Negative Lattice Mismatch Research Articles

Behavior of Microstrain in Nd3+-Sensitized Near-Infrared Upconverting Core-Shell Nanocrystals for Defect-Induced Tailoring of Luminescence Intensity.

In an attempt to optimize the upconversion luminescence (UCL) output of a Nd3+-sensitized near-infrared (808 nm) upconverting core-shell (CS) nanocrystal through deliberate incorporation of lattice defects, a comprehensive analysis of microstrain both at the CS interface and within the core layer was performed using integral breadth calculation of high-energy synchrotron X-ray (λ = 0.568551 Å) diffraction. An atomic level interpretation of such microstrain was performed using pair distribution function analysis of the high-energy total scattering. The core NC developed compressive microstrain, which gradually transformed into tensile microstrain with the growth of the epitaxial shell. Such a reversal was rationalized in terms of a consistent negative lattice mismatch. Upon introduction of lattice defects into the CS systems upon incorporation of Li+, the corresponding UCL intensity was maximized at some specific Li+ incorporation, where the tensile microstrain of CS, compressive microstrain of the core, and atomic level disorders exhibited their respective extreme values irrespective of the activator ions.

Spontaneous pattern of orthogonal ferroelectric domains in epitaxial KNN films

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.

Determination of lateral strain in InGaAsSb alloys and its effect on structural and optical properties

In_xGa_{1-x}As_ySb_{1-y} epilayers with a fixed In content of textit{x} = 0.145 were grown on GaSb(100) substrates using liquid-phase epitaxy (LPE). The lattice mismatch between the quaternary epilayers and substrates was analyzed for different As contents (textit{y}) by X-ray omega-2theta. Epilayers with As content between textit{y} = 0.120 and textit{y} = 0.124 exhibited a positive lattice mismatch, leading to compressive strain. These samples showed a high crystalline quality and flat surfaces, as confirmed by high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). Quaternary alloys with As content between textit{y} = 0.133 and textit{y} = 0.141 displayed a negative lattice mismatch, resulting in tensile strain. Structural defects in these samples were evidenced by HR-XRD and on AFM micrographs. Raman measurements also revealed that lateral strain has a direct impact on the intensities of the LO-like, phonon–plasmon and disorder-activated longitudinal acoustic modes. For all In_xGa_{1-x}As_ySb_{1-y} films, photoluminescence (PL) spectra showed a bound exciton (BE) transition, with additional features observed in samples under tensile strain, indicating the involvement of native defect centers and donor–acceptor pairs. This study provides new insights into the effect of lateral strain on the crystalline and surface quality, and optical properties of quaternary alloys, relevant for novel optoelectronic applications.

Influence of rafted microstructures on creep in Ni-base single crystal superalloys: a 3D discrete dislocation dynamics study

Ni-base single-crystal superalloys exhibit a dynamic evolution of their microstructure during operation at elevated temperatures. The rafting of γ′ precipitates changes the mechanical behavior in a way that was understood insufficiently. In this work, we combine a phase-field method with a discrete dislocation dynamics model to clarify the influence of different rafted microstructures with the same initial dislocation density and configuration on creep behavior. The unrafted and rafted microstructures of Ni-base single crystal superalloys are simulated by a phase-field crystal plasticity method. By introducing these microstructures into a 3D discrete dislocation dynamics (DDD) model, the creep behavior under uniaxial loads of 350 and 250 MPa along [100] direction at 950 °C is studied. Due to the negative lattice mismatch of Ni-base superalloys, the N-type rafting with the formation of plate-like γ′ precipitates occurs under uniaxial tensile loads along {100} direction at high temperatures, while the P-type rafting with the formation of rod-like γ′ precipitates occurs under compressive loads. Taking the cuboidal, N-type rafted and P-type rafted microstructures as the initial and fixed microstructures for the same loading conditions, it is found from DDD simulations that the rafted microstructures result in smaller creep deformation than the cuboidal microstructure. The reason for this is that the coalescence of γ′ precipitates during the rafting diminishes the width of some γ channels, so as to increase the local Orowan stresses which retard the dislocation glide. For tensile loads, the N-type rafted microstructure has the best creep resistance. For a low compressive load, the P-type rafting shows a better creep resistance than N-type rafting.

Property of YBCO Films Doping with Positive and Negative Lattice Mismatch Nanoparticles

Property of YBCO Films Doping with Positive and Negative Lattice Mismatch Nanoparticles

Influence of Stress and Strain on <inline-formula> <tex-math notation="LaTeX">$L 1_{0}$ </tex-math> </inline-formula>-Ordered Phase Formation in FePt Thin Film

Effects of stress and strain in FePt thin film on $L 1_{0}$ ordering are investigated by employing single-crystal substrate of which lattice mismatch with FePt crystal is varied in a range from −10.3% to +0.8% for 10 nm thick FePt thin film. Thin films are prepared under different conditions; direct deposition at 600 °C, two-step process consisting of 200 °C deposition followed by 600 °C annealing, and the two-step process with MgO cap layer formed on top of the FePt layer at 200 °C. Formation of $L 1_{0}$ -FePt(001) crystal and $L 1_{0}$ ordering is enhanced on a substrate with larger negative lattice mismatch, though the lattice mismatch with substrate is decreased greatly by introduction of misfit dislocation and by lattice bending in FePt material. The lateral in-plane strain in FePt material associated with several factors: substrate material, film morphology, and cap layer has given a strong influence in $L 1_{0}$ -ordered phase formation in FePt thin film.

Lattice Mismatch in Crystalline Nanoparticle Thin Films.

For atomic thin films, lattice mismatch during heteroepitaxy leads to an accumulation of strain energy, generally causing the films to irreversibly deform and generate defects. In contrast, more elastically malleable building blocks should be better able to accommodate this mismatch and the resulting strain. Herein, that hypothesis is tested by utilizing DNA-modified nanoparticles as "soft," programmable atom equivalents to grow a heteroepitaxial colloidal thin film. Calculations of interaction potentials, small-angle X-ray scattering data, and electron microscopy images show that the oligomer corona surrounding a particle core can deform and rearrange to store elastic strain up to ±7.7% lattice mismatch, substantially exceeding the ±1% mismatch tolerated by atomic thin films. Importantly, these DNA-coated particles dissipate strain both elastically through a gradual and coherent relaxation/broadening of the mismatched lattice parameter and plastically (irreversibly) through the formation of dislocations or vacancies. These data also suggest that the DNA cannot be extended as readily as compressed, and thus the thin films exhibit distinctly different relaxation behavior in the positive and negative lattice mismatch regimes. These observations provide a more general understanding of how utilizing rigid building blocks coated with soft compressible polymeric materials can be used to control nano- and microstructure.

The behavior of commensurate-incommensurate transitions using the phase field crystal model

We study the behavior of the commensurate-incommensurate (CI) transitions by using a phase field crystal model. The model is capable of modeling both elastic and plastic deformation and can simulate the evolution of the microstructure of the material at the atomic scale and the diffusive time scale, such as for adsorbed monolayer. Specifically, we study the behavior of the CI transitions as a function of lattice mismatch and the amplitude of substrate pinning potential. The behavior of CI phase transitions is revealed with the increase of the amplitude of pinning potential in some certain lattice mismatches. We find that for the negative lattice mismatch absorbed monolayer undergoes division, reorganization and displacement as increasing the amplitude of substrate pinning potential. In addition, for the positive mismatch absorbed monolayer undergoes a progress of phase transformation after a complete grain is split. Our results accord with simulations for atomic models of absorbed monolayer on a substrate surface.

Converse magnetoelectric effect in NiFe2O4/BaTiO3 heterostructure by electric field induced inter-ferroelectric phase transition

Converse magnetoelectric effect was experimentally demonstrated in NiFe2O4/BaTiO3 (NFO/BT) heterostructure. Electric field induced inter-ferroelectric phase transition in BT substrate and its impact on magnetization of NFO was studied for the first time. Presence of meta-stable orthorhombic phase (Amm2) along with tetragonal (P4mm) BT was revealed from Rietveld refinement in unpoled state itself. Negative lattice mismatch between NFO film and BT substrate induces compressive stress along in-plane direction of NFO and reduces magnetoelastic anisotropy energy. Hence, large magnetization was obtained along in-plane direction compared to that of out of plane. Poling stabilizes orthorhombic phase that enhances compressive stress further along in-plane direction. Consequently, increase and decrease of magnetization was observed along in and out-of plane respectively. Observations from the present work revealed that magnetization in such heterostructures can be controlled via strain-mediated coupling by voltage which provides an opportunity in spintronic devices for low power consumption.

Preparation of SrCoOx thin films on LaAlO3 substrate and their reversible redox process at moderate temperatures

Using magnetron sputtering and annealing techniques, we have prepared SrCoOx films on LaAlO3 and SrTiO3 substrates. Distinctly different structures of the films have been found on the two substrates. It is suggested that positive lattice mismatch between film and substrate promotes SrCoO2.5 films with an orthorhombic structure grown on SrTiO3 substrate, whereas negative lattice mismatch from LaAlO3 substrate is in favor of increasing the valence state of Co and thus the growth of oxygen-rich SrCoOx with a tetragonal structure. In addition to the structural characterization, magnetic and electrical measurements confirm that the oxygen content x is between 2.75 and 2.88 for the latter. Reversibility of the topotactic phase transformation between SrCoO2.5 and the oxygen-rich SrCoOx films has also been studied by changing the oxygen pressure during annealing process. Even in the presence of a negative lattice mismatch, the results reveal that the tetragonal SrCoOx films on LaAlO3 substrate retain high oxygen mobility identified before in cubic SrCoOx films.

Influence of misfit stresses on dislocation glide in single crystal superalloys: A three-dimensional discrete dislocation dynamics study

In the characteristic γ/γ′ microstructure of single crystal superalloys, misfit stresses occur due to a significant lattice mismatch of those two phases. The magnitude of this lattice mismatch depends on the chemical composition of both phases as well as on temperature. Furthermore, the lattice mismatch of γ and γ′ phases can be either positive or negative in sign. The internal stresses caused by such lattice mismatch play a decisive role for the micromechanical processes that lead to the observed macroscopic athermal deformation behavior of these high-temperature alloys. Three-dimensional discrete dislocation dynamics (DDD) simulations are applied to investigate dislocation glide in γ matrix channels and shearing of γ′ precipitates by superdislocations under externally applied uniaxial stresses, by fully taking into account internal misfit stresses. Misfit stress fields are calculated by the fast Fourier transformation (FFT) method and hybridized with DDD simulations. For external loading along the crystallographic [001] direction of the single crystal, it was found that the different internal stress states for negative and positive lattice mismatch result in non-uniform dislocation movement and different dislocation patterns in horizontal and vertical γ matrix channels. Furthermore, positive lattice mismatch produces a lower deformation rate than negative lattice mismatch under the same tensile loading, but for an increasing magnitude of lattice mismatch, the deformation resistance always diminishes. Hence, the best deformation performance is expected to result from alloys with either small positive, or even better, vanishing lattice mismatch between γ and γ′ phase.

Interface layer control and optimization of InAs/GaSb type-II superlattices grown by molecular beam epitaxy

We reported in the paper the optimization and control of InSb interface properties during molecular beam epitaxy growth of InAs/GaSb superlattice structures. Samples with identical structure but different growth approaches, conventional molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE), for interface layers were first prepared and their structural, morphological and optical properties were compared. The MEE samples had significant higher As composition in InSb interface layers and higher luminescence efficiency. Samples with different InSb interface layer thickness were then prepared. By changing the interface layer thickness, one can effectively tune the lattice mismatch and photoluminescence peak wavelength. Though X-ray diffraction satellite peak linewidth and surface roughness of the grown samples changed little, the one with smallest negative lattice mismatch showed the highest luminescence efficiency. Finally a P-I-N superlattice detector structure was grown with controlled interfaces. The full width at half maximum (FWHM) of the 1st-order X-ray diffraction satellite peak of the absorption layers was only 19″. The detector structure showed a cutoff wavelength of 6.3µm at 77K. The dark current density at −50mV bias was 4.3×10−5A/cm2 and the peak detectivity was 4.2×1011cmHz1/2/W.

Microstructure-sensitive creep models for nickel-base superalloy single crystals

Microstructure-sensitive creep models have been developed for Ni-base superalloy single crystals. Creep rupture testing was conducted on fourteen single crystal alloys at two applied stress levels at each of two temperatures, 982 and 1093°C. The variation in creep lives among the different alloys could be explained with regression models containing relatively few microstructural parameters. At 982°C, γ−γ′ lattice mismatch, γ′ volume fraction, and initial γ′ size were statistically significant in explaining the creep rupture lives. At 1093°C, only lattice mismatch and γ′ volume fraction were significant. These models could explain from 84% to 94% of the variation in creep lives, depending on test condition. Longer creep lives were associated with alloys having more negative lattice mismatch, lower γ′ volume fractions, and finer γ′ sizes. The γ−γ′ lattice mismatch exhibited the strongest influence of all the microstructural parameters at both temperatures. Although a majority of the alloys in this study were stable with respect to topologically close packed (TCP) phases, it appeared that up to ∼2vol% TCP phase did not affect the 1093°C creep lives under applied stresses that produced lives of ∼200–300h. In contrast, TCP phase contents of ∼2vol% were detrimental at lower applied stresses where creep lives were longer.

Effect of silicon on relaxation of the crystal lattice in MOCVD–hydride Al x Ga1 − x As:Si/GaAs(100) heterostructures

The X-ray diffraction and infrared spectroscopy data for MOCVD-hydride AlxGa1 − xAs:Si/GaAs(100) heterostructures and homoepitaxial GaAs:Si/GaAs(100) structures doped with Si to a content of up to ∼1 at % are reported. It is shown that, in the homoepitaxial heterostructures, the formation of alloys with Si yields a decrease in the crystal lattice parameters of the epitaxial layer and a negative lattice mismatch with the single-crystal substrate (Δa < 0). At the same time, the formation of quaternary alloys in the AlxGa1 − xAs:Si/GaAs(100) heterostructures is not accompanied by any pronounced strains in the crystal lattice. By introducing Si into the epitaxial layers of these heterostructures, it is possible to attain complete matching of crystal lattice parameters of the film and substrate in the appropriately chosen technological conditions of growth of the epitaxial layers.

Monoclinic double tungstate lattice matched epitaxial layers for integrated optics applications

In this work we report on the activation of KY 1−x−yGdxLuy(WO4)2 lattice matched epitaxial layers with Er3+ and Tm3+ ions. The epilayers posses low enough lattice mismatch as well as high enough refractive index contrast with respect to the KY (WO4)2 substrates, which are key parameters for the fabrication of high quality optical waveguides. A structural study revealed that there are two types of lattice matched KY 1−x−yGdxLuy(WO4)2 compositions, one with a negative lattice mismatch, - 0.53×10−3, corresponding to KY 0.58Gd0.22Lu0.20(WO4)2, and the second one with a positive lattice mismatch, 0.55×10−3, which corresponds to KY 0.75Gd0.18Lu0.07(WO4)2. However, the refractive indices corresponding to the first one were greater than those of the second composition. We then chose the composition with a negative lattice mismatch for doping with Er3+ and Tm3+ active ions, both with percentages of 1% at. and 3% at. Epitaxial growth of the active lattice matched layers on b oriented KY (WO4)2 substrates was obtained without loosing structural quality compared with the passive compositions. Waveguiding demonstration was observed by recording the dark mode spectra at different wavelengths.

Parasitic Interlayer at the GaAs-on-InGaP Interface in MOVPE InGaP ∕ GaAs : A Study by the Chemically Sensitive (200) Diffraction

The chemically sensitive (200) diffraction in the dark field (DF) mode of transmission electron microscopy has been used to detect, identify, and evaluate the composition of the parasitic interlayer at the GaAs-on-InGaP interface in metallorganic vapor-phase epitaxy (MOVPE) heterostructures. The latter were grown at with no growth interruption. The composition range determined by (200) DF was further refined by using the X-ray diffraction result that the interlayer has a negative lattice mismatch to GaAs. The parasitic interlayer can be either or with and . intermixing and In segregation are assumed to drive the formation of the interlayer. The low In content in the quaternary is ascribed to the reduced In segregation at . In segregation is likely favored by the tensile strain associated with the interlayer in its process of growth.

Neutron diffraction study of stress and lattice mismatch induced by thermo-mechanical fatigue in single crystal superalloys

The center points of the thermomechanical fatigued single crystal Nibased superalloys were measured by neutron diffraction and the results have been treated in two methods: two overlapped peaks and a single peak. The macroscopic equivalent stress, deviatoric stress components, γ/γ′ phase equivalent stress and lattice mismatch are determined according to the experimental results. The stress results show that the macroscopic equivalent stresses are basically consistent when using two analysis methods, the maximum of macroscopic equivalent stress and deviatoric stress components appeared at thermomechanical fatigue that of 100 cycles, and at this point, the microdefects， such as dislocations, are saturated and the stress of γ′ phase away from the center was reduced remarkably. The calculated results also indicate that the negative lattice mismatch is reduce markedly at the beginning of the thermomechanical fatigue. Subsequently, it increases linearly with the fatigue cycles at the speed 81×10-6 per cycle because of the accumulated plastic strain in the matrix.

On the creep behavior at 1033K of new generation single-crystal superalloys

The various parameters influencing the creep behavior at 1033 K of the MC544 and MC534 new generation single crystal nickel-based superalloys were analyzed in comparison with that of the first generation AM1 superalloy. A clear relationship was evidenced between the main deformation process and the amplitude of primary creep stage. A large amplitude of primary creep is always associated with heterogeneous deformation resulting from the low mobility of the a/2〈1 1 0〉 perfect matrix dislocation and shearing of γ′ precipitates by 〈1 1 2〉{1 1 1} slip. On the contrary, limited amplitude of primary creep is systematically associated with homogeneous deformation within the γ matrix channels by 〈1 1 0〉{1 1 1} slip. The differences of γ/γ′ microstructure, stacking fault energy of the γ phase, antiphase boundary energy, lattice mismatch amplitude and solid solution strengthening have been taken into account to explain the transition between these two deformation modes. The comparison between the respective solid solution strengthening effects in the γ and γ′ phases of the AM1 and MC544 alloys allows to explain their difference in creep behavior. On the other hand, the beneficial role of a high negative lattice mismatch regarding the creep behavior at 1033 K was clearly evidenced in the MC534 alloy.

On the creep behavior at 1033K of new generation single-crystal superalloys

The various parameters influencing the creep behavior at 1033 K of the MC544 and MC534 new generation single crystal nickel-based superalloys were analyzed in comparison with that of the first generation AM1 superalloy. A clear relationship was evidenced between the main deformation process and the amplitude of primary creep stage. A large amplitude of primary creep is always associated with heterogeneous deformation resulting from the low mobility of the a/2〈1 1 0〉 perfect matrix dislocation and shearing of γ′ precipitates by 〈1 1 2〉{1 1 1} slip. On the contrary, limited amplitude of primary creep is systematically associated with homogeneous deformation within the γ matrix channels by 〈1 1 0〉{1 1 1} slip. The differences of γ/γ′ microstructure, stacking fault energy of the γ phase, antiphase boundary energy, lattice mismatch amplitude and solid solution strengthening have been taken into account to explain the transition between these two deformation modes. The comparison between the respective solid solution strengthening effects in the γ and γ′ phases of the AM1 and MC544 alloys allows to explain their difference in creep behavior. On the other hand, the beneficial role of a high negative lattice mismatch regarding the creep behavior at 1033 K was clearly evidenced in the MC534 alloy.

Growth of heavily Zn-doped InGaAs at low temperature by LP-MOCVD

Epitaxial growth of Zn-doped InGaAs on InP substrates has been carried out at 550°C by LP-MOCVD. Hole concentration as high as 6×10 19 cm −3 has been achieved at the H 2 flow rate of 20 sccm through DEZn bubbler. The lattice constant of Zn-doped InGaAs was found to be dependent on the flow rate of DEZn, and the tensile strain mismatch increases with increasing H 2 flow rate of DEZn. The negative lattice mismatch of heavily Zn-dopped InGaAs may be due to the small covalent bonding radius of zinc and the combination of butane from ethyl of DEZn and TEGa. And the latter accelerates the pyrolysis of TEGa, which is the dominant mechanism in determining the negative mismatch of Zn-doped InGaAs.