Georgios I. Sfyris

Theory of surface segregation in ternary semiconductor quantum dots

We present a phenomenological species transport theory for surface segregation of constituent and dopant atoms in ternary semiconductor quantum dots. The theory employs continuum elasticity to describe species-surface interactions and is used to fit atomistic simulation results according to a first-principles-based force-field parameterization. The theory predicts equilibrium concentration profiles in the form of Maxwellian atmospheres near nanocrystal surfaces. The theory is accurate in the dilute limit and it is validated by fitting results of Monte Carlo simulations of compositional relaxation in InxGa1−xAs and ZnSe1−xTex nanocrystals for various representative sizes (diameters≤5.7 nm) and compositions (x≤0.2).

Electromigration-driven surface morphological stabilization of a coherently strained epitaxial thin film on a substrate

We analyze the surface morphological stability of a coherently strained thin film grown epitaxially on a thick elastic substrate under the simultaneous action of an electric field that drives surface electromigration. A model of driven film surface morphological evolution is developed and a linear stability analysis is carried out of the planar film surface morphology. The analysis reveals that surface electromigration can inhibit surface morphological instabilities due to the lattice mismatch between the film and the substrate. The critical electric-field strength for surface stabilization and the optimal electric-field direction are determined and the effects of surface diffusional anisotropy are examined.

Analysis of current-driven surface morphological stabilization of a coherently strained epitaxial thin film on a finite-thickness deformable substrate

We report results for the surface morphological stability of a thin film that lies on a finite-thickness substrate and is subjected simultaneously to an external electric field. The film is grown epitaxially on the substrate and may undergo a Stranski–Krastanow instability due to its lattice mismatch with the substrate material. We develop a model for the surface morphological evolution of the thin film and conduct a linear stability analysis to examine the morphological stability of the epitaxial film’s planar surface state. Our analysis shows that surface electromigration due to a properly applied and sufficiently strong electric field can inhibit Stranski–Krastanow-type instabilities, which can be used to control the onset of island formation on the film surface. We find that using a finite-thickness substrate can have the beneficial effect of reducing the critical strength of the electric field required to stabilize the planar surface morphology of the epitaxial film with respect to the field strength...

Surface morphological stabilization of stressed crystalline solids by simultaneous action of applied electric and thermal fields

We examine the surface morphological stability of electrically and thermally conducting crystalline elastic solids in uniaxial tension under the simultaneous action of an electric field and a temperature gradient. We use linear stability analysis of a surface mass transport model that accounts for surface electromigration and thermomigration induced by the applied fields and for surface diffusional anisotropy. We find that a properly oriented applied thermal gradient can reduce the critical electric-field strength requirement for stabilization of the planar surface morphology. The temperature dependence of the solid material’s properties does not affect the critical strength requirement of the applied fields.

The effect of a thermal gradient on the electromigration-driven surface morphological stabilization of an epitaxial thin film on a compliant substrate

We report a theoretical analysis on the surface morphological stability of a coherently strained thin film that has been grown epitaxially on a deformable substrate and is simultaneously subjected to an external electric field and a temperature gradient. Using well justified approximations, we develop a three-dimensional model for the surface morphological evolution of the thin film and conduct a linear stability analysis of the heteroepitaxial films planar surface state. The effect of the simultaneous action of multiple external fields on the surface diffusional anisotropy tensor is accounted for. Various substrate types are considered, but emphasis is placed on a compliant substrate that has the ability to accommodate elastically some of the misfit strain in the film due to its lattice mismatch with the substrate. We derive the condition for the synergy or competition of the two externally applied fields and determine the optimal alignment of the external fields that minimizes the critical electric fie...

The effect of a compliant substrate on the electromigration-driven surface morphological stabilization of an epitaxial thin film

We analyze the surface morphological stability of a coherently strained thin film that has been grown epitaxially on a compliant substrate of finite thickness and is subjected simultaneously to an external electric field, which drives surface electromigration. The compliant substrate has the ability to accommodate elastically some of the misfit strain that is developed in the epitaxial film due to the lattice mismatch between the film and substrate materials. We develop a three-dimensional model for the surface morphological evolution of the thin film and conduct a linear stability analysis for the morphological stability of the heteroepitaxial film’s planar state; of particular importance for the analysis is the elastostatic boundary-value problem for the heteroepitaxial film/substrate system. The analysis shows that surface electromigration due to a properly applied and sufficiently strong electric field can inhibit Stranski-Krastanow-type instabilities. Furthermore, we determine the critical electric-f...

Electromigration-driven complex dynamics of void surfaces in stressed metallic thin films under a general biaxial mechanical loading

We report results of a systematic computational study of the electromigration-driven complex surface dynamics of voids in mechanically stressed thin films of face-centered cubic metals with 〈100〉-oriented film planes. The films are subjected to an external electric field simultaneously with biaxial mechanical loading, which can be either purely compressive, ranging from purely isotropic to strongly anisotropic including uniaxial, or a mixed type of loading with both tensile and compressive stress components in the applied stress tensor. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological evolution following a well validated, two-dimensional, and fully nonlinear model. We find that depending on the electromechanical conditions, void size, and surface diffusional anisotropy, two types of asymptotic states can be stabilized in the void surface dynamical response, namely, morphologically steady or time-periodic traveling voids, and film failure can be caused by ...

Effect of applied stress tensor anisotropy on the electromechanically driven complex dynamics of void surfaces in metallic thin films

We present a systematic computational analysis of the complex, electromechanically driven surface dynamics of voids in thin films of face-centered cubic metals for 〈100〉-oriented film planes characterized by four-fold symmetry of surface diffusional anisotropy. The voids are located at an edge of the metallic thin film, and the film is subjected simultaneously to an external electric field and an anisotropic biaxial tensile stress. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological response according to a well-validated, two-dimensional, and fully nonlinear model. We examine thoroughly the effects of the anisotropic mechanical loading on the morphological evolution of the electromigration-driven void surface and the resulting asymptotic states of the surface morphological response. We have found supercritical Hopf-bifurcation transitions from stable steady to stable time-periodic states. For such films and over the range of electromechanical conditions exam...