Misfit Strain Relaxation Research Articles

Configurable topological textures in strain graded ferroelectric nanoplates

Topological defects in matter behave collectively to form highly non-trivial structures called topological textures that are characterised by conserved quantities such as the winding number. Here we show that an epitaxial ferroelectric square nanoplate of bismuth ferrite subjected to a large strain gradient (as much as 105 m−1) associated with misfit strain relaxation enables five discrete levels for the ferroelectric topological invariant of the entire system because of its peculiar radial quadrant domain texture and its inherent domain wall chirality. The total winding number of the topological texture can be configured from − 1 to 3 by selective non-local electric switching of the quadrant domains. By using angle-resolved piezoresponse force microscopy in conjunction with local winding number analysis, we directly identify the existence of vortices and anti-vortices, observe pair creation and annihilation and manipulate the net number of vortices. Our findings offer a useful concept for multi-level topological defect memory.

X-ray Bragg Ptychography on a Single InGaN/GaN Core-Shell Nanowire.

The future of solid-state lighting can be potentially driven by applications of InGaN/GaN core-shell nanowires. These heterostructures provide the possibility for fine-tuning of functional properties by controlling a strain state between mismatched layers. We present a nondestructive study of a single 400 nm-thick InGaN/GaN core-shell nanowire using two-dimensional (2D) X-ray Bragg ptychography (XBP) with a nanofocused X-ray beam. The XBP reconstruction enabled the determination of a detailed three-dimensional (3D) distribution of the strain in the particular nanowire using a model based on finite element method. We observed the strain induced by the lattice mismatch between the GaN core and InGaN shell to be in the range from -0.1% to 0.15% for an In concentration of 30%. The maximum value of the strain component normal to the facets was concentrated at the transition region between the main part of the nanowire and the GaN tip. In addition, a variation in misfit strain relaxation between the axial growth and in-plane directions was revealed.

Relationship between dislocations and misfit strain relaxation in InGaAs/GaAs heterostructures

Large lattice mismatched heterostructures In0.78Ga0.22As/GaAs were grown with a low-temperature (LT) InGaAs buffer. Misfit dislocation arrays were observed at the LT-buffer/GaAs interfaces. Transmission electron microscopy (TEM) was used to investigate the microstructures of the heterointerfaces. The relationship between misfit dislocations and strain relaxation at the interface was analysed. It was found that strain redistribution gives rise to the discrepancy of interfacial structure and misfit strain relaxation for different samples. The experimental results are in favor of the conclusion and confirm the analysis we proposed.

Distribution of dislocations in AlN crystals grown on evaporating SiC substrates

By the use of high-resolution X-ray diffractometry and synchrotron radiation topography, the distribution of dislocations in AlN crystals grown on evaporating SiC substrates is studied. The growth of AlN layer in one process with the evaporation of SiC substrate gave the possibility to prevent the relaxation of thermal stresses through cracking of AlN during cooling the structure. The continuous 0.2-1.5 mm thick plates were used as the model objects for the study of dislocation structure near the AlN/SiC interface. Analysis of the broadening of the Bragg reflection peaks, the shape of scattering maps in reciprocal space and topographs showed that dislocations formed the mosaic structure, different from the distribution of threading dislocations in GaN epilayers. A theoretical model for misfit strain relaxation in the growing AlN layer is suggested. The results enable to clarify the dislocation processes during sublimation growth of industrial quality AlN crystals.

Crystal structure and polarization hysteresis properties of ferroelectric BaTiO3 thin-film capacitors on (Ba,Sr)TiO3-buffered substrates

Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1−xSrx)TiO3 (BST) have been fabricated on (001) SrTiO3 (STO) single-crystal substrates by a pulsed laser deposition method, and the crystal structure and polarization hysteresis properties have been investigated. X-ray diffraction reciprocal space mapping shows that the BST buffer effectively reduces the misfit strain relaxation of the BT films on SrRuO3 (SRO) electrodes. The BT capacitor with the SRO electrodes on the BST (x = 0.3) buffer exhibits a well-saturated hysteresis loop with a remanent polarization of 29 µC/cm2. The hysteresis loop displays a shift toward a specific field direction, which is suggested to stem from the flexoelectric coupling between the out-of-plane polarization and the strain gradient adjacent to the bottom interface.

Growth and optical properties of FePt:BaTiO3 nanocomposite films

FePt:BaTiO3 nanocomposite films consisting of a regular stacking of pure BaTiO3 (BTO) layers and BTO layers embedding FePt nanocrystals were grown on (001) MgO substrates by pulsed laser deposition. Transmission electron microscopy experiments on cross-sectional specimens demonstrated the high quality of the epitaxial growth of the different layers, whatever their composition. The structural analysis also gave evidence of a misfit strain relaxation at the BTO/MgO interface. We report and discuss the optical and pholuminescence properties of the nanocomposite films in comparison with the ones recorded in pure BTO films. Our results emphasize the role of nanostructuration and growth processes on the evolution of the observed properties.

Improved optical properties of InAs quantum dots for intermediate band solar cells by suppression of misfit strain relaxation

The properties of InAs quantum dots (QDs) have been studied for application in intermediate band solar cells. It is found that suppression of plastic relaxation in the QDs has a significant effect on the optoelectronic properties. Partial capping plus annealing is shown to be effective in controlling the height of the QDs and in suppressing plastic relaxation. A force balancing model is used to explain the relationship between plastic relaxation and QD height. A strong luminescence has been observed from strained QDs, indicating the presence of localized states in the desired energy range. No luminescence has been observed from plastically relaxed QDs.

Low-temperature growth of InGaN films over the entire composition range by MBE

The surface morphology, microstructural, and optical properties of indium gallium nitride (InGaN) films grown by plasma-assisted molecular beam epitaxy under low growth temperatures and slightly nitrogen-rich growth conditions are studied. The single-phase InGaN films exhibit improved defect density, an absence of stacking faults, efficient In incorporation, enhanced optical properties, but a grain-like morphology. With increasing In content, we observe an increase in the degree of relaxation and a complete misfit strain relaxation through the formation of a uniform array of misfit dislocations at the InGaN/GaN interface for InGaN films with indium contents higher than 55–60%.

Thermal Misfit Strain Relaxation in Ge/(001)Si Heterostructures

We used x-ray diffraction and transmission electron microscopy to study the mechanism of thermal misfit strain relaxation in epitaxial Ge films grown on Si(001) substrates by the two-step growth technique. Lattice misfit strain associated with Ge/Si(001)Si mismatched epitaxy is largely relieved by a network of Lomer edge misfit dislocations during the first step of growth. However, thermal misfit strain during growth is relieved primarily by interdiffusion at the Si/Ge heterointerface. Two SiGe compositions containing 0.5 at.% and 6.0 at.% Si detected at the interface relieve thermal mismatch strain associated with the two growth steps. A thermodynamic model has been proposed to explain the state of strain in the films.

Phase-field modeling of displacive phase transformations in elastically anisotropic and inhomogeneous polycrystals

We integrate the inhomogeneous elasticity model and the phase-field equations for displacive phase transformations in polycrystalline materials. The relaxation of the misfit strain between parent and transformed product phases or among different structural variants of transformed product phases near grain boundaries is taken into account. It is applied to the fcc to bcc martensitic transformation described by a Bain strain in a polycrystalline Fe-31at.%Ni metallic alloy. The focus is on the effect of grain boundaries on the displacive transformation behaviors. We first study nucleation of the bcc product phase at a grain boundary of a bicrystal. The predicted microstructures through nucleation near grain boundaries are compared to existing experimental observations in literature. The effects of grain boundary characteristics such as the degree and range of the misfit strain relaxation at the grain boundary and grain boundary curvature on the phase behaviors near a grain boundary are then examined for both a flat or a curved grain boundary. The model is also applied to polycrystals containing multiple grains. The effects of the misfit strain relaxation at grain boundaries, elastic anisotropy, and applied stress on the kinetics and the microstructures of displacive transformations are discussed.

Highly luminescent, high-indium-content InGaN film with uniform composition and full misfit-strain relaxation

We have studied the properties of thick InxGa1−xN films, with indium content ranging from x ∼ 0.22 to 0.67, grown by metal-modulated epitaxy. While the low indium-content films exhibit high density of stacking faults and dislocations, a significant improvement in the crystalline quality and optical properties has been observed starting at x ∼ 0.6. Surprisingly, the InxGa1−xN film with x ∼ 0.67 exhibits high luminescence intensity, low defect density, and uniform full lattice-mismatch strain relaxation. The efficient strain relaxation is shown to be due to a critical thickness close to the monolayer range. These films were grown at low temperatures (∼400 °C) to facilitate indium incorporation and with precursor modulation to enhance surface morphology and metal adlayer diffusion. These findings should contribute to the development of growth techniques for nitride semiconductors under high lattice misfit conditions.

Perfect crystals grown from imperfect interfaces

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.

Determination of heteroepitaxial layer relaxation at growth temperature from room temperature X-ray reciprocal space maps

Lattice-mismatched heteroepitaxial growth in compound semiconductor layer structures, e.g. metamorphic buffers, is frequently used to combine materials with favorable properties. A detailed understanding of the strain relaxation during growth is important to optimize the buffer layers and to achieve high material quality. We present a method to determine the epilayer relaxation at growth temperature from room temperature X-ray diffraction measurements. For this the lattice parameters are measured with (004) and (224) reciprocal space maps along two orthogonal <110> directions to account for anisotropic misfit strain relaxation. Dissimilar thermal expansion coefficients of substrate and epilayer are taken into consideration when calculating the lattice parameters and epilayer relaxation parameters at growth temperature. The application of this method is discussed for GaAsxP1−x metamorphic buffer structure on Si with eight lattice-mismatched layers showing a relaxation parameter of over 130% at room temperature. All layers were found to be nearly 100% relaxed at growth temperature. The use of this method for other compound semiconductors is discussed.

Self-assembled pit arrays as templates for the integration of Au nanocrystals in oxide surfaces

We report on the fabrication of long-range ordered arrays of Au nanocrystals (sub-50 nm range) on top of manganite (La(2/3)Sr(1/3)MnO(3)) thin films achieving area densities around 2 × 10(10) gold nanocrystals per cm(2), well above the densities achievable by using conventional nanofabrication techniques. The gold-manganite interface exhibits excellent conduction properties. Long-range order is achieved by a guided self-assembling process of Au nanocrystals on self-organized pit-arrays acting as a template for the nucleation of gold nanocrystals. Self-organization of pits on the manganite film surface promoted by the underlying stepped SrTiO(3) substrate is achieved by a fine tuning of the growth kinetic pathway, taking advantage of the unusual misfit strain relaxation behaviour of manganite films.

Atomic interface sequence, misfit strain relaxation and intrinsic flux-pinning defects in different YBa2Cu3O7−δ heterogeneous systems

To explore the interface effects on enhancing critical current density in YBa2Cu3O7−δ (YBCO) thin films, four heterogeneous systems, of which two have small opposite lattice misfits (i.e., the YBCO/STO (∼1.6%) and the YBCO/LAO (∼−1.6%)), and the other two have relatively large opposite mismatch (i.e., the YBCO/MgO (∼8.9%) and the YBCO/YSZ (∼−5.9%)), were selected and synthesized by pulsed-laser deposition. A detailed microstructure analysis including XRD, TEM and the state-of-the-art Cs-corrected STEM imaging, and a thorough superconducting property characterization including critical transition temperature (Tc) and critical current density ( and (H ‖ c) at various temperatures), were conducted for all four heterogeneous systems. The results reveal that the YBCO intrinsic defects, driven by the interface mismatch of different strain states, present a diverse nature in their distribution and density, and thus different flux-pinning performances under various measurement conditions. This study provides an in-depth insight into the microstructure–property correlation in a strongly correlated electronic system, the YBCO heterogeneous structures.

Accommodation at the interface of highly dissimilar GaN(0 0 0 1)/Sc2O3(1 1 1) heteroepitaxial systems

We present first-principles calculations to study the heterojunction between a wurtzite GaN(0 0 0 1) film and a cubic Sc 2 O 3 (1 1 1) substrate. We report that the most favorable interface consists of the graphitic-like GaN nanofilms, indicating that the nanofilms can accommodate the misfit strain relaxation at heteroepitaxial interfaces. This interface structure gives rise to Ga polarity in the GaN(0 0 0 1) epitaxial film. Our findings agree with previously reported experimental results.

Misfit dislocations of anisotropic magnetoresistant Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (1 1 0) substrates

Nd0.45Sr0.55MnO3 is an A-type antiferromagnetic manganite showing obvious angular-dependent magnetoresistance, which can be tuned by misfit strain. The misfit strain relaxation of Nd0.45Sr0.55MnO3 thin films is of both fundamental and technical importance. In this paper, microstructures of epitaxial Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (110) substrates by pulsed laser deposition were investigated by means of (scanning) transmission electron microscopy. The Nd0.45Sr0.55MnO3 thin films exhibit a two-layered structure: a continuous perovskite layer epitaxial grown on the substrate followed by epitaxially grown columnar nanostructures. An approximately periodic array of misfit dislocations is found along the interface with line directions of both〈111〉and [001]. High-resolution (scanning) transmission electron microscopy reveals that all the misfit dislocations possess a〈110〉-type Burgers vectors. A formation mechanism based on gliding or climbing of the dislocations is proposed to elucidate this novel misfit dislocation configuration. These misfit dislocations have complex effects on the strain relaxation and microstructure of the films, and thus their influence needs further consideration for heteroepitaxial perovskite thin film systems, especially for films grown on substrates with low-symmetry surfaces such as SrTiO3 (110) and (111), which are attracting attention for their potentially new functions.

Strain relief and growth optimization of GaSb on GaP by molecular beam epitaxy

In this paper, the impact of growth parameters on the strain relaxation of highly lattice mismatched (11.8%) GaSb grown on GaP substrate by molecular beam epitaxy has been investigated. The surface morphology, misfit dislocation and strain relaxation of the GaSb islands are shown to be highly related to the initial surface treatment, growth rate and temperature. More specifically, Sb-rich surface treatment is shown to promote the formation of Lomer misfit dislocations. Analysis of the misfit dislocation and strain relaxation as functions of the growth temperature and rate led to an optimal growth window for a high quality GaSb epitaxial layer on (001) GaP. With this demonstrated optimized growth, a high mobility (25 500 cm2 V −1 s−1 at room temperature) AlSb/InAs heterostructure on a semi-insulating (001) GaP substrate has been achieved.

Evolution of self-assembled type-II ZnTe/ZnSe nanostructures: Structural and electronic properties

The strain-mediated evolution of epitaxial ZnTe/ZnSe quantum structures is studied at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy, coupled with electronic properties characterized by photoluminescence spectroscopy. The growth development of these buried quantum dots clearly demonstrates a homogeneous profile with similar pyramidal geometry rather than bi-modal distribution; contradicting prior reports on ZnTe/ZnSe quantum dots. The result is consistent with atomistic theoretical calculations on strain distribution and electronic structure of a modeled quantum dot of similar geometry using a valence force field model. It is also found that the transition from 2-D islands to 3-D quantum dots involves thermally activated carrier transfer process and follows up with formation of extended defects at the quantum dot surface, acting as an effective source for remnant misfit strain relaxation. The new physical understanding concerning the growth of self-assembled ZnTe/ZnSe quantum dots embedded in the active regions provides important information for the measures to control the properties of buried ZnTe quantum dots, setting up a key footstep in developing novel materials of energy conversion.

Thermal Stability and Surface Passivation of Ge Nanowires Coated by Epitaxial SiGe Shells

Epitaxial growth of a highly strained, coherent SiGe alloy shell around a Ge nanowire core is investigated as a method to achieve surface passivation and carrier confinement, important in realizing nanowire devices. The high photoluminescence intensity observed from the core-shell nanowires with spectral features similar to that of bulk Ge indicates effective surface passivation. Thermal stability of these core-shell heterostructures has been systematically investigated, with a method demonstrated to avoid misfit strain relaxation during postgrowth annealing.