth

The onset of misfit dislocation formation, i.e., the critical thickness for heteroepitaxy, is studied for selective epitaxial growth of high Ge-content, strained SiGe on oxide-patterned Si wafers. Misfit dislocation spacing was analyzed as a function of film thickness using plan-view transmission-electron microscopy. For selective epitaxial growth at 450 °C, the critical thickness for Si0.33Ge0.67 is found to be 8.5 nm. This is a twofold increase compared to the 4.0 nm theoretical equilibrium critical thickness and the 4.5 nm critical thickness measured for growth on bare Si wafers. The misfit dislocation density for selective epitaxial growth is strongly influenced by the shape and orientation of the growth area.

Equilibrium strain and dislocation density in exponentially graded Si1−xGex/Si (001)

We have calculated the equilibrium strain and misfit dislocation density profiles for heteroepitaxial Si1−xGex/Si (001) with convex exponential grading of composition. A graded layer of this type exhibits two regions free from misfit dislocations, one near the interface of thickness y1 and another near the free surface of thickness h−yd, where h is the layer thickness. The intermediate region contains an exponentially tapered density of misfit dislocations. We report approximate analytical models for the strain and dislocation density profile in exponentially graded Si1−xGex/Si (001) which may be used to calculate the effective stress and rate of lattice relaxation. The results of this work are readily extended to other semiconductor material systems and may be applied to the design of exponentially graded buffer layers for metamorphic device structures including transistors and light emitting diodes.

Identification of atomic steps at AlSb/GaAs hetero-epitaxial interface using geometric phase method by high-resolution electron microscopy

Geometric phase analysis (GPA) is applied to determining the strain fields in AlSb/GaAs hetero-epitaxial film from high-resolution electron microscopy (HREM) images. The misfit dislocations along the hetero-interface are shown to be predominantly 90° Lomer dislocations. The epitaxial film is almost fully relaxed by a high density of misfit dislocations. The 90° Lomer dislocations are assumed to be formed by either recombination of two 60° mixed misfit dislocations through a glide and climb process or direct nucleation at the interface. The atomic steps (ASs) are visualized in the strain map, providing a new method for identifying the ASs at the interface.

Low‐Temperature Superionic Conductivity in Strained Yttria‐Stabilized Zirconia

AbstractVery high lateral ionic conductivities in epitaxial cubic yttria‐stabilized zirconia (YSZ) synthesized on single‐crystal SrTiO3 and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order ∼1 Ω−1cm−1) are observed at 500 °C for 58‐nm‐thick films on MgO. The results indicate a superposition of two parallel contributions – one due to bulk conductivity and one attributable to conduction along the film–substrate interface. Interfacial effects dominate the conductivity at low temperatures (&lt;350 °C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk‐like conductivity is observed. The films have a negligible grain‐boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150–500 °C are of great fundamental importance but may also be technologically relevant for low‐temperature applications.

Growth of low‐dislocation‐density AlGaN using Mg‐doped AlN underlying layer

AbstractWe report on a new technology for growing low‐dislocation‐density AlGaN in which a Mg‐doped AlN underlying layer is used. By growing AlxGa1‐xN on AlN:Mg with AlN molar fractions x of 0.3, 0.5 and 0.7, the density of misfit dislocations is much reduced compared with that in the case of growing AlGaN on undoped AlN. In addition, the surface becomes atomically flat. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Tunnel magnetoresistance with improved bias voltage dependence in lattice-matched Fe/spinel MgAl2O4/Fe(001) junctions

We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

Control of asymmetric strain relaxation in InGaAs grown by molecular-beam epitaxy

InGaAs strain relaxation is studied by an in situ multibeam optical stress sensor (MOSS). Strain relaxation during growth of InGaAs on GaAs occurs at different thicknesses and rates along the directions perpendicular to its misfit dislocations, [110] and [11¯0]. We show the asymmetry of relaxation between these directions in real time by aligning the MOSS laser array along [110] and [11¯0]. This asymmetric relaxation data from the MOSS correlates with both x-ray diffraction relaxation analysis and an estimation of the misfit dislocation density from transmission electron microscopy images. Lowering the V/III ratio or raising the growth temperature lowers the thickness of the onset of dislocation formation, changes the relaxation rate, lowers the final relaxation during 2 μm of growth, and shifts the initial direction of relaxation from [110] to [11¯0]. We identify two phases of relaxation that occur at different growth thicknesses. Lowering the V/III ratio changes the relative contribution of each of these phases to the total relaxation of the epilayer.

Dislocations and strain relief in reverse-graded semiconductor layers

We have calculated the equilibrium misfit dislocation density and strain profiles in reverse-graded heteroepitaxial layers, using Si1−xGex/Si (0 0 1) as a model material system. In these structures there is a finite lattice mismatch at the substrate interface and the composition is graded linearly in such a way that the lattice mismatch decreases with distance from this interface. Reverse-graded layers exhibit two distinct behaviors which may be referred to as unkinked and kinked. In the unkinked reverse-graded layer the misfit dislocations are confined to a thin region near the substrate interface, the residual strain is a linear function of distance from the interface and the strain exhibits a sign reversal within the layer. Kinked reverse-graded layers exhibit a kink in the strain versus distance characteristic, and this kinked region contains a nearly constant density of misfit dislocations. We have developed models for the behavior of both types of reverse-graded layers, and we show that these models provide accurate predictions for Si1−xGex/Si (0 0 1) reverse-graded layers.

Current transport through InP/InSb heterojunction: Effect of lattice mismatch

Semiconductor heterojunctions of MOCVD grown InP were fabricated on n-InSb to study some features of a current transport in strained heterojunctions. The MOCVD grown undoped InP samples on InP substrates were characterized by XRD and Hall measurements whereas the InP/InSb heterojunction was characterized by XRD, TEM and I– V measurements in the temperature range 160–305 K. On increasing the voltage, first the current through the heterojunction is found to increase linearly and then it gets saturate due to surface states saturation. When the misfit dislocation density was increased, overlapping in the depletion regions gave rise to a barrier to the current flow there by saturating the current.

Misfit dislocation density and strain relaxation in graded semiconductor heterostructures with arbitrary composition profiles

We present a computational approach for the determination of the equilibrium misfit dislocation density and strain in a semiconductor heterostructure with an arbitrary compositional profile. We demonstrate that there is good agreement between our computed results and known analytical solutions for heterostructures containing a single linearly graded layer or a single uniform composition layer. We have calculated the dislocation density and strain profiles in Si1−xGex/Si(001), InxGa1−xAs/GaAs(001), and ZnSySe1−y/GaAs(001) heterostructures, each containing a uniform composition layer (uniform layer) on a linearly graded buffer layer (graded layer). The density of misfit dislocations in the graded layer is inversely proportional to its grading coefficient and is unchanged by the presence of the uniform layer, but the dislocated thickness increases with the uniform layer thickness. If the uniform layer is sufficiently thick, misfit dislocations will exist throughout the graded layer, but additional misfit dislocations are not produced in the uniform layer. The biaxial strain in the uniform layer is inversely proportional to its thickness and is unchanged by the graded layer. We have also calculated the equilibrium configuration in a convex, exponentially graded Si1−xGex/Si(001) layer, for which the misfit dislocation density is tapered with distance from the interface. Other nonlinear grading profiles offer opportunities to tailor the misfit dislocation density and strain profile.

MgO-Based Epitaxial Magnetic Tunnel Junctions Using Fe-V Electrodes

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> To examine the influence of the barrier quality in fully epitaxial Fe/MgO/Fe(001) magnetic tunnel junctions (MTJs), we propose to use Fe-V alloys as magnetic electrodes. This leads to a reduced misfit with MgO. We actually observe, by high-resolution electron microscopy (HREM) and local strain measurements, that the misfit dislocations density in the MgO barrier is lower when it is grown on Fe-V(001). This improvement of the crystalline quality of the MgO barrier actually leads to a significant increase of the tunnel magneto-resistance (TMR), despite the loss of spin polarization (SP) in these alloys, which was measured by spin-polarized X-ray photoelectron spectroscopy (SR-XPS). </para>

Switching of magnetic anisotropy in epitaxial CoFe2O4 thin films induced by SrRuO3 buffer layer

The magnetic anisotropy of epitaxial spinel ferrite CoFe2O4 films grown on SrTiO3 substrates by pulsed laser deposition can be reoriented by inserting a thin SrRuO3 buffer layer. Without SrRuO3, the CoFe2O4 films show a uniaxial anisotropy with the easy axis perpendicular to the film plane, while inserting a SrRuO3 buffer layer results in the switching of the easy axis into the in-plane orientation. This is associated with a tensile and a compressive strain for the films without and with buffer layer, respectively, which is also correlated with the different density of interfacial misfit dislocations. As a result, the ferrite films can be effectively tailored by using SrRuO3.

The reduction of the misfit dislocation in non-doped AlAs/GaAs DBRs

The non-doped AlAs/GaAs distributed Bragg reflectors (DBRs) with density of misfit dislocation (MD) close to zero had been obtained. The reduction of MD density was achieved by increasing temperature distribution homogeneity on the growing crystal in consequence of higher rotation rate of the wafer. Two structures of DBR were crystallized using molecular beam epitaxy (MBE) under the same optimal growth condition. The growth runs differ only in the rotation rate of the wafers. X-ray topograph showed no residual MDs in case of faster rotation. The DBR structure with residual MD density is still highly strained. No additional relaxation process has occurred, what was confirmed by an angular position of DBR zeroth-order peak on high-resolution X-ray diffractometry (HRXRD) rocking curve.

Influence of interface structure on mass transport in phase boundaries between different ionic materials: Experimental studies and formal considerations.

Internal and external interfaces in solids exhibit completely different transport properties compared to the bulk. Transport parallel to grain or phase boundaries is usually strongly enhanced. Transport perpendicular to an interface is usually blocked, i.e., transport across an interface is often much slower. Due to the high density of interfaces in modern micro- and nanoscaled devices, a severe influence on the total transport properties can be expected. In contrast to diffusion in metal grain boundaries, transport phenomena in boundaries of ionic materials are still less understood. The specific transport properties along metal grain boundaries are explained by structural factors like packing densities or dislocation densities in the interface region. In most studies dealing with ionic materials, the interfacial transport properties are merely explained by the influence of space charge regions. In this study the influence of the interface structure on the interfacial transport properties of ionic materials is discussed in analogy to metallic materials. A qualitative model based on the density of misfit dislocations and on interfacial strain is introduced for (untilted and untwisted) phase boundaries. For experimental verification, the interfacial ionic conductivity of different multilayer systems consisting of stabilised ZrO2 and an insulating oxide is investigated as a funtion of structural mismatch. As predicted by the model, the interfacial conductivity increases when the lattice mismatch is increased.Graphical abstract

Comparison of structural, microstructural, and electrical analyses of barium strontium titanate thin films

The results of structural and electrical characterizations of a barium strontium titanate (Ba0.5Sr0.5TiO3) film, including low temperature x-ray diffraction, low temperature, field dependent Raman spectroscopy, transmission electron microscopy, and low temperature, field dependent microwave measurements, are compared and contrasted. The structural characterization showed the film to be a good single crystal, epitaxial with the (001) MgO substrate. In the ferroelectric state, the tetragonal axis lies in plane leading to a 90° domain structure. A high density of misfit and threading dislocations was observed in the film. The transition to the paraelectric state on warming was shown to be diffuse and appeared to be strongly influenced by local variations in strain which were attributed to the defective microstructure. The dielectric response was also dominated by such effects.