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- Journal of materials research
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- National Science Foundation
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In this work, we investigate misfit dislocations in PbTe/PbSe heteroepitaxial systems using the concurrent atomistic–continuum (CAC) method. A potential model containing the long-range Coulombic interaction and short-range Buckingham potential is developed for the system. By considering the minimum potential energy of relaxed interface structures for various initial conditions and PbTe layer thicknesses, the equilibrium structure of misfit dislocations and the dislocation spacings in PbTe/PbSe(001) heteroepitaxial thin films are obtained as a function of the PbTe layer thicknesses grown on a PbSe substrate. The critical layer thickness above which misfit dislocations inevitably form, the structure of the misfit dislocations at the interfaces, and the dependence of average dislocation spacing on PbTe layer thickness are obtained and discussed. The simulation results provide an explanation for the narrowing of the spread of the distribution of misfit dislocation spacing as layer thickness increases in PbTe/PbSe(001) heteroepitaxy.
Ti0.5Mg0.5N has recently been predicted to be a semiconductor with a 1.3 eV band gap and promising properties for thermoelectric and plasmonic devices. As a first step towards experimental validation, epitaxial Ti0.5Mg0.5N(001) layers are deposited on MgO(001) by reactive magnetron co-sputtering from titanium and magnesium targets at 600 °C in pure N2 atmospheres. X-ray diffraction ω-2θ scans, ω-rocking curves, φ-scans, and high resolution reciprocal space maps show that Ti0.5Mg0.5N alloys form a pseudobinary rocksalt structure and are single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)TiMgN║(001)MgO and TiMgN║MgO. A 275-nm-thick Ti0.5Mg0.5N layer is fully relaxed and exhibits a 002 ω-rocking curve width ω = 0.73°, while a 36-nm-thick layer is fully strained and has a ω = 0.49°. These results indicate a thickness-dependent strain state which suggests a critical thickness for misfit dislocation nucleation and glide which is between 36 and 275 nm. A measured negative temperature coefficient of resistivity in combination with a low optical absorption coefficient of 0.25 × 105 cm 1 for λ = 740 nm, and a vanishing density of states at the Fermi level measured by x-ray photoelectron spectroscopy support the prediction that Ti0.5Mg0.5N is a semiconductor.
Abstract We focus on the existence and rigidity problems of the vectorial Peierls–Nabarro (PN) model for dislocations. Under the assumption that the misfit potential on the slip plane only depends on the shear displacement along the Burgers vector, a reduced non-local scalar Ginzburg–Landau equation with an anisotropic positive (if Poisson ratio belongs to (−1/2, 1/3)) singular kernel is derived on the slip plane. We first prove that minimizers of the PN energy for this reduced scalar problem exist. Starting from H 1/2 regularity, we prove that these minimizers are smooth 1D profiles only depending on the shear direction, monotonically and uniformly converge to two stable states at far fields in the direction of the Burgers vector. Then a De Giorgi-type conjecture of single-variable symmetry for both minimizers and layer solutions is established. As a direct corollary, minimizers and layer solutions are unique up to translations. The proof of this De Giorgi-type conjecture relies on a delicate spectral analysis which is especially powerful for nonlocal pseudo-differential operators with strong maximal principle. All these results hold in any dimension since we work on the domain periodic in the transverse directions of the slip plane. The physical interpretation of this rigidity result ismore »
Influence of strain and dislocations on GaSb/GaAs quantum dots: From nested to staggered band alignment
We investigate the influence of strain and dislocations on band alignment in GaSb/GaAs quantum dot systems. Composition profiles from cross-sectional scanning tunneling microscopy images are interpolated onto a finite element mesh in order to calculate the distribution of local elastic strain, which is converted to a spatially varying band alignment using deformation potential theory. Our calculations predict that dislocation-induced strain relaxation and charging lead to significant local variations in band alignment. Furthermore, misfit strain induces a transition from a nested (type I) to a staggered (type II) band alignment. Although dislocation-induced strain relaxation prevents the type I to type II transition, electrostatic charging at dislocations induces the staggered band alignment once again.
The mechanical properties of core–shell bimetallic composite nanowires, forming the bases of nanoporous metallic foams, have been investigated and compared with pure metal nanowires using molecular dynamics simulations. In the current study, pure copper and gold nanowires under uniaxial loading were tested at room temperature and compared to composite nanowires of the same materials (core) with a nickel coating (shell). The core radius ranged from 1 to 15 nm, and the shell thickness ranged from 0.1 to 5 nm. The tension strain was performed along the  direction under room temperature. Both coherent and semi-coherent composite nanowires were studied, and the effect of coating layer thickness was investigated. The strengthening mechanisms of the core–shell structures due to the presence of the two different types of interfaces were investigated for various nickel thicknesses. The atomistic simulation results revealed that the addition of the nickel shell strengthens the structure when the layer thickness exceeds a critical value.