Influence Of Lattice Misfit Research Articles

The influence of lattice misfit on screw and edge dislocation-controlled solid solution strengthening in Mo-Ti alloys

Mo-Ti alloys form solid solutions over a wide range of compositions, with lattice misfit parameters increasing significantly with titanium content. This indicates a strong increase in the critical stress for edge dislocation motion. Here, we probe the transition from screw to edge dislocation-dominated strengthening in Mo-Ti solid solutions with titanium content up to 80 at%. The alloys were scale-bridging characterized to isolate the impact of substitutional solid solution strengthening. Mechanical testing yielded no significant influence of grain boundaries or grain orientation. The results were corrected for the strengthening by unavoidable interstitial oxygen. Modelling of screw and edge dislocation-controlled solid solution strengthening was applied to the results to evaluate the contributions of both dislocation types. The analysis reveals that screw dislocation motion controls the strength in allows with less than 40 at% titanium, while edge dislocation motion provides comparable strength for 60–80 at% titanium. These results in a system of reduced chemical complexity support the recent investigations of edge dislocation-controlled strengthening found in high-entropy alloys.

Influence of lattice misfit on crack formation during the epitaxy of InyAl1-yN on GaN

InyAl1−yN epifilms with alloy compositions (y) ranging from 0.13 to 0.47 were grown on a GaN template using a low-temperature molecular-beam-epitaxy technique. Because of the larger lattice constant of high-indium-content InyAl1−yN epifilm, the compressive stress accumulated during epitaxy. When the compressive stress exceeded the elastic limit of the InyAl1−yN epifilms, a “concave-valley” crack with an equivalent width (~25 nm) was generated to form a prismatic domain. Our calculation and fitting results showed that no cracking occurred when the lattice mismatch was<1% because the stress was within the elastic limit. By contrast, the cracks appeared when the lattice mismatch was>2.4% because the stress exceeded the elastic limit.

Analytical solutions of electroelastic fields in piezoelectric thin-film multilayer: applications to piezoelectric sensors and actuators

Analytical expressions are derived for the stresses and electric fields induced in a piezoelectric multilayer deposited on a substrate with lattice misfit and thermal expansion coefficient mismatch. The piezoelectric multilayer can be subjected to an externally applied force, moment and electric potential. The derived formulations can model any number of layers using recursive relations that minimize the computation time. A proper rotation matrix is utilized to generalize the derived expressions to accommodate various orientations allowing each layer to have hexagonal crystal symmetry. The influence of lattice misfit and thermal expansion coefficient mismatch in the presence of externally applied force, moment and electric potential on the state of the electroelastic fields in each layer is evaluated for various applications. Comparison with finite element analysis results shows excellent agreement. The analytical expressions developed here can be useful in designing electromechanical sensors, actuators and optoelectronic devices made from piezoelectric multilayers.

Strain relaxation in GaN/AlN superlattices on GaN(0001) substrate: Combined superlattice-to-substrate lattice misfit and thickness-dependent effects

The relaxation of built-up strain in the heteroepitaxial GaN/AlN superlattices (SLs) leads to defect-related undesirable changes of the optical and electrical properties of SL-based devices. In the present study, the influence of lattice misfit between the GaN/AlN SL and GaN(0001) substrate on the mechanism of strain relaxation in the SLs is comprehensively analyzed. A strain/thickness-dependent effect on the densities of threading dislocations (TDs) and cracks in the SLs is observed. At the initial stage of growth of the SLs with relatively small lattice misfit, the formation of TDs dominates over the post-grown cracking triggered by the thermal stress during the sample cooling. Cracking during the growth is the main mechanism of strain relaxation for the SLs with large lattice misfit. It is established that for the GaN/AlN SLs with thicknesses >115 ± 15 nm, which corresponds to a SL relaxation degree >40 ± 5%, the density of defects drastically decreases. This result offers an essential guidance for excluding the strong influence of the substrate on generation of a large number of defects in the SLs. The quantitative interpretation and discussions of the presented results are important toward the design of a high-quality heteroepitaxial GaN/AlN SLs.

Heterogeneous nucleation of pure gold on highly smooth ceramic substrates and the influence of lattice misfit and cooling rate

The present paper considers the heterogeneous nucleation of pure Au droplet solidified on a single crystal substrate plate under the influence of lattice misfit and cooling rate. The undercooling of droplet was measured by DSC. The experimental results present that the undercooling of Au droplet is directly proportional to both the lattice misfit and the cooling rate. It means that the heterogeneous nucleation can be significantly driven by lattice misfit and cooling rate. In addition, the related results show that the relationship between the undercooling and the cooling rate of Au droplet is independent on the lattice misfit. The results with respect to the interface of solidified Au and substrate analyzed by HRTEM prove that the orientation relationship between the substrate and the solidified Au matches with the minimum lattice misfit.

Modeling of stresses and electric fields in piezoelectric multilayer: Application to multi quantum wells

Exact closed-form expressions have been derived for the stresses and the electric fields induced in piezoelectric multilayers deposited on a substrate with lattice misfit and thermal expansion coefficient mismatch. The derived formulations can model any number of layers using recursive relations that minimize the computation time. A proper rotation matrix has been utilized to generalize the expressions so that they can be used for various growth orientations with each layer having hexagonal crystal symmetry. As an example, the influence of lattice misfit and thermal expansion coefficient mismatch on the state of electroelastic fields in different layers of GaN multi quantum wells has been examined. A comparison with the finite element analysis results showed very close agreement. The analytical expressions developed herein will be useful in designing optoelectronic devices as well as in predicting defect density in multi quantum wells.

The First-principles Study on the Occupation Behavior and the Ductility Mechanism of Zr in Ni–Ni3Al System with Lattice Misfit

The influence of lattice misfit on the occupation behavior and the ductility effect of Zr in Ni–Ni3Al alloys were explored. It is found in energy analysis that the preferable site of Zr between Ni sublattice and Al sublattice will change under different lattice misfit, however, the Zr prefers to segregate Ni phase rather than Ni3Al phase in all lattice misfit range, which makes it impossible for Zr to go into Ni3Al phase to occupy Al sublattice in Ni–Ni3Al system. Bond order (BO) analysis shows that the localized ductility effect of Zr differs in different region, and the comparison between Zr-free and Zr-doped BO analysis successfully explain the mechanism of the embrittlement of Ni–Ni3Al alloys and the ductility effect of Zr.

The Influence of Lattice Misfit on the Atomic Structures and Defect Energetics of Face Centered Cubic–Body Centered Cubic Interfaces

Using “tunable” interatomic potentials, the lattice misfits for a fcc–bcc metal system have been varied in atomistic models, while keeping other properties essentially unchanged. The procedure and the fitting results of such tunable interatomic potentials for fcc–bcc systems are presented. Varying lattice misfits were found to significantly alter the atomic structure of fcc–bcc interfaces in Kurdjumov–Sachs crystallographic orientation. Defect formation energies at the interfaces were calculated. For vacancies, in general, high numbers of low energy sites are associated with high dislocation junction densities. For interstitials, the formation energies are all substantially below the bulk value, regardless of lattice misfits. These results are relevant to understanding the sink strength of interfaces with different atomic structures.

Influence of lattice misfit on the internal stress and strain states before and after creep investigated in nickel-base superalloys containing rhenium and ruthenium

The microstructure of nickel-base superalloys changes significantly during creep. The strengthening γ′-precipitates coarsen directionally and dislocation networks develop at the interfaces between γ′-precipitates and γ-matrix. Coherency stresses due to the lattice misfit between γ and γ′ phase act as the driving force for these microstructural changes. The measurement of the lattice constant of the γ-matrix and γ′-phase by X-ray diffraction before and after creep experiments allows the changes of internal stress and strain states during creep to be revealed. In this work, three experimental alloys with systematically varied amounts of lattice misfit were studied. It was found that the separation of X-ray peaks of γ and γ′ after creep is directly dependent on the amount of lattice misfit. Also the distortion of the lattice, visible in the broadening of the respective rocking curves, was measured and found to be dependent on the lattice misfit as long as the γ/γ′-interfaces stay coherent. The lattice misfit is directly influenced by the alloy composition. Rhenium and ruthenium decrease the lattice misfit and modify the coherency stresses between γ- and γ′-phase accordingly.

Influence of lattice misfit on diffusion coefficients in thin films polycrystalline Pd–Ag and Pd–Au systems

The dependence of effective interdiffusion coefficients versus the misfit of the parameters of crystalline lattices of film and substrate for double polycrystalline thin films of Pd–Ag and Pd–Au systems has been studied with the use of X-ray analysis. Silver, gold, Pd–Ag and Pd–Au alloys were used as substrates. Palladium was used as a film. The magnitude of the misfit was changed with the use of a Pd concentration change in the substrate. It was established that the decrease of misfit from 4.9 to 0.94% leads to subsidence of the effective diffusion coefficients in Pd film more than several orders of magnitudes.

Titanium thin film growth on small and large misfit substrates

The influence of lattice misfit on the growth of Ti (0001) is investigated in the limit of small negative (−1%) and large positive (+6.8%) misfit by choosing MgO (111) and Al2O3 (0001) as substrate materials. Reflection high energy electron diffraction imaging and intensity measurements during growth reveal two-dimensional nucleation of islands on MgO, in contrast to three-dimensional nucleation on Al2O3. X-ray analysis of 30-nm-thick films on MgO shows a two-component line shape in transverse scans of the (0002) and (0004) reflections, pointing to a high degree of structural coherence in the weak disorder limit. The surface morphology of films grown on MgO depends strongly on the substrate temperature during growth.

Analysis of strain and impurity distribution in II–VI epilayers with optical methods

The detailed knowledge of strain and impurity distribution is important for the production and application of thin epilayers. In this paper, it is shown that the combination of different methods as reflection, excitation and resonant Raman scattering spectroscopy is needed to investigate these problems satisfactorily. Two II–VI epilayer systems are chosen: ZnSe and ZnTe, grown on GaAs by different growth methods (MBE, HWE, MOVPE) and at various substrate temperatures. The influence of lattice-misfit and thermally induced strain in different layer regions is analysed and described in a layer-thickness dependent strain model. In comparison with spectroscopy of the strain-split free excitons, that of the bound excitons and their excitation channels in dependence on the layer thickness and strain magnitude allows statements on the nature of the impurities and their local distribution. While in ZnSe/GaAs the main impurities (donors) lie in the lattice-misfit induced dislocation region, in ZnTe/GaAs three differently distributed acceptor-bound-exciton types are identified bound to atoms originating from source contaminations, diffused atoms stemming from the substrate material, and dislocation-related centers.

Influence of lattice misfit on heterojunction bipolar transistors with lattice-mismatched InGaAs bases

AlGaAs/InGaAs/GaAs heterojunction bipolar transistors (HBTs) with lattice-mismatched InGaAs bases have been studied for determining the effect of misfit defects on device performance. Defect structures were investigated using scanning cathodoluminescence and transmission electron microscopy. Misfit dislocations were found to form at the bottom interface between n-InGaAs and n-GaAs in the collector. Electrical properties of lattice-misfitted n-GaAs/n-InGaAs heterojunctions were studied using p+-GaAs/n-GaAs/n-InGaAs diode structures. Deep-level transient spectroscopy measurements revealed electron traps (Et=0.49–0.55 eV) and hole traps associated with the lattice-misfitted interface. Devices showed high dc current gains in spite of the presence of misfit dislocations. Poorer saturation of collector current was observed with increasing defect density. HBTS with lattice-mismatched InGaAs bases are considered to be relatively insensitive to the introduction of misfit dislocations in terms of dc characteristics.