Search

The mechanical behavior of thin films subjected to laser irradiation is described by a dynamical model that is based on coupled evolution equations for the deformation and vacancy density fields. Lattice vacancies are generated in a thin layer as a result of shallow absorption of electromagnetic laser radiation. The strain field associated with lattice dilatation due to vacancies is shown to couple with bending and stretching mechanical deformation fields. The dynamical model developed here is an extension of the work of Emel'yanov in two respects: (1) the coupling between the diffusion and mechanical deformation fields is rigorously developed with additional cross-field contributions; (2) new equations for reduced dynamics are derived from this model, and are used to analyze the physical conditions for the onset of a deformational instability. For a given material, the threshold for this instability is correlated mainly with laser power. We also show that, although the instability threshold and critical wavelengths are given by the linear part of the dynamics, the selection and type of deformation patterns induced by this instability require a nonlinear formulation. Both numerical and analytical analysis are performed here. According to the relative importance of nonlinearities arising from the defect or from the bending dynamics, square or hexagonal planforms are shown to be selected. Furthermore, it appears that one-dimensional gratings are always unstable in isotropic systems. The results for square patterns are consistent with experimental observations, while those for hexagonal and one-dimensional gratings show the importance of anisotropies on their final selection.

We investigate here the evolution of self-organized monolayer atomic clusters on atomically flat substrates during epitaxial deposition. A phase field-model is developed for the free energy of the system, which includes short-range as well as long-range interactions between deposited atom clusters mediated by the substrate, using the elastic theory of intrinsic surface stress. Natural self-organization of surface atomic clusters is shown to result from reaction-diffusion kinetics, where patterns are either dots or stripes, at low (high) and intermediate coverage, respectively. Cluster-cluster interactions slightly favor stripes. The length scale of natural self-organized structures is in the tens of nanometers range. Imposition of a substrate periodic strain field by subsurface interfacial dislocations is shown to dramatically change the self-organized pattern and its length scale. Qualitative agreements between model predictions and experimental observations on self-organized Ge quantum dots on Si substrate are demonstrated. © 2007 The American Physical Society.

We analyze the formation and selection of self-organized defect microstructure in irradiated materials within the framework of a kinetic model for point and clustered defects. We take explicitly into account the influence of glissile interstitial clusters on the stability and morphology of ordered microstructure. Under void growth conditions, we find that the anisotropic motion of interstitial clusters provides a key element for microstructure morphology selection. In particular, it results in the formation of the void lattice in parallel orientation with the underlying crystal structure, in agreement with experimental observations. We also find that bcc and fcc void lattices develop in bcc and fcc crystals, respectively, while in hcp crystals, voids form ordered arrays parallel to basal planes. It is also predicted that a fcc void lattice is unstable, explaining the experimental difficulty for void lattice formation in fcc crystals.

The effect of crystal structure on laser induced deformation patterns in thin films and surfaces is analyzed within the framework of a dynamical model for the coupled evolution of defect densities and deformation fields. In crystals with covalent bonding, such as Si and SiC, preferential bond breaking may occur, as a result of the relative orientation of the laser electric field and crystallographic axes. We extend here our theoretical framework to incorporate the effects of anisotropic defect diffusion, and the influence of film-substrate adhesion on deformation pattern selection and stability of thin films subjected to laser beams. We also compare theoretical predictions to experimental observations on single crystal silicon wafer surfaces. Furthermore, it is predicted that the laser induced damage threshold for SiC single crystals can be in excess of 200 J/cm 2 .

The formation of inhomogeneous distributions of vacancy loops in irradiated materials is discussed in the framework of a dynamical model based on the rate theory of radiation damage. Dislocation structures are associated with dynamical instabilities due to the competition between defect motion and interactions. The dependence of the critical wavelength of the microstructures on material variables, such as the displacement-damage rate, network-dislocation density, or temperature, is obtained. The postbifurcation analysis is performed in the weakly nonlinear regime, where the selection and stability properties of three-dimensional structures are investigated.

SCOPUS: ar.j, info:eu-repo/semantics/published