Influence Of Misfit Dislocations Research Articles

Influence of misfit dislocations on ionic conductivity at oxide interfaces

Mismatched complex oxide thin films and heterostructures have gained significant traction for use as electrolytes in intermediate temperature solid oxide fuel cells, wherein interfaces exhibit variation in ionic conductivity as...

Hydrogen redistribution during hydride precipitation in Zr-2.5Nb pressure tubes

Hydrided furnace-cooled and water-quenched samples of Zr-2.5Nb pressure tube were studied in order to aid comprehension as regards the hydrogen redistribution that occurs during hydride precipitation in this material. Results showed a high fraction of γ-hydrides grown at hoop-grains in the quenched condition, resulting from the strong reduction in diffusion times imposed by quenching. This also lead to variations in δ-hydride distribution, δ-hydride populations and hydrogen solubility (TSS) during subsequent thermal cycling, for which an interpretation based on the memory effect of hydride precipitation was proposed. Experimental observations suggest that hydrogen redistribution during cool-down from high temperature is controlled not only by certain preferential grains, but also by the presence of misfit dislocations left by pre-existing hydride particles. A hypothesis was presented to explain the differences in precipitation solubility (TSSP) between furnace-cooled and water-quenched material, which was associated to the possible influence of misfit dislocations on the driving force for hydrogen diffusion.

Influence of misfit dislocations on the magnetoresistance of MgO-based epitaxial magnetic tunnel junctions

Influence of misfit dislocations on the magnetoresistance of MgO-based epitaxial magnetic tunnel junctions

Ferroelectric thin films phase diagrams with self-polarized phase and electret state

We calculated the three components of polarization in phenomenological theory framework by consideration of three Euler-Lagrange equations allowing for mismatch effect and influence of misfit dislocations, surface piezoelectric effect caused by broken symmetry on the film surface, surface tension, and depolarization field. The equations were solved with the help of variational method proposed earlier [M. D. Glinchuk et al., Physica B 332, 356 (2002)]. This approach leads to the free energy in the form of algebraic expression for different powers of polarization components with the coefficients dependent on film thickness, misfit strain, temperature, etc. The odd powers of polarization related to built-in electric field normal to the surface originated from misfit strain and piezoelectricity in the vicinity of the surface which appeared in the free energy expansion. The obtained free energy opens the way for the calculation of the ferroelectric film properties by conventional procedure of minimization. As an example we calculated phase diagrams of PbZr0.5Ti0.5O3 [PZT(50∕50)] compressive- or tensile-strained films. The temperature behavior of pyroelectric coefficient and dielectric permittivity lead to the forecast of the electretlike polar state, i.e., existence of pyroelectricity below the critical thickness of ferroelectric-paraelectric phase transitions. The mismatch-induced electric field was shown to be the physical nature of the self-polarization phenomenon in ferroelectric thin films.

Influence of misfit dislocations on the electrical properties of CdTe layers grown by molecular beam epitaxy on InSb

The electrical properties of n-CdTe films grown by molecular beam epitaxy on (001) InSb substrates have been studied with a view to examining the influence of dislocations arising from strain relaxation in these materials. It is shown that the observed nonuniformity in free carrier density as a function of depth through the films can be explained by electron trapping at dislocations assuming that the density of dislocations diminishes with distance from the substrate surface in proportion to the spatially distributed strain within the films. From a comparison of the experimentally deduced and theoretically derived values for the maximum dislocation density, it is concluded that CdTe layers grown using a stoichiometric Cd/Te flux are characterized by an interfacial layer, probably In2Te3, consistent with independent evidence from x-ray diffraction studies.

Abrupt changes and hysteretic behavior of90°domains in epitaxial ferroelectric thin films with misfit dislocations

The influence of misfit dislocations on the statics and dynamics of 90° domains (twins) in epitaxial thin films is studied theoretically. The misfit dislocations create additional potential barriers, which hinder the movements of domain walls along the film/substrate interface. For ferroelastic 90° walls, these barriers are considerable because an elastic interaction exists between 90° domains and misfit dislocations owing to the coupling of the dislocation stresses to the spontaneous strains generated at the proper or improper ferroelastic phase transition. The energy of elastic interaction is evaluated for a single 90° domain embedded into a tetragonal film containing a periodic array of edge misfit dislocations. On this basis. the equilibrium width of a single embedded 90° domain is calculated as a function of the misfit strain in the film/substrate systems. It is found that, at some threshold values of the misfit strain, the gradual variation of the domain width is interrupted by steplike changes. The evolution of the 90° domain pattern during the polarization switching in ferroelectric films with misfit dislocations is analyzed in detail. The calculations show that the electric-field-induced growth of an embedded 90° domain involves three different stages. Remarkably, at some critical value of the applied electric field, one of the 90° domain walls jumps over the neighboring misfit dislocation, which is accompanied by abrupt changes of the domain width and position in the film. The steplike movements of 90° domain walls over the misfit dislocations are proposed as a possible microscopic mechanism of the Barkhausen effect occurring in ferroelectric thin films during the polarization reversal. The hysteretic behavior of 90° domains at the cyclic variation of an applied electric field is also demonstrated.

Effect of interface defect formation on carrier diffusion and luminescence in In0.2Ga0.8As/AlxGa1−xAs quantum wells

We have examined the influence of strain relaxation on the excitonic recombination and diffusion in In0.2Ga0.8As/AlxGa1−xAs quantum-well (QW) samples designed for high-electron-mobility transistors, using spectrally and spatially resolved polarized cathodoluminescence (CL). Six molecular-beam epitaxial grown samples, with varying channel thicknesses ranging from 75 to 300 Å, were examined at various temperatures between 87 and 300 K. An increase in misfit dislocation density occurred with increasing channel thicknesses and resulted in changes in the dark line defect (DLD) density, polarization anisotropy, QW excitonic luminescence energy, and luminescence activation energy, as observed in CL. The influence of misfit dislocations on the ambipolar diffusion of excess carriers in a direction parallel to the dislocation line, in varying proximity to the DLDs, was examined with a CL-based diffusion experiment. The temperature dependence of the CL imaging was examined, enabling a study of the spatial variation of the activation energies associated with thermal quenching of the GaAs/Al0.25Ga0.75As multiple QW and In0.2Ga0.8As QW luminescence. The CL intensity exhibits an Arrenhius-type dependence on temperature and is controlled by thermally activated nonradiative recombination. The activation energies for both the In0.2Ga0.8As QW and Al0.25Ga0.75As MQW luminescence are found to vary spatially in close proximity to the misfit dislocations. We have utilized a new approach to obtain 2D images of the activation energies. The influence of the strain relaxation on the polarization and energy of the In0.2Ga0.8As QW excitonic luminescence was examined with linearly polarized CL and CL wavelength imaging. A strain-induced modification of the luminescence energy and an increase in the polarization anisotropy was measured near DLDs. Thus, we find that certain DLDs exhibit significant polarization and energy variations in their optical properties, in addition to their more familiar nonradiative behavior.

Influence of misfit dislocations on thermal quenching of luminescence in InxGa1−xAs/GaAs multiple quantum wells

The temperature dependence of the cathodoluminescence (CL) originating from In0.21Ga0.79As/GaAs multiple quantum wells has been studied between 86 and 250 K. The CL intensity exhibits an Arrenhius-type dependence on temperature (T), characterized by two different activation energies. The influence of misfit dislocations and point defects associated with strain relaxation on the thermal quenching of luminescence has been investigated, and the spatial variation in the activation energies has been examined. The CL intensity dependence on temperature for T≲150 K is controlled by thermally activated nonradiative recombination. For T≳150 K the decrease in CL intensity is largely influenced by thermal re-emission of carriers out of the quantum wells.

Influence of misfit dislocations on the surface morphology of Si1−xGex films

The influence of misfit dislocations on the surface morphology of partially strain relaxed Si1−xGex films is studied by atomic force microscopy and transmission electron microscopy. Surface steps arising from the formation of single and multiple 60° dislocations are identified. The role of such steps in the development of a cross-hatch pattern in surface morphology is discussed.

GaAs/Ga0.47In0.53As lattice-mismatched Schottky barrier gates: Influence of misfit dislocations on reverse leakage currents

Epitaxial GaAs has been grown on Ga0.47In0.53As by molecular beam epitaxy to form a lattice-mismatched Schottky barrier gate for field-effect transistor applications. The study of cross-sectional transmission electron microscopy shows that the misfit dislocations accommodating the 3.7% lattice mismatch between these two materials are confined in a GaAs interfacial layer of 300–400-Å thickness. As a result of significant reduction in dislocation densities, Schottky barrier gates made on a sample having a 960-Å-thick GaAs layer exhibit reverse leakage currents two orders of magnitude lower than those made on a sample with a 580-Å-thick GaAs layer. This approach looks promising for fabricating Ga0.47In0.53As field-effect transistors for optoelectronic integration as well as high-speed logic applications.

Electrical and recombination characteristics of GeGaAs and SixGe1−x-GaAs heterojunctions

The structural, electrical, and recombination properties of GeGaAs and SixGe1−x-GaAs hetero-junctions are investigated experimentally at various temperatures. For comparison these characteristics are measured also on Schottky diodes prepared on the same GaAs substrate. The defining influence of misfit dislocations on the electrical and recombination characteristics of hetero-junctions is shown.

Influence of misfit dislocations on the alignment of epitaxial thin films

Many epitaxial deposits are elastically strained to fit their substrate lattice. This strain persists until a critical deposit thickness is reached. Thereafter, misfit is shared between dislocations and strain. The formation of misfit dislocations after the critical thickness is reached is often accompanied by a deterioration in the alignment of the deposit. The magnitude of this deterioration depends on several factors, among the most important of which is the misfit itself. This paper describes the dependence of misalignment on misfit in several simple epitaxial systems.